This part of the reference documentation covers the Spring Framework’s integration with a number of Java EE (and related) technologies.

1. Remoting and web services using Spring

1.1. Introduction

Spring features integration classes for remoting support using various technologies. The remoting support eases the development of remote-enabled services, implemented by your usual (Spring) POJOs. Currently, Spring supports the following remoting technologies:

  • Remote Method Invocation (RMI). Through the use of the RmiProxyFactoryBean and the RmiServiceExporter Spring supports both traditional RMI (with java.rmi.Remote interfaces and java.rmi.RemoteException) and transparent remoting via RMI invokers (with any Java interface).

  • Spring’s HTTP invoker. Spring provides a special remoting strategy which allows for Java serialization via HTTP, supporting any Java interface (just like the RMI invoker). The corresponding support classes are HttpInvokerProxyFactoryBean and HttpInvokerServiceExporter.

  • Hessian. By using Spring’s HessianProxyFactoryBean and the HessianServiceExporter you can transparently expose your services using the lightweight binary HTTP-based protocol provided by Caucho.

  • JAX-WS. Spring provides remoting support for web services via JAX-WS (the successor of JAX-RPC, as introduced in Java EE 5 and Java 6).

  • JMS. Remoting using JMS as the underlying protocol is supported via the JmsInvokerServiceExporter and JmsInvokerProxyFactoryBean classes.

  • AMQP. Remoting using AMQP as the underlying protocol is supported by the Spring AMQP project.

While discussing the remoting capabilities of Spring, we’ll use the following domain model and corresponding services:

public class Account implements Serializable{

        private String name;

        public String getName(){
                return name;
        }

        public void setName(String name) {
                this.name = name;
        }

}
public interface AccountService {

        public void insertAccount(Account account);

        public List<Account> getAccounts(String name);

}
// the implementation doing nothing at the moment
public class AccountServiceImpl implements AccountService {

        public void insertAccount(Account acc) {
                // do something...
        }

        public List<Account> getAccounts(String name) {
                // do something...
        }

}

We will start exposing the service to a remote client by using RMI and talk a bit about the drawbacks of using RMI. We’ll then continue to show an example using Hessian as the protocol.

1.2. Exposing services using RMI

Using Spring’s support for RMI, you can transparently expose your services through the RMI infrastructure. After having this set up, you basically have a configuration similar to remote EJBs, except for the fact that there is no standard support for security context propagation or remote transaction propagation. Spring does provide hooks for such additional invocation context when using the RMI invoker, so you can for example plug in security frameworks or custom security credentials here.

1.2.1. Exporting the service using the RmiServiceExporter

Using the RmiServiceExporter, we can expose the interface of our AccountService object as RMI object. The interface can be accessed by using RmiProxyFactoryBean, or via plain RMI in case of a traditional RMI service. The RmiServiceExporter explicitly supports the exposing of any non-RMI services via RMI invokers.

Of course, we first have to set up our service in the Spring container:

<bean id="accountService" class="example.AccountServiceImpl">
        <!-- any additional properties, maybe a DAO? -->
</bean>

Next we’ll have to expose our service using the RmiServiceExporter:

<bean class="org.springframework.remoting.rmi.RmiServiceExporter">
        <!-- does not necessarily have to be the same name as the bean to be exported -->
        <property name="serviceName" value="AccountService"/>
        <property name="service" ref="accountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
        <!-- defaults to 1099 -->
        <property name="registryPort" value="1199"/>
</bean>

As you can see, we’re overriding the port for the RMI registry. Often, your application server also maintains an RMI registry and it is wise to not interfere with that one. Furthermore, the service name is used to bind the service under. So right now, the service will be bound at 'rmi://HOST:1199/AccountService'. We’ll use the URL later on to link in the service at the client side.

The servicePort property has been omitted (it defaults to 0). This means that an anonymous port will be used to communicate with the service.

1.2.2. Linking in the service at the client

Our client is a simple object using the AccountService to manage accounts:

public class SimpleObject {

        private AccountService accountService;

        public void setAccountService(AccountService accountService) {
                this.accountService = accountService;
        }

        // additional methods using the accountService

}

To link in the service on the client, we’ll create a separate Spring container, containing the simple object and the service linking configuration bits:

<bean class="example.SimpleObject">
        <property name="accountService" ref="accountService"/>
</bean>

<bean id="accountService" class="org.springframework.remoting.rmi.RmiProxyFactoryBean">
        <property name="serviceUrl" value="rmi://HOST:1199/AccountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

That’s all we need to do to support the remote account service on the client. Spring will transparently create an invoker and remotely enable the account service through the RmiServiceExporter. At the client we’re linking it in using the RmiProxyFactoryBean.

1.3. Using Hessian to remotely call services via HTTP

Hessian offers a binary HTTP-based remoting protocol. It is developed by Caucho and more information about Hessian itself can be found at http://www.caucho.com.

1.3.1. Wiring up the DispatcherServlet for Hessian and co.

Hessian communicates via HTTP and does so using a custom servlet. Using Spring’s DispatcherServlet principles, as known from Spring Web MVC usage, you can easily wire up such a servlet exposing your services. First we’ll have to create a new servlet in your application (this is an excerpt from 'web.xml'):

<servlet>
        <servlet-name>remoting</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
        <servlet-name>remoting</servlet-name>
        <url-pattern>/remoting/*</url-pattern>
</servlet-mapping>

You’re probably familiar with Spring’s DispatcherServlet principles and if so, you know that now you’ll have to create a Spring container configuration resource named 'remoting-servlet.xml' (after the name of your servlet) in the 'WEB-INF' directory. The application context will be used in the next section.

Alternatively, consider the use of Spring’s simpler HttpRequestHandlerServlet. This allows you to embed the remote exporter definitions in your root application context (by default in 'WEB-INF/applicationContext.xml'), with individual servlet definitions pointing to specific exporter beans. Each servlet name needs to match the bean name of its target exporter in this case.

1.3.2. Exposing your beans by using the HessianServiceExporter

In the newly created application context called remoting-servlet.xml, we’ll create a HessianServiceExporter exporting your services:

<bean id="accountService" class="example.AccountServiceImpl">
        <!-- any additional properties, maybe a DAO? -->
</bean>

<bean name="/AccountService" class="org.springframework.remoting.caucho.HessianServiceExporter">
        <property name="service" ref="accountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

Now we’re ready to link in the service at the client. No explicit handler mapping is specified, mapping request URLs onto services, so BeanNameUrlHandlerMapping will be used: Hence, the service will be exported at the URL indicated through its bean name within the containing DispatcherServlet's mapping (as defined above): 'http://HOST:8080/remoting/AccountService'.

Alternatively, create a HessianServiceExporter in your root application context (e.g. in 'WEB-INF/applicationContext.xml'):

<bean name="accountExporter" class="org.springframework.remoting.caucho.HessianServiceExporter">
        <property name="service" ref="accountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

In the latter case, define a corresponding servlet for this exporter in 'web.xml', with the same end result: The exporter getting mapped to the request path /remoting/AccountService. Note that the servlet name needs to match the bean name of the target exporter.

<servlet>
        <servlet-name>accountExporter</servlet-name>
        <servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class>
</servlet>

<servlet-mapping>
        <servlet-name>accountExporter</servlet-name>
        <url-pattern>/remoting/AccountService</url-pattern>
</servlet-mapping>

1.3.3. Linking in the service on the client

Using the HessianProxyFactoryBean we can link in the service at the client. The same principles apply as with the RMI example. We’ll create a separate bean factory or application context and mention the following beans where the SimpleObject is using the AccountService to manage accounts:

<bean class="example.SimpleObject">
        <property name="accountService" ref="accountService"/>
</bean>

<bean id="accountService" class="org.springframework.remoting.caucho.HessianProxyFactoryBean">
        <property name="serviceUrl" value="http://remotehost:8080/remoting/AccountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

1.3.4. Applying HTTP basic authentication to a service exposed through Hessian

One of the advantages of Hessian is that we can easily apply HTTP basic authentication, because both protocols are HTTP-based. Your normal HTTP server security mechanism can easily be applied through using the web.xml security features, for example. Usually, you don’t use per-user security credentials here, but rather shared credentials defined at the HessianProxyFactoryBean level (similar to a JDBC DataSource).

<bean class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping">
        <property name="interceptors" ref="authorizationInterceptor"/>
</bean>

<bean id="authorizationInterceptor"
                class="org.springframework.web.servlet.handler.UserRoleAuthorizationInterceptor">
        <property name="authorizedRoles" value="administrator,operator"/>
</bean>

This is an example where we explicitly mention the BeanNameUrlHandlerMapping and set an interceptor allowing only administrators and operators to call the beans mentioned in this application context.

Of course, this example doesn’t show a flexible kind of security infrastructure. For more options as far as security is concerned, have a look at the Spring Security project at http://projects.spring.io/spring-security/.

1.4. Exposing services using HTTP invokers

As opposed to Hessian, which are both lightweight protocols using their own slim serialization mechanisms, Spring HTTP invokers use the standard Java serialization mechanism to expose services through HTTP. This has a huge advantage if your arguments and return types are complex types that cannot be serialized using the serialization mechanisms Hessian uses (refer to the next section for more considerations when choosing a remoting technology).

Under the hood, Spring uses either the standard facilities provided by the JDK or Apache HttpComponents to perform HTTP calls. Use the latter if you need more advanced and easier-to-use functionality. Refer to hc.apache.org/httpcomponents-client-ga/ for more information.

Be aware of vulnerabilities due to unsafe Java deserialization: Manipulated input streams could lead to unwanted code execution on the server during the deserialization step. As a consequence, do not expose HTTP invoker endpoints to untrusted clients but rather just between your own services. In general, we strongly recommend any other message format (e.g. JSON) instead.

If you are concerned about security vulnerabilities due to Java serialization, consider the general-purpose serialization filter mechanism at the core JVM level, originally developed for JDK 9 but backported to JDK 8, 7 and 6 in the meantime: https://blogs.oracle.com/java-platform-group/entry/incoming_filter_serialization_data_a http://openjdk.java.net/jeps/290

1.4.1. Exposing the service object

Setting up the HTTP invoker infrastructure for a service object resembles closely the way you would do the same using Hessian. Just as Hessian support provides the HessianServiceExporter, Spring’s HttpInvoker support provides the org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter.

To expose the AccountService (mentioned above) within a Spring Web MVC DispatcherServlet, the following configuration needs to be in place in the dispatcher’s application context:

<bean name="/AccountService" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter">
        <property name="service" ref="accountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

Such an exporter definition will be exposed through the DispatcherServlet's standard mapping facilities, as explained in the section on Hessian.

Alternatively, create an HttpInvokerServiceExporter in your root application context (e.g. in 'WEB-INF/applicationContext.xml'):

<bean name="accountExporter" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter">
        <property name="service" ref="accountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

In addition, define a corresponding servlet for this exporter in 'web.xml', with the servlet name matching the bean name of the target exporter:

<servlet>
        <servlet-name>accountExporter</servlet-name>
        <servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class>
</servlet>

<servlet-mapping>
        <servlet-name>accountExporter</servlet-name>
        <url-pattern>/remoting/AccountService</url-pattern>
</servlet-mapping>

If you are running outside of a servlet container and are using Oracle’s Java 6, then you can use the built-in HTTP server implementation. You can configure the SimpleHttpServerFactoryBean together with a SimpleHttpInvokerServiceExporter as is shown in this example:

<bean name="accountExporter"
                class="org.springframework.remoting.httpinvoker.SimpleHttpInvokerServiceExporter">
        <property name="service" ref="accountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

<bean id="httpServer"
                class="org.springframework.remoting.support.SimpleHttpServerFactoryBean">
        <property name="contexts">
                <util:map>
                        <entry key="/remoting/AccountService" value-ref="accountExporter"/>
                </util:map>
        </property>
        <property name="port" value="8080" />
</bean>

1.4.2. Linking in the service at the client

Again, linking in the service from the client much resembles the way you would do it when using Hessian. Using a proxy, Spring will be able to translate your calls to HTTP POST requests to the URL pointing to the exported service.

<bean id="httpInvokerProxy" class="org.springframework.remoting.httpinvoker.HttpInvokerProxyFactoryBean">
        <property name="serviceUrl" value="http://remotehost:8080/remoting/AccountService"/>
        <property name="serviceInterface" value="example.AccountService"/>
</bean>

As mentioned before, you can choose what HTTP client you want to use. By default, the HttpInvokerProxy uses the JDK’s HTTP functionality, but you can also use the Apache HttpComponents client by setting the httpInvokerRequestExecutor property:

<property name="httpInvokerRequestExecutor">
        <bean class="org.springframework.remoting.httpinvoker.HttpComponentsHttpInvokerRequestExecutor"/>
</property>

1.5. Web services

Spring provides full support for standard Java web services APIs:

  • Exposing web services using JAX-WS

  • Accessing web services using JAX-WS

In addition to stock support for JAX-WS in Spring Core, the Spring portfolio also features Spring Web Services, a solution for contract-first, document-driven web services - highly recommended for building modern, future-proof web services.

1.5.1. Exposing servlet-based web services using JAX-WS

Spring provides a convenient base class for JAX-WS servlet endpoint implementations - SpringBeanAutowiringSupport. To expose our AccountService we extend Spring’s SpringBeanAutowiringSupport class and implement our business logic here, usually delegating the call to the business layer. We’ll simply use Spring’s @Autowired annotation for expressing such dependencies on Spring-managed beans.

/**
 * JAX-WS compliant AccountService implementation that simply delegates
 * to the AccountService implementation in the root web application context.
 *
 * This wrapper class is necessary because JAX-WS requires working with dedicated
 * endpoint classes. If an existing service needs to be exported, a wrapper that
 * extends SpringBeanAutowiringSupport for simple Spring bean autowiring (through
 * the @Autowired annotation) is the simplest JAX-WS compliant way.
 *
 * This is the class registered with the server-side JAX-WS implementation.
 * In the case of a Java EE 5 server, this would simply be defined as a servlet
 * in web.xml, with the server detecting that this is a JAX-WS endpoint and reacting
 * accordingly. The servlet name usually needs to match the specified WS service name.
 *
 * The web service engine manages the lifecycle of instances of this class.
 * Spring bean references will just be wired in here.
 */
import org.springframework.web.context.support.SpringBeanAutowiringSupport;

@WebService(serviceName="AccountService")
public class AccountServiceEndpoint extends SpringBeanAutowiringSupport {

        @Autowired
        private AccountService biz;

        @WebMethod
        public void insertAccount(Account acc) {
                biz.insertAccount(acc);
        }

        @WebMethod
        public Account[] getAccounts(String name) {
                return biz.getAccounts(name);
        }

}

Our AccountServiceEndpoint needs to run in the same web application as the Spring context to allow for access to Spring’s facilities. This is the case by default in Java EE 5 environments, using the standard contract for JAX-WS servlet endpoint deployment. See Java EE 5 web service tutorials for details.

1.5.2. Exporting standalone web services using JAX-WS

The built-in JAX-WS provider that comes with Oracle’s JDK supports exposure of web services using the built-in HTTP server that’s included in the JDK as well. Spring’s SimpleJaxWsServiceExporter detects all @WebService annotated beans in the Spring application context, exporting them through the default JAX-WS server (the JDK HTTP server).

In this scenario, the endpoint instances are defined and managed as Spring beans themselves; they will be registered with the JAX-WS engine but their lifecycle will be up to the Spring application context. This means that Spring functionality like explicit dependency injection may be applied to the endpoint instances. Of course, annotation-driven injection through @Autowired will work as well.

<bean class="org.springframework.remoting.jaxws.SimpleJaxWsServiceExporter">
        <property name="baseAddress" value="http://localhost:8080/"/>
</bean>

<bean id="accountServiceEndpoint" class="example.AccountServiceEndpoint">
        ...
</bean>

...

The AccountServiceEndpoint may derive from Spring’s SpringBeanAutowiringSupport but doesn’t have to since the endpoint is a fully Spring-managed bean here. This means that the endpoint implementation may look like as follows, without any superclass declared - and Spring’s @Autowired configuration annotation still being honored:

@WebService(serviceName="AccountService")
public class AccountServiceEndpoint {

        @Autowired
        private AccountService biz;

        @WebMethod
        public void insertAccount(Account acc) {
                biz.insertAccount(acc);
        }

        @WebMethod
        public List<Account> getAccounts(String name) {
                return biz.getAccounts(name);
        }

}

1.5.3. Exporting web services using the JAX-WS RI’s Spring support

Oracle’s JAX-WS RI, developed as part of the GlassFish project, ships Spring support as part of its JAX-WS Commons project. This allows for defining JAX-WS endpoints as Spring-managed beans, similar to the standalone mode discussed in the previous section - but this time in a Servlet environment. Note that this is not portable in a Java EE 5 environment; it is mainly intended for non-EE environments such as Tomcat, embedding the JAX-WS RI as part of the web application.

The difference to the standard style of exporting servlet-based endpoints is that the lifecycle of the endpoint instances themselves will be managed by Spring here, and that there will be only one JAX-WS servlet defined in web.xml. With the standard Java EE 5 style (as illustrated above), you’ll have one servlet definition per service endpoint, with each endpoint typically delegating to Spring beans (through the use of @Autowired, as shown above).

Check out https://jax-ws-commons.java.net/spring/ for details on setup and usage style.

1.5.4. Accessing web services using JAX-WS

Spring provides two factory beans to create JAX-WS web service proxies, namely LocalJaxWsServiceFactoryBean and JaxWsPortProxyFactoryBean. The former can only return a JAX-WS service class for us to work with. The latter is the full-fledged version that can return a proxy that implements our business service interface. In this example we use the latter to create a proxy for the AccountService endpoint (again):

<bean id="accountWebService" class="org.springframework.remoting.jaxws.JaxWsPortProxyFactoryBean">
        <property name="serviceInterface" value="example.AccountService"/>
        <property name="wsdlDocumentUrl" value="http://localhost:8888/AccountServiceEndpoint?WSDL"/>
        <property name="namespaceUri" value="http://example/"/>
        <property name="serviceName" value="AccountService"/>
        <property name="portName" value="AccountServiceEndpointPort"/>
</bean>

Where serviceInterface is our business interface the clients will use. wsdlDocumentUrl is the URL for the WSDL file. Spring needs this a startup time to create the JAX-WS Service. namespaceUri corresponds to the targetNamespace in the .wsdl file. serviceName corresponds to the service name in the .wsdl file. portName corresponds to the port name in the .wsdl file.

Accessing the web service is now very easy as we have a bean factory for it that will expose it as AccountService interface. We can wire this up in Spring:

<bean id="client" class="example.AccountClientImpl">
        ...
        <property name="service" ref="accountWebService"/>
</bean>

From the client code we can access the web service just as if it was a normal class:

public class AccountClientImpl {

        private AccountService service;

        public void setService(AccountService service) {
                this.service = service;
        }

        public void foo() {
                service.insertAccount(...);
        }
}

The above is slightly simplified in that JAX-WS requires endpoint interfaces and implementation classes to be annotated with @WebService, @SOAPBinding etc annotations. This means that you cannot (easily) use plain Java interfaces and implementation classes as JAX-WS endpoint artifacts; you need to annotate them accordingly first. Check the JAX-WS documentation for details on those requirements.

1.6. JMS

It is also possible to expose services transparently using JMS as the underlying communication protocol. The JMS remoting support in the Spring Framework is pretty basic - it sends and receives on the same thread and in the same non-transactional Session, and as such throughput will be very implementation dependent. Note that these single-threaded and non-transactional constraints apply only to Spring’s JMS remoting support. See JMS (Java Message Service) for information on Spring’s rich support for JMS-based messaging.

The following interface is used on both the server and the client side.

package com.foo;

public interface CheckingAccountService {

        public void cancelAccount(Long accountId);

}

The following simple implementation of the above interface is used on the server-side.

package com.foo;

public class SimpleCheckingAccountService implements CheckingAccountService {

        public void cancelAccount(Long accountId) {
                System.out.println("Cancelling account [" + accountId + "]");
        }

}

This configuration file contains the JMS-infrastructure beans that are shared on both the client and server.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.springframework.org/schema/beans
                http://www.springframework.org/schema/beans/spring-beans.xsd">

        <bean id="connectionFactory" class="org.apache.activemq.ActiveMQConnectionFactory">
                <property name="brokerURL" value="tcp://ep-t43:61616"/>
        </bean>

        <bean id="queue" class="org.apache.activemq.command.ActiveMQQueue">
                <constructor-arg value="mmm"/>
        </bean>

</beans>

1.6.1. Server-side configuration

On the server, you just need to expose the service object using the JmsInvokerServiceExporter.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.springframework.org/schema/beans
                http://www.springframework.org/schema/beans/spring-beans.xsd">

        <bean id="checkingAccountService"
                        class="org.springframework.jms.remoting.JmsInvokerServiceExporter">
                <property name="serviceInterface" value="com.foo.CheckingAccountService"/>
                <property name="service">
                        <bean class="com.foo.SimpleCheckingAccountService"/>
                </property>
        </bean>

        <bean class="org.springframework.jms.listener.SimpleMessageListenerContainer">
                <property name="connectionFactory" ref="connectionFactory"/>
                <property name="destination" ref="queue"/>
                <property name="concurrentConsumers" value="3"/>
                <property name="messageListener" ref="checkingAccountService"/>
        </bean>

</beans>
package com.foo;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Server {

        public static void main(String[] args) throws Exception {
                new ClassPathXmlApplicationContext(new String[]{"com/foo/server.xml", "com/foo/jms.xml"});
        }

}

1.6.2. Client-side configuration

The client merely needs to create a client-side proxy that will implement the agreed upon interface ( CheckingAccountService). The resulting object created off the back of the following bean definition can be injected into other client side objects, and the proxy will take care of forwarding the call to the server-side object via JMS.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.springframework.org/schema/beans
                http://www.springframework.org/schema/beans/spring-beans.xsd">

        <bean id="checkingAccountService"
                        class="org.springframework.jms.remoting.JmsInvokerProxyFactoryBean">
                <property name="serviceInterface" value="com.foo.CheckingAccountService"/>
                <property name="connectionFactory" ref="connectionFactory"/>
                <property name="queue" ref="queue"/>
        </bean>

</beans>
package com.foo;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Client {

        public static void main(String[] args) throws Exception {
                ApplicationContext ctx = new ClassPathXmlApplicationContext(
                                new String[] {"com/foo/client.xml", "com/foo/jms.xml"});
                CheckingAccountService service = (CheckingAccountService) ctx.getBean("checkingAccountService");
                service.cancelAccount(new Long(10));
        }

}

1.7. AMQP

1.8. Auto-detection is not implemented for remote interfaces

The main reason why auto-detection of implemented interfaces does not occur for remote interfaces is to avoid opening too many doors to remote callers. The target object might implement internal callback interfaces like InitializingBean or DisposableBean which one would not want to expose to callers.

Offering a proxy with all interfaces implemented by the target usually does not matter in the local case. But when exporting a remote service, you should expose a specific service interface, with specific operations intended for remote usage. Besides internal callback interfaces, the target might implement multiple business interfaces, with just one of them intended for remote exposure. For these reasons, we require such a service interface to be specified.

This is a trade-off between configuration convenience and the risk of accidental exposure of internal methods. Always specifying a service interface is not too much effort, and puts you on the safe side regarding controlled exposure of specific methods.

1.9. Considerations when choosing a technology

Each and every technology presented here has its drawbacks. You should carefully consider your needs, the services you are exposing and the objects you’ll be sending over the wire when choosing a technology.

When using RMI, it’s not possible to access the objects through the HTTP protocol, unless you’re tunneling the RMI traffic. RMI is a fairly heavy-weight protocol in that it supports full-object serialization which is important when using a complex data model that needs serialization over the wire. However, RMI-JRMP is tied to Java clients: It is a Java-to-Java remoting solution.

Spring’s HTTP invoker is a good choice if you need HTTP-based remoting but also rely on Java serialization. It shares the basic infrastructure with RMI invokers, just using HTTP as transport. Note that HTTP invokers are not only limited to Java-to-Java remoting but also to Spring on both the client and server side. (The latter also applies to Spring’s RMI invoker for non-RMI interfaces.)

Hessian might provide significant value when operating in a heterogeneous environment, because they explicitly allow for non-Java clients. However, non-Java support is still limited. Known issues include the serialization of Hibernate objects in combination with lazily-initialized collections. If you have such a data model, consider using RMI or HTTP invokers instead of Hessian.

JMS can be useful for providing clusters of services and allowing the JMS broker to take care of load balancing, discovery and auto-failover. By default: Java serialization is used when using JMS remoting but the JMS provider could use a different mechanism for the wire formatting, such as XStream to allow servers to be implemented in other technologies.

Last but not least, EJB has an advantage over RMI in that it supports standard role-based authentication and authorization and remote transaction propagation. It is possible to get RMI invokers or HTTP invokers to support security context propagation as well, although this is not provided by core Spring: There are just appropriate hooks for plugging in third-party or custom solutions here.

1.10. Accessing REST endpoints

The Spring Framework has two choices for client-side access to REST endpoints:

  • RestTemplate — the original Spring REST client with an API similar to other template classes in Spring such as JdbcTemplate, JmsTemplate and others. RestTemplate has a synchronous API and relies on blocking I/O which is okay for client scenarios with low concurrency.

  • WebClient — reactive client with a functional, fluent API from the spring-webflux module. It relies on non-blocking I/O which allows it to support high concurrency more efficiently (i.e. using a small number of threads) than the RestTemplate. WebClient is a natural fit for streaming scenarios.

1.10.1. RestTemplate

The RestTemplate provides a higher level API over HTTP client libraries with methods that correspond to each of the six main HTTP methods that make invoking many RESTful services a one-liner and enforce REST best practices.

Table 1. Overview of RestTemplate methods
HTTP Method RestTemplate Method

DELETE

delete

GET

getForObject getForEntity

HEAD

headForHeaders(String url, String…​ uriVariables)

OPTIONS

optionsForAllow(String url, String…​ uriVariables)

POST

postForLocation(String url, Object request, String…​ uriVariables) postForObject(String url, Object request, Class<T> responseType, String…​ uriVariables)

PUT

put(String url, Object request, String…​uriVariables)

PATCH and others

exchange execute

The names of RestTemplate methods follow a naming convention, the first part indicates what HTTP method is being invoked and the second part indicates what is returned. For example, the method getForObject() will perform a GET, convert the HTTP response into an object type of your choice and return that object. The method postForLocation() will do a POST, converting the given object into a HTTP request and return the response HTTP Location header where the newly created object can be found. In case of an exception processing the HTTP request, an exception of the type RestClientException will be thrown; this behavior can be changed by plugging in another ResponseErrorHandler implementation into the RestTemplate.

The exchange and execute methods are generalized versions of the more specific methods listed above them and can support additional combinations and methods, like HTTP PATCH. However, note that the underlying HTTP library must also support the desired combination. The JDK HttpURLConnection does not support the PATCH method, but Apache HttpComponents HttpClient version 4.2 or later does. They also enable RestTemplate to read an HTTP response to a generic type (e.g. List<Account>), using a ParameterizedTypeReference, a new class that enables capturing and passing generic type info.

Objects passed to and returned from these methods are converted to and from HTTP messages by HttpMessageConverter instances. Converters for the main mime types are registered by default, but you can also write your own converter and register it via the messageConverters() bean property. The default converter instances registered with the template are ByteArrayHttpMessageConverter, StringHttpMessageConverter, FormHttpMessageConverter and SourceHttpMessageConverter. You can override these defaults using the messageConverters() bean property as would be required if using the MarshallingHttpMessageConverter or MappingJackson2HttpMessageConverter.

Each method takes URI template arguments in two forms, either as a String variable-length argument or a Map<String,String>. For example,

String result = restTemplate.getForObject(
                "http://example.com/hotels/{hotel}/bookings/{booking}", String.class,"42", "21");

using variable-length arguments and

Map<String, String> vars = Collections.singletonMap("hotel", "42");
String result = restTemplate.getForObject(
                "http://example.com/hotels/{hotel}/rooms/{hotel}", String.class, vars);

using a Map<String,String>.

To create an instance of RestTemplate you can simply call the default no-arg constructor. This will use standard Java classes from the java.net package as the underlying implementation to create HTTP requests. This can be overridden by specifying an implementation of ClientHttpRequestFactory. Spring provides the implementation HttpComponentsClientHttpRequestFactory that uses the Apache HttpComponents HttpClient to create requests. HttpComponentsClientHttpRequestFactory is configured using an instance of org.apache.http.client.HttpClient which can in turn be configured with credentials information or connection pooling functionality.

Note that the java.net implementation for HTTP requests may raise an exception when accessing the status of a response that represents an error (e.g. 401). If this is an issue, switch to HttpComponentsClientHttpRequestFactory instead.

The previous example using Apache HttpComponents HttpClient directly rewritten to use the RestTemplate is shown below

uri = "http://example.com/hotels/{id}/bookings";

RestTemplate template = new RestTemplate();

Booking booking = // create booking object

URI location = template.postForLocation(uri, booking, "1");

To use Apache HttpComponents instead of the native java.net functionality, construct the RestTemplate as follows:

RestTemplate template = new RestTemplate(new HttpComponentsClientHttpRequestFactory());

Apache HttpClient supports gzip encoding. To use it, construct a HttpComponentsClientHttpRequestFactory like so:

HttpClient httpClient = HttpClientBuilder.create().build();
   ClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);
   RestTemplate restTemplate = new RestTemplate(requestFactory);

The general callback interface is RequestCallback and is called when the execute method is invoked.

public <T> T execute(String url, HttpMethod method, RequestCallback requestCallback,
                ResponseExtractor<T> responseExtractor, String... uriVariables)

// also has an overload with uriVariables as a Map<String, String>.

The RequestCallback interface is defined as

public interface RequestCallback {
 void doWithRequest(ClientHttpRequest request) throws IOException;
}

and allows you to manipulate the request headers and write to the request body. When using the execute method you do not have to worry about any resource management, the template will always close the request and handle any errors. Refer to the API documentation for more information on using the execute method and the meaning of its other method arguments.

Working with the URI

For each of the main HTTP methods, the RestTemplate provides variants that either take a String URI or java.net.URI as the first argument.

The String URI variants accept template arguments as a String variable-length argument or as a Map<String,String>. They also assume the URL String is not encoded and needs to be encoded. For example the following:

restTemplate.getForObject("http://example.com/hotel list", String.class);

will perform a GET on http://example.com/hotel%20list. That means if the input URL String is already encoded, it will be encoded twice — i.e. http://example.com/hotel%20list will become http://example.com/hotel%2520list. If this is not the intended effect, use the java.net.URI method variant, which assumes the URL is already encoded is also generally useful if you want to reuse a single (fully expanded) URI multiple times.

The UriComponentsBuilder class can be used to build and encode the URI including support for URI templates. For example you can start with a URL String:

UriComponents uriComponents = UriComponentsBuilder.fromUriString(
                "http://example.com/hotels/{hotel}/bookings/{booking}").build()
                .expand("42", "21")
                .encode();

URI uri = uriComponents.toUri();

Or specify each URI component individually:

UriComponents uriComponents = UriComponentsBuilder.newInstance()
                .scheme("http").host("example.com").path("/hotels/{hotel}/bookings/{booking}").build()
                .expand("42", "21")
                .encode();

URI uri = uriComponents.toUri();
Dealing with request and response headers

Besides the methods described above, the RestTemplate also has the exchange() method, which can be used for arbitrary HTTP method execution based on the HttpEntity class.

Perhaps most importantly, the exchange() method can be used to add request headers and read response headers. For example:

HttpHeaders requestHeaders = new HttpHeaders();
requestHeaders.set("MyRequestHeader", "MyValue");
HttpEntity<?> requestEntity = new HttpEntity(requestHeaders);

HttpEntity<String> response = template.exchange(
                "http://example.com/hotels/{hotel}",
                HttpMethod.GET, requestEntity, String.class, "42");

String responseHeader = response.getHeaders().getFirst("MyResponseHeader");
String body = response.getBody();

In the above example, we first prepare a request entity that contains the MyRequestHeader header. We then retrieve the response, and read the MyResponseHeader and body.

Jackson JSON Views support

It is possible to specify a Jackson JSON View to serialize only a subset of the object properties. For example:

MappingJacksonValue value = new MappingJacksonValue(new User("eric", "7!jd#h23"));
value.setSerializationView(User.WithoutPasswordView.class);
HttpEntity<MappingJacksonValue> entity = new HttpEntity<MappingJacksonValue>(value);
String s = template.postForObject("http://example.com/user", entity, String.class);

1.10.2. HTTP message conversion

Objects passed to and returned from the methods getForObject(), postForLocation(), and put() are converted to HTTP requests and from HTTP responses by HttpMessageConverters. The HttpMessageConverter interface is shown below to give you a better feel for its functionality

public interface HttpMessageConverter<T> {

        // Indicate whether the given class and media type can be read by this converter.
        boolean canRead(Class<?> clazz, MediaType mediaType);

        // Indicate whether the given class and media type can be written by this converter.
        boolean canWrite(Class<?> clazz, MediaType mediaType);

        // Return the list of MediaType objects supported by this converter.
        List<MediaType> getSupportedMediaTypes();

        // Read an object of the given type from the given input message, and returns it.
        T read(Class<T> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException;

        // Write an given object to the given output message.
        void write(T t, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException;

}

Concrete implementations for the main media (mime) types are provided in the framework and are registered by default with the RestTemplate on the client-side and with RequestMethodHandlerAdapter on the server-side.

The implementations of HttpMessageConverters are described in the following sections. For all converters a default media type is used but can be overridden by setting the supportedMediaTypes bean property

StringHttpMessageConverter

An HttpMessageConverter implementation that can read and write Strings from the HTTP request and response. By default, this converter supports all text media types ( text/*), and writes with a Content-Type of text/plain.

FormHttpMessageConverter

An HttpMessageConverter implementation that can read and write form data from the HTTP request and response. By default, this converter reads and writes the media type application/x-www-form-urlencoded. Form data is read from and written into a MultiValueMap<String, String>.

ByteArrayHttpMessageConverter

An HttpMessageConverter implementation that can read and write byte arrays from the HTTP request and response. By default, this converter supports all media types ( */*), and writes with a Content-Type of application/octet-stream. This can be overridden by setting the supportedMediaTypes property, and overriding getContentType(byte[]).

MarshallingHttpMessageConverter

An HttpMessageConverter implementation that can read and write XML using Spring’s Marshaller and Unmarshaller abstractions from the org.springframework.oxm package. This converter requires a Marshaller and Unmarshaller before it can be used. These can be injected via constructor or bean properties. By default this converter supports ( text/xml) and ( application/xml).

MappingJackson2HttpMessageConverter

An HttpMessageConverter implementation that can read and write JSON using Jackson’s ObjectMapper. JSON mapping can be customized as needed through the use of Jackson’s provided annotations. When further control is needed, a custom ObjectMapper can be injected through the ObjectMapper property for cases where custom JSON serializers/deserializers need to be provided for specific types. By default this converter supports ( application/json).

MappingJackson2XmlHttpMessageConverter

An HttpMessageConverter implementation that can read and write XML using Jackson XML extension’s XmlMapper. XML mapping can be customized as needed through the use of JAXB or Jackson’s provided annotations. When further control is needed, a custom XmlMapper can be injected through the ObjectMapper property for cases where custom XML serializers/deserializers need to be provided for specific types. By default this converter supports ( application/xml).

SourceHttpMessageConverter

An HttpMessageConverter implementation that can read and write javax.xml.transform.Source from the HTTP request and response. Only DOMSource, SAXSource, and StreamSource are supported. By default, this converter supports ( text/xml) and ( application/xml).

BufferedImageHttpMessageConverter

An HttpMessageConverter implementation that can read and write java.awt.image.BufferedImage from the HTTP request and response. This converter reads and writes the media type supported by the Java I/O API.

1.10.3. Async RestTemplate

The AsyncRestTemplate is deprecated. Please use the WebClient instead.

2. Enterprise JavaBeans (EJB) integration

2.1. Introduction

As a lightweight container, Spring is often considered an EJB replacement. We do believe that for many if not most applications and use cases, Spring as a container, combined with its rich supporting functionality in the area of transactions, ORM and JDBC access, is a better choice than implementing equivalent functionality via an EJB container and EJBs.

However, it is important to note that using Spring does not prevent you from using EJBs. In fact, Spring makes it much easier to access EJBs and implement EJBs and functionality within them. Additionally, using Spring to access services provided by EJBs allows the implementation of those services to later transparently be switched between local EJB, remote EJB, or POJO (plain old Java object) variants, without the client code having to be changed.

In this chapter, we look at how Spring can help you access and implement EJBs. Spring provides particular value when accessing stateless session beans (SLSBs), so we’ll begin by discussing this.

2.2. Accessing EJBs

2.2.1. Concepts

To invoke a method on a local or remote stateless session bean, client code must normally perform a JNDI lookup to obtain the (local or remote) EJB Home object, then use a 'create' method call on that object to obtain the actual (local or remote) EJB object. One or more methods are then invoked on the EJB.

To avoid repeated low-level code, many EJB applications use the Service Locator and Business Delegate patterns. These are better than spraying JNDI lookups throughout client code, but their usual implementations have significant disadvantages. For example:

  • Typically code using EJBs depends on Service Locator or Business Delegate singletons, making it hard to test.

  • In the case of the Service Locator pattern used without a Business Delegate, application code still ends up having to invoke the create() method on an EJB home, and deal with the resulting exceptions. Thus it remains tied to the EJB API and the complexity of the EJB programming model.

  • Implementing the Business Delegate pattern typically results in significant code duplication, where we have to write numerous methods that simply call the same method on the EJB.

The Spring approach is to allow the creation and use of proxy objects, normally configured inside a Spring container, which act as codeless business delegates. You do not need to write another Service Locator, another JNDI lookup, or duplicate methods in a hand-coded Business Delegate unless you are actually adding real value in such code.

2.2.2. Accessing local SLSBs

Assume that we have a web controller that needs to use a local EJB. We’ll follow best practice and use the EJB Business Methods Interface pattern, so that the EJB’s local interface extends a non EJB-specific business methods interface. Let’s call this business methods interface MyComponent.

public interface MyComponent {
        ...
}

One of the main reasons to use the Business Methods Interface pattern is to ensure that synchronization between method signatures in local interface and bean implementation class is automatic. Another reason is that it later makes it much easier for us to switch to a POJO (plain old Java object) implementation of the service if it makes sense to do so. Of course we’ll also need to implement the local home interface and provide an implementation class that implements SessionBean and the MyComponent business methods interface. Now the only Java coding we’ll need to do to hook up our web tier controller to the EJB implementation is to expose a setter method of type MyComponent on the controller. This will save the reference as an instance variable in the controller:

private MyComponent myComponent;

public void setMyComponent(MyComponent myComponent) {
        this.myComponent = myComponent;
}

We can subsequently use this instance variable in any business method in the controller. Now assuming we are obtaining our controller object out of a Spring container, we can (in the same context) configure a LocalStatelessSessionProxyFactoryBean instance, which will be the EJB proxy object. The configuration of the proxy, and setting of the myComponent property of the controller is done with a configuration entry such as:

<bean id="myComponent"
                class="org.springframework.ejb.access.LocalStatelessSessionProxyFactoryBean">
        <property name="jndiName" value="ejb/myBean"/>
        <property name="businessInterface" value="com.mycom.MyComponent"/>
</bean>

<bean id="myController" class="com.mycom.myController">
        <property name="myComponent" ref="myComponent"/>
</bean>

There’s a lot of work happening behind the scenes, courtesy of the Spring AOP framework, although you aren’t forced to work with AOP concepts to enjoy the results. The myComponent bean definition creates a proxy for the EJB, which implements the business method interface. The EJB local home is cached on startup, so there’s only a single JNDI lookup. Each time the EJB is invoked, the proxy invokes the classname method on the local EJB and invokes the corresponding business method on the EJB.

The myController bean definition sets the myComponent property of the controller class to the EJB proxy.

Alternatively (and preferably in case of many such proxy definitions), consider using the <jee:local-slsb> configuration element in Spring’s "jee" namespace:

<jee:local-slsb id="myComponent" jndi-name="ejb/myBean"
                business-interface="com.mycom.MyComponent"/>

<bean id="myController" class="com.mycom.myController">
        <property name="myComponent" ref="myComponent"/>
</bean>

This EJB access mechanism delivers huge simplification of application code: the web tier code (or other EJB client code) has no dependence on the use of EJB. If we want to replace this EJB reference with a POJO or a mock object or other test stub, we could simply change the myComponent bean definition without changing a line of Java code. Additionally, we haven’t had to write a single line of JNDI lookup or other EJB plumbing code as part of our application.

Benchmarks and experience in real applications indicate that the performance overhead of this approach (which involves reflective invocation of the target EJB) is minimal, and is typically undetectable in typical use. Remember that we don’t want to make fine-grained calls to EJBs anyway, as there’s a cost associated with the EJB infrastructure in the application server.

There is one caveat with regards to the JNDI lookup. In a bean container, this class is normally best used as a singleton (there simply is no reason to make it a prototype). However, if that bean container pre-instantiates singletons (as do the various XML ApplicationContext variants) you may have a problem if the bean container is loaded before the EJB container loads the target EJB. That is because the JNDI lookup will be performed in the init() method of this class and then cached, but the EJB will not have been bound at the target location yet. The solution is to not pre-instantiate this factory object, but allow it to be created on first use. In the XML containers, this is controlled via the lazy-init attribute.

Although this will not be of interest to the majority of Spring users, those doing programmatic AOP work with EJBs may want to look at LocalSlsbInvokerInterceptor.

2.2.3. Accessing remote SLSBs

Accessing remote EJBs is essentially identical to accessing local EJBs, except that the SimpleRemoteStatelessSessionProxyFactoryBean or <jee:remote-slsb> configuration element is used. Of course, with or without Spring, remote invocation semantics apply; a call to a method on an object in another VM in another computer does sometimes have to be treated differently in terms of usage scenarios and failure handling.

Spring’s EJB client support adds one more advantage over the non-Spring approach. Normally it is problematic for EJB client code to be easily switched back and forth between calling EJBs locally or remotely. This is because the remote interface methods must declare that they throw RemoteException, and client code must deal with this, while the local interface methods don’t. Client code written for local EJBs which needs to be moved to remote EJBs typically has to be modified to add handling for the remote exceptions, and client code written for remote EJBs which needs to be moved to local EJBs, can either stay the same but do a lot of unnecessary handling of remote exceptions, or needs to be modified to remove that code. With the Spring remote EJB proxy, you can instead not declare any thrown RemoteException in your Business Method Interface and implementing EJB code, have a remote interface which is identical except that it does throw RemoteException, and rely on the proxy to dynamically treat the two interfaces as if they were the same. That is, client code does not have to deal with the checked RemoteException class. Any actual RemoteException that is thrown during the EJB invocation will be re-thrown as the non-checked RemoteAccessException class, which is a subclass of RuntimeException. The target service can then be switched at will between a local EJB or remote EJB (or even plain Java object) implementation, without the client code knowing or caring. Of course, this is optional; there is nothing stopping you from declaring RemoteExceptions in your business interface.

2.2.4. Accessing EJB 2.x SLSBs versus EJB 3 SLSBs

Accessing EJB 2.x Session Beans and EJB 3 Session Beans via Spring is largely transparent. Spring’s EJB accessors, including the <jee:local-slsb> and <jee:remote-slsb> facilities, transparently adapt to the actual component at runtime. They handle a home interface if found (EJB 2.x style), or perform straight component invocations if no home interface is available (EJB 3 style).

Note: For EJB 3 Session Beans, you could effectively use a JndiObjectFactoryBean / <jee:jndi-lookup> as well, since fully usable component references are exposed for plain JNDI lookups there. Defining explicit <jee:local-slsb> / <jee:remote-slsb> lookups simply provides consistent and more explicit EJB access configuration.

3. JMS (Java Message Service)

3.1. Introduction

Spring provides a JMS integration framework that simplifies the use of the JMS API much like Spring’s integration does for the JDBC API.

JMS can be roughly divided into two areas of functionality, namely the production and consumption of messages. The JmsTemplate class is used for message production and synchronous message reception. For asynchronous reception similar to Java EE’s message-driven bean style, Spring provides a number of message listener containers that are used to create Message-Driven POJOs (MDPs). Spring also provides a declarative way of creating message listeners.

The package org.springframework.jms.core provides the core functionality for using JMS. It contains JMS template classes that simplify the use of the JMS by handling the creation and release of resources, much like the JdbcTemplate does for JDBC. The design principle common to Spring template classes is to provide helper methods to perform common operations and for more sophisticated usage, delegate the essence of the processing task to user implemented callback interfaces. The JMS template follows the same design. The classes offer various convenience methods for the sending of messages, consuming a message synchronously, and exposing the JMS session and message producer to the user.

The package org.springframework.jms.support provides JMSException translation functionality. The translation converts the checked JMSException hierarchy to a mirrored hierarchy of unchecked exceptions. If there are any provider specific subclasses of the checked javax.jms.JMSException, this exception is wrapped in the unchecked UncategorizedJmsException.

The package org.springframework.jms.support.converter provides a MessageConverter abstraction to convert between Java objects and JMS messages.

The package org.springframework.jms.support.destination provides various strategies for managing JMS destinations, such as providing a service locator for destinations stored in JNDI.

The package org.springframework.jms.annotation provides the necessary infrastructure to support annotation-driven listener endpoints using @JmsListener.

The package org.springframework.jms.config provides the parser implementation for the jms namespace as well the java config support to configure listener containers and create listener endpoints.

Finally, the package org.springframework.jms.connection provides an implementation of the ConnectionFactory suitable for use in standalone applications. It also contains an implementation of Spring’s PlatformTransactionManager for JMS (the cunningly named JmsTransactionManager). This allows for seamless integration of JMS as a transactional resource into Spring’s transaction management mechanisms.

3.2. Using Spring JMS

3.2.1. JmsTemplate

The JmsTemplate class is the central class in the JMS core package. It simplifies the use of JMS since it handles the creation and release of resources when sending or synchronously receiving messages.

Code that uses the JmsTemplate only needs to implement callback interfaces giving them a clearly defined high level contract. The MessageCreator callback interface creates a message given a Session provided by the calling code in JmsTemplate. In order to allow for more complex usage of the JMS API, the callback SessionCallback provides the user with the JMS session and the callback ProducerCallback exposes a Session and MessageProducer pair.

The JMS API exposes two types of send methods, one that takes delivery mode, priority, and time-to-live as Quality of Service (QOS) parameters and one that takes no QOS parameters which uses default values. Since there are many send methods in JmsTemplate, the setting of the QOS parameters have been exposed as bean properties to avoid duplication in the number of send methods. Similarly, the timeout value for synchronous receive calls is set using the property setReceiveTimeout.

Some JMS providers allow the setting of default QOS values administratively through the configuration of the ConnectionFactory. This has the effect that a call to MessageProducer's send method send(Destination destination, Message message) will use different QOS default values than those specified in the JMS specification. In order to provide consistent management of QOS values, the JmsTemplate must therefore be specifically enabled to use its own QOS values by setting the boolean property isExplicitQosEnabled to true.

For convenience, JmsTemplate also exposes a basic request-reply operation that allows to send a message and wait for a reply on a temporary queue that is created as part of the operation.

Instances of the JmsTemplate class are thread-safe once configured. This is important because it means that you can configure a single instance of a JmsTemplate and then safely inject this shared reference into multiple collaborators. To be clear, the JmsTemplate is stateful, in that it maintains a reference to a ConnectionFactory, but this state is not conversational state.

As of Spring Framework 4.1, JmsMessagingTemplate is built on top of JmsTemplate and provides an integration with the messaging abstraction, i.e. org.springframework.messaging.Message. This allows you to create the message to send in generic manner.

3.2.2. Connections

The JmsTemplate requires a reference to a ConnectionFactory. The ConnectionFactory is part of the JMS specification and serves as the entry point for working with JMS. It is used by the client application as a factory to create connections with the JMS provider and encapsulates various configuration parameters, many of which are vendor specific such as SSL configuration options.

When using JMS inside an EJB, the vendor provides implementations of the JMS interfaces so that they can participate in declarative transaction management and perform pooling of connections and sessions. In order to use this implementation, Java EE containers typically require that you declare a JMS connection factory as a resource-ref inside the EJB or servlet deployment descriptors. To ensure the use of these features with the JmsTemplate inside an EJB, the client application should ensure that it references the managed implementation of the ConnectionFactory.

Caching Messaging Resources

The standard API involves creating many intermediate objects. To send a message the following 'API' walk is performed

ConnectionFactory->Connection->Session->MessageProducer->send

Between the ConnectionFactory and the Send operation there are three intermediate objects that are created and destroyed. To optimise the resource usage and increase performance two implementations of ConnectionFactory are provided.

SingleConnectionFactory

Spring provides an implementation of the ConnectionFactory interface, SingleConnectionFactory, that will return the same Connection on all createConnection() calls and ignore calls to close(). This is useful for testing and standalone environments so that the same connection can be used for multiple JmsTemplate calls that may span any number of transactions. SingleConnectionFactory takes a reference to a standard ConnectionFactory that would typically come from JNDI.

CachingConnectionFactory

The CachingConnectionFactory extends the functionality of SingleConnectionFactory and adds the caching of Sessions, MessageProducers, and MessageConsumers. The initial cache size is set to 1, use the property sessionCacheSize to increase the number of cached sessions. Note that the number of actual cached sessions will be more than that number as sessions are cached based on their acknowledgment mode, so there can be up to 4 cached session instances when sessionCacheSize is set to one, one for each acknowledgment mode. MessageProducers and MessageConsumers are cached within their owning session and also take into account the unique properties of the producers and consumers when caching. MessageProducers are cached based on their destination. MessageConsumers are cached based on a key composed of the destination, selector, noLocal delivery flag, and the durable subscription name (if creating durable consumers).

3.2.3. Destination Management

Destinations, like ConnectionFactories, are JMS administered objects that can be stored and retrieved in JNDI. When configuring a Spring application context you can use the JNDI factory class JndiObjectFactoryBean / <jee:jndi-lookup> to perform dependency injection on your object’s references to JMS destinations. However, often this strategy is cumbersome if there are a large number of destinations in the application or if there are advanced destination management features unique to the JMS provider. Examples of such advanced destination management would be the creation of dynamic destinations or support for a hierarchical namespace of destinations. The JmsTemplate delegates the resolution of a destination name to a JMS destination object to an implementation of the interface DestinationResolver. DynamicDestinationResolver is the default implementation used by JmsTemplate and accommodates resolving dynamic destinations. A JndiDestinationResolver is also provided that acts as a service locator for destinations contained in JNDI and optionally falls back to the behavior contained in DynamicDestinationResolver.

Quite often the destinations used in a JMS application are only known at runtime and therefore cannot be administratively created when the application is deployed. This is often because there is shared application logic between interacting system components that create destinations at runtime according to a well-known naming convention. Even though the creation of dynamic destinations is not part of the JMS specification, most vendors have provided this functionality. Dynamic destinations are created with a name defined by the user which differentiates them from temporary destinations and are often not registered in JNDI. The API used to create dynamic destinations varies from provider to provider since the properties associated with the destination are vendor specific. However, a simple implementation choice that is sometimes made by vendors is to disregard the warnings in the JMS specification and to use the TopicSession method createTopic(String topicName) or the QueueSession method createQueue(String queueName) to create a new destination with default destination properties. Depending on the vendor implementation, DynamicDestinationResolver may then also create a physical destination instead of only resolving one.

The boolean property pubSubDomain is used to configure the JmsTemplate with knowledge of what JMS domain is being used. By default the value of this property is false, indicating that the point-to-point domain, Queues, will be used. This property used by JmsTemplate determines the behavior of dynamic destination resolution via implementations of the DestinationResolver interface.

You can also configure the JmsTemplate with a default destination via the property defaultDestination. The default destination will be used with send and receive operations that do not refer to a specific destination.

3.2.4. Message Listener Containers

One of the most common uses of JMS messages in the EJB world is to drive message-driven beans (MDBs). Spring offers a solution to create message-driven POJOs (MDPs) in a way that does not tie a user to an EJB container. (See Asynchronous reception: Message-Driven POJOs for detailed coverage of Spring’s MDP support.) As from Spring Framework 4.1, endpoint methods can be simply annotated using @JmsListener see Annotation-driven listener endpoints for more details.

A message listener container is used to receive messages from a JMS message queue and drive the MessageListener that is injected into it. The listener container is responsible for all threading of message reception and dispatches into the listener for processing. A message listener container is the intermediary between an MDP and a messaging provider, and takes care of registering to receive messages, participating in transactions, resource acquisition and release, exception conversion and suchlike. This allows you as an application developer to write the (possibly complex) business logic associated with receiving a message (and possibly responding to it), and delegates boilerplate JMS infrastructure concerns to the framework.

There are two standard JMS message listener containers packaged with Spring, each with its specialised feature set.

SimpleMessageListenerContainer

This message listener container is the simpler of the two standard flavors. It creates a fixed number of JMS sessions and consumers at startup, registers the listener using the standard JMS MessageConsumer.setMessageListener() method, and leaves it up the JMS provider to perform listener callbacks. This variant does not allow for dynamic adaption to runtime demands or for participation in externally managed transactions. Compatibility-wise, it stays very close to the spirit of the standalone JMS specification - but is generally not compatible with Java EE’s JMS restrictions.

While SimpleMessageListenerContainer does not allow for the participation in externally managed transactions, it does support native JMS transactions: simply switch the 'sessionTransacted' flag to 'true' or, in the namespace, set the 'acknowledge' attribute to 'transacted': Exceptions thrown from your listener will lead to a rollback then, with the message getting redelivered. Alternatively, consider using 'CLIENT_ACKNOWLEDGE' mode which provides redelivery in case of an exception as well but does not use transacted Sessions and therefore does not include any other Session operations (such as sending response messages) in the transaction protocol.

DefaultMessageListenerContainer

This message listener container is the one used in most cases. In contrast to SimpleMessageListenerContainer, this container variant allows for dynamic adaptation to runtime demands and is able to participate in externally managed transactions. Each received message is registered with an XA transaction when configured with a JtaTransactionManager; so processing may take advantage of XA transaction semantics. This listener container strikes a good balance between low requirements on the JMS provider, advanced functionality such as the participation in externally managed transactions, and compatibility with Java EE environments.

The cache level of the container can be customized. Note that when no caching is enabled, a new connection and a new session is created for each message reception. Combining this with a non durable subscription with high loads may lead to message lost. Make sure to use a proper cache level in such case.

This container also has recoverable capabilities when the broker goes down. By default, a simple BackOff implementation retries every 5 seconds. It is possible to specify a custom BackOff implementation for more fine-grained recovery options, see ExponentialBackOff for an example.

Like its sibling SimpleMessageListenerContainer, DefaultMessageListenerContainer supports native JMS transactions and also allows for customizing the acknowledgment mode. This is strongly recommended over externally managed transactions if feasible for your scenario: that is, if you can live with occasional duplicate messages in case of the JVM dying. Custom duplicate message detection steps in your business logic may cover such situations, e.g. in the form of a business entity existence check or a protocol table check. Any such arrangements will be significantly more efficient than the alternative: wrapping your entire processing with an XA transaction (through configuring your DefaultMessageListenerContainer with an JtaTransactionManager), covering the reception of the JMS message as well as the execution of the business logic in your message listener (including database operations etc).

3.2.5. Transaction management

Spring provides a JmsTransactionManager that manages transactions for a single JMS ConnectionFactory. This allows JMS applications to leverage the managed transaction features of Spring as described in Transaction Management. The JmsTransactionManager performs local resource transactions, binding a JMS Connection/Session pair from the specified ConnectionFactory to the thread. JmsTemplate automatically detects such transactional resources and operates on them accordingly.

In a Java EE environment, the ConnectionFactory will pool Connections and Sessions, so those resources are efficiently reused across transactions. In a standalone environment, using Spring’s SingleConnectionFactory will result in a shared JMS Connection, with each transaction having its own independent Session. Alternatively, consider the use of a provider-specific pooling adapter such as ActiveMQ’s PooledConnectionFactory class.

JmsTemplate can also be used with the JtaTransactionManager and an XA-capable JMS ConnectionFactory for performing distributed transactions. Note that this requires the use of a JTA transaction manager as well as a properly XA-configured ConnectionFactory! (Check your Java EE server’s / JMS provider’s documentation.)

Reusing code across a managed and unmanaged transactional environment can be confusing when using the JMS API to create a Session from a Connection. This is because the JMS API has only one factory method to create a Session and it requires values for the transaction and acknowledgment modes. In a managed environment, setting these values is the responsibility of the environment’s transactional infrastructure, so these values are ignored by the vendor’s wrapper to the JMS Connection. When using the JmsTemplate in an unmanaged environment you can specify these values through the use of the properties sessionTransacted and sessionAcknowledgeMode. When using a PlatformTransactionManager with JmsTemplate, the template will always be given a transactional JMS Session.

3.3. Sending a Message

The JmsTemplate contains many convenience methods to send a message. There are send methods that specify the destination using a javax.jms.Destination object and those that specify the destination using a string for use in a JNDI lookup. The send method that takes no destination argument uses the default destination.

import javax.jms.ConnectionFactory;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.Queue;
import javax.jms.Session;

import org.springframework.jms.core.MessageCreator;
import org.springframework.jms.core.JmsTemplate;

public class JmsQueueSender {

        private JmsTemplate jmsTemplate;
        private Queue queue;

        public void setConnectionFactory(ConnectionFactory cf) {
                this.jmsTemplate = new JmsTemplate(cf);
        }

        public void setQueue(Queue queue) {
                this.queue = queue;
        }

        public void simpleSend() {
                this.jmsTemplate.send(this.queue, new MessageCreator() {
                        public Message createMessage(Session session) throws JMSException {
                                return session.createTextMessage("hello queue world");
                        }
                });
        }
}

This example uses the MessageCreator callback to create a text message from the supplied Session object. The JmsTemplate is constructed by passing a reference to a ConnectionFactory. As an alternative, a zero argument constructor and connectionFactory is provided and can be used for constructing the instance in JavaBean style (using a BeanFactory or plain Java code). Alternatively, consider deriving from Spring’s JmsGatewaySupport convenience base class, which provides pre-built bean properties for JMS configuration.

The method send(String destinationName, MessageCreator creator) lets you send a message using the string name of the destination. If these names are registered in JNDI, you should set the destinationResolver property of the template to an instance of JndiDestinationResolver.

If you created the JmsTemplate and specified a default destination, the send(MessageCreator c) sends a message to that destination.

3.3.1. Using Message Converters

In order to facilitate the sending of domain model objects, the JmsTemplate has various send methods that take a Java object as an argument for a message’s data content. The overloaded methods convertAndSend() and receiveAndConvert() in JmsTemplate delegate the conversion process to an instance of the MessageConverter interface. This interface defines a simple contract to convert between Java objects and JMS messages. The default implementation SimpleMessageConverter supports conversion between String and TextMessage, byte[] and BytesMesssage, and java.util.Map and MapMessage. By using the converter, you and your application code can focus on the business object that is being sent or received via JMS and not be concerned with the details of how it is represented as a JMS message.

The sandbox currently includes a MapMessageConverter which uses reflection to convert between a JavaBean and a MapMessage. Other popular implementation choices you might implement yourself are Converters that use an existing XML marshalling package, such as JAXB, Castor or XStream, to create a TextMessage representing the object.

To accommodate the setting of a message’s properties, headers, and body that can not be generically encapsulated inside a converter class, the MessagePostProcessor interface gives you access to the message after it has been converted, but before it is sent. The example below demonstrates how to modify a message header and a property after a java.util.Map is converted to a message.

public void sendWithConversion() {
        Map map = new HashMap();
        map.put("Name", "Mark");
        map.put("Age", new Integer(47));
        jmsTemplate.convertAndSend("testQueue", map, new MessagePostProcessor() {
                public Message postProcessMessage(Message message) throws JMSException {
                        message.setIntProperty("AccountID", 1234);
                        message.setJMSCorrelationID("123-00001");
                        return message;
                }
        });
}

This results in a message of the form:

MapMessage={
	Header={
		... standard headers ...
		CorrelationID={123-00001}
	}
	Properties={
		AccountID={Integer:1234}
	}
	Fields={
		Name={String:Mark}
		Age={Integer:47}
	}
}

3.3.2. SessionCallback and ProducerCallback

While the send operations cover many common usage scenarios, there are cases when you want to perform multiple operations on a JMS Session or MessageProducer. The SessionCallback and ProducerCallback expose the JMS Session and Session / MessageProducer pair respectively. The execute() methods on JmsTemplate execute these callback methods.

3.4. Receiving a message

3.4.1. Synchronous reception

While JMS is typically associated with asynchronous processing, it is possible to consume messages synchronously. The overloaded receive(..) methods provide this functionality. During a synchronous receive, the calling thread blocks until a message becomes available. This can be a dangerous operation since the calling thread can potentially be blocked indefinitely. The property receiveTimeout specifies how long the receiver should wait before giving up waiting for a message.

3.4.2. Asynchronous reception: Message-Driven POJOs

Spring also supports annotated-listener endpoints through the use of the @JmsListener annotation and provides an open infrastructure to register endpoints programmatically. This is by far the most convenient way to setup an asynchronous receiver, see Enable listener endpoint annotations for more details.

In a fashion similar to a Message-Driven Bean (MDB) in the EJB world, the Message-Driven POJO (MDP) acts as a receiver for JMS messages. The one restriction (but see also below for the discussion of the MessageListenerAdapter class) on an MDP is that it must implement the javax.jms.MessageListener interface. Please also be aware that in the case where your POJO will be receiving messages on multiple threads, it is important to ensure that your implementation is thread-safe.

Below is a simple implementation of an MDP:

import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageListener;
import javax.jms.TextMessage;

public class ExampleListener implements MessageListener {

        public void onMessage(Message message) {
                if (message instanceof TextMessage) {
                        try {
                                System.out.println(((TextMessage) message).getText());
                        }
                        catch (JMSException ex) {
                                throw new RuntimeException(ex);
                        }
                }
                else {
                        throw new IllegalArgumentException("Message must be of type TextMessage");
                }
        }

}

Once you’ve implemented your MessageListener, it’s time to create a message listener container.

Find below an example of how to define and configure one of the message listener containers that ships with Spring (in this case the DefaultMessageListenerContainer).

<!-- this is the Message Driven POJO (MDP) -->
<bean id="messageListener" class="jmsexample.ExampleListener" />

<!-- and this is the message listener container -->
<bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer">
        <property name="connectionFactory" ref="connectionFactory"/>
        <property name="destination" ref="destination"/>
        <property name="messageListener" ref="messageListener" />
</bean>

Please refer to the Spring javadocs of the various message listener containers for a full description of the features supported by each implementation.

3.4.3. SessionAwareMessageListener interface

The SessionAwareMessageListener interface is a Spring-specific interface that provides a similar contract to the JMS MessageListener interface, but also provides the message handling method with access to the JMS Session from which the Message was received.

package org.springframework.jms.listener;

public interface SessionAwareMessageListener {

        void onMessage(Message message, Session session) throws JMSException;

}

You can choose to have your MDPs implement this interface (in preference to the standard JMS MessageListener interface) if you want your MDPs to be able to respond to any received messages (using the Session supplied in the onMessage(Message, Session) method). All of the message listener container implementations that ship with Spring have support for MDPs that implement either the MessageListener or SessionAwareMessageListener interface. Classes that implement the SessionAwareMessageListener come with the caveat that they are then tied to Spring through the interface. The choice of whether or not to use it is left entirely up to you as an application developer or architect.

Please note that the 'onMessage(..)' method of the SessionAwareMessageListener interface throws JMSException. In contrast to the standard JMS MessageListener interface, when using the SessionAwareMessageListener interface, it is the responsibility of the client code to handle any exceptions thrown.

3.4.4. MessageListenerAdapter

The MessageListenerAdapter class is the final component in Spring’s asynchronous messaging support: in a nutshell, it allows you to expose almost any class as a MDP (there are of course some constraints).

Consider the following interface definition. Notice that although the interface extends neither the MessageListener nor SessionAwareMessageListener interfaces, it can still be used as a MDP via the use of the MessageListenerAdapter class. Notice also how the various message handling methods are strongly typed according to the contents of the various Message types that they can receive and handle.

public interface MessageDelegate {

        void handleMessage(String message);

        void handleMessage(Map message);

        void handleMessage(byte[] message);

        void handleMessage(Serializable message);

}
public class DefaultMessageDelegate implements MessageDelegate {
        // implementation elided for clarity...
}

In particular, note how the above implementation of the MessageDelegate interface (the above DefaultMessageDelegate class) has no JMS dependencies at all. It truly is a POJO that we will make into an MDP via the following configuration.

<!-- this is the Message Driven POJO (MDP) -->
<bean id="messageListener" class="org.springframework.jms.listener.adapter.MessageListenerAdapter">
        <constructor-arg>
                <bean class="jmsexample.DefaultMessageDelegate"/>
        </constructor-arg>
</bean>

<!-- and this is the message listener container... -->
<bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer">
        <property name="connectionFactory" ref="connectionFactory"/>
        <property name="destination" ref="destination"/>
        <property name="messageListener" ref="messageListener" />
</bean>

Below is an example of another MDP that can only handle the receiving of JMS TextMessage messages. Notice how the message handling method is actually called 'receive' (the name of the message handling method in a MessageListenerAdapter defaults to 'handleMessage'), but it is configurable (as you will see below). Notice also how the 'receive(..)' method is strongly typed to receive and respond only to JMS TextMessage messages.

public interface TextMessageDelegate {

        void receive(TextMessage message);

}
public class DefaultTextMessageDelegate implements TextMessageDelegate {
        // implementation elided for clarity...
}

The configuration of the attendant MessageListenerAdapter would look like this:

<bean id="messageListener" class="org.springframework.jms.listener.adapter.MessageListenerAdapter">
        <constructor-arg>
                <bean class="jmsexample.DefaultTextMessageDelegate"/>
        </constructor-arg>
        <property name="defaultListenerMethod" value="receive"/>
        <!-- we don't want automatic message context extraction -->
        <property name="messageConverter">
                <null/>
        </property>
</bean>

Please note that if the above 'messageListener' receives a JMS Message of a type other than TextMessage, an IllegalStateException will be thrown (and subsequently swallowed). Another of the capabilities of the MessageListenerAdapter class is the ability to automatically send back a response Message if a handler method returns a non-void value. Consider the interface and class:

public interface ResponsiveTextMessageDelegate {

        // notice the return type...
        String receive(TextMessage message);

}
public class DefaultResponsiveTextMessageDelegate implements ResponsiveTextMessageDelegate {
        // implementation elided for clarity...
}

If the above DefaultResponsiveTextMessageDelegate is used in conjunction with a MessageListenerAdapter then any non-null value that is returned from the execution of the 'receive(..)' method will (in the default configuration) be converted into a TextMessage. The resulting TextMessage will then be sent to the Destination (if one exists) defined in the JMS Reply-To property of the original Message, or the default Destination set on the MessageListenerAdapter (if one has been configured); if no Destination is found then an InvalidDestinationException will be thrown (and please note that this exception will not be swallowed and will propagate up the call stack).

3.4.5. Processing messages within transactions

Invoking a message listener within a transaction only requires reconfiguration of the listener container.

Local resource transactions can simply be activated through the sessionTransacted flag on the listener container definition. Each message listener invocation will then operate within an active JMS transaction, with message reception rolled back in case of listener execution failure. Sending a response message (via SessionAwareMessageListener) will be part of the same local transaction, but any other resource operations (such as database access) will operate independently. This usually requires duplicate message detection in the listener implementation, covering the case where database processing has committed but message processing failed to commit.

<bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer">
        <property name="connectionFactory" ref="connectionFactory"/>
        <property name="destination" ref="destination"/>
        <property name="messageListener" ref="messageListener"/>
        <property name="sessionTransacted" value="true"/>
</bean>

For participating in an externally managed transaction, you will need to configure a transaction manager and use a listener container which supports externally managed transactions: typically DefaultMessageListenerContainer.

To configure a message listener container for XA transaction participation, you’ll want to configure a JtaTransactionManager (which, by default, delegates to the Java EE server’s transaction subsystem). Note that the underlying JMS ConnectionFactory needs to be XA-capable and properly registered with your JTA transaction coordinator! (Check your Java EE server’s configuration of JNDI resources.) This allows message reception as well as e.g. database access to be part of the same transaction (with unified commit semantics, at the expense of XA transaction log overhead).

<bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>

Then you just need to add it to our earlier container configuration. The container will take care of the rest.

<bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer">
        <property name="connectionFactory" ref="connectionFactory"/>
        <property name="destination" ref="destination"/>
        <property name="messageListener" ref="messageListener"/>
        <property name="transactionManager" ref="transactionManager"/>
</bean>

3.5. Support for JCA Message Endpoints

Beginning with version 2.5, Spring also provides support for a JCA-based MessageListener container. The JmsMessageEndpointManager will attempt to automatically determine the ActivationSpec class name from the provider’s ResourceAdapter class name. Therefore, it is typically possible to just provide Spring’s generic JmsActivationSpecConfig as shown in the following example.

<bean class="org.springframework.jms.listener.endpoint.JmsMessageEndpointManager">
        <property name="resourceAdapter" ref="resourceAdapter"/>
        <property name="activationSpecConfig">
                <bean class="org.springframework.jms.listener.endpoint.JmsActivationSpecConfig">
                        <property name="destinationName" value="myQueue"/>
                </bean>
        </property>
        <property name="messageListener" ref="myMessageListener"/>
</bean>

Alternatively, you may set up a JmsMessageEndpointManager with a given ActivationSpec object. The ActivationSpec object may also come from a JNDI lookup (using <jee:jndi-lookup>).

<bean class="org.springframework.jms.listener.endpoint.JmsMessageEndpointManager">
        <property name="resourceAdapter" ref="resourceAdapter"/>
        <property name="activationSpec">
                <bean class="org.apache.activemq.ra.ActiveMQActivationSpec">
                        <property name="destination" value="myQueue"/>
                        <property name="destinationType" value="javax.jms.Queue"/>
                </bean>
        </property>
        <property name="messageListener" ref="myMessageListener"/>
</bean>

Using Spring’s ResourceAdapterFactoryBean, the target ResourceAdapter may be configured locally as depicted in the following example.

<bean id="resourceAdapter" class="org.springframework.jca.support.ResourceAdapterFactoryBean">
        <property name="resourceAdapter">
                <bean class="org.apache.activemq.ra.ActiveMQResourceAdapter">
                        <property name="serverUrl" value="tcp://localhost:61616"/>
                </bean>
        </property>
        <property name="workManager">
                <bean class="org.springframework.jca.work.SimpleTaskWorkManager"/>
        </property>
</bean>

The specified WorkManager may also point to an environment-specific thread pool - typically through SimpleTaskWorkManager’s "asyncTaskExecutor" property. Consider defining a shared thread pool for all your ResourceAdapter instances if you happen to use multiple adapters.

In some environments (e.g. WebLogic 9 or above), the entire ResourceAdapter object may be obtained from JNDI instead (using <jee:jndi-lookup>). The Spring-based message listeners can then interact with the server-hosted ResourceAdapter, also using the server’s built-in WorkManager.

Please consult the javadoc for JmsMessageEndpointManager, JmsActivationSpecConfig, and ResourceAdapterFactoryBean for more details.

Spring also provides a generic JCA message endpoint manager which is not tied to JMS: org.springframework.jca.endpoint.GenericMessageEndpointManager. This component allows for using any message listener type (e.g. a CCI MessageListener) and any provider-specific ActivationSpec object. Check out your JCA provider’s documentation to find out about the actual capabilities of your connector, and consult GenericMessageEndpointManager's javadoc for the Spring-specific configuration details.

JCA-based message endpoint management is very analogous to EJB 2.1 Message-Driven Beans; it uses the same underlying resource provider contract. Like with EJB 2.1 MDBs, any message listener interface supported by your JCA provider can be used in the Spring context as well. Spring nevertheless provides explicit 'convenience' support for JMS, simply because JMS is the most common endpoint API used with the JCA endpoint management contract.

3.6. Annotation-driven listener endpoints

The easiest way to receive a message asynchronously is to use the annotated listener endpoint infrastructure. In a nutshell, it allows you to expose a method of a managed bean as a JMS listener endpoint.

@Component
public class MyService {

        @JmsListener(destination = "myDestination")
        public void processOrder(String data) { ... }
}

The idea of the example above is that whenever a message is available on the javax.jms.Destination "myDestination", the processOrder method is invoked accordingly (in this case, with the content of the JMS message similarly to what the MessageListenerAdapter provides).

The annotated endpoint infrastructure creates a message listener container behind the scenes for each annotated method, using a JmsListenerContainerFactory. Such a container is not registered against the application context but can be easily located for management purposes using the JmsListenerEndpointRegistry bean.

@JmsListener is a repeatable annotation on Java 8, so it is possible to associate several JMS destinations to the same method by adding additional @JmsListener declarations to it.

3.6.1. Enable listener endpoint annotations

To enable support for @JmsListener annotations add @EnableJms to one of your @Configuration classes.

@Configuration
@EnableJms
public class AppConfig {

        @Bean
        public DefaultJmsListenerContainerFactory jmsListenerContainerFactory() {
                DefaultJmsListenerContainerFactory factory =
                                new DefaultJmsListenerContainerFactory();
                factory.setConnectionFactory(connectionFactory());
                factory.setDestinationResolver(destinationResolver());
                factory.setConcurrency("3-10");
                return factory;
        }
}

By default, the infrastructure looks for a bean named jmsListenerContainerFactory as the source for the factory to use to create message listener containers. In this case, and ignoring the JMS infrastructure setup, the processOrder method can be invoked with a core poll size of 3 threads and a maximum pool size of 10 threads.

It is possible to customize the listener container factory to use per annotation or an explicit default can be configured by implementing the JmsListenerConfigurer interface. The default is only required if at least one endpoint is registered without a specific container factory. See the javadoc for full details and examples.

If you prefer XML configuration use the <jms:annotation-driven> element.

<jms:annotation-driven/>

   <bean id="jmsListenerContainerFactory"
           class="org.springframework.jms.config.DefaultJmsListenerContainerFactory">
       <property name="connectionFactory" ref="connectionFactory"/>
       <property name="destinationResolver" ref="destinationResolver"/>
       <property name="concurrency" value="3-10"/>
   </bean>

3.6.2. Programmatic endpoints registration

JmsListenerEndpoint provides a model of an JMS endpoint and is responsible for configuring the container for that model. The infrastructure allows you to configure endpoints programmatically in addition to the ones that are detected by the JmsListener annotation.

@Configuration
@EnableJms
public class AppConfig implements JmsListenerConfigurer {

        @Override
        public void configureJmsListeners(JmsListenerEndpointRegistrar registrar) {
                SimpleJmsListenerEndpoint endpoint = new SimpleJmsListenerEndpoint();
                endpoint.setId("myJmsEndpoint");
                endpoint.setDestination("anotherQueue");
                endpoint.setMessageListener(message -> {
                        // processing
                });
                registrar.registerEndpoint(endpoint);
        }
}

In the example above, we used SimpleJmsListenerEndpoint which provides the actual MessageListener to invoke but you could just as well build your own endpoint variant describing a custom invocation mechanism.

It should be noted that you could just as well skip the use of @JmsListener altogether and only register your endpoints programmatically through JmsListenerConfigurer.

3.6.3. Annotated endpoint method signature

So far, we have been injecting a simple String in our endpoint but it can actually have a very flexible method signature. Let’s rewrite it to inject the Order with a custom header:

@Component
public class MyService {

           @JmsListener(destination = "myDestination")
           public void processOrder(Order order, @Header("order_type") String orderType) {
               ...
           }
}

These are the main elements you can inject in JMS listener endpoints:

  • The raw javax.jms.Message or any of its subclasses (provided of course that it matches the incoming message type).

  • The javax.jms.Session for optional access to the native JMS API e.g. for sending a custom reply.

  • The org.springframework.messaging.Message representing the incoming JMS message. Note that this message holds both the custom and the standard headers (as defined by JmsHeaders).

  • @Header-annotated method arguments to extract a specific header value, including standard JMS headers.

  • @Headers-annotated argument that must also be assignable to java.util.Map for getting access to all headers.

  • A non-annotated element that is not one of the supported types (i.e. Message and Session) is considered to be the payload. You can make that explicit by annotating the parameter with @Payload. You can also turn on validation by adding an extra @Valid.

The ability to inject Spring’s Message abstraction is particularly useful to benefit from all the information stored in the transport-specific message without relying on transport-specific API.

@JmsListener(destination = "myDestination")
public void processOrder(Message<Order> order) { ... }

Handling of method arguments is provided by DefaultMessageHandlerMethodFactory which can be further customized to support additional method arguments. The conversion and validation support can be customized there as well.

For instance, if we want to make sure our Order is valid before processing it, we can annotate the payload with @Valid and configure the necessary validator as follows:

@Configuration
@EnableJms
public class AppConfig implements JmsListenerConfigurer {

           @Override
           public void configureJmsListeners(JmsListenerEndpointRegistrar registrar) {
               registrar.setMessageHandlerMethodFactory(myJmsHandlerMethodFactory());
           }

           @Bean
           public DefaultMessageHandlerMethodFactory myHandlerMethodFactory() {
               DefaultMessageHandlerMethodFactory factory = new DefaultMessageHandlerMethodFactory();
               factory.setValidator(myValidator());
               return factory;
           }
}

3.6.4. Response management

The existing support in MessageListenerAdapter already allows your method to have a non-void return type. When that’s the case, the result of the invocation is encapsulated in a javax.jms.Message sent either in the destination specified in the JMSReplyTo header of the original message or in the default destination configured on the listener. That default destination can now be set using the @SendTo annotation of the messaging abstraction.

Assuming our processOrder method should now return an OrderStatus, it is possible to write it as follow to automatically send a response:

@JmsListener(destination = "myDestination")
@SendTo("status")
public OrderStatus processOrder(Order order) {
           // order processing
           return status;
}

If you have several @JmsListener-annotated methods, you can also place the @SendTo annotation at the class level to share a default reply destination.

If you need to set additional headers in a transport-independent manner, you could return a Message instead, something like:

@JmsListener(destination = "myDestination")
@SendTo("status")
public Message<OrderStatus> processOrder(Order order) {
           // order processing
           return MessageBuilder
                   .withPayload(status)
                   .setHeader("code", 1234)
                   .build();
}

If you need to compute the response destination at runtime, you can encapsulate your response in a JmsResponse instance that also provides the destination to use at runtime. The previous example can be rewritten as follows:

@JmsListener(destination = "myDestination")
public JmsResponse<Message<OrderStatus>> processOrder(Order order) {
           // order processing
           Message<OrderStatus> response = MessageBuilder
                   .withPayload(status)
                   .setHeader("code", 1234)
                   .build();
        return JmsResponse.forQueue(response, "status");
}

Finally if you need to specify some QoS values for the response such as the priority or the time to live, you can configure the JmsListenerContainerFactory accordingly:

@Configuration
@EnableJms
public class AppConfig {

        @Bean
        public DefaultJmsListenerContainerFactory jmsListenerContainerFactory() {
                DefaultJmsListenerContainerFactory factory =
                                new DefaultJmsListenerContainerFactory();
                factory.setConnectionFactory(connectionFactory());
                QosSettings replyQosSettings = new ReplyQosSettings();
                replyQosSettings.setPriority(2);
                replyQosSettings.setTimeToLive(10000);
                factory.setReplyQosSettings(replyQosSettings);
                return factory;
        }
}

3.7. JMS namespace support

Spring provides an XML namespace for simplifying JMS configuration. To use the JMS namespace elements you will need to reference the JMS schema:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xmlns:jms="http://www.springframework.org/schema/jms"
                xsi:schemaLocation="
                        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
                        http://www.springframework.org/schema/jms http://www.springframework.org/schema/jms/spring-jms.xsd">

        <!-- bean definitions here -->

</beans>

The namespace consists of three top-level elements: <annotation-driven/>, <listener-container/> and <jca-listener-container/>. <annotation-driven enables the use of annotation-driven listener endpoints. <listener-container/> and <jca-listener-container/> defines shared listener container configuration and may contain <listener/> child elements. Here is an example of a basic configuration for two listeners.

<jms:listener-container>

        <jms:listener destination="queue.orders" ref="orderService" method="placeOrder"/>

        <jms:listener destination="queue.confirmations" ref="confirmationLogger" method="log"/>

</jms:listener-container>

The example above is equivalent to creating two distinct listener container bean definitions and two distinct MessageListenerAdapter bean definitions as demonstrated in MessageListenerAdapter. In addition to the attributes shown above, the listener element may contain several optional ones. The following table describes all available attributes:

Table 2. Attributes of the JMS <listener> element
Attribute Description

id

A bean name for the hosting listener container. If not specified, a bean name will be automatically generated.

destination (required)

The destination name for this listener, resolved through the DestinationResolver strategy.

ref (required)

The bean name of the handler object.

method

The name of the handler method to invoke. If the ref points to a MessageListener or Spring SessionAwareMessageListener, this attribute may be omitted.

response-destination

The name of the default response destination to send response messages to. This will be applied in case of a request message that does not carry a "JMSReplyTo" field. The type of this destination will be determined by the listener-container’s "response-destination-type" attribute. Note: This only applies to a listener method with a return value, for which each result object will be converted into a response message.

subscription

The name of the durable subscription, if any.

selector

An optional message selector for this listener.

concurrency

The number of concurrent sessions/consumers to start for this listener. Can either be a simple number indicating the maximum number (e.g. "5") or a range indicating the lower as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a hint and might be ignored at runtime. Default is the value provided by the container

The <listener-container/> element also accepts several optional attributes. This allows for customization of the various strategies (for example, taskExecutor and destinationResolver) as well as basic JMS settings and resource references. Using these attributes, it is possible to define highly-customized listener containers while still benefiting from the convenience of the namespace.

Such settings can be automatically exposed as a JmsListenerContainerFactory by specifying the id of the bean to expose through the factory-id attribute.

<jms:listener-container connection-factory="myConnectionFactory"
                task-executor="myTaskExecutor"
                destination-resolver="myDestinationResolver"
                transaction-manager="myTransactionManager"
                concurrency="10">

        <jms:listener destination="queue.orders" ref="orderService" method="placeOrder"/>

        <jms:listener destination="queue.confirmations" ref="confirmationLogger" method="log"/>

</jms:listener-container>

The following table describes all available attributes. Consult the class-level javadocs of the AbstractMessageListenerContainer and its concrete subclasses for more details on the individual properties. The javadocs also provide a discussion of transaction choices and message redelivery scenarios.

Table 3. Attributes of the JMS <listener-container> element
Attribute Description

container-type

The type of this listener container. Available options are: default, simple, default102, or simple102 (the default value is 'default').

container-class

A custom listener container implementation class as fully qualified class name. Default is Spring’s standard DefaultMessageListenerContainer or SimpleMessageListenerContainer, according to the "container-type" attribute.

factory-id

Exposes the settings defined by this element as a JmsListenerContainerFactory with the specified id so that they can be reused with other endpoints.

connection-factory

A reference to the JMS ConnectionFactory bean (the default bean name is 'connectionFactory').

task-executor

A reference to the Spring TaskExecutor for the JMS listener invokers.

destination-resolver

A reference to the DestinationResolver strategy for resolving JMS Destinations.

message-converter

A reference to the MessageConverter strategy for converting JMS Messages to listener method arguments. Default is a SimpleMessageConverter.

error-handler

A reference to an ErrorHandler strategy for handling any uncaught Exceptions that may occur during the execution of the MessageListener.

destination-type

The JMS destination type for this listener: queue, topic, durableTopic, sharedTopic or sharedDurableTopic. This enables potentially the pubSubDomain, subscriptionDurable and subscriptionShared properties of the container. The default is queue (i.e. disabling those 3 properties).

response-destination-type

The JMS destination type for responses: "queue", "topic". Default is the value of the "destination-type" attribute.

client-id

The JMS client id for this listener container. Needs to be specified when using durable subscriptions.

cache

The cache level for JMS resources: none, connection, session, consumer or auto. By default ( auto), the cache level will effectively be "consumer", unless an external transaction manager has been specified - in which case the effective default will be none (assuming Java EE-style transaction management where the given ConnectionFactory is an XA-aware pool).

acknowledge

The native JMS acknowledge mode: auto, client, dups-ok or transacted. A value of transacted activates a locally transacted Session. As an alternative, specify the transaction-manager attribute described below. Default is auto.

transaction-manager

A reference to an external PlatformTransactionManager (typically an XA-based transaction coordinator, e.g. Spring’s JtaTransactionManager). If not specified, native acknowledging will be used (see "acknowledge" attribute).

concurrency

The number of concurrent sessions/consumers to start for each listener. Can either be a simple number indicating the maximum number (e.g. "5") or a range indicating the lower as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a hint and might be ignored at runtime. Default is 1; keep concurrency limited to 1 in case of a topic listener or if queue ordering is important; consider raising it for general queues.

prefetch

The maximum number of messages to load into a single session. Note that raising this number might lead to starvation of concurrent consumers!

receive-timeout

The timeout to use for receive calls (in milliseconds). The default is 1000 ms (1 sec); -1 indicates no timeout at all.

back-off

Specify the BackOff instance to use to compute the interval between recovery attempts. If the BackOffExecution implementation returns BackOffExecution#STOP, the listener container will not further attempt to recover. The recovery-interval value is ignored when this property is set. The default is a FixedBackOff with an interval of 5000 ms, that is 5 seconds.

recovery-interval

Specify the interval between recovery attempts, in milliseconds. Convenience way to create a FixedBackOff with the specified interval. For more recovery options, consider specifying a BackOff instance instead. The default is 5000 ms, that is 5 seconds.

phase

The lifecycle phase within which this container should start and stop. The lower the value the earlier this container will start and the later it will stop. The default is Integer.MAX_VALUE meaning the container will start as late as possible and stop as soon as possible.

Configuring a JCA-based listener container with the "jms" schema support is very similar.

<jms:jca-listener-container resource-adapter="myResourceAdapter"
                destination-resolver="myDestinationResolver"
                transaction-manager="myTransactionManager"
                concurrency="10">

        <jms:listener destination="queue.orders" ref="myMessageListener"/>

</jms:jca-listener-container>

The available configuration options for the JCA variant are described in the following table:

Table 4. Attributes of the JMS <jca-listener-container/> element
Attribute Description

factory-id

Exposes the settings defined by this element as a JmsListenerContainerFactory with the specified id so that they can be reused with other endpoints.

resource-adapter

A reference to the JCA ResourceAdapter bean (the default bean name is 'resourceAdapter').

activation-spec-factory

A reference to the JmsActivationSpecFactory. The default is to autodetect the JMS provider and its ActivationSpec class (see DefaultJmsActivationSpecFactory)

destination-resolver

A reference to the DestinationResolver strategy for resolving JMS Destinations.

message-converter

A reference to the MessageConverter strategy for converting JMS Messages to listener method arguments. Default is a SimpleMessageConverter.

destination-type

The JMS destination type for this listener: queue, topic, durableTopic, sharedTopic or sharedDurableTopic. This enables potentially the pubSubDomain, subscriptionDurable and subscriptionShared properties of the container. The default is queue (i.e. disabling those 3 properties).

response-destination-type

The JMS destination type for responses: "queue", "topic". Default is the value of the "destination-type" attribute.

client-id

The JMS client id for this listener container. Needs to be specified when using durable subscriptions.

acknowledge

The native JMS acknowledge mode: auto, client, dups-ok or transacted. A value of transacted activates a locally transacted Session. As an alternative, specify the transaction-manager attribute described below. Default is auto.

transaction-manager

A reference to a Spring JtaTransactionManager or a javax.transaction.TransactionManager for kicking off an XA transaction for each incoming message. If not specified, native acknowledging will be used (see the "acknowledge" attribute).

concurrency

The number of concurrent sessions/consumers to start for each listener. Can either be a simple number indicating the maximum number (e.g. "5") or a range indicating the lower as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a hint and will typically be ignored at runtime when using a JCA listener container. Default is 1.

prefetch

The maximum number of messages to load into a single session. Note that raising this number might lead to starvation of concurrent consumers!

4. JMX

4.1. Introduction

The JMX support in Spring provides you with the features to easily and transparently integrate your Spring application into a JMX infrastructure.

JMX?

This chapter is not an introduction to JMX…​ it doesn’t try to explain the motivations of why one might want to use JMX (or indeed what the letters JMX actually stand for). If you are new to JMX, check out Further resources at the end of this chapter.

Specifically, Spring’s JMX support provides four core features:

  • The automatic registration of any Spring bean as a JMX MBean

  • A flexible mechanism for controlling the management interface of your beans

  • The declarative exposure of MBeans over remote, JSR-160 connectors

  • The simple proxying of both local and remote MBean resources

These features are designed to work without coupling your application components to either Spring or JMX interfaces and classes. Indeed, for the most part your application classes need not be aware of either Spring or JMX in order to take advantage of the Spring JMX features.

4.2. Exporting your beans to JMX

The core class in Spring’s JMX framework is the MBeanExporter. This class is responsible for taking your Spring beans and registering them with a JMX MBeanServer. For example, consider the following class:

package org.springframework.jmx;

public class JmxTestBean implements IJmxTestBean {

        private String name;
        private int age;
        private boolean isSuperman;

        public int getAge() {
                return age;
        }

        public void setAge(int age) {
                this.age = age;
        }

        public void setName(String name) {
                this.name = name;
        }

        public String getName() {
                return name;
        }

        public int add(int x, int y) {
                return x + y;
        }

        public void dontExposeMe() {
                throw new RuntimeException();
        }
}

To expose the properties and methods of this bean as attributes and operations of an MBean you simply configure an instance of the MBeanExporter class in your configuration file and pass in the bean as shown below:

<beans>
        <!-- this bean must not be lazily initialized if the exporting is to happen -->
        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter" lazy-init="false">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean1" value-ref="testBean"/>
                        </map>
                </property>
        </bean>
        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>
</beans>

The pertinent bean definition from the above configuration snippet is the exporter bean. The beans property tells the MBeanExporter exactly which of your beans must be exported to the JMX MBeanServer. In the default configuration, the key of each entry in the beans Map is used as the ObjectName for the bean referenced by the corresponding entry value. This behavior can be changed as described in Controlling the ObjectNames for your beans.

With this configuration the testBean bean is exposed as an MBean under the ObjectName bean:name=testBean1. By default, all public properties of the bean are exposed as attributes and all public methods (bar those inherited from the Object class) are exposed as operations.

MBeanExporter is a Lifecycle bean (see Startup and shutdown callbacks) and MBeans are exported as late as possible during the application lifecycle by default. It is possible to configure the phase at which the export happens or disable automatic registration by setting the autoStartup flag.

4.2.1. Creating an MBeanServer

The above configuration assumes that the application is running in an environment that has one (and only one) MBeanServer already running. In this case, Spring will attempt to locate the running MBeanServer and register your beans with that server (if any). This behavior is useful when your application is running inside a container such as Tomcat or IBM WebSphere that has its own MBeanServer.

However, this approach is of no use in a standalone environment, or when running inside a container that does not provide an MBeanServer. To address this you can create an MBeanServer instance declaratively by adding an instance of the org.springframework.jmx.support.MBeanServerFactoryBean class to your configuration. You can also ensure that a specific MBeanServer is used by setting the value of the MBeanExporter's server property to the MBeanServer value returned by an MBeanServerFactoryBean; for example:

<beans>

        <bean id="mbeanServer" class="org.springframework.jmx.support.MBeanServerFactoryBean"/>

        <!--
        this bean needs to be eagerly pre-instantiated in order for the exporting to occur;
        this means that it must not be marked as lazily initialized
        -->
        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean1" value-ref="testBean"/>
                        </map>
                </property>
                <property name="server" ref="mbeanServer"/>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

</beans>

Here an instance of MBeanServer is created by the MBeanServerFactoryBean and is supplied to the MBeanExporter via the server property. When you supply your own MBeanServer instance, the MBeanExporter will not attempt to locate a running MBeanServer and will use the supplied MBeanServer instance. For this to work correctly, you must (of course) have a JMX implementation on your classpath.

4.2.2. Reusing an existing MBeanServer

If no server is specified, the MBeanExporter tries to automatically detect a running MBeanServer. This works in most environment where only one MBeanServer instance is used, however when multiple instances exist, the exporter might pick the wrong server. In such cases, one should use the MBeanServer agentId to indicate which instance to be used:

<beans>
        <bean id="mbeanServer" class="org.springframework.jmx.support.MBeanServerFactoryBean">
                <!-- indicate to first look for a server -->
                <property name="locateExistingServerIfPossible" value="true"/>
                <!-- search for the MBeanServer instance with the given agentId -->
                <property name="agentId" value="MBeanServer_instance_agentId>"/>
        </bean>
        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="server" ref="mbeanServer"/>
                ...
        </bean>
</beans>

For platforms/cases where the existing MBeanServer has a dynamic (or unknown) agentId which is retrieved through lookup methods, one should use factory-method:

<beans>
        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="server">
                        <!-- Custom MBeanServerLocator -->
                        <bean class="platform.package.MBeanServerLocator" factory-method="locateMBeanServer"/>
                </property>
        </bean>

        <!-- other beans here -->

</beans>

4.2.3. Lazy-initialized MBeans

If you configure a bean with the MBeanExporter that is also configured for lazy initialization, then the MBeanExporter will not break this contract and will avoid instantiating the bean. Instead, it will register a proxy with the MBeanServer and will defer obtaining the bean from the container until the first invocation on the proxy occurs.

4.2.4. Automatic registration of MBeans

Any beans that are exported through the MBeanExporter and are already valid MBeans are registered as-is with the MBeanServer without further intervention from Spring. MBeans can be automatically detected by the MBeanExporter by setting the autodetect property to true:

<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
        <property name="autodetect" value="true"/>
</bean>

<bean name="spring:mbean=true" class="org.springframework.jmx.export.TestDynamicMBean"/>

Here, the bean called spring:mbean=true is already a valid JMX MBean and will be automatically registered by Spring. By default, beans that are autodetected for JMX registration have their bean name used as the ObjectName. This behavior can be overridden as detailed in Controlling the ObjectNames for your beans.

4.2.5. Controlling the registration behavior

Consider the scenario where a Spring MBeanExporter attempts to register an MBean with an MBeanServer using the ObjectName 'bean:name=testBean1'. If an MBean instance has already been registered under that same ObjectName, the default behavior is to fail (and throw an InstanceAlreadyExistsException).

It is possible to control the behavior of exactly what happens when an MBean is registered with an MBeanServer. Spring’s JMX support allows for three different registration behaviors to control the registration behavior when the registration process finds that an MBean has already been registered under the same ObjectName; these registration behaviors are summarized on the following table:

Table 5. Registration Behaviors
Registration behavior Explanation

REGISTRATION_FAIL_ON_EXISTING

This is the default registration behavior. If an MBean instance has already been registered under the same ObjectName, the MBean that is being registered will not be registered and an InstanceAlreadyExistsException will be thrown. The existing MBean is unaffected.

REGISTRATION_IGNORE_EXISTING

If an MBean instance has already been registered under the same ObjectName, the MBean that is being registered will not be registered. The existing MBean is unaffected, and no Exception will be thrown. This is useful in settings where multiple applications want to share a common MBean in a shared MBeanServer.

REGISTRATION_REPLACE_EXISTING

If an MBean instance has already been registered under the same ObjectName, the existing MBean that was previously registered will be unregistered and the new MBean will be registered in its place (the new MBean effectively replaces the previous instance).

The above values are defined as constants on the MBeanRegistrationSupport class (the MBeanExporter class derives from this superclass). If you want to change the default registration behavior, you simply need to set the value of the registrationBehaviorName property on your MBeanExporter definition to one of those values.

The following example illustrates how to effect a change from the default registration behavior to the REGISTRATION_REPLACE_EXISTING behavior:

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean1" value-ref="testBean"/>
                        </map>
                </property>
                <property name="registrationBehaviorName" value="REGISTRATION_REPLACE_EXISTING"/>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

</beans>

4.3. Controlling the management interface of your beans

In the previous example, you had little control over the management interface of your bean; all of the public properties and methods of each exported bean was exposed as JMX attributes and operations respectively. To exercise finer-grained control over exactly which properties and methods of your exported beans are actually exposed as JMX attributes and operations, Spring JMX provides a comprehensive and extensible mechanism for controlling the management interfaces of your beans.

4.3.1. MBeanInfoAssembler interface

Behind the scenes, the MBeanExporter delegates to an implementation of the org.springframework.jmx.export.assembler.MBeanInfoAssembler interface which is responsible for defining the management interface of each bean that is being exposed. The default implementation, org.springframework.jmx.export.assembler.SimpleReflectiveMBeanInfoAssembler, simply defines a management interface that exposes all public properties and methods (as you saw in the previous examples). Spring provides two additional implementations of the MBeanInfoAssembler interface that allow you to control the generated management interface using either source-level metadata or any arbitrary interface.

4.3.2. Using source-level metadata: Java annotations

Using the MetadataMBeanInfoAssembler you can define the management interfaces for your beans using source level metadata. The reading of metadata is encapsulated by the org.springframework.jmx.export.metadata.JmxAttributeSource interface. Spring JMX provides a default implementation which uses Java annotations, namely org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource. The MetadataMBeanInfoAssembler must be configured with an implementation instance of the JmxAttributeSource interface for it to function correctly (there is no default).

To mark a bean for export to JMX, you should annotate the bean class with the ManagedResource annotation. Each method you wish to expose as an operation must be marked with the ManagedOperation annotation and each property you wish to expose must be marked with the ManagedAttribute annotation. When marking properties you can omit either the annotation of the getter or the setter to create a write-only or read-only attribute respectively.

A ManagedResource annotated bean must be public as well as the methods exposing an operation or an attribute.

The example below shows the annotated version of the JmxTestBean class that you saw earlier:

package org.springframework.jmx;

import org.springframework.jmx.export.annotation.ManagedResource;
import org.springframework.jmx.export.annotation.ManagedOperation;
import org.springframework.jmx.export.annotation.ManagedAttribute;

@ManagedResource(
                objectName="bean:name=testBean4",
                description="My Managed Bean",
                log=true,
                logFile="jmx.log",
                currencyTimeLimit=15,
                persistPolicy="OnUpdate",
                persistPeriod=200,
                persistLocation="foo",
                persistName="bar")
public class AnnotationTestBean implements IJmxTestBean {

        private String name;
        private int age;

        @ManagedAttribute(description="The Age Attribute", currencyTimeLimit=15)
        public int getAge() {
                return age;
        }

        public void setAge(int age) {
                this.age = age;
        }

        @ManagedAttribute(description="The Name Attribute",
                        currencyTimeLimit=20,
                        defaultValue="bar",
                        persistPolicy="OnUpdate")
        public void setName(String name) {
                this.name = name;
        }

        @ManagedAttribute(defaultValue="foo", persistPeriod=300)
        public String getName() {
                return name;
        }

        @ManagedOperation(description="Add two numbers")
        @ManagedOperationParameters({
                @ManagedOperationParameter(name = "x", description = "The first number"),
                @ManagedOperationParameter(name = "y", description = "The second number")})
        public int add(int x, int y) {
                return x + y;
        }

        public void dontExposeMe() {
                throw new RuntimeException();
        }

}

Here you can see that the JmxTestBean class is marked with the ManagedResource annotation and that this ManagedResource annotation is configured with a set of properties. These properties can be used to configure various aspects of the MBean that is generated by the MBeanExporter, and are explained in greater detail later in section entitled Source-level metadata types.

You will also notice that both the age and name properties are annotated with the ManagedAttribute annotation, but in the case of the age property, only the getter is marked. This will cause both of these properties to be included in the management interface as attributes, but the age attribute will be read-only.

Finally, you will notice that the add(int, int) method is marked with the ManagedOperation attribute whereas the dontExposeMe() method is not. This will cause the management interface to contain only one operation, add(int, int), when using the MetadataMBeanInfoAssembler.

The configuration below shows how you configure the MBeanExporter to use the MetadataMBeanInfoAssembler:

<beans>
        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="assembler" ref="assembler"/>
                <property name="namingStrategy" ref="namingStrategy"/>
                <property name="autodetect" value="true"/>
        </bean>

        <bean id="jmxAttributeSource"
                        class="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/>

        <!-- will create management interface using annotation metadata -->
        <bean id="assembler"
                        class="org.springframework.jmx.export.assembler.MetadataMBeanInfoAssembler">
                <property name="attributeSource" ref="jmxAttributeSource"/>
        </bean>

        <!-- will pick up the ObjectName from the annotation -->
        <bean id="namingStrategy"
                        class="org.springframework.jmx.export.naming.MetadataNamingStrategy">
                <property name="attributeSource" ref="jmxAttributeSource"/>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.AnnotationTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>
</beans>

Here you can see that an MetadataMBeanInfoAssembler bean has been configured with an instance of the AnnotationJmxAttributeSource class and passed to the MBeanExporter through the assembler property. This is all that is required to take advantage of metadata-driven management interfaces for your Spring-exposed MBeans.

4.3.3. Source-level metadata types

The following source level metadata types are available for use in Spring JMX:

Table 6. Source-level metadata types
Purpose Annotation Annotation Type

Mark all instances of a Class as JMX managed resources

@ManagedResource

Class

Mark a method as a JMX operation

@ManagedOperation

Method

Mark a getter or setter as one half of a JMX attribute

@ManagedAttribute

Method (only getters and setters)

Define descriptions for operation parameters

@ManagedOperationParameter and @ManagedOperationParameters

Method

The following configuration parameters are available for use on these source-level metadata types:

Table 7. Source-level metadata parameters
Parameter Description Applies to

ObjectName

Used by MetadataNamingStrategy to determine the ObjectName of a managed resource

ManagedResource

description

Sets the friendly description of the resource, attribute or operation

ManagedResource, ManagedAttribute, ManagedOperation, ManagedOperationParameter

currencyTimeLimit

Sets the value of the currencyTimeLimit descriptor field

ManagedResource, ManagedAttribute

defaultValue

Sets the value of the defaultValue descriptor field

ManagedAttribute

log

Sets the value of the log descriptor field

ManagedResource

logFile

Sets the value of the logFile descriptor field

ManagedResource

persistPolicy

Sets the value of the persistPolicy descriptor field

ManagedResource

persistPeriod

Sets the value of the persistPeriod descriptor field

ManagedResource

persistLocation

Sets the value of the persistLocation descriptor field

ManagedResource

persistName

Sets the value of the persistName descriptor field

ManagedResource

name

Sets the display name of an operation parameter

ManagedOperationParameter

index

Sets the index of an operation parameter

ManagedOperationParameter

4.3.4. AutodetectCapableMBeanInfoAssembler interface

To simplify configuration even further, Spring introduces the AutodetectCapableMBeanInfoAssembler interface which extends the MBeanInfoAssembler interface to add support for autodetection of MBean resources. If you configure the MBeanExporter with an instance of AutodetectCapableMBeanInfoAssembler then it is allowed to "vote" on the inclusion of beans for exposure to JMX.

Out of the box, the only implementation of the AutodetectCapableMBeanInfo interface is the MetadataMBeanInfoAssembler which will vote to include any bean which is marked with the ManagedResource attribute. The default approach in this case is to use the bean name as the ObjectName which results in a configuration like this:

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <!-- notice how no 'beans' are explicitly configured here -->
                <property name="autodetect" value="true"/>
                <property name="assembler" ref="assembler"/>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

        <bean id="assembler" class="org.springframework.jmx.export.assembler.MetadataMBeanInfoAssembler">
                <property name="attributeSource">
                        <bean class="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/>
                </property>
        </bean>

</beans>

Notice that in this configuration no beans are passed to the MBeanExporter; however, the JmxTestBean will still be registered since it is marked with the ManagedResource attribute and the MetadataMBeanInfoAssembler detects this and votes to include it. The only problem with this approach is that the name of the JmxTestBean now has business meaning. You can address this issue by changing the default behavior for ObjectName creation as defined in Controlling the ObjectNames for your beans.

4.3.5. Defining management interfaces using Java interfaces

In addition to the MetadataMBeanInfoAssembler, Spring also includes the InterfaceBasedMBeanInfoAssembler which allows you to constrain the methods and properties that are exposed based on the set of methods defined in a collection of interfaces.

Although the standard mechanism for exposing MBeans is to use interfaces and a simple naming scheme, the InterfaceBasedMBeanInfoAssembler extends this functionality by removing the need for naming conventions, allowing you to use more than one interface and removing the need for your beans to implement the MBean interfaces.

Consider this interface that is used to define a management interface for the JmxTestBean class that you saw earlier:

public interface IJmxTestBean {

        public int add(int x, int y);

        public long myOperation();

        public int getAge();

        public void setAge(int age);

        public void setName(String name);

        public String getName();

}

This interface defines the methods and properties that will be exposed as operations and attributes on the JMX MBean. The code below shows how to configure Spring JMX to use this interface as the definition for the management interface:

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean5" value-ref="testBean"/>
                        </map>
                </property>
                <property name="assembler">
                        <bean class="org.springframework.jmx.export.assembler.InterfaceBasedMBeanInfoAssembler">
                                <property name="managedInterfaces">
                                        <value>org.springframework.jmx.IJmxTestBean</value>
                                </property>
                        </bean>
                </property>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
          </bean>

</beans>

Here you can see that the InterfaceBasedMBeanInfoAssembler is configured to use the IJmxTestBean interface when constructing the management interface for any bean. It is important to understand that beans processed by the InterfaceBasedMBeanInfoAssembler are not required to implement the interface used to generate the JMX management interface.

In the case above, the IJmxTestBean interface is used to construct all management interfaces for all beans. In many cases this is not the desired behavior and you may want to use different interfaces for different beans. In this case, you can pass InterfaceBasedMBeanInfoAssembler a Properties instance via the interfaceMappings property, where the key of each entry is the bean name and the value of each entry is a comma-separated list of interface names to use for that bean.

If no management interface is specified through either the managedInterfaces or interfaceMappings properties, then the InterfaceBasedMBeanInfoAssembler will reflect on the bean and use all of the interfaces implemented by that bean to create the management interface.

4.3.6. Using MethodNameBasedMBeanInfoAssembler

The MethodNameBasedMBeanInfoAssembler allows you to specify a list of method names that will be exposed to JMX as attributes and operations. The code below shows a sample configuration for this:

<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
        <property name="beans">
                <map>
                        <entry key="bean:name=testBean5" value-ref="testBean"/>
                </map>
        </property>
        <property name="assembler">
                <bean class="org.springframework.jmx.export.assembler.MethodNameBasedMBeanInfoAssembler">
                        <property name="managedMethods">
                                <value>add,myOperation,getName,setName,getAge</value>
                        </property>
                </bean>
        </property>
</bean>

Here you can see that the methods add and myOperation will be exposed as JMX operations and getName(), setName(String) and getAge() will be exposed as the appropriate half of a JMX attribute. In the code above, the method mappings apply to beans that are exposed to JMX. To control method exposure on a bean-by-bean basis, use the methodMappings property of MethodNameMBeanInfoAssembler to map bean names to lists of method names.

4.4. Controlling the ObjectNames for your beans

Behind the scenes, the MBeanExporter delegates to an implementation of the ObjectNamingStrategy to obtain ObjectNames for each of the beans it is registering. The default implementation, KeyNamingStrategy, will, by default, use the key of the beans Map as the ObjectName. In addition, the KeyNamingStrategy can map the key of the beans Map to an entry in a Properties file (or files) to resolve the ObjectName. In addition to the KeyNamingStrategy, Spring provides two additional ObjectNamingStrategy implementations: the IdentityNamingStrategy that builds an ObjectName based on the JVM identity of the bean and the MetadataNamingStrategy that uses source level metadata to obtain the ObjectName.

4.4.1. Reading ObjectNames from Properties

You can configure your own KeyNamingStrategy instance and configure it to read ObjectNames from a Properties instance rather than use bean key. The KeyNamingStrategy will attempt to locate an entry in the Properties with a key corresponding to the bean key. If no entry is found or if the Properties instance is null then the bean key itself is used.

The code below shows a sample configuration for the KeyNamingStrategy:

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="testBean" value-ref="testBean"/>
                        </map>
                </property>
                <property name="namingStrategy" ref="namingStrategy"/>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

        <bean id="namingStrategy" class="org.springframework.jmx.export.naming.KeyNamingStrategy">
                <property name="mappings">
                        <props>
                                <prop key="testBean">bean:name=testBean1</prop>
                        </props>
                </property>
                <property name="mappingLocations">
                        <value>names1.properties,names2.properties</value>
                </property>
        </bean>

</beans>

Here an instance of KeyNamingStrategy is configured with a Properties instance that is merged from the Properties instance defined by the mapping property and the properties files located in the paths defined by the mappings property. In this configuration, the testBean bean will be given the ObjectName bean:name=testBean1 since this is the entry in the Properties instance that has a key corresponding to the bean key.

If no entry in the Properties instance can be found then the bean key name is used as the ObjectName.

4.4.2. Using the MetadataNamingStrategy

The MetadataNamingStrategy uses the objectName property of the ManagedResource attribute on each bean to create the ObjectName. The code below shows the configuration for the MetadataNamingStrategy:

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="testBean" value-ref="testBean"/>
                        </map>
                </property>
                <property name="namingStrategy" ref="namingStrategy"/>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

        <bean id="namingStrategy" class="org.springframework.jmx.export.naming.MetadataNamingStrategy">
                <property name="attributeSource" ref="attributeSource"/>
        </bean>

        <bean id="attributeSource"
                        class="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/>

</beans>

If no objectName has been provided for the ManagedResource attribute, then an ObjectName will be created with the following format:[fully-qualified-package-name]:type=[short-classname],name=[bean-name]. For example, the generated ObjectName for the following bean would be: com.foo:type=MyClass,name=myBean.

<bean id="myBean" class="com.foo.MyClass"/>

4.4.3. Configuring annotation based MBean export

If you prefer using the annotation based approach to define your management interfaces, then a convenience subclass of MBeanExporter is available: AnnotationMBeanExporter. When defining an instance of this subclass, the namingStrategy, assembler, and attributeSource configuration is no longer needed, since it will always use standard Java annotation-based metadata (autodetection is always enabled as well). In fact, rather than defining an MBeanExporter bean, an even simpler syntax is supported by the @EnableMBeanExport @Configuration annotation.

@Configuration
@EnableMBeanExport
public class AppConfig {

}

If you prefer XML based configuration the 'context:mbean-export' element serves the same purpose.

<context:mbean-export/>

You can provide a reference to a particular MBean server if necessary, and the defaultDomain attribute (a property of AnnotationMBeanExporter) accepts an alternate value for the generated MBean `ObjectNames’ domains. This would be used in place of the fully qualified package name as described in the previous section on MetadataNamingStrategy.

@EnableMBeanExport(server="myMBeanServer", defaultDomain="myDomain")
@Configuration
ContextConfiguration {

}
<context:mbean-export server="myMBeanServer" default-domain="myDomain"/>

Do not use interface-based AOP proxies in combination with autodetection of JMX annotations in your bean classes. Interface-based proxies 'hide' the target class, which also hides the JMX managed resource annotations. Hence, use target-class proxies in that case: through setting the 'proxy-target-class' flag on <aop:config/>, <tx:annotation-driven/>, etc. Otherwise, your JMX beans might be silently ignored at startup…​

4.5. JSR-160 Connectors

For remote access, Spring JMX module offers two FactoryBean implementations inside the org.springframework.jmx.support package for creating both server- and client-side connectors.

4.5.1. Server-side connectors

To have Spring JMX create, start and expose a JSR-160 JMXConnectorServer use the following configuration:

<bean id="serverConnector" class="org.springframework.jmx.support.ConnectorServerFactoryBean"/>

By default ConnectorServerFactoryBean creates a JMXConnectorServer bound to "service:jmx:jmxmp://localhost:9875". The serverConnector bean thus exposes the local MBeanServer to clients through the JMXMP protocol on localhost, port 9875. Note that the JMXMP protocol is marked as optional by the JSR 160 specification: currently, the main open-source JMX implementation, MX4J, and the one provided with the JDK do not support JMXMP.

To specify another URL and register the JMXConnectorServer itself with the MBeanServer use the serviceUrl and ObjectName properties respectively:

<bean id="serverConnector"
                class="org.springframework.jmx.support.ConnectorServerFactoryBean">
        <property name="objectName" value="connector:name=rmi"/>
        <property name="serviceUrl"
                        value="service:jmx:rmi://localhost/jndi/rmi://localhost:1099/myconnector"/>
</bean>

If the ObjectName property is set Spring will automatically register your connector with the MBeanServer under that ObjectName. The example below shows the full set of parameters which you can pass to the ConnectorServerFactoryBean when creating a JMXConnector:

<bean id="serverConnector"
                class="org.springframework.jmx.support.ConnectorServerFactoryBean">
        <property name="objectName" value="connector:name=iiop"/>
        <property name="serviceUrl"
                value="service:jmx:iiop://localhost/jndi/iiop://localhost:900/myconnector"/>
        <property name="threaded" value="true"/>
        <property name="daemon" value="true"/>
        <property name="environment">
                <map>
                        <entry key="someKey" value="someValue"/>
                </map>
        </property>
</bean>

Note that when using a RMI-based connector you need the lookup service (tnameserv or rmiregistry) to be started in order for the name registration to complete. If you are using Spring to export remote services for you via RMI, then Spring will already have constructed an RMI registry. If not, you can easily start a registry using the following snippet of configuration:

<bean id="registry" class="org.springframework.remoting.rmi.RmiRegistryFactoryBean">
        <property name="port" value="1099"/>
</bean>

4.5.2. Client-side connectors

To create an MBeanServerConnection to a remote JSR-160 enabled MBeanServer use the MBeanServerConnectionFactoryBean as shown below:

<bean id="clientConnector" class="org.springframework.jmx.support.MBeanServerConnectionFactoryBean">
        <property name="serviceUrl" value="service:jmx:rmi://localhost/jndi/rmi://localhost:1099/jmxrmi"/>
</bean>

4.5.3. JMX over Hessian or SOAP

JSR-160 permits extensions to the way in which communication is done between the client and the server. The examples above are using the mandatory RMI-based implementation required by the JSR-160 specification (IIOP and JRMP) and the (optional) JMXMP. By using other providers or JMX implementations (such as MX4J) you can take advantage of protocols like SOAP or Hessian over simple HTTP or SSL and others:

<bean id="serverConnector" class="org.springframework.jmx.support.ConnectorServerFactoryBean">
        <property name="objectName" value="connector:name=burlap"/>
        <property name="serviceUrl" value="service:jmx:burlap://localhost:9874"/>
</bean>

In the case of the above example, MX4J 3.0.0 was used; see the official MX4J documentation for more information.

4.6. Accessing MBeans via proxies

Spring JMX allows you to create proxies that re-route calls to MBeans registered in a local or remote MBeanServer. These proxies provide you with a standard Java interface through which you can interact with your MBeans. The code below shows how to configure a proxy for an MBean running in a local MBeanServer:

<bean id="proxy" class="org.springframework.jmx.access.MBeanProxyFactoryBean">
        <property name="objectName" value="bean:name=testBean"/>
        <property name="proxyInterface" value="org.springframework.jmx.IJmxTestBean"/>
</bean>

Here you can see that a proxy is created for the MBean registered under the ObjectName: bean:name=testBean. The set of interfaces that the proxy will implement is controlled by the proxyInterfaces property and the rules for mapping methods and properties on these interfaces to operations and attributes on the MBean are the same rules used by the InterfaceBasedMBeanInfoAssembler.

The MBeanProxyFactoryBean can create a proxy to any MBean that is accessible via an MBeanServerConnection. By default, the local MBeanServer is located and used, but you can override this and provide an MBeanServerConnection pointing to a remote MBeanServer to cater for proxies pointing to remote MBeans:

<bean id="clientConnector"
                class="org.springframework.jmx.support.MBeanServerConnectionFactoryBean">
        <property name="serviceUrl" value="service:jmx:rmi://remotehost:9875"/>
</bean>

<bean id="proxy" class="org.springframework.jmx.access.MBeanProxyFactoryBean">
        <property name="objectName" value="bean:name=testBean"/>
        <property name="proxyInterface" value="org.springframework.jmx.IJmxTestBean"/>
        <property name="server" ref="clientConnector"/>
</bean>

Here you can see that we create an MBeanServerConnection pointing to a remote machine using the MBeanServerConnectionFactoryBean. This MBeanServerConnection is then passed to the MBeanProxyFactoryBean via the server property. The proxy that is created will forward all invocations to the MBeanServer via this MBeanServerConnection.

4.7. Notifications

Spring’s JMX offering includes comprehensive support for JMX notifications.

4.7.1. Registering listeners for notifications

Spring’s JMX support makes it very easy to register any number of NotificationListeners with any number of MBeans (this includes MBeans exported by Spring’s MBeanExporter and MBeans registered via some other mechanism). By way of an example, consider the scenario where one would like to be informed (via a Notification) each and every time an attribute of a target MBean changes.

package com.example;

import javax.management.AttributeChangeNotification;
import javax.management.Notification;
import javax.management.NotificationFilter;
import javax.management.NotificationListener;

public class ConsoleLoggingNotificationListener
                implements NotificationListener, NotificationFilter {

        public void handleNotification(Notification notification, Object handback) {
                System.out.println(notification);
                System.out.println(handback);
        }

        public boolean isNotificationEnabled(Notification notification) {
                return AttributeChangeNotification.class.isAssignableFrom(notification.getClass());
        }

}
<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean1" value-ref="testBean"/>
                        </map>
                </property>
                <property name="notificationListenerMappings">
                        <map>
                                <entry key="bean:name=testBean1">
                                        <bean class="com.example.ConsoleLoggingNotificationListener"/>
                                </entry>
                        </map>
                </property>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

</beans>

With the above configuration in place, every time a JMX Notification is broadcast from the target MBean ( bean:name=testBean1), the ConsoleLoggingNotificationListener bean that was registered as a listener via the notificationListenerMappings property will be notified. The ConsoleLoggingNotificationListener bean can then take whatever action it deems appropriate in response to the Notification.

You can also use straight bean names as the link between exported beans and listeners:

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean1" value-ref="testBean"/>
                        </map>
                </property>
                <property name="notificationListenerMappings">
                        <map>
                                <entry key="testBean">
                                        <bean class="com.example.ConsoleLoggingNotificationListener"/>
                                </entry>
                        </map>
                </property>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

</beans>

If one wants to register a single NotificationListener instance for all of the beans that the enclosing MBeanExporter is exporting, one can use the special wildcard '*' (sans quotes) as the key for an entry in the notificationListenerMappings property map; for example:

<property name="notificationListenerMappings">
        <map>
                <entry key="*">
                        <bean class="com.example.ConsoleLoggingNotificationListener"/>
                </entry>
        </map>
</property>

If one needs to do the inverse (that is, register a number of distinct listeners against an MBean), then one has to use the notificationListeners list property instead (and in preference to the notificationListenerMappings property). This time, instead of configuring simply a NotificationListener for a single MBean, one configures NotificationListenerBean instances…​ a NotificationListenerBean encapsulates a NotificationListener and the ObjectName (or ObjectNames) that it is to be registered against in an MBeanServer. The NotificationListenerBean also encapsulates a number of other properties such as a NotificationFilter and an arbitrary handback object that can be used in advanced JMX notification scenarios.

The configuration when using NotificationListenerBean instances is not wildly different to what was presented previously:

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean1" value-ref="testBean"/>
                        </map>
                </property>
                <property name="notificationListeners">
                        <list>
                                <bean class="org.springframework.jmx.export.NotificationListenerBean">
                                        <constructor-arg>
                                                <bean class="com.example.ConsoleLoggingNotificationListener"/>
                                        </constructor-arg>
                                        <property name="mappedObjectNames">
                                                <list>
                                                        <value>bean:name=testBean1</value>
                                                </list>
                                        </property>
                                </bean>
                        </list>
                </property>
        </bean>

        <bean id="testBean" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

</beans>

The above example is equivalent to the first notification example. Lets assume then that we want to be given a handback object every time a Notification is raised, and that additionally we want to filter out extraneous Notifications by supplying a NotificationFilter. (For a full discussion of just what a handback object is, and indeed what a NotificationFilter is, please do consult that section of the JMX specification (1.2) entitled 'The JMX Notification Model'.)

<beans>

        <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
                <property name="beans">
                        <map>
                                <entry key="bean:name=testBean1" value-ref="testBean1"/>
                                <entry key="bean:name=testBean2" value-ref="testBean2"/>
                        </map>
                </property>
                <property name="notificationListeners">
                        <list>
                                <bean class="org.springframework.jmx.export.NotificationListenerBean">
                                        <constructor-arg ref="customerNotificationListener"/>
                                        <property name="mappedObjectNames">
                                                <list>
                                                        <!-- handles notifications from two distinct MBeans -->
                                                        <value>bean:name=testBean1</value>
                                                        <value>bean:name=testBean2</value>
                                                </list>
                                        </property>
                                        <property name="handback">
                                                <bean class="java.lang.String">
                                                        <constructor-arg value="This could be anything..."/>
                                                </bean>
                                        </property>
                                        <property name="notificationFilter" ref="customerNotificationListener"/>
                                </bean>
                        </list>
                </property>
        </bean>

        <!-- implements both the NotificationListener and NotificationFilter interfaces -->
        <bean id="customerNotificationListener" class="com.example.ConsoleLoggingNotificationListener"/>

        <bean id="testBean1" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="TEST"/>
                <property name="age" value="100"/>
        </bean>

        <bean id="testBean2" class="org.springframework.jmx.JmxTestBean">
                <property name="name" value="ANOTHER TEST"/>
                <property name="age" value="200"/>
        </bean>

</beans>

4.7.2. Publishing Notifications

Spring provides support not just for registering to receive Notifications, but also for publishing Notifications.

Please note that this section is really only relevant to Spring managed beans that have been exposed as MBeans via an MBeanExporter; any existing, user-defined MBeans should use the standard JMX APIs for notification publication.

The key interface in Spring’s JMX notification publication support is the NotificationPublisher interface (defined in the org.springframework.jmx.export.notification package). Any bean that is going to be exported as an MBean via an MBeanExporter instance can implement the related NotificationPublisherAware interface to gain access to a NotificationPublisher instance. The NotificationPublisherAware interface simply supplies an instance of a NotificationPublisher to the implementing bean via a simple setter method, which the bean can then use to publish Notifications.

As stated in the javadocs of the NotificationPublisher class, managed beans that are publishing events via the NotificationPublisher mechanism are not responsible for the state management of any notification listeners and the like …​ Spring’s JMX support will take care of handling all the JMX infrastructure issues. All one need do as an application developer is implement the NotificationPublisherAware interface and start publishing events using the supplied NotificationPublisher instance. Note that the NotificationPublisher will be set after the managed bean has been registered with an MBeanServer.

Using a NotificationPublisher instance is quite straightforward…​ one simply creates a JMX Notification instance (or an instance of an appropriate Notification subclass), populates the notification with the data pertinent to the event that is to be published, and one then invokes the sendNotification(Notification) on the NotificationPublisher instance, passing in the Notification.

Find below a simple example…​ in this scenario, exported instances of the JmxTestBean are going to publish a NotificationEvent every time the add(int, int) operation is invoked.

package org.springframework.jmx;

import org.springframework.jmx.export.notification.NotificationPublisherAware;
import org.springframework.jmx.export.notification.NotificationPublisher;
import javax.management.Notification;

public class JmxTestBean implements IJmxTestBean, NotificationPublisherAware {

        private String name;
        private int age;
        private boolean isSuperman;
        private NotificationPublisher publisher;

        // other getters and setters omitted for clarity

        public int add(int x, int y) {
                int answer = x + y;
                this.publisher.sendNotification(new Notification("add", this, 0));
                return answer;
        }

        public void dontExposeMe() {
                throw new RuntimeException();
        }

        public void setNotificationPublisher(NotificationPublisher notificationPublisher) {
                this.publisher = notificationPublisher;
        }

}

The NotificationPublisher interface and the machinery to get it all working is one of the nicer features of Spring’s JMX support. It does however come with the price tag of coupling your classes to both Spring and JMX; as always, the advice here is to be pragmatic…​ if you need the functionality offered by the NotificationPublisher and you can accept the coupling to both Spring and JMX, then do so.

4.8. Further resources

This section contains links to further resources about JMX.

5. JCA CCI

5.1. Introduction

Java EE provides a specification to standardize access to enterprise information systems (EIS): the JCA (Java EE Connector Architecture). This specification is divided into several different parts:

  • SPI (Service provider interfaces) that the connector provider must implement. These interfaces constitute a resource adapter which can be deployed on a Java EE application server. In such a scenario, the server manages connection pooling, transaction and security (managed mode). The application server is also responsible for managing the configuration, which is held outside the client application. A connector can be used without an application server as well; in this case, the application must configure it directly (non-managed mode).

  • CCI (Common Client Interface) that an application can use to interact with the connector and thus communicate with an EIS. An API for local transaction demarcation is provided as well.

The aim of the Spring CCI support is to provide classes to access a CCI connector in typical Spring style, leveraging the Spring Framework’s general resource and transaction management facilities.

The client side of connectors doesn’t alway use CCI. Some connectors expose their own APIs, only providing JCA resource adapter to use the system contracts of a Java EE container (connection pooling, global transactions, security). Spring does not offer special support for such connector-specific APIs.

5.2. Configuring CCI

5.2.1. Connector configuration

The base resource to use JCA CCI is the ConnectionFactory interface. The connector used must provide an implementation of this interface.

To use your connector, you can deploy it on your application server and fetch the ConnectionFactory from the server’s JNDI environment (managed mode). The connector must be packaged as a RAR file (resource adapter archive) and contain a ra.xml file to describe its deployment characteristics. The actual name of the resource is specified when you deploy it. To access it within Spring, simply use Spring’s JndiObjectFactoryBean / <jee:jndi-lookup> fetch the factory by its JNDI name.

Another way to use a connector is to embed it in your application (non-managed mode), not using an application server to deploy and configure it. Spring offers the possibility to configure a connector as a bean, through a provided FactoryBean ( LocalConnectionFactoryBean). In this manner, you only need the connector library in the classpath (no RAR file and no ra.xml descriptor needed). The library must be extracted from the connector’s RAR file, if necessary.

Once you have got access to your ConnectionFactory instance, you can inject it into your components. These components can either be coded against the plain CCI API or leverage Spring’s support classes for CCI access (e.g. CciTemplate).

When you use a connector in non-managed mode, you can’t use global transactions because the resource is never enlisted / delisted in the current global transaction of the current thread. The resource is simply not aware of any global Java EE transactions that might be running.

5.2.2. ConnectionFactory configuration in Spring

In order to make connections to the EIS, you need to obtain a ConnectionFactory from the application server if you are in a managed mode, or directly from Spring if you are in a non-managed mode.

In a managed mode, you access a ConnectionFactory from JNDI; its properties will be configured in the application server.

<jee:jndi-lookup id="eciConnectionFactory" jndi-name="eis/cicseci"/>

In non-managed mode, you must configure the ConnectionFactory you want to use in the configuration of Spring as a JavaBean. The LocalConnectionFactoryBean class offers this setup style, passing in the ManagedConnectionFactory implementation of your connector, exposing the application-level CCI ConnectionFactory.

<bean id="eciManagedConnectionFactory" class="com.ibm.connector2.cics.ECIManagedConnectionFactory">
        <property name="serverName" value="TXSERIES"/>
        <property name="connectionURL" value="tcp://localhost/"/>
        <property name="portNumber" value="2006"/>
</bean>

<bean id="eciConnectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean">
        <property name="managedConnectionFactory" ref="eciManagedConnectionFactory"/>
</bean>

You can’t directly instantiate a specific ConnectionFactory. You need to go through the corresponding implementation of the ManagedConnectionFactory interface for your connector. This interface is part of the JCA SPI specification.

5.2.3. Configuring CCI connections

JCA CCI allow the developer to configure the connections to the EIS using the ConnectionSpec implementation of your connector. In order to configure its properties, you need to wrap the target connection factory with a dedicated adapter, ConnectionSpecConnectionFactoryAdapter. So, the dedicated ConnectionSpec can be configured with the property connectionSpec (as an inner bean).

This property is not mandatory because the CCI ConnectionFactory interface defines two different methods to obtain a CCI connection. Some of the ConnectionSpec properties can often be configured in the application server (in managed mode) or on the corresponding local ManagedConnectionFactory implementation.

public interface ConnectionFactory implements Serializable, Referenceable {
        ...
        Connection getConnection() throws ResourceException;
        Connection getConnection(ConnectionSpec connectionSpec) throws ResourceException;
        ...
}

Spring provides a ConnectionSpecConnectionFactoryAdapter that allows for specifying a ConnectionSpec instance to use for all operations on a given factory. If the adapter’s connectionSpec property is specified, the adapter uses the getConnection variant with the ConnectionSpec argument, otherwise the variant without argument.

<bean id="managedConnectionFactory"
                class="com.sun.connector.cciblackbox.CciLocalTxManagedConnectionFactory">
        <property name="connectionURL" value="jdbc:hsqldb:hsql://localhost:9001"/>
        <property name="driverName" value="org.hsqldb.jdbcDriver"/>
</bean>

<bean id="targetConnectionFactory"
                class="org.springframework.jca.support.LocalConnectionFactoryBean">
        <property name="managedConnectionFactory" ref="managedConnectionFactory"/>
</bean>

<bean id="connectionFactory"
                class="org.springframework.jca.cci.connection.ConnectionSpecConnectionFactoryAdapter">
        <property name="targetConnectionFactory" ref="targetConnectionFactory"/>
        <property name="connectionSpec">
                <bean class="com.sun.connector.cciblackbox.CciConnectionSpec">
                        <property name="user" value="sa"/>
                        <property name="password" value=""/>
                </bean>
        </property>
</bean>

5.2.4. Using a single CCI connection

If you want to use a single CCI connection, Spring provides a further ConnectionFactory adapter to manage this. The SingleConnectionFactory adapter class will open a single connection lazily and close it when this bean is destroyed at application shutdown. This class will expose special Connection proxies that behave accordingly, all sharing the same underlying physical connection.

<bean id="eciManagedConnectionFactory"
                class="com.ibm.connector2.cics.ECIManagedConnectionFactory">
        <property name="serverName" value="TEST"/>
        <property name="connectionURL" value="tcp://localhost/"/>
        <property name="portNumber" value="2006"/>
</bean>

<bean id="targetEciConnectionFactory"
                class="org.springframework.jca.support.LocalConnectionFactoryBean">
        <property name="managedConnectionFactory" ref="eciManagedConnectionFactory"/>
</bean>

<bean id="eciConnectionFactory"
                class="org.springframework.jca.cci.connection.SingleConnectionFactory">
        <property name="targetConnectionFactory" ref="targetEciConnectionFactory"/>
</bean>

This ConnectionFactory adapter cannot directly be configured with a ConnectionSpec. Use an intermediary ConnectionSpecConnectionFactoryAdapter that the SingleConnectionFactory talks to if you require a single connection for a specific ConnectionSpec.

5.3. Using Spring’s CCI access support

5.3.1. Record conversion

One of the aims of the JCA CCI support is to provide convenient facilities for manipulating CCI records. The developer can specify the strategy to create records and extract datas from records, for use with Spring’s CciTemplate. The following interfaces will configure the strategy to use input and output records if you don’t want to work with records directly in your application.

In order to create an input Record, the developer can use a dedicated implementation of the RecordCreator interface.

public interface RecordCreator {

        Record createRecord(RecordFactory recordFactory) throws ResourceException, DataAccessException;

}

As you can see, the createRecord(..) method receives a RecordFactory instance as parameter, which corresponds to the RecordFactory of the ConnectionFactory used. This reference can be used to create IndexedRecord or MappedRecord instances. The following sample shows how to use the RecordCreator interface and indexed/mapped records.

public class MyRecordCreator implements RecordCreator {

        public Record createRecord(RecordFactory recordFactory) throws ResourceException {
                IndexedRecord input = recordFactory.createIndexedRecord("input");
                input.add(new Integer(id));
                return input;
        }

}

An output Record can be used to receive data back from the EIS. Hence, a specific implementation of the RecordExtractor interface can be passed to Spring’s CciTemplate for extracting data from the output Record.

public interface RecordExtractor {

        Object extractData(Record record) throws ResourceException, SQLException, DataAccessException;

}

The following sample shows how to use the RecordExtractor interface.

public class MyRecordExtractor implements RecordExtractor {

        public Object extractData(Record record) throws ResourceException {
                CommAreaRecord commAreaRecord = (CommAreaRecord) record;
                String str = new String(commAreaRecord.toByteArray());
                String field1 = string.substring(0,6);
                String field2 = string.substring(6,1);
                return new OutputObject(Long.parseLong(field1), field2);
        }

}

5.3.2. CciTemplate

The CciTemplate is the central class of the core CCI support package ( org.springframework.jca.cci.core). It simplifies the use of CCI since it handles the creation and release of resources. This helps to avoid common errors like forgetting to always close the connection. It cares for the lifecycle of connection and interaction objects, letting application code focus on generating input records from application data and extracting application data from output records.

The JCA CCI specification defines two distinct methods to call operations on an EIS. The CCI Interaction interface provides two execute method signatures:

public interface javax.resource.cci.Interaction {

        ...

        boolean execute(InteractionSpec spec, Record input, Record output) throws ResourceException;

        Record execute(InteractionSpec spec, Record input) throws ResourceException;

        ...

}

Depending on the template method called, CciTemplate will know which execute method to call on the interaction. In any case, a correctly initialized InteractionSpec instance is mandatory.

CciTemplate.execute(..) can be used in two ways:

  • With direct Record arguments. In this case, you simply need to pass the CCI input record in, and the returned object be the corresponding CCI output record.

  • With application objects, using record mapping. In this case, you need to provide corresponding RecordCreator and RecordExtractor instances.

With the first approach, the following methods of the template will be used. These methods directly correspond to those on the Interaction interface.

public class CciTemplate implements CciOperations {

        public Record execute(InteractionSpec spec, Record inputRecord)
                        throws DataAccessException { ... }

        public void execute(InteractionSpec spec, Record inputRecord, Record outputRecord)
                        throws DataAccessException { ... }

}

With the second approach, we need to specify the record creation and record extraction strategies as arguments. The interfaces used are those describe in the previous section on record conversion. The corresponding CciTemplate methods are the following:

public class CciTemplate implements CciOperations {

        public Record execute(InteractionSpec spec,
                        RecordCreator inputCreator) throws DataAccessException {
                // ...
        }

        public Object execute(InteractionSpec spec, Record inputRecord,
                        RecordExtractor outputExtractor) throws DataAccessException {
                // ...
        }

        public Object execute(InteractionSpec spec, RecordCreator creator,
                        RecordExtractor extractor) throws DataAccessException {
                // ...
        }

}

Unless the outputRecordCreator property is set on the template (see the following section), every method will call the corresponding execute method of the CCI Interaction with two parameters: InteractionSpec and input Record, receiving an output Record as return value.

CciTemplate also provides methods to create IndexRecord and MappedRecord outside a RecordCreator implementation, through its createIndexRecord(..) and createMappedRecord(..) methods. This can be used within DAO implementations to create Record instances to pass into corresponding CciTemplate.execute(..) methods.

public class CciTemplate implements CciOperations {

        public IndexedRecord createIndexedRecord(String name) throws DataAccessException { ... }

        public MappedRecord createMappedRecord(String name) throws DataAccessException { ... }

}

5.3.3. DAO support

Spring’s CCI support provides a abstract class for DAOs, supporting injection of a ConnectionFactory or a CciTemplate instances. The name of the class is CciDaoSupport: It provides simple setConnectionFactory and setCciTemplate methods. Internally, this class will create a CciTemplate instance for a passed-in ConnectionFactory, exposing it to concrete data access implementations in subclasses.

public abstract class CciDaoSupport {

        public void setConnectionFactory(ConnectionFactory connectionFactory) {
                // ...
        }

        public ConnectionFactory getConnectionFactory() {
                // ...
        }

        public void setCciTemplate(CciTemplate cciTemplate) {
                // ...
        }

        public CciTemplate getCciTemplate() {
                // ...
        }

}

5.3.4. Automatic output record generation

If the connector used only supports the Interaction.execute(..) method with input and output records as parameters (that is, it requires the desired output record to be passed in instead of returning an appropriate output record), you can set the outputRecordCreator property of the CciTemplate to automatically generate an output record to be filled by the JCA connector when the response is received. This record will be then returned to the caller of the template.

This property simply holds an implementation of the RecordCreator interface, used for that purpose. The RecordCreator interface has already been discussed in Record conversion. The outputRecordCreator property must be directly specified on the CciTemplate. This could be done in the application code like so:

cciTemplate.setOutputRecordCreator(new EciOutputRecordCreator());

Or (recommended) in the Spring configuration, if the CciTemplate is configured as a dedicated bean instance:

<bean id="eciOutputRecordCreator" class="eci.EciOutputRecordCreator"/>

<bean id="cciTemplate" class="org.springframework.jca.cci.core.CciTemplate">
        <property name="connectionFactory" ref="eciConnectionFactory"/>
        <property name="outputRecordCreator" ref="eciOutputRecordCreator"/>
</bean>

As the CciTemplate class is thread-safe, it will usually be configured as a shared instance.

5.3.5. Summary

The following table summarizes the mechanisms of the CciTemplate class and the corresponding methods called on the CCI Interaction interface:

Table 8. Usage of Interaction execute methods
CciTemplate method signature CciTemplate outputRecordCreator property execute method called on the CCI Interaction

Record execute(InteractionSpec, Record)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, Record)

set

boolean execute(InteractionSpec, Record, Record)

void execute(InteractionSpec, Record, Record)

not set

void execute(InteractionSpec, Record, Record)

void execute(InteractionSpec, Record, Record)

set

void execute(InteractionSpec, Record, Record)

Record execute(InteractionSpec, RecordCreator)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, RecordCreator)

set

void execute(InteractionSpec, Record, Record)

Record execute(InteractionSpec, Record, RecordExtractor)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, Record, RecordExtractor)

set

void execute(InteractionSpec, Record, Record)

Record execute(InteractionSpec, RecordCreator, RecordExtractor)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, RecordCreator, RecordExtractor)

set

void execute(InteractionSpec, Record, Record)

5.3.6. Using a CCI Connection and Interaction directly

CciTemplate also offers the possibility to work directly with CCI connections and interactions, in the same manner as JdbcTemplate and JmsTemplate. This is useful when you want to perform multiple operations on a CCI connection or interaction, for example.

The interface ConnectionCallback provides a CCI Connection as argument, in order to perform custom operations on it, plus the CCI ConnectionFactory which the Connection was created with. The latter can be useful for example to get an associated RecordFactory instance and create indexed/mapped records, for example.

public interface ConnectionCallback {

        Object doInConnection(Connection connection, ConnectionFactory connectionFactory)
                        throws ResourceException, SQLException, DataAccessException;

}

The interface InteractionCallback provides the CCI Interaction, in order to perform custom operations on it, plus the corresponding CCI ConnectionFactory.

public interface InteractionCallback {

        Object doInInteraction(Interaction interaction, ConnectionFactory connectionFactory)
                throws ResourceException, SQLException, DataAccessException;

}

InteractionSpec objects can either be shared across multiple template calls or newly created inside every callback method. This is completely up to the DAO implementation.

5.3.7. Example for CciTemplate usage

In this section, the usage of the CciTemplate will be shown to acces to a CICS with ECI mode, with the IBM CICS ECI connector.

Firstly, some initializations on the CCI InteractionSpec must be done to specify which CICS program to access and how to interact with it.

ECIInteractionSpec interactionSpec = new ECIInteractionSpec();
interactionSpec.setFunctionName("MYPROG");
interactionSpec.setInteractionVerb(ECIInteractionSpec.SYNC_SEND_RECEIVE);

Then the program can use CCI via Spring’s template and specify mappings between custom objects and CCI Records.

public class MyDaoImpl extends CciDaoSupport implements MyDao {

        public OutputObject getData(InputObject input) {
                ECIInteractionSpec interactionSpec = ...;

        OutputObject output = (ObjectOutput) getCciTemplate().execute(interactionSpec,
                new RecordCreator() {
                        public Record createRecord(RecordFactory recordFactory) throws ResourceException {
                                return new CommAreaRecord(input.toString().getBytes());
                        }
                },
                new RecordExtractor() {
                        public Object extractData(Record record) throws ResourceException {
                                CommAreaRecord commAreaRecord = (CommAreaRecord)record;
                                String str = new String(commAreaRecord.toByteArray());
                                String field1 = string.substring(0,6);
                                String field2 = string.substring(6,1);
                                return new OutputObject(Long.parseLong(field1), field2);
                        }
                });

                return output;
        }
}

As discussed previously, callbacks can be used to work directly on CCI connections or interactions.

public class MyDaoImpl extends CciDaoSupport implements MyDao {

        public OutputObject getData(InputObject input) {
                ObjectOutput output = (ObjectOutput) getCciTemplate().execute(
                        new ConnectionCallback() {
                                public Object doInConnection(Connection connection,
                                                ConnectionFactory factory) throws ResourceException {

                                        // do something...

                                }
                        });
                }
                return output;
        }

}

With a ConnectionCallback, the Connection used will be managed and closed by the CciTemplate, but any interactions created on the connection must be managed by the callback implementation.

For a more specific callback, you can implement an InteractionCallback. The passed-in Interaction will be managed and closed by the CciTemplate in this case.

public class MyDaoImpl extends CciDaoSupport implements MyDao {

        public String getData(String input) {
                ECIInteractionSpec interactionSpec = ...;
                String output = (String) getCciTemplate().execute(interactionSpec,
                        new InteractionCallback() {
                                public Object doInInteraction(Interaction interaction,
                                                ConnectionFactory factory) throws ResourceException {
                                        Record input = new CommAreaRecord(inputString.getBytes());
                                        Record output = new CommAreaRecord();
                                        interaction.execute(holder.getInteractionSpec(), input, output);
                                        return new String(output.toByteArray());
                                }
                        });
                return output;
        }

}

For the examples above, the corresponding configuration of the involved Spring beans could look like this in non-managed mode:

<bean id="managedConnectionFactory" class="com.ibm.connector2.cics.ECIManagedConnectionFactory">
        <property name="serverName" value="TXSERIES"/>
        <property name="connectionURL" value="local:"/>
        <property name="userName" value="CICSUSER"/>
        <property name="password" value="CICS"/>
</bean>

<bean id="connectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean">
        <property name="managedConnectionFactory" ref="managedConnectionFactory"/>
</bean>

<bean id="component" class="mypackage.MyDaoImpl">
        <property name="connectionFactory" ref="connectionFactory"/>
</bean>

In managed mode (that is, in a Java EE environment), the configuration could look as follows:

<jee:jndi-lookup id="connectionFactory" jndi-name="eis/cicseci"/>

<bean id="component" class="MyDaoImpl">
        <property name="connectionFactory" ref="connectionFactory"/>
</bean>

5.4. Modeling CCI access as operation objects

The org.springframework.jca.cci.object package contains support classes that allow you to access the EIS in a different style: through reusable operation objects, analogous to Spring’s JDBC operation objects (see JDBC chapter). This will usually encapsulate the CCI API: an application-level input object will be passed to the operation object, so it can construct the input record and then convert the received record data to an application-level output object and return it.

This approach is internally based on the CciTemplate class and the RecordCreator / RecordExtractor interfaces, reusing the machinery of Spring’s core CCI support.

5.4.1. MappingRecordOperation

MappingRecordOperation essentially performs the same work as CciTemplate, but represents a specific, pre-configured operation as an object. It provides two template methods to specify how to convert an input object to a input record, and how to convert an output record to an output object (record mapping):

  • createInputRecord(..) to specify how to convert an input object to an input Record

  • extractOutputData(..) to specify how to extract an output object from an output Record

Here are the signatures of these methods:

public abstract class MappingRecordOperation extends EisOperation {

        ...

        protected abstract Record createInputRecord(RecordFactory recordFactory,
                        Object inputObject) throws ResourceException, DataAccessException {
                // ...
        }

        protected abstract Object extractOutputData(Record outputRecord)
                        throws ResourceException, SQLException, DataAccessException {
                // ...
        }

        ...

}

Thereafter, in order to execute an EIS operation, you need to use a single execute method, passing in an application-level input object and receiving an application-level output object as result:

public abstract class MappingRecordOperation extends EisOperation {

        ...

        public Object execute(Object inputObject) throws DataAccessException {
        }

        ...
}

As you can see, contrary to the CciTemplate class, this execute(..) method does not have an InteractionSpec as argument. Instead, the InteractionSpec is global to the operation. The following constructor must be used to instantiate an operation object with a specific InteractionSpec:

InteractionSpec spec = ...;
MyMappingRecordOperation eisOperation = new MyMappingRecordOperation(getConnectionFactory(), spec);
...

5.4.2. MappingCommAreaOperation

Some connectors use records based on a COMMAREA which represents an array of bytes containing parameters to send to the EIS and data returned by it. Spring provides a special operation class for working directly on COMMAREA rather than on records. The MappingCommAreaOperation class extends the MappingRecordOperation class to provide such special COMMAREA support. It implicitly uses the CommAreaRecord class as input and output record type, and provides two new methods to convert an input object into an input COMMAREA and the output COMMAREA into an output object.

public abstract class MappingCommAreaOperation extends MappingRecordOperation {

        ...

        protected abstract byte[] objectToBytes(Object inObject)
                        throws IOException, DataAccessException;

        protected abstract Object bytesToObject(byte[] bytes)
                throws IOException, DataAccessException;

        ...

}

5.4.3. Automatic output record generation

As every MappingRecordOperation subclass is based on CciTemplate internally, the same way to automatically generate output records as with CciTemplate is available. Every operation object provides a corresponding setOutputRecordCreator(..) method. For further information, see Automatic output record generation.

5.4.4. Summary

The operation object approach uses records in the same manner as the CciTemplate class.

Table 9. Usage of Interaction execute methods
MappingRecordOperation method signature MappingRecordOperation outputRecordCreator property execute method called on the CCI Interaction

Object execute(Object)

not set

Record execute(InteractionSpec, Record)

Object execute(Object)

set

boolean execute(InteractionSpec, Record, Record)

5.4.5. Example for MappingRecordOperation usage

In this section, the usage of the MappingRecordOperation will be shown to access a database with the Blackbox CCI connector.

The original version of this connector is provided by the Java EE SDK (version 1.3), available from Oracle.

Firstly, some initializations on the CCI InteractionSpec must be done to specify which SQL request to execute. In this sample, we directly define the way to convert the parameters of the request to a CCI record and the way to convert the CCI result record to an instance of the Person class.

public class PersonMappingOperation extends MappingRecordOperation {

        public PersonMappingOperation(ConnectionFactory connectionFactory) {
                setConnectionFactory(connectionFactory);
                CciInteractionSpec interactionSpec = new CciConnectionSpec();
                interactionSpec.setSql("select * from person where person_id=?");
                setInteractionSpec(interactionSpec);
        }

        protected Record createInputRecord(RecordFactory recordFactory,
                        Object inputObject) throws ResourceException {
                Integer id = (Integer) inputObject;
                IndexedRecord input = recordFactory.createIndexedRecord("input");
                input.add(new Integer(id));
                return input;
        }

        protected Object extractOutputData(Record outputRecord)
                        throws ResourceException, SQLException {
                ResultSet rs = (ResultSet) outputRecord;
                Person person = null;
                if (rs.next()) {
                        Person person = new Person();
                        person.setId(rs.getInt("person_id"));
                        person.setLastName(rs.getString("person_last_name"));
                        person.setFirstName(rs.getString("person_first_name"));
                }
                return person;
        }
}

Then the application can execute the operation object, with the person identifier as argument. Note that operation object could be set up as shared instance, as it is thread-safe.

public class MyDaoImpl extends CciDaoSupport implements MyDao {

        public Person getPerson(int id) {
                PersonMappingOperation query = new PersonMappingOperation(getConnectionFactory());
                Person person = (Person) query.execute(new Integer(id));
                return person;
        }
}

The corresponding configuration of Spring beans could look as follows in non-managed mode:

<bean id="managedConnectionFactory"
                class="com.sun.connector.cciblackbox.CciLocalTxManagedConnectionFactory">
        <property name="connectionURL" value="jdbc:hsqldb:hsql://localhost:9001"/>
        <property name="driverName" value="org.hsqldb.jdbcDriver"/>
</bean>

<bean id="targetConnectionFactory"
                class="org.springframework.jca.support.LocalConnectionFactoryBean">
        <property name="managedConnectionFactory" ref="managedConnectionFactory"/>
</bean>

<bean id="connectionFactory"
                class="org.springframework.jca.cci.connection.ConnectionSpecConnectionFactoryAdapter">
        <property name="targetConnectionFactory" ref="targetConnectionFactory"/>
        <property name="connectionSpec">
                <bean class="com.sun.connector.cciblackbox.CciConnectionSpec">
                        <property name="user" value="sa"/>
                        <property name="password" value=""/>
                </bean>
        </property>
</bean>

<bean id="component" class="MyDaoImpl">
        <property name="connectionFactory" ref="connectionFactory"/>
</bean>

In managed mode (that is, in a Java EE environment), the configuration could look as follows:

<jee:jndi-lookup id="targetConnectionFactory" jndi-name="eis/blackbox"/>

<bean id="connectionFactory"
                class="org.springframework.jca.cci.connection.ConnectionSpecConnectionFactoryAdapter">
        <property name="targetConnectionFactory" ref="targetConnectionFactory"/>
        <property name="connectionSpec">
                <bean class="com.sun.connector.cciblackbox.CciConnectionSpec">
                        <property name="user" value="sa"/>
                        <property name="password" value=""/>
                </bean>
        </property>
</bean>

<bean id="component" class="MyDaoImpl">
        <property name="connectionFactory" ref="connectionFactory"/>
</bean>

5.4.6. Example for MappingCommAreaOperation usage

In this section, the usage of the MappingCommAreaOperation will be shown: accessing a CICS with ECI mode with the IBM CICS ECI connector.

Firstly, the CCI InteractionSpec needs to be initialized to specify which CICS program to access and how to interact with it.

public abstract class EciMappingOperation extends MappingCommAreaOperation {

        public EciMappingOperation(ConnectionFactory connectionFactory, String programName) {
                setConnectionFactory(connectionFactory);
                ECIInteractionSpec interactionSpec = new ECIInteractionSpec(),
                interactionSpec.setFunctionName(programName);
                interactionSpec.setInteractionVerb(ECIInteractionSpec.SYNC_SEND_RECEIVE);
                interactionSpec.setCommareaLength(30);
                setInteractionSpec(interactionSpec);
                setOutputRecordCreator(new EciOutputRecordCreator());
        }

        private static class EciOutputRecordCreator implements RecordCreator {
                public Record createRecord(RecordFactory recordFactory) throws ResourceException {
                        return new CommAreaRecord();
                }
        }

}

The abstract EciMappingOperation class can then be subclassed to specify mappings between custom objects and Records.

public class MyDaoImpl extends CciDaoSupport implements MyDao {

        public OutputObject getData(Integer id) {
                EciMappingOperation query = new EciMappingOperation(getConnectionFactory(), "MYPROG") {

                        protected abstract byte[] objectToBytes(Object inObject) throws IOException {
                                Integer id = (Integer) inObject;
                                return String.valueOf(id);
                        }

                        protected abstract Object bytesToObject(byte[] bytes) throws IOException;
                                String str = new String(bytes);
                                String field1 = str.substring(0,6);
                                String field2 = str.substring(6,1);
                                String field3 = str.substring(7,1);
                                return new OutputObject(field1, field2, field3);
                        }
                });

                return (OutputObject) query.execute(new Integer(id));
        }

}

The corresponding configuration of Spring beans could look as follows in non-managed mode:

<bean id="managedConnectionFactory" class="com.ibm.connector2.cics.ECIManagedConnectionFactory">
        <property name="serverName" value="TXSERIES"/>
        <property name="connectionURL" value="local:"/>
        <property name="userName" value="CICSUSER"/>
        <property name="password" value="CICS"/>
</bean>

<bean id="connectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean">
        <property name="managedConnectionFactory" ref="managedConnectionFactory"/>
</bean>

<bean id="component" class="MyDaoImpl">
        <property name="connectionFactory" ref="connectionFactory"/>
</bean>

In managed mode (that is, in a Java EE environment), the configuration could look as follows:

<jee:jndi-lookup id="connectionFactory" jndi-name="eis/cicseci"/>

<bean id="component" class="MyDaoImpl">
        <property name="connectionFactory" ref="connectionFactory"/>
</bean>

5.5. Transactions

JCA specifies several levels of transaction support for resource adapters. The kind of transactions that your resource adapter supports is specified in its ra.xml file. There are essentially three options: none (for example with CICS EPI connector), local transactions (for example with a CICS ECI connector), global transactions (for example with an IMS connector).

<connector>
        <resourceadapter>
                <!-- <transaction-support>NoTransaction</transaction-support> -->
                <!-- <transaction-support>LocalTransaction</transaction-support> -->
                <transaction-support>XATransaction</transaction-support>
        <resourceadapter>
<connector>

For global transactions, you can use Spring’s generic transaction infrastructure to demarcate transactions, with JtaTransactionManager as backend (delegating to the Java EE server’s distributed transaction coordinator underneath).

For local transactions on a single CCI ConnectionFactory, Spring provides a specific transaction management strategy for CCI, analogous to the DataSourceTransactionManager for JDBC. The CCI API defines a local transaction object and corresponding local transaction demarcation methods. Spring’s CciLocalTransactionManager executes such local CCI transactions, fully compliant with Spring’s generic PlatformTransactionManager abstraction.

<jee:jndi-lookup id="eciConnectionFactory" jndi-name="eis/cicseci"/>

<bean id="eciTransactionManager"
                class="org.springframework.jca.cci.connection.CciLocalTransactionManager">
        <property name="connectionFactory" ref="eciConnectionFactory"/>
</bean>

Both transaction strategies can be used with any of Spring’s transaction demarcation facilities, be it declarative or programmatic. This is a consequence of Spring’s generic PlatformTransactionManager abstraction, which decouples transaction demarcation from the actual execution strategy. Simply switch between JtaTransactionManager and CciLocalTransactionManager as needed, keeping your transaction demarcation as-is.

For more information on Spring’s transaction facilities, see the chapter entitled Transaction Management.

6. Email

6.1. Introduction

Library dependencies

The following JAR needs to be on the classpath of your application in order to use the Spring Framework’s email library.

This library is freely available on the web — for example, in Maven Central as com.sun.mail:javax.mail.

The Spring Framework provides a helpful utility library for sending email that shields the user from the specifics of the underlying mailing system and is responsible for low level resource handling on behalf of the client.

The org.springframework.mail package is the root level package for the Spring Framework’s email support. The central interface for sending emails is the MailSender interface; a simple value object encapsulating the properties of a simple mail such as from and to (plus many others) is the SimpleMailMessage class. This package also contains a hierarchy of checked exceptions which provide a higher level of abstraction over the lower level mail system exceptions with the root exception being MailException. Please refer to the javadocs for more information on the rich mail exception hierarchy.

The org.springframework.mail.javamail.JavaMailSender interface adds specialized JavaMail features such as MIME message support to the MailSender interface (from which it inherits). JavaMailSender also provides a callback interface for preparation of JavaMail MIME messages, called org.springframework.mail.javamail.MimeMessagePreparator

6.2. Usage

Let’s assume there is a business interface called OrderManager:

public interface OrderManager {

        void placeOrder(Order order);

}

Let us also assume that there is a requirement stating that an email message with an order number needs to be generated and sent to a customer placing the relevant order.

6.2.1. Basic MailSender and SimpleMailMessage usage

import org.springframework.mail.MailException;
import org.springframework.mail.MailSender;
import org.springframework.mail.SimpleMailMessage;

public class SimpleOrderManager implements OrderManager {

        private MailSender mailSender;
        private SimpleMailMessage templateMessage;

        public void setMailSender(MailSender mailSender) {
                this.mailSender = mailSender;
        }

        public void setTemplateMessage(SimpleMailMessage templateMessage) {
                this.templateMessage = templateMessage;
        }

        public void placeOrder(Order order) {

                // Do the business calculations...

                // Call the collaborators to persist the order...

                // Create a thread safe "copy" of the template message and customize it
                SimpleMailMessage msg = new SimpleMailMessage(this.templateMessage);
                msg.setTo(order.getCustomer().getEmailAddress());
                msg.setText(
                        "Dear " + order.getCustomer().getFirstName()
                                + order.getCustomer().getLastName()
                                + ", thank you for placing order. Your order number is "
                                + order.getOrderNumber());
                try{
                        this.mailSender.send(msg);
                }
                catch (MailException ex) {
                        // simply log it and go on...
                        System.err.println(ex.getMessage());
                }
        }

}

Find below the bean definitions for the above code:

<bean id="mailSender" class="org.springframework.mail.javamail.JavaMailSenderImpl">
        <property name="host" value="mail.mycompany.com"/>
</bean>

<!-- this is a template message that we can pre-load with default state -->
<bean id="templateMessage" class="org.springframework.mail.SimpleMailMessage">
        <property name="from" value="customerservice@mycompany.com"/>
        <property name="subject" value="Your order"/>
</bean>

<bean id="orderManager" class="com.mycompany.businessapp.support.SimpleOrderManager">
        <property name="mailSender" ref="mailSender"/>
        <property name="templateMessage" ref="templateMessage"/>
</bean>

6.2.2. Using the JavaMailSender and the MimeMessagePreparator

Here is another implementation of OrderManager using the MimeMessagePreparator callback interface. Please note in this case that the mailSender property is of type JavaMailSender so that we are able to use the JavaMail MimeMessage class:

import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;

import javax.mail.internet.MimeMessage;
import org.springframework.mail.MailException;
import org.springframework.mail.javamail.JavaMailSender;
import org.springframework.mail.javamail.MimeMessagePreparator;

public class SimpleOrderManager implements OrderManager {

        private JavaMailSender mailSender;

        public void setMailSender(JavaMailSender mailSender) {
                this.mailSender = mailSender;
        }

        public void placeOrder(final Order order) {

                // Do the business calculations...

                // Call the collaborators to persist the order...

                MimeMessagePreparator preparator = new MimeMessagePreparator() {

                        public void prepare(MimeMessage mimeMessage) throws Exception {

                                mimeMessage.setRecipient(Message.RecipientType.TO,
                                                new InternetAddress(order.getCustomer().getEmailAddress()));
                                mimeMessage.setFrom(new InternetAddress("mail@mycompany.com"));
                                mimeMessage.setText(
                                                "Dear " + order.getCustomer().getFirstName() + " "
                                                        + order.getCustomer().getLastName()
                                                        + ", thank you for placing order. Your order number is "
                                                        + order.getOrderNumber());
                        }
                };

                try {
                        this.mailSender.send(preparator);
                }
                catch (MailException ex) {
                        // simply log it and go on...
                        System.err.println(ex.getMessage());
                }
        }

}

The mail code is a crosscutting concern and could well be a candidate for refactoring into a custom Spring AOP aspect, which then could be executed at appropriate joinpoints on the OrderManager target.

The Spring Framework’s mail support ships with the standard JavaMail implementation. Please refer to the relevant javadocs for more information.

6.3. Using the JavaMail MimeMessageHelper

A class that comes in pretty handy when dealing with JavaMail messages is the org.springframework.mail.javamail.MimeMessageHelper class, which shields you from having to use the verbose JavaMail API. Using the MimeMessageHelper it is pretty easy to create a MimeMessage:

// of course you would use DI in any real-world cases
JavaMailSenderImpl sender = new JavaMailSenderImpl();
sender.setHost("mail.host.com");

MimeMessage message = sender.createMimeMessage();
MimeMessageHelper helper = new MimeMessageHelper(message);
helper.setTo("test@host.com");
helper.setText("Thank you for ordering!");

sender.send(message);

6.3.1. Sending attachments and inline resources

Multipart email messages allow for both attachments and inline resources. Examples of inline resources would be images or a stylesheet you want to use in your message, but that you don’t want displayed as an attachment.

Attachments

The following example shows you how to use the MimeMessageHelper to send an email along with a single JPEG image attachment.

JavaMailSenderImpl sender = new JavaMailSenderImpl();
sender.setHost("mail.host.com");

MimeMessage message = sender.createMimeMessage();

// use the true flag to indicate you need a multipart message
MimeMessageHelper helper = new MimeMessageHelper(message, true);
helper.setTo("test@host.com");

helper.setText("Check out this image!");

// let's attach the infamous windows Sample file (this time copied to c:/)
FileSystemResource file = new FileSystemResource(new File("c:/Sample.jpg"));
helper.addAttachment("CoolImage.jpg", file);

sender.send(message);
Inline resources

The following example shows you how to use the MimeMessageHelper to send an email along with an inline image.

JavaMailSenderImpl sender = new JavaMailSenderImpl();
sender.setHost("mail.host.com");

MimeMessage message = sender.createMimeMessage();

// use the true flag to indicate you need a multipart message
MimeMessageHelper helper = new MimeMessageHelper(message, true);
helper.setTo("test@host.com");

// use the true flag to indicate the text included is HTML
helper.setText("<html><body><img src='cid:identifier1234'></body></html>", true);

// let's include the infamous windows Sample file (this time copied to c:/)
FileSystemResource res = new FileSystemResource(new File("c:/Sample.jpg"));
helper.addInline("identifier1234", res);

sender.send(message);

Inline resources are added to the mime message using the specified Content-ID ( identifier1234 in the above example). The order in which you are adding the text and the resource are very important. Be sure to first add the text and after that the resources. If you are doing it the other way around, it won’t work!

6.3.2. Creating email content using a templating library

The code in the previous examples explicitly created the content of the email message, using methods calls such as message.setText(..). This is fine for simple cases, and it is okay in the context of the aforementioned examples, where the intent was to show you the very basics of the API.

In your typical enterprise application though, you are not going to create the content of your emails using the above approach for a number of reasons.

  • Creating HTML-based email content in Java code is tedious and error prone

  • There is no clear separation between display logic and business logic

  • Changing the display structure of the email content requires writing Java code, recompiling, redeploying…​

Typically the approach taken to address these issues is to use a template library such as FreeMarker to define the display structure of email content. This leaves your code tasked only with creating the data that is to be rendered in the email template and sending the email. It is definitely a best practice for when the content of your emails becomes even moderately complex, and with the Spring Framework’s support classes for FreeMarker becomes quite easy to do.

7. Task Execution and Scheduling

7.1. Introduction

The Spring Framework provides abstractions for asynchronous execution and scheduling of tasks with the TaskExecutor and TaskScheduler interfaces, respectively. Spring also features implementations of those interfaces that support thread pools or delegation to CommonJ within an application server environment. Ultimately the use of these implementations behind the common interfaces abstracts away the differences between Java SE 5, Java SE 6 and Java EE environments.

Spring also features integration classes for supporting scheduling with the Timer, part of the JDK since 1.3, and the Quartz Scheduler ( http://quartz-scheduler.org). Both of those schedulers are set up using a FactoryBean with optional references to Timer or Trigger instances, respectively. Furthermore, a convenience class for both the Quartz Scheduler and the Timer is available that allows you to invoke a method of an existing target object (analogous to the normal MethodInvokingFactoryBean operation).

7.2. The Spring TaskExecutor abstraction

Executors are the JDK name for the concept of thread pools. The "executor" naming is due to the fact that there is no guarantee that the underlying implementation is actually a pool; an executor may be single-threaded or even synchronous. Spring’s abstraction hides implementation details between Java SE and Java EE environments.

Spring’s TaskExecutor interface is identical to the java.util.concurrent.Executor interface. In fact, originally, its primary reason for existence was to abstract away the need for Java 5 when using thread pools. The interface has a single method execute(Runnable task) that accepts a task for execution based on the semantics and configuration of the thread pool.

The TaskExecutor was originally created to give other Spring components an abstraction for thread pooling where needed. Components such as the ApplicationEventMulticaster, JMS’s AbstractMessageListenerContainer, and Quartz integration all use the TaskExecutor abstraction to pool threads. However, if your beans need thread pooling behavior, it is possible to use this abstraction for your own needs.

7.2.1. TaskExecutor types

There are a number of pre-built implementations of TaskExecutor included with the Spring distribution. In all likelihood, you shouldn’t ever need to implement your own.

  • SimpleAsyncTaskExecutor This implementation does not reuse any threads, rather it starts up a new thread for each invocation. However, it does support a concurrency limit which will block any invocations that are over the limit until a slot has been freed up. If you are looking for true pooling, see the discussions of SimpleThreadPoolTaskExecutor and ThreadPoolTaskExecutor below.

  • SyncTaskExecutor This implementation doesn’t execute invocations asynchronously. Instead, each invocation takes place in the calling thread. It is primarily used in situations where multi-threading isn’t necessary such as simple test cases.

  • ConcurrentTaskExecutor This implementation is an adapter for a java.util.concurrent.Executor object. There is an alternative, ThreadPoolTaskExecutor, that exposes the Executor configuration parameters as bean properties. It is rare to need to use the ConcurrentTaskExecutor, but if the ThreadPoolTaskExecutor isn’t flexible enough for your needs, the ConcurrentTaskExecutor is an alternative.

  • SimpleThreadPoolTaskExecutor This implementation is actually a subclass of Quartz’s SimpleThreadPool which listens to Spring’s lifecycle callbacks. This is typically used when you have a thread pool that may need to be shared by both Quartz and non-Quartz components.

  • ThreadPoolTaskExecutor This implementation is the most commonly used one. It exposes bean properties for configuring a java.util.concurrent.ThreadPoolExecutor and wraps it in a TaskExecutor. If you need to adapt to a different kind of java.util.concurrent.Executor, it is recommended that you use a ConcurrentTaskExecutor instead.

  • WorkManagerTaskExecutor

    CommonJ is a set of specifications jointly developed between BEA and IBM. These specifications are not Java EE standards, but are standard across BEA’s and IBM’s Application Server implementations.

    This implementation uses the CommonJ WorkManager as its backing implementation and is the central convenience class for setting up a CommonJ WorkManager reference in a Spring context. Similar to the SimpleThreadPoolTaskExecutor, this class implements the WorkManager interface and therefore can be used directly as a WorkManager as well.

7.2.2. Using a TaskExecutor

Spring’s TaskExecutor implementations are used as simple JavaBeans. In the example below, we define a bean that uses the ThreadPoolTaskExecutor to asynchronously print out a set of messages.

import org.springframework.core.task.TaskExecutor;

public class TaskExecutorExample {

        private class MessagePrinterTask implements Runnable {

                private String message;

                public MessagePrinterTask(String message) {
                        this.message = message;
                }

                public void run() {
                        System.out.println(message);
                }

        }

        private TaskExecutor taskExecutor;

        public TaskExecutorExample(TaskExecutor taskExecutor) {
                this.taskExecutor = taskExecutor;
        }

        public void printMessages() {
                for(int i = 0; i < 25; i++) {
                        taskExecutor.execute(new MessagePrinterTask("Message" + i));
                }
        }

}

As you can see, rather than retrieving a thread from the pool and executing yourself, you add your Runnable to the queue and the TaskExecutor uses its internal rules to decide when the task gets executed.

To configure the rules that the TaskExecutor will use, simple bean properties have been exposed.

<bean id="taskExecutor" class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor">
        <property name="corePoolSize" value="5" />
        <property name="maxPoolSize" value="10" />
        <property name="queueCapacity" value="25" />
</bean>

<bean id="taskExecutorExample" class="TaskExecutorExample">
        <constructor-arg ref="taskExecutor" />
</bean>

7.3. The Spring TaskScheduler abstraction

In addition to the TaskExecutor abstraction, Spring 3.0 introduces a TaskScheduler with a variety of methods for scheduling tasks to run at some point in the future.

public interface TaskScheduler {

        ScheduledFuture schedule(Runnable task, Trigger trigger);

        ScheduledFuture schedule(Runnable task, Date startTime);

        ScheduledFuture scheduleAtFixedRate(Runnable task, Date startTime, long period);

        ScheduledFuture scheduleAtFixedRate(Runnable task, long period);

        ScheduledFuture scheduleWithFixedDelay(Runnable task, Date startTime, long delay);

        ScheduledFuture scheduleWithFixedDelay(Runnable task, long delay);

}

The simplest method is the one named 'schedule' that takes a Runnable and Date only. That will cause the task to run once after the specified time. All of the other methods are capable of scheduling tasks to run repeatedly. The fixed-rate and fixed-delay methods are for simple, periodic execution, but the method that accepts a Trigger is much more flexible.

7.3.1. Trigger interface

The Trigger interface is essentially inspired by JSR-236, which, as of Spring 3.0, has not yet been officially implemented. The basic idea of the Trigger is that execution times may be determined based on past execution outcomes or even arbitrary conditions. If these determinations do take into account the outcome of the preceding execution, that information is available within a TriggerContext. The Trigger interface itself is quite simple:

public interface Trigger {

        Date nextExecutionTime(TriggerContext triggerContext);

}

As you can see, the TriggerContext is the most important part. It encapsulates all of the relevant data, and is open for extension in the future if necessary. The TriggerContext is an interface (a SimpleTriggerContext implementation is used by default). Here you can see what methods are available for Trigger implementations.

public interface TriggerContext {

        Date lastScheduledExecutionTime();

        Date lastActualExecutionTime();

        Date lastCompletionTime();

}

7.3.2. Trigger implementations

Spring provides two implementations of the Trigger interface. The most interesting one is the CronTrigger. It enables the scheduling of tasks based on cron expressions. For example, the following task is being scheduled to run 15 minutes past each hour but only during the 9-to-5 "business hours" on weekdays.

scheduler.schedule(task, new CronTrigger("0 15 9-17 * * MON-FRI"));

The other out-of-the-box implementation is a PeriodicTrigger that accepts a fixed period, an optional initial delay value, and a boolean to indicate whether the period should be interpreted as a fixed-rate or a fixed-delay. Since the TaskScheduler interface already defines methods for scheduling tasks at a fixed-rate or with a fixed-delay, those methods should be used directly whenever possible. The value of the PeriodicTrigger implementation is that it can be used within components that rely on the Trigger abstraction. For example, it may be convenient to allow periodic triggers, cron-based triggers, and even custom trigger implementations to be used interchangeably. Such a component could take advantage of dependency injection so that such Triggers could be configured externally and therefore easily modified or extended.

7.3.3. TaskScheduler implementations

As with Spring’s TaskExecutor abstraction, the primary benefit of the TaskScheduler is that code relying on scheduling behavior need not be coupled to a particular scheduler implementation. The flexibility this provides is particularly relevant when running within Application Server environments where threads should not be created directly by the application itself. For such cases, Spring provides a TimerManagerTaskScheduler that delegates to a CommonJ TimerManager instance, typically configured with a JNDI-lookup.

A simpler alternative, the ThreadPoolTaskScheduler, can be used whenever external thread management is not a requirement. Internally, it delegates to a ScheduledExecutorService instance. ThreadPoolTaskScheduler actually implements Spring’s TaskExecutor interface as well, so that a single instance can be used for asynchronous execution as soon as possible as well as scheduled, and potentially recurring, executions.

7.4. Annotation Support for Scheduling and Asynchronous Execution

Spring provides annotation support for both task scheduling and asynchronous method execution.

7.4.1. Enable scheduling annotations

To enable support for @Scheduled and @Async annotations add @EnableScheduling and @EnableAsync to one of your @Configuration classes:

@Configuration
@EnableAsync
@EnableScheduling
public class AppConfig {
}

You are free to pick and choose the relevant annotations for your application. For example, if you only need support for @Scheduled, simply omit @EnableAsync. For more fine-grained control you can additionally implement the SchedulingConfigurer and/or AsyncConfigurer interfaces. See the javadocs for full details.

If you prefer XML configuration use the <task:annotation-driven> element.

<task:annotation-driven executor="myExecutor" scheduler="myScheduler"/>
<task:executor id="myExecutor" pool-size="5"/>
<task:scheduler id="myScheduler" pool-size="10"/>

Notice with the above XML that an executor reference is provided for handling those tasks that correspond to methods with the @Async annotation, and the scheduler reference is provided for managing those methods annotated with @Scheduled.

The default advice mode for processing @Async annotations is "proxy" which allows for interception of calls through the proxy only; local calls within the same class cannot get intercepted that way. For a more advanced mode of interception, consider switching to "aspectj" mode in combination with compile-time or load-time weaving.

7.4.2. The @Scheduled annotation

The @Scheduled annotation can be added to a method along with trigger metadata. For example, the following method would be invoked every 5 seconds with a fixed delay, meaning that the period will be measured from the completion time of each preceding invocation.

@Scheduled(fixedDelay=5000)
public void doSomething() {
        // something that should execute periodically
}

If a fixed rate execution is desired, simply change the property name specified within the annotation. The following would be executed every 5 seconds measured between the successive start times of each invocation.

@Scheduled(fixedRate=5000)
public void doSomething() {
        // something that should execute periodically
}

For fixed-delay and fixed-rate tasks, an initial delay may be specified indicating the number of milliseconds to wait before the first execution of the method.

@Scheduled(initialDelay=1000, fixedRate=5000)
public void doSomething() {
        // something that should execute periodically
}

If simple periodic scheduling is not expressive enough, then a cron expression may be provided. For example, the following will only execute on weekdays.

@Scheduled(cron="*/5 * * * * MON-FRI")
public void doSomething() {
        // something that should execute on weekdays only
}

You can additionally use the zone attribute to specify the time zone in which the cron expression will be resolved.

Notice that the methods to be scheduled must have void returns and must not expect any arguments. If the method needs to interact with other objects from the Application Context, then those would typically have been provided through dependency injection.

As of Spring Framework 4.3, @Scheduled methods are supported on beans of any scope.

Make sure that you are not initializing multiple instances of the same @Scheduled annotation class at runtime, unless you do want to schedule callbacks to each such instance. Related to this, make sure that you do not use @Configurable on bean classes which are annotated with @Scheduled and registered as regular Spring beans with the container: You would get double initialization otherwise, once through the container and once through the @Configurable aspect, with the consequence of each @Scheduled method being invoked twice.

7.4.3. The @Async annotation

The @Async annotation can be provided on a method so that invocation of that method will occur asynchronously. In other words, the caller will return immediately upon invocation and the actual execution of the method will occur in a task that has been submitted to a Spring TaskExecutor. In the simplest case, the annotation may be applied to a void-returning method.

@Async
void doSomething() {
        // this will be executed asynchronously
}

Unlike the methods annotated with the @Scheduled annotation, these methods can expect arguments, because they will be invoked in the "normal" way by callers at runtime rather than from a scheduled task being managed by the container. For example, the following is a legitimate application of the @Async annotation.

@Async
void doSomething(String s) {
        // this will be executed asynchronously
}

Even methods that return a value can be invoked asynchronously. However, such methods are required to have a Future typed return value. This still provides the benefit of asynchronous execution so that the caller can perform other tasks prior to calling get() on that Future.

@Async
Future<String> returnSomething(int i) {
        // this will be executed asynchronously
}

@Async methods may not only declare a regular java.util.concurrent.Future return type but also Spring’s org.springframework.util.concurrent.ListenableFuture or, as of Spring 4.2, JDK 8’s java.util.concurrent.CompletableFuture: for richer interaction with the asynchronous task and for immediate composition with further processing steps.

@Async can not be used in conjunction with lifecycle callbacks such as @PostConstruct. To asynchronously initialize Spring beans you currently have to use a separate initializing Spring bean that invokes the @Async annotated method on the target then.

public class SampleBeanImpl implements SampleBean {

        @Async
        void doSomething() {
                // ...
        }

}

public class SampleBeanInitializer {

        private final SampleBean bean;

        public SampleBeanInitializer(SampleBean bean) {
                this.bean = bean;
        }

        @PostConstruct
        public void initialize() {
                bean.doSomething();
        }

}

There is no direct XML equivalent for @Async since such methods should be designed for asynchronous execution in the first place, not externally re-declared to be async. However, you may manually set up Spring’s AsyncExecutionInterceptor with Spring AOP, in combination with a custom pointcut.

7.4.4. Executor qualification with @Async

By default when specifying @Async on a method, the executor that will be used is the one supplied to the 'annotation-driven' element as described above. However, the value attribute of the @Async annotation can be used when needing to indicate that an executor other than the default should be used when executing a given method.

@Async("otherExecutor")
void doSomething(String s) {
        // this will be executed asynchronously by "otherExecutor"
}

In this case, "otherExecutor" may be the name of any Executor bean in the Spring container, or may be the name of a qualifier associated with any Executor, e.g. as specified with the <qualifier> element or Spring’s @Qualifier annotation.

7.4.5. Exception management with @Async

When an @Async method has a Future typed return value, it is easy to manage an exception that was thrown during the method execution as this exception will be thrown when calling get on the Future result. With a void return type however, the exception is uncaught and cannot be transmitted. For those cases, an AsyncUncaughtExceptionHandler can be provided to handle such exceptions.

public class MyAsyncUncaughtExceptionHandler implements AsyncUncaughtExceptionHandler {

           @Override
           public void handleUncaughtException(Throwable ex, Method method, Object... params) {
                // handle exception
           }
}

By default, the exception is simply logged. A custom AsyncUncaughtExceptionHandler can be defined via AsyncConfigurer or the task:annotation-driven XML element.

7.5. The task namespace

Beginning with Spring 3.0, there is an XML namespace for configuring TaskExecutor and TaskScheduler instances. It also provides a convenient way to configure tasks to be scheduled with a trigger.

7.5.1. The 'scheduler' element

The following element will create a ThreadPoolTaskScheduler instance with the specified thread pool size.

<task:scheduler id="scheduler" pool-size="10"/>

The value provided for the 'id' attribute will be used as the prefix for thread names within the pool. The 'scheduler' element is relatively straightforward. If you do not provide a 'pool-size' attribute, the default thread pool will only have a single thread. There are no other configuration options for the scheduler.

7.5.2. The 'executor' element

The following will create a ThreadPoolTaskExecutor instance:

<task:executor id="executor" pool-size="10"/>

As with the scheduler above, the value provided for the 'id' attribute will be used as the prefix for thread names within the pool. As far as the pool size is concerned, the 'executor' element supports more configuration options than the 'scheduler' element. For one thing, the thread pool for a ThreadPoolTaskExecutor is itself more configurable. Rather than just a single size, an executor’s thread pool may have different values for the core and the max size. If a single value is provided then the executor will have a fixed-size thread pool (the core and max sizes are the same). However, the 'executor' element’s 'pool-size' attribute also accepts a range in the form of "min-max".

<task:executor
                id="executorWithPoolSizeRange"
                pool-size="5-25"
                queue-capacity="100"/>

As you can see from that configuration, a 'queue-capacity' value has also been provided. The configuration of the thread pool should also be considered in light of the executor’s queue capacity. For the full description of the relationship between pool size and queue capacity, consult the documentation for ThreadPoolExecutor. The main idea is that when a task is submitted, the executor will first try to use a free thread if the number of active threads is currently less than the core size. If the core size has been reached, then the task will be added to the queue as long as its capacity has not yet been reached. Only then, if the queue’s capacity has been reached, will the executor create a new thread beyond the core size. If the max size has also been reached, then the executor will reject the task.

By default, the queue is unbounded, but this is rarely the desired configuration, because it can lead to OutOfMemoryErrors if enough tasks are added to that queue while all pool threads are busy. Furthermore, if the queue is unbounded, then the max size has no effect at all. Since the executor will always try the queue before creating a new thread beyond the core size, a queue must have a finite capacity for the thread pool to grow beyond the core size (this is why a fixed size pool is the only sensible case when using an unbounded queue).

In a moment, we will review the effects of the keep-alive setting which adds yet another factor to consider when providing a pool size configuration. First, let’s consider the case, as mentioned above, when a task is rejected. By default, when a task is rejected, a thread pool executor will throw a TaskRejectedException. However, the rejection policy is actually configurable. The exception is thrown when using the default rejection policy which is the AbortPolicy implementation. For applications where some tasks can be skipped under heavy load, either the DiscardPolicy or DiscardOldestPolicy may be configured instead. Another option that works well for applications that need to throttle the submitted tasks under heavy load is the CallerRunsPolicy. Instead of throwing an exception or discarding tasks, that policy will simply force the thread that is calling the submit method to run the task itself. The idea is that such a caller will be busy while running that task and not able to submit other tasks immediately. Therefore it provides a simple way to throttle the incoming load while maintaining the limits of the thread pool and queue. Typically this allows the executor to "catch up" on the tasks it is handling and thereby frees up some capacity on the queue, in the pool, or both. Any of these options can be chosen from an enumeration of values available for the 'rejection-policy' attribute on the 'executor' element.

<task:executor
                id="executorWithCallerRunsPolicy"
                pool-size="5-25"
                queue-capacity="100"
                rejection-policy="CALLER_RUNS"/>

Finally, the keep-alive setting determines the time limit (in seconds) for which threads may remain idle before being terminated. If there are more than the core number of threads currently in the pool, after waiting this amount of time without processing a task, excess threads will get terminated. A time value of zero will cause excess threads to terminate immediately after executing a task without remaining follow-up work in the task queue.

<task:executor
                id="executorWithKeepAlive"
                pool-size="5-25"
                keep-alive="120"/>

7.5.3. The 'scheduled-tasks' element

The most powerful feature of Spring’s task namespace is the support for configuring tasks to be scheduled within a Spring Application Context. This follows an approach similar to other "method-invokers" in Spring, such as that provided by the JMS namespace for configuring Message-driven POJOs. Basically a "ref" attribute can point to any Spring-managed object, and the "method" attribute provides the name of a method to be invoked on that object. Here is a simple example.

<task:scheduled-tasks scheduler="myScheduler">
        <task:scheduled ref="beanA" method="methodA" fixed-delay="5000"/>
</task:scheduled-tasks>

<task:scheduler id="myScheduler" pool-size="10"/>

As you can see, the scheduler is referenced by the outer element, and each individual task includes the configuration of its trigger metadata. In the preceding example, that metadata defines a periodic trigger with a fixed delay indicating the number of milliseconds to wait after each task execution has completed. Another option is 'fixed-rate', indicating how often the method should be executed regardless of how long any previous execution takes. Additionally, for both fixed-delay and fixed-rate tasks an 'initial-delay' parameter may be specified indicating the number of milliseconds to wait before the first execution of the method. For more control, a "cron" attribute may be provided instead. Here is an example demonstrating these other options.

<task:scheduled-tasks scheduler="myScheduler">
        <task:scheduled ref="beanA" method="methodA" fixed-delay="5000" initial-delay="1000"/>
        <task:scheduled ref="beanB" method="methodB" fixed-rate="5000"/>
        <task:scheduled ref="beanC" method="methodC" cron="*/5 * * * * MON-FRI"/>
</task:scheduled-tasks>

<task:scheduler id="myScheduler" pool-size="10"/>

7.6. Using the Quartz Scheduler

Quartz uses Trigger, Job and JobDetail objects to realize scheduling of all kinds of jobs. For the basic concepts behind Quartz, have a look at http://quartz-scheduler.org. For convenience purposes, Spring offers a couple of classes that simplify the usage of Quartz within Spring-based applications.

7.6.1. Using the JobDetailFactoryBean

Quartz JobDetail objects contain all information needed to run a job. Spring provides a JobDetailFactoryBean which provides bean-style properties for XML configuration purposes. Let’s have a look at an example:

<bean name="exampleJob" class="org.springframework.scheduling.quartz.JobDetailFactoryBean">
        <property name="jobClass" value="example.ExampleJob"/>
        <property name="jobDataAsMap">
                <map>
                        <entry key="timeout" value="5"/>
                </map>
        </property>
</bean>

The job detail configuration has all information it needs to run the job (ExampleJob). The timeout is specified in the job data map. The job data map is available through the JobExecutionContext (passed to you at execution time), but the JobDetail also gets its properties from the job data mapped to properties of the job instance. So in this case, if the ExampleJob contains a bean property named timeout, the JobDetail will have it applied automatically:

package example;

public class ExampleJob extends QuartzJobBean {

        private int timeout;

        /**
         * Setter called after the ExampleJob is instantiated
         * with the value from the JobDetailFactoryBean (5)
         */
        public void setTimeout(int timeout) {
                this.timeout = timeout;
        }

        protected void executeInternal(JobExecutionContext ctx) throws JobExecutionException {
                // do the actual work
        }

}

All additional properties from the job data map are of course available to you as well.

Using the name and group properties, you can modify the name and the group of the job, respectively. By default, the name of the job matches the bean name of the JobDetailFactoryBean (in the example above, this is exampleJob).

7.6.2. Using the MethodInvokingJobDetailFactoryBean

Often you just need to invoke a method on a specific object. Using the MethodInvokingJobDetailFactoryBean you can do exactly this:

<bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean">
        <property name="targetObject" ref="exampleBusinessObject"/>
        <property name="targetMethod" value="doIt"/>
</bean>

The above example will result in the doIt method being called on the exampleBusinessObject method (see below):

public class ExampleBusinessObject {

        // properties and collaborators

        public void doIt() {
                // do the actual work
        }
}
<bean id="exampleBusinessObject" class="examples.ExampleBusinessObject"/>

Using the MethodInvokingJobDetailFactoryBean, you don’t need to create one-line jobs that just invoke a method, and you only need to create the actual business object and wire up the detail object.

By default, Quartz Jobs are stateless, resulting in the possibility of jobs interfering with each other. If you specify two triggers for the same JobDetail, it might be possible that before the first job has finished, the second one will start. If JobDetail classes implement the Stateful interface, this won’t happen. The second job will not start before the first one has finished. To make jobs resulting from the MethodInvokingJobDetailFactoryBean non-concurrent, set the concurrent flag to false.

<bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean">
        <property name="targetObject" ref="exampleBusinessObject"/>
        <property name="targetMethod" value="doIt"/>
        <property name="concurrent" value="false"/>
</bean>

By default, jobs will run in a concurrent fashion.

7.6.3. Wiring up jobs using triggers and the SchedulerFactoryBean

We’ve created job details and jobs. We’ve also reviewed the convenience bean that allows you to invoke a method on a specific object. Of course, we still need to schedule the jobs themselves. This is done using triggers and a SchedulerFactoryBean. Several triggers are available within Quartz and Spring offers two Quartz FactoryBean implementations with convenient defaults: CronTriggerFactoryBean and SimpleTriggerFactoryBean.

Triggers need to be scheduled. Spring offers a SchedulerFactoryBean that exposes triggers to be set as properties. SchedulerFactoryBean schedules the actual jobs with those triggers.

Find below a couple of examples:

<bean id="simpleTrigger" class="org.springframework.scheduling.quartz.SimpleTriggerFactoryBean">
        <!-- see the example of method invoking job above -->
        <property name="jobDetail" ref="jobDetail"/>
        <!-- 10 seconds -->
        <property name="startDelay" value="10000"/>
        <!-- repeat every 50 seconds -->
        <property name="repeatInterval" value="50000"/>
</bean>

<bean id="cronTrigger" class="org.springframework.scheduling.quartz.CronTriggerFactoryBean">
        <property name="jobDetail" ref="exampleJob"/>
        <!-- run every morning at 6 AM -->
        <property name="cronExpression" value="0 0 6 * * ?"/>
</bean>

Now we’ve set up two triggers, one running every 50 seconds with a starting delay of 10 seconds and one every morning at 6 AM. To finalize everything, we need to set up the SchedulerFactoryBean:

<bean class="org.springframework.scheduling.quartz.SchedulerFactoryBean">
        <property name="triggers">
                <list>
                        <ref bean="cronTrigger"/>
                        <ref bean="simpleTrigger"/>
                </list>
        </property>
</bean>

More properties are available for the SchedulerFactoryBean for you to set, such as the calendars used by the job details, properties to customize Quartz with, etc. Have a look at the SchedulerFactoryBean javadocs for more information.

8. Cache Abstraction

8.1. Introduction

Since version 3.1, Spring Framework provides support for transparently adding caching into an existing Spring application. Similar to the transaction support, the caching abstraction allows consistent use of various caching solutions with minimal impact on the code.

As from Spring 4.1, the cache abstraction has been significantly improved with the support of JSR-107 annotations and more customization options.

8.2. Understanding the cache abstraction

Cache vs Buffer

The terms "buffer" and "cache" tend to be used interchangeably; note however they represent different things. A buffer is used traditionally as an intermediate temporary store for data between a fast and a slow entity. As one party would have to wait for the other affecting performance, the buffer alleviates this by allowing entire blocks of data to move at once rather then in small chunks. The data is written and read only once from the buffer. Furthermore, the buffers are visible to at least one party which is aware of it.

A cache on the other hand by definition is hidden and neither party is aware that caching occurs.It as well improves performance but does that by allowing the same data to be read multiple times in a fast fashion.

A further explanation of the differences between two can be found here.

At its core, the abstraction applies caching to Java methods, reducing thus the number of executions based on the information available in the cache. That is, each time a targeted method is invoked, the abstraction will apply a caching behavior checking whether the method has been already executed for the given arguments. If it has, then the cached result is returned without having to execute the actual method; if it has not, then method is executed, the result cached and returned to the user so that, the next time the method is invoked, the cached result is returned. This way, expensive methods (whether CPU or IO bound) can be executed only once for a given set of parameters and the result reused without having to actually execute the method again. The caching logic is applied transparently without any interference to the invoker.

Obviously this approach works only for methods that are guaranteed to return the same output (result) for a given input (or arguments) no matter how many times it is being executed.

Other cache-related operations are provided by the abstraction such as the ability to update the content of the cache or remove one of all entries. These are useful if the cache deals with data that can change during the course of the application.

Just like other services in the Spring Framework, the caching service is an abstraction (not a cache implementation) and requires the use of an actual storage to store the cache data - that is, the abstraction frees the developer from having to write the caching logic but does not provide the actual stores. This abstraction is materialized by the org.springframework.cache.Cache and org.springframework.cache.CacheManager interfaces.

There are a few implementations of that abstraction available out of the box: JDK java.util.concurrent.ConcurrentMap based caches, Ehcache 2.x, Gemfire cache, Caffeine and JSR-107 compliant caches (e.g. Ehcache 3.x). See Plugging-in different back-end caches for more information on plugging in other cache stores/providers.

The caching abstraction has no special handling of multi-threaded and multi-process environments as such features are handled by the cache implementation. .

If you have a multi-process environment (i.e. an application deployed on several nodes), you will need to configure your cache provider accordingly. Depending on your use cases, a copy of the same data on several nodes may be enough but if you change the data during the course of the application, you may need to enable other propagation mechanisms.

Caching a particular item is a direct equivalent of the typical get-if-not-found-then- proceed-and-put-eventually code blocks found with programmatic cache interaction: no locks are applied and several threads may try to load the same item concurrently. The same applies to eviction: if several threads are trying to update or evict data concurrently, you may use stale data. Certain cache providers offer advanced features in that area, refer to the documentation of the cache provider that you are using for more details.

To use the cache abstraction, the developer needs to take care of two aspects:

  • caching declaration - identify the methods that need to be cached and their policy

  • cache configuration - the backing cache where the data is stored and read from

8.3. Declarative annotation-based caching

For caching declaration, the abstraction provides a set of Java annotations:

  • @Cacheable triggers cache population

  • @CacheEvict triggers cache eviction

  • @CachePut updates the cache without interfering with the method execution

  • @Caching regroups multiple cache operations to be applied on a method

  • @CacheConfig shares some common cache-related settings at class-level

Let us take a closer look at each annotation:

8.3.1. @Cacheable annotation

As the name implies, @Cacheable is used to demarcate methods that are cacheable - that is, methods for whom the result is stored into the cache so on subsequent invocations (with the same arguments), the value in the cache is returned without having to actually execute the method. In its simplest form, the annotation declaration requires the name of the cache associated with the annotated method:

@Cacheable("books")
public Book findBook(ISBN isbn) {...}

In the snippet above, the method findBook is associated with the cache named books. Each time the method is called, the cache is checked to see whether the invocation has been already executed and does not have to be repeated. While in most cases, only one cache is declared, the annotation allows multiple names to be specified so that more than one cache are being used. In this case, each of the caches will be checked before executing the method - if at least one cache is hit, then the associated value will be returned:

All the other caches that do not contain the value will be updated as well even though the cached method was not actually executed.

@Cacheable({"books", "isbns"})
public Book findBook(ISBN isbn) {...}
Default Key Generation

Since caches are essentially key-value stores, each invocation of a cached method needs to be translated into a suitable key for cache access. Out of the box, the caching abstraction uses a simple KeyGenerator based on the following algorithm:

  • If no params are given, return SimpleKey.EMPTY.

  • If only one param is given, return that instance.

  • If more the one param is given, return a SimpleKey containing all parameters.

This approach works well for most use-cases; As long as parameters have natural keys and implement valid hashCode() and equals() methods. If that is not the case then the strategy needs to be changed.

To provide a different default key generator, one needs to implement the org.springframework.cache.interceptor.KeyGenerator interface.

The default key generation strategy changed with the release of Spring 4.0. Earlier versions of Spring used a key generation strategy that, for multiple key parameters, only considered the hashCode() of parameters and not equals(); this could cause unexpected key collisions (see SPR-10237 for background). The new 'SimpleKeyGenerator' uses a compound key for such scenarios.

If you want to keep using the previous key strategy, you can configure the deprecated org.springframework.cache.interceptor.DefaultKeyGenerator class or create a custom hash-based 'KeyGenerator' implementation.

Custom Key Generation Declaration

Since caching is generic, it is quite likely the target methods have various signatures that cannot be simply mapped on top of the cache structure. This tends to become obvious when the target method has multiple arguments out of which only some are suitable for caching (while the rest are used only by the method logic). For example:

@Cacheable("books")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

At first glance, while the two boolean arguments influence the way the book is found, they are no use for the cache. Further more what if only one of the two is important while the other is not?

For such cases, the @Cacheable annotation allows the user to specify how the key is generated through its key attribute. The developer can use SpEL to pick the arguments of interest (or their nested properties), perform operations or even invoke arbitrary methods without having to write any code or implement any interface. This is the recommended approach over the default generator since methods tend to be quite different in signatures as the code base grows; while the default strategy might work for some methods, it rarely does for all methods.

Below are some examples of various SpEL declarations - if you are not familiar with it, do yourself a favor and read Spring Expression Language:

@Cacheable(cacheNames="books", key="#isbn")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

@Cacheable(cacheNames="books", key="#isbn.rawNumber")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

@Cacheable(cacheNames="books", key="T(someType).hash(#isbn)")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

The snippets above show how easy it is to select a certain argument, one of its properties or even an arbitrary (static) method.

If the algorithm responsible to generate the key is too specific or if it needs to be shared, you may define a custom keyGenerator on the operation. To do this, specify the name of the KeyGenerator bean implementation to use:

@Cacheable(cacheNames="books", keyGenerator="myKeyGenerator")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

The key and keyGenerator parameters are mutually exclusive and an operation specifying both will result in an exception.

Default Cache Resolution

Out of the box, the caching abstraction uses a simple CacheResolver that retrieves the cache(s) defined at the operation level using the configured CacheManager.

To provide a different default cache resolver, one needs to implement the org.springframework.cache.interceptor.CacheResolver interface.

Custom cache resolution

The default cache resolution fits well for applications working with a single CacheManager and with no complex cache resolution requirements.

For applications working with several cache managers, it is possible to set the cacheManager to use per operation:

@Cacheable(cacheNames="books", cacheManager="anotherCacheManager")
public Book findBook(ISBN isbn) {...}

It is also possible to replace the CacheResolver entirely in a similar fashion as for key generation. The resolution is requested for every cache operation, giving a chance to the implementation to actually resolve the cache(s) to use based on runtime arguments:

@Cacheable(cacheResolver="runtimeCacheResolver")
public Book findBook(ISBN isbn) {...}

Since Spring 4.1, the value attribute of the cache annotations are no longer mandatory since this particular information can be provided by the CacheResolver regardless of the content of the annotation.

Similarly to key and keyGenerator, the cacheManager and cacheResolver parameters are mutually exclusive and an operation specifying both will result in an exception as a custom CacheManager will be ignored by the CacheResolver implementation. This is probably not what you expect.

Synchronized caching

In a multi-threaded environment, certain operations might be concurrently invoked for the same argument (typically on startup). By default, the cache abstraction does not lock anything and the same value may be computed several times, defeating the purpose of caching.

For those particular cases, the sync attribute can be used to instruct the underlying cache provider to lock the cache entry while the value is being computed. As a result, only one thread will be busy computing the value while the others are blocked until the entry is updated in the cache.

@Cacheable(cacheNames="foos", sync=true)
public Foo executeExpensiveOperation(String id) {...}

This is an optional feature and your favorite cache library may not support it. All CacheManager implementations provided by the core framework support it. Check the documentation of your cache provider for more details.

Conditional caching

Sometimes, a method might not be suitable for caching all the time (for example, it might depend on the given arguments). The cache annotations support such functionality through the condition parameter which takes a SpEL expression that is evaluated to either true or false. If true, the method is cached - if not, it behaves as if the method is not cached, that is executed every time no matter what values are in the cache or what arguments are used. A quick example - the following method will be cached only if the argument name has a length shorter than 32:

@Cacheable(cacheNames="book", condition="#name.length() < 32")
public Book findBook(String name)

In addition the condition parameter, the unless parameter can be used to veto the adding of a value to the cache. Unlike condition, unless expressions are evaluated after the method has been called. Expanding on the previous example - perhaps we only want to cache paperback books:

@Cacheable(cacheNames="book", condition="#name.length() < 32", unless="#result.hardback")
public Book findBook(String name)

The cache abstraction supports java.util.Optional, using its content as cached value only if it present. #result always refers to the business entity and never on a supported wrapper so the previous example can be rewritten as follows:

@Cacheable(cacheNames="book", condition="#name.length() < 32", unless="#result?.hardback")
public Optional<Book> findBook(String name)

Note that result still refers to Book and not Optional. As it might be null, we should use the safe navigation operator.

Available caching SpEL evaluation context

Each SpEL expression evaluates again a dedicated context. In addition to the build in parameters, the framework provides dedicated caching related metadata such as the argument names. The next table lists the items made available to the context so one can use them for key and conditional computations:

Table 10. Cache SpEL available metadata
Name Location Description Example

methodName

root object

The name of the method being invoked

#root.methodName

method

root object

The method being invoked

#root.method.name

target

root object

The target object being invoked

#root.target

targetClass

root object

The class of the target being invoked

#root.targetClass

args

root object

The arguments (as array) used for invoking the target

#root.args[0]

caches

root object

Collection of caches against which the current method is executed

#root.caches[0].name

argument name

evaluation context

Name of any of the method arguments. If for some reason the names are not available (e.g. no debug information), the argument names are also available under the #a<#arg> where #arg stands for the argument index (starting from 0).

#iban or #a0 (one can also use #p0 or #p<#arg> notation as an alias).

result

evaluation context

The result of the method call (the value to be cached). Only available in unless expressions, cache put expressions (to compute the key), or cache evict expressions (when beforeInvocation is false). For supported wrappers such as Optional, #result refers to the actual object, not the wrapper.

#result

8.3.2. @CachePut annotation

For cases where the cache needs to be updated without interfering with the method execution, one can use the @CachePut annotation. That is, the method will always be executed and its result placed into the cache (according to the @CachePut options). It supports the same options as @Cacheable and should be used for cache population rather than method flow optimization:

@CachePut(cacheNames="book", key="#isbn")
public Book updateBook(ISBN isbn, BookDescriptor descriptor)

Note that using @CachePut and @Cacheable annotations on the same method is generally strongly discouraged because they have different behaviors. While the latter causes the method execution to be skipped by using the cache, the former forces the execution in order to execute a cache update. This leads to unexpected behavior and with the exception of specific corner-cases (such as annotations having conditions that exclude them from each other), such declaration should be avoided. Note also that such condition should not rely on the result object (i.e. the #result variable) as these are validated upfront to confirm the exclusion.

8.3.3. @CacheEvict annotation

The cache abstraction allows not just population of a cache store but also eviction. This process is useful for removing stale or unused data from the cache. Opposed to @Cacheable, annotation @CacheEvict demarcates methods that perform cache eviction, that is methods that act as triggers for removing data from the cache. Just like its sibling, @CacheEvict requires specifying one (or multiple) caches that are affected by the action, allows a custom cache and key resolution or a condition to be specified but in addition, features an extra parameter allEntries which indicates whether a cache-wide eviction needs to be performed rather then just an entry one (based on the key):

@CacheEvict(cacheNames="books", allEntries=true)
public void loadBooks(InputStream batch)

This option comes in handy when an entire cache region needs to be cleared out - rather then evicting each entry (which would take a long time since it is inefficient), all the entries are removed in one operation as shown above. Note that the framework will ignore any key specified in this scenario as it does not apply (the entire cache is evicted not just one entry).

One can also indicate whether the eviction should occur after (the default) or before the method executes through the beforeInvocation attribute. The former provides the same semantics as the rest of the annotations - once the method completes successfully, an action (in this case eviction) on the cache is executed. If the method does not execute (as it might be cached) or an exception is thrown, the eviction does not occur. The latter ( beforeInvocation=true) causes the eviction to occur always, before the method is invoked - this is useful in cases where the eviction does not need to be tied to the method outcome.

It is important to note that void methods can be used with @CacheEvict - as the methods act as triggers, the return values are ignored (as they don’t interact with the cache) - this is not the case with @Cacheable which adds/updates data into the cache and thus requires a result.

8.3.4. @Caching annotation

There are cases when multiple annotations of the same type, such as @CacheEvict or @CachePut need to be specified, for example because the condition or the key expression is different between different caches. @Caching allows multiple nested @Cacheable, @CachePut and @CacheEvict to be used on the same method:

@Caching(evict = { @CacheEvict("primary"), @CacheEvict(cacheNames="secondary", key="#p0") })
public Book importBooks(String deposit, Date date)

8.3.5. @CacheConfig annotation

So far we have seen that caching operations offered many customization options and these can be set on an operation basis. However, some of the customization options can be tedious to configure if they apply to all operations of the class. For instance, specifying the name of the cache to use for every cache operation of the class could be replaced by a single class-level definition. This is where @CacheConfig comes into play.

@CacheConfig("books")
public class BookRepositoryImpl implements BookRepository {

        @Cacheable
        public Book findBook(ISBN isbn) {...}
}

@CacheConfig is a class-level annotation that allows to share the cache names, the custom KeyGenerator, the custom CacheManager and finally the custom CacheResolver. Placing this annotation on the class does not turn on any caching operation.

An operation-level customization will always override a customization set on @CacheConfig. This gives therefore three levels of customizations per cache operation:

  • Globally configured, available for CacheManager, KeyGenerator

  • At class level, using @CacheConfig

  • At the operation level

8.3.6. Enable caching annotations

It is important to note that even though declaring the cache annotations does not automatically trigger their actions - like many things in Spring, the feature has to be declaratively enabled (which means if you ever suspect caching is to blame, you can disable it by removing only one configuration line rather than all the annotations in your code).

To enable caching annotations add the annotation @EnableCaching to one of your @Configuration classes:

@Configuration
@EnableCaching
public class AppConfig {
}

Alternatively for XML configuration use the cache:annotation-driven element:

<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:cache="http://www.springframework.org/schema/cache"
        xsi:schemaLocation="
                http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
                http://www.springframework.org/schema/cache http://www.springframework.org/schema/cache/spring-cache.xsd">

                <cache:annotation-driven />

</beans>

Both the cache:annotation-driven element and @EnableCaching annotation allow various options to be specified that influence the way the caching behavior is added to the application through AOP. The configuration is intentionally similar with that of @Transactional:

The default advice mode for processing caching annotations is "proxy" which allows for interception of calls through the proxy only; local calls within the same class cannot get intercepted that way. For a more advanced mode of interception, consider switching to "aspectj" mode in combination with compile-time or load-time weaving.

Advanced customizations using Java config require to implement CachingConfigurer: Please refer to the javadoc for more details.

Table 11. Cache annotation settings
XML Attribute Annotation Attribute Default Description

cache-manager

N/A (See CachingConfigurer javadocs)

cacheManager

Name of cache manager to use. A default CacheResolver will be initialized behind the scenes with this cache manager (or `cacheManager`if not set). For more fine-grained management of the cache resolution, consider setting the 'cache-resolver' attribute.

cache-resolver

N/A (See CachingConfigurer javadocs)

A SimpleCacheResolver using the configured cacheManager.

The bean name of the CacheResolver that is to be used to resolve the backing caches. This attribute is not required, and only needs to be specified as an alternative to the 'cache-manager' attribute.

key-generator

N/A (See CachingConfigurer javadocs)

SimpleKeyGenerator

Name of the custom key generator to use.

error-handler

N/A (See CachingConfigurer javadocs)

SimpleCacheErrorHandler

Name of the custom cache error handler to use. By default, any exception throw during a cache related operations are thrown back at the client.

mode

mode

proxy

The default mode "proxy" processes annotated beans to be proxied using Spring’s AOP framework (following proxy semantics, as discussed above, applying to method calls coming in through the proxy only). The alternative mode "aspectj" instead weaves the affected classes with Spring’s AspectJ caching aspect, modifying the target class byte code to apply to any kind of method call. AspectJ weaving requires spring-aspects.jar in the classpath as well as load-time weaving (or compile-time weaving) enabled. (See Spring configuration for details on how to set up load-time weaving.)

proxy-target-class

proxyTargetClass

false

Applies to proxy mode only. Controls what type of caching proxies are created for classes annotated with the @Cacheable or @CacheEvict annotations. If the proxy-target-class attribute is set to true, then class-based proxies are created. If proxy-target-class is false or if the attribute is omitted, then standard JDK interface-based proxies are created. (See Proxying mechanisms for a detailed examination of the different proxy types.)

order

order

Ordered.LOWEST_PRECEDENCE

Defines the order of the cache advice that is applied to beans annotated with @Cacheable or @CacheEvict. (For more information about the rules related to ordering of AOP advice, see Advice ordering.) No specified ordering means that the AOP subsystem determines the order of the advice.

<cache:annotation-driven/> only looks for @Cacheable/@CachePut/@CacheEvict/@Caching on beans in the same application context it is defined in. This means that, if you put <cache:annotation-driven/> in a WebApplicationContext for a DispatcherServlet, it only checks for beans in your controllers, and not your services. See the MVC section for more information.

Method visibility and cache annotations

When using proxies, you should apply the cache annotations only to methods with public visibility. If you do annotate protected, private or package-visible methods with these annotations, no error is raised, but the annotated method does not exhibit the configured caching settings. Consider the use of AspectJ (see below) if you need to annotate non-public methods as it changes the bytecode itself.

Spring recommends that you only annotate concrete classes (and methods of concrete classes) with the @Cache* annotation, as opposed to annotating interfaces. You certainly can place the @Cache* annotation on an interface (or an interface method), but this works only as you would expect it to if you are using interface-based proxies. The fact that Java annotations are not inherited from interfaces means that if you are using class-based proxies ( proxy-target-class="true") or the weaving-based aspect ( mode="aspectj"), then the caching settings are not recognized by the proxying and weaving infrastructure, and the object will not be wrapped in a caching proxy, which would be decidedly bad.

In proxy mode (which is the default), only external method calls coming in through the proxy are intercepted. This means that self-invocation, in effect, a method within the target object calling another method of the target object, will not lead to an actual caching at runtime even if the invoked method is marked with @Cacheable - considering using the aspectj mode in this case. Also, the proxy must be fully initialized to provide the expected behaviour so you should not rely on this feature in your initialization code, i.e. @PostConstruct.

8.3.7. Using custom annotations

Custom annotation and AspectJ

This feature only works out-of-the-box with the proxy-based approach but can be enabled with a bit of extra effort using AspectJ.

The spring-aspects module defines an aspect for the standard annotations only. If you have defined your own annotations, you also need to define an aspect for those. Check AnnotationCacheAspect for an example.

The caching abstraction allows you to use your own annotations to identify what method triggers cache population or eviction. This is quite handy as a template mechanism as it eliminates the need to duplicate cache annotation declarations (especially useful if the key or condition are specified) or if the foreign imports (org.springframework) are not allowed in your code base. Similar to the rest of the stereotype annotations, @Cacheable, @CachePut, @CacheEvict and @CacheConfig can be used as meta-annotations, that is annotations that can annotate other annotations. To wit, let us replace a common @Cacheable declaration with our own, custom annotation:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
@Cacheable(cacheNames="books", key="#isbn")
public @interface SlowService {
}

Above, we have defined our own SlowService annotation which itself is annotated with @Cacheable - now we can replace the following code:

@Cacheable(cacheNames="books", key="#isbn")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

with:

@SlowService
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

Even though @SlowService is not a Spring annotation, the container automatically picks up its declaration at runtime and understands its meaning. Note that as mentioned above, the annotation-driven behavior needs to be enabled.

8.4. JCache (JSR-107) annotations

Since the Spring Framework 4.1, the caching abstraction fully supports the JCache standard annotations: these are @CacheResult, @CachePut, @CacheRemove and @CacheRemoveAll as well as the @CacheDefaults, @CacheKey and @CacheValue companions. These annotations can be used right the way without migrating your cache store to JSR-107: the internal implementation uses Spring’s caching abstraction and provides default CacheResolver and KeyGenerator implementations that are compliant with the specification. In other words, if you are already using Spring’s caching abstraction, you can switch to these standard annotations without changing your cache storage (or configuration, for that matter).

8.4.1. Feature summary

For those who are familiar with Spring’s caching annotations, the following table describes the main differences between the Spring annotations and the JSR-107 counterpart:

Table 12. Spring vs. JSR-107 caching annotations
Spring JSR-107 Remark

@Cacheable

@CacheResult

Fairly similar. @CacheResult can cache specific exceptions and force the execution of the method regardless of the content of the cache.

@CachePut

@CachePut

While Spring updates the cache with the result of the method invocation, JCache requires to pass it as an argument that is annotated with @CacheValue. Due to this difference, JCache allows to update the cache before or after the actual method invocation.

@CacheEvict

@CacheRemove

Fairly similar. @CacheRemove supports a conditional evict in case the method invocation results in an exception.

@CacheEvict(allEntries=true)

@CacheRemoveAll

See @CacheRemove.

@CacheConfig

@CacheDefaults

Allows to configure the same concepts, in a similar fashion.

JCache has the notion of javax.cache.annotation.CacheResolver that is identical to the Spring’s CacheResolver interface, except that JCache only supports a single cache. By default, a simple implementation retrieves the cache to use based on the name declared on the annotation. It should be noted that if no cache name is specified on the annotation, a default is automatically generated, check the javadoc of @CacheResult#cacheName() for more information.

CacheResolver instances are retrieved by a CacheResolverFactory. It is possible to customize the factory per cache operation:

@CacheResult(cacheNames="books", cacheResolverFactory=MyCacheResolverFactory.class)
public Book findBook(ISBN isbn)

For all referenced classes, Spring tries to locate a bean with the given type. If more than one match exists, a new instance is created and can use the regular bean lifecycle callbacks such as dependency injection.

Keys are generated by a javax.cache.annotation.CacheKeyGenerator that serves the same purpose as Spring’s KeyGenerator. By default, all method arguments are taken into account unless at least one parameter is annotated with @CacheKey. This is similar to Spring’s custom key generation declaration. For instance these are identical operations, one using Spring’s abstraction and the other with JCache:

@Cacheable(cacheNames="books", key="#isbn")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

@CacheResult(cacheName="books")
public Book findBook(@CacheKey ISBN isbn, boolean checkWarehouse, boolean includeUsed)

The CacheKeyResolver to use can also be specified on the operation, in a similar fashion as the CacheResolverFactory.

JCache can manage exceptions thrown by annotated methods: this can prevent an update of the cache but it can also cache the exception as an indicator of the failure instead of calling the method again. Let’s assume that InvalidIsbnNotFoundException is thrown if the structure of the ISBN is invalid. This is a permanent failure, no book could ever be retrieved with such parameter. The following caches the exception so that further calls with the same, invalid ISBN, throws the cached exception directly instead of invoking the method again.

@CacheResult(cacheName="books", exceptionCacheName="failures"
             cachedExceptions = InvalidIsbnNotFoundException.class)
public Book findBook(ISBN isbn)

8.4.2. Enabling JSR-107 support

Nothing specific needs to be done to enable the JSR-107 support alongside Spring’s declarative annotation support. Both @EnableCaching and the cache:annotation-driven element will enable automatically the JCache support if both the JSR-107 API and the spring-context-support module are present in the classpath.

Depending of your use case, the choice is basically yours. You can even mix and match services using the JSR-107 API and others using Spring’s own annotations. Be aware however that if these services are impacting the same caches, a consistent and identical key generation implementation should be used.

8.5. Declarative XML-based caching

If annotations are not an option (no access to the sources or no external code), one can use XML for declarative caching. So instead of annotating the methods for caching, one specifies the target method and the caching directives externally (similar to the declarative transaction management advice). The previous example can be translated into:

<!-- the service we want to make cacheable -->
<bean id="bookService" class="x.y.service.DefaultBookService"/>

<!-- cache definitions -->
<cache:advice id="cacheAdvice" cache-manager="cacheManager">
        <cache:caching cache="books">
                <cache:cacheable method="findBook" key="#isbn"/>
                <cache:cache-evict method="loadBooks" all-entries="true"/>
        </cache:caching>
</cache:advice>

<!-- apply the cacheable behavior to all BookService interfaces -->
<aop:config>
        <aop:advisor advice-ref="cacheAdvice" pointcut="execution(* x.y.BookService.*(..))"/>
</aop:config>

<!-- cache manager definition omitted -->

In the configuration above, the bookService is made cacheable. The caching semantics to apply are encapsulated in the cache:advice definition which instructs method findBooks to be used for putting data into the cache while method loadBooks for evicting data. Both definitions are working against the books cache.

The aop:config definition applies the cache advice to the appropriate points in the program by using the AspectJ pointcut expression (more information is available in Aspect Oriented Programming with Spring). In the example above, all methods from the BookService are considered and the cache advice applied to them.

The declarative XML caching supports all of the annotation-based model so moving between the two should be fairly easy - further more both can be used inside the same application. The XML based approach does not touch the target code however it is inherently more verbose; when dealing with classes with overloaded methods that are targeted for caching, identifying the proper methods does take an extra effort since the method argument is not a good discriminator - in these cases, the AspectJ pointcut can be used to cherry pick the target methods and apply the appropriate caching functionality. However through XML, it is easier to apply a package/group/interface-wide caching (again due to the AspectJ pointcut) and to create template-like definitions (as we did in the example above by defining the target cache through the cache:definitions cache attribute).

8.6. Configuring the cache storage

Out of the box, the cache abstraction provides several storage integration. To use them, one needs to simply declare an appropriate CacheManager - an entity that controls and manages Caches and can be used to retrieve these for storage.

8.6.1. JDK ConcurrentMap-based Cache

The JDK-based Cache implementation resides under org.springframework.cache.concurrent package. It allows one to use ConcurrentHashMap as a backing Cache store.

<!-- simple cache manager -->
<bean id="cacheManager" class="org.springframework.cache.support.SimpleCacheManager">
        <property name="caches">
                <set>
                        <bean class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean" p:name="default"/>
                        <bean class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean" p:name="books"/>
                </set>
        </property>
</bean>

The snippet above uses the SimpleCacheManager to create a CacheManager for the two nested ConcurrentMapCache instances named default and books. Note that the names are configured directly for each cache.

As the cache is created by the application, it is bound to its lifecycle, making it suitable for basic use cases, tests or simple applications. The cache scales well and is very fast but it does not provide any management or persistence capabilities nor eviction contracts.

8.6.2. Ehcache-based Cache

Ehcache 3.x is fully JSR-107 compliant and no dedicated support is required for it.

The Ehcache 2.x implementation is located under org.springframework.cache.ehcache package. Again, to use it, one simply needs to declare the appropriate CacheManager:

<bean id="cacheManager"
      class="org.springframework.cache.ehcache.EhCacheCacheManager" p:cache-manager-ref="ehcache"/>

<!-- EhCache library setup -->
<bean id="ehcache"
      class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean" p:config-location="ehcache.xml"/>

This setup bootstraps the ehcache library inside Spring IoC (through the ehcache bean) which is then wired into the dedicated CacheManager implementation. Note the entire ehcache-specific configuration is read from ehcache.xml.

8.6.3. Caffeine Cache

Caffeine is a Java 8 rewrite of Guava’s cache and its implementation is located under org.springframework.cache.caffeine package and provides access to several features of Caffeine.

Configuring a CacheManager that creates the cache on demand is straightforward:

<bean id="cacheManager"
      class="org.springframework.cache.caffeine.CaffeineCacheManager"/>

It is also possible to provide the caches to use explicitly. In that case, only those will be made available by the manager:

<bean id="cacheManager" class="org.springframework.cache.caffeine.CaffeineCacheManager">
        <property name="caches">
                <set>
                        <value>default</value>
                        <value>books</value>
                </set>
        </property>
</bean>

The Caffeine CacheManager also supports customs Caffeine and CacheLoader. See the Caffeine documentation for more information about those.

8.6.4. GemFire-based Cache

GemFire is a memory-oriented/disk-backed, elastically scalable, continuously available, active (with built-in pattern-based subscription notifications), globally replicated database and provides fully-featured edge caching. For further information on how to use GemFire as a CacheManager (and more), please refer to the Spring Data GemFire reference documentation.

8.6.5. JSR-107 Cache

JSR-107 compliant caches can also be used by Spring’s caching abstraction. The JCache implementation is located under org.springframework.cache.jcache package.

Again, to use it, one simply needs to declare the appropriate CacheManager:

<bean id="cacheManager"
      class="org.springframework.cache.jcache.JCacheCacheManager"
      p:cache-manager-ref="jCacheManager"/>

<!-- JSR-107 cache manager setup  -->
<bean id="jCacheManager" .../>

8.6.6. Dealing with caches without a backing store

Sometimes when switching environments or doing testing, one might have cache declarations without an actual backing cache configured. As this is an invalid configuration, at runtime an exception will be thrown since the caching infrastructure is unable to find a suitable store. In situations like this, rather then removing the cache declarations (which can prove tedious), one can wire in a simple, dummy cache that performs no caching - that is, forces the cached methods to be executed every time:

<bean id="cacheManager" class="org.springframework.cache.support.CompositeCacheManager">
        <property name="cacheManagers">
                <list>
                        <ref bean="jdkCache"/>
                        <ref bean="gemfireCache"/>
                </list>
        </property>
        <property name="fallbackToNoOpCache" value="true"/>
</bean>

The CompositeCacheManager above chains multiple CacheManagers and additionally, through the fallbackToNoOpCache flag, adds a no op cache that for all the definitions not handled by the configured cache managers. That is, every cache definition not found in either jdkCache or gemfireCache (configured above) will be handled by the no op cache, which will not store any information causing the target method to be executed every time.

8.7. Plugging-in different back-end caches

Clearly there are plenty of caching products out there that can be used as a backing store. To plug them in, one needs to provide a CacheManager and Cache implementation since unfortunately there is no available standard that we can use instead. This may sound harder than it is since in practice, the classes tend to be simple adapters that map the caching abstraction framework on top of the storage API as the ehcache classes can show. Most CacheManager classes can use the classes in org.springframework.cache.support package, such as AbstractCacheManager which takes care of the boiler-plate code leaving only the actual mapping to be completed. We hope that in time, the libraries that provide integration with Spring can fill in this small configuration gap.

8.8. How can I set the TTL/TTI/Eviction policy/XXX feature?

Directly through your cache provider. The cache abstraction is…​ well, an abstraction not a cache implementation. The solution you are using might support various data policies and different topologies which other solutions do not (take for example the JDK ConcurrentHashMap) - exposing that in the cache abstraction would be useless simply because there would no backing support. Such functionality should be controlled directly through the backing cache, when configuring it or through its native API.

9. Appendix

9.1. XML Schemas

This part of the appendix lists XML schemas related to integration technologies.

9.1.1. The jee schema

The jee tags deal with Java EE (Java Enterprise Edition)-related configuration issues, such as looking up a JNDI object and defining EJB references.

To use the tags in the jee schema, you need to have the following preamble at the top of your Spring XML configuration file; the text in the following snippet references the correct schema so that the tags in the jee namespace are available to you.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:jee="http://www.springframework.org/schema/jee" xsi:schemaLocation="
                http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
                http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee.xsd"> <!-- bean definitions here -->

</beans>
<jee:jndi-lookup/> (simple)

Before…​

<bean id="dataSource" class="org.springframework.jndi.JndiObjectFactoryBean">
        <property name="jndiName" value="jdbc/MyDataSource"/>
</bean>
<bean id="userDao" class="com.foo.JdbcUserDao">
        <!-- Spring will do the cast automatically (as usual) -->
        <property name="dataSource" ref="dataSource"/>
</bean>

After…​

<jee:jndi-lookup id="dataSource" jndi-name="jdbc/MyDataSource"/>

<bean id="userDao" class="com.foo.JdbcUserDao">
        <!-- Spring will do the cast automatically (as usual) -->
        <property name="dataSource" ref="dataSource"/>
</bean>
<jee:jndi-lookup/> (with single JNDI environment setting)

Before…​

<bean id="simple" class="org.springframework.jndi.JndiObjectFactoryBean">
        <property name="jndiName" value="jdbc/MyDataSource"/>
        <property name="jndiEnvironment">
                <props>
                        <prop key="foo">bar</prop>
                </props>
        </property>
</bean>

After…​

<jee:jndi-lookup id="simple" jndi-name="jdbc/MyDataSource">
        <jee:environment>foo=bar</jee:environment>
</jee:jndi-lookup>
<jee:jndi-lookup/> (with multiple JNDI environment settings)

Before…​

<bean id="simple" class="org.springframework.jndi.JndiObjectFactoryBean">
        <property name="jndiName" value="jdbc/MyDataSource"/>
        <property name="jndiEnvironment">
                <props>
                        <prop key="foo">bar</prop>
                        <prop key="ping">pong</prop>
                </props>
        </property>
</bean>

After…​

<jee:jndi-lookup id="simple" jndi-name="jdbc/MyDataSource">
        <!-- newline-separated, key-value pairs for the environment (standard Properties format) -->
        <jee:environment>
                foo=bar
                ping=pong
        </jee:environment>
</jee:jndi-lookup>
<jee:jndi-lookup/> (complex)

Before…​

<bean id="simple" class="org.springframework.jndi.JndiObjectFactoryBean">
        <property name="jndiName" value="jdbc/MyDataSource"/>
        <property name="cache" value="true"/>
        <property name="resourceRef" value="true"/>
        <property name="lookupOnStartup" value="false"/>
        <property name="expectedType" value="com.myapp.DefaultFoo"/>
        <property name="proxyInterface" value="com.myapp.Foo"/>
</bean>

After…​

<jee:jndi-lookup id="simple"
                jndi-name="jdbc/MyDataSource"
                cache="true"
                resource-ref="true"
                lookup-on-startup="false"
                expected-type="com.myapp.DefaultFoo"
                proxy-interface="com.myapp.Foo"/>
<jee:local-slsb/> (simple)

The <jee:local-slsb/> tag configures a reference to an EJB Stateless SessionBean.

Before…​

<bean id="simple"
                class="org.springframework.ejb.access.LocalStatelessSessionProxyFactoryBean">
        <property name="jndiName" value="ejb/RentalServiceBean"/>
        <property name="businessInterface" value="com.foo.service.RentalService"/>
</bean>

After…​

<jee:local-slsb id="simpleSlsb" jndi-name="ejb/RentalServiceBean"
                business-interface="com.foo.service.RentalService"/>
<jee:local-slsb/> (complex)
<bean id="complexLocalEjb"
                class="org.springframework.ejb.access.LocalStatelessSessionProxyFactoryBean">
        <property name="jndiName" value="ejb/RentalServiceBean"/>
        <property name="businessInterface" value="com.foo.service.RentalService"/>
        <property name="cacheHome" value="true"/>
        <property name="lookupHomeOnStartup" value="true"/>
        <property name="resourceRef" value="true"/>
</bean>

After…​

<jee:local-slsb id="complexLocalEjb"
                jndi-name="ejb/RentalServiceBean"
                business-interface="com.foo.service.RentalService"
                cache-home="true"
                lookup-home-on-startup="true"
                resource-ref="true">
<jee:remote-slsb/>

The <jee:remote-slsb/> tag configures a reference to a remote EJB Stateless SessionBean.

Before…​

<bean id="complexRemoteEjb"
                class="org.springframework.ejb.access.SimpleRemoteStatelessSessionProxyFactoryBean">
        <property name="jndiName" value="ejb/MyRemoteBean"/>
        <property name="businessInterface" value="com.foo.service.RentalService"/>
        <property name="cacheHome" value="true"/>
        <property name="lookupHomeOnStartup" value="true"/>
        <property name="resourceRef" value="true"/>
        <property name="homeInterface" value="com.foo.service.RentalService"/>
        <property name="refreshHomeOnConnectFailure" value="true"/>
</bean>

After…​

<jee:remote-slsb id="complexRemoteEjb"
                jndi-name="ejb/MyRemoteBean"
                business-interface="com.foo.service.RentalService"
                cache-home="true"
                lookup-home-on-startup="true"
                resource-ref="true"
                home-interface="com.foo.service.RentalService"
                refresh-home-on-connect-failure="true">

9.1.2. The jms schema

The jms tags deal with configuring JMS-related beans such as Spring’s MessageListenerContainers. These tags are detailed in the section of the JMS chapter entitled JMS namespace support. Please do consult that chapter for full details on this support and the jms tags themselves.

In the interest of completeness, to use the tags in the jms schema, you need to have the following preamble at the top of your Spring XML configuration file; the text in the following snippet references the correct schema so that the tags in the jms namespace are available to you.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:jms="http://www.springframework.org/schema/jms" xsi:schemaLocation="
                http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
                http://www.springframework.org/schema/jms http://www.springframework.org/schema/jms/spring-jms.xsd"> <!-- bean definitions here -->

</beans>

9.1.3. <context:mbean-export/>

This element is detailed in Configuring annotation based MBean export.

9.1.4. The cache schema

The cache tags can be used to enable support for Spring’s @CacheEvict, @CachePut and @Caching annotations. It it also supports declarative XML-based caching. See Enable caching annotations and Declarative XML-based caching for details.

To use the tags in the cache schema, you need to have the following preamble at the top of your Spring XML configuration file; the text in the following snippet references the correct schema so that the tags in the cache namespace are available to you.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:cache="http://www.springframework.org/schema/cache" xsi:schemaLocation="
                http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
                http://www.springframework.org/schema/cache http://www.springframework.org/schema/cache/spring-cache.xsd"> <!-- bean definitions here -->

</beans>