The first part of this chapter covers some background theory and reveals quite a bit about the underlying API that drives Spring Integration's various messaging components. This information can be helpful if you want to really understand what's going on behind the scenes. However, if you want to get up and running with the simplified namespace-based configuration of the various elements, feel free to skip ahead to Section 7.1.4, “Namespace Support” for now.
As mentioned in the overview, Message Endpoints are responsible for connecting the various messaging components to channels. Over the next several chapters, you will see a number of different components that consume Messages. Some of these are also capable of sending reply Messages. Sending Messages is quite straightforward. As shown above in Section 3.1, “Message Channels”, it's easy to send a Message to a Message Channel. However, receiving is a bit more complicated. The main reason is that there are two types of consumers: Polling Consumers and Event Driven Consumers.
Of the two, Event Driven Consumers are much simpler. Without any need to manage and schedule a separate poller thread, they are essentially just listeners with a callback method. When connecting to one of Spring Integration's subscribable Message Channels, this simple option works great. However, when connecting to a buffering, pollable Message Channel, some component has to schedule and manage the polling thread(s). Spring Integration provides two different endpoint implementations to accommodate these two types of consumers. Therefore, the consumers themselves can simply implement the callback interface. When polling is required, the endpoint acts as a container for the consumer instance. The benefit is similar to that of using a container for hosting Message Driven Beans, but since these consumers are simply Spring-managed Objects running within an ApplicationContext, it more closely resembles Spring's own MessageListener containers.
Spring Integration's MessageHandler
interface is implemented by many of the
components within the framework. In other words, this is not part of the public API, and a developer would not
typically implement MessageHandler
directly. Nevertheless, it is used by a Message
Consumer for actually handling the consumed Messages, and so being aware of this strategy interface does help in
terms of understanding the overall role of a consumer. The interface is defined as follows:
public interface MessageHandler { void handleMessage(Message<?> message); }
Despite its simplicity, this provides the foundation for most of the components that will be covered in the following chapters (Routers, Transformers, Splitters, Aggregators, Service Activators, etc). Those components each perform very different functionality with the Messages they handle, but the requirements for actually receiving a Message are the same, and the choice between polling and event-driven behavior is also the same. Spring Integration provides two endpoint implementations that host these callback-based handlers and allow them to be connected to Message Channels.
Because it is the simpler of the two, we will cover the Event Driven Consumer endpoint first. You may recall that
the SubscribableChannel
interface provides a subscribe()
method and that the method accepts a MessageHandler
parameter (as shown in
the section called “SubscribableChannel”):
subscribableChannel.subscribe(messageHandler);
Since a handler that is subscribed to a channel does not have to actively poll that channel, this is an
Event Driven Consumer, and the implementation provided by Spring Integration accepts a
a SubscribableChannel
and a MessageHandler
:
SubscribableChannel channel = context.getBean("subscribableChannel", SubscribableChannel.class); EventDrivenConsumer consumer = new EventDrivenConsumer(channel, exampleHandler);
Spring Integration also provides a PollingConsumer
, and it can be instantiated in
the same way except that the channel must implement PollableChannel
:
PollableChannel channel = context.getBean("pollableChannel", PollableChannel.class); PollingConsumer consumer = new PollingConsumer(channel, exampleHandler);
Note | |
---|---|
For more information regarding Polling Consumers, please also read Section 3.2, “Poller (Polling Consumer)” as well as Section 3.3, “Channel Adapter”. |
There are many other configuration options for the Polling Consumer. For example, the trigger is a required property:
PollingConsumer consumer = new PollingConsumer(channel, handler); consumer.setTrigger(new IntervalTrigger(30, TimeUnit.SECONDS));
Spring Integration currently provides two implementations of the Trigger
interface: IntervalTrigger
and CronTrigger
. The
IntervalTrigger
is typically defined with a simple interval (in milliseconds), but
also supports an initialDelay property and a boolean fixedRate property (the default is false, i.e.
fixed delay):
IntervalTrigger trigger = new IntervalTrigger(1000); trigger.setInitialDelay(5000); trigger.setFixedRate(true);
The CronTrigger
simply requires a valid cron
expression (see the Javadoc for details):
CronTrigger trigger = new CronTrigger("*/10 * * * * MON-FRI");
In addition to the trigger, several other polling-related configuration properties may be specified:
PollingConsumer consumer = new PollingConsumer(channel, handler); consumer.setMaxMessagesPerPoll(10); consumer.setReceiveTimeout(5000);
The maxMessagesPerPoll property specifies the maximum number of messages to receive within a given poll
operation. This means that the poller will continue calling receive() without waiting
until either null
is returned or that max is reached. For example, if a poller has a 10 second
interval trigger and a maxMessagesPerPoll setting of 25, and it is polling a channel that has 100 messages
in its queue, all 100 messages can be retrieved within 40 seconds. It grabs 25, waits 10 seconds, grabs the
next 25, and so on.
The receiveTimeout property specifies the amount of time the poller should wait if no messages are available when it invokes the receive operation. For example, consider two options that seem similar on the surface but are actually quite different: the first has an interval trigger of 5 seconds and a receive timeout of 50 milliseconds while the second has an interval trigger of 50 milliseconds and a receive timeout of 5 seconds. The first one may receive a message up to 4950 milliseconds later than it arrived on the channel (if that message arrived immediately after one of its poll calls returned). On the other hand, the second configuration will never miss a message by more than 50 milliseconds. The difference is that the second option requires a thread to wait, but as a result it is able to respond much more quickly to arriving messages. This technique, known as long polling, can be used to emulate event-driven behavior on a polled source.
A Polling Consumer may also delegate to a Spring TaskExecutor
,
as illustrated in the following example:
PollingConsumer consumer = new PollingConsumer(channel, handler); TaskExecutor taskExecutor = context.getBean("exampleExecutor", TaskExecutor.class); consumer.setTaskExecutor(taskExecutor);
Furthermore, a PollingConsumer
has a property called
adviceChain. This property allows you to specify a
List
of AOP Advices for handling additional
cross cutting concerns including transactions. These advices are applied
around the doPoll()
method. For more in-depth
information, please see the sections AOP Advice chains
and Transaction Support under Section 7.1.4, “Namespace Support”.
The examples above show dependency lookups, but keep in mind that these
consumers will most often be configured as Spring bean definitions.
In fact, Spring Integration also provides a FactoryBean
called ConsumerEndpointFactoryBean
that creates
the appropriate consumer type based on the type of channel, and there is
full XML namespace support to even further hide those details. The
namespace-based configuration will be featured as each component type is
introduced.
Note | |
---|---|
Many of the MessageHandler implementations are also capable of generating reply
Messages. As mentioned above, sending Messages is trivial when compared to the Message reception. Nevertheless,
when and how many reply Messages are sent depends on the handler
type. For example, an Aggregator waits for a number of Messages to arrive and is often
configured as a downstream consumer for a Splitter which may generate multiple
replies for each Message it handles. When using the namespace configuration, you do not strictly need to know
all of the details, but it still might be worth knowing that several of these components share a common base
class, the AbstractReplyProducingMessageHandler , and it provides a
setOutputChannel(..) method.
|
Throughout the reference manual, you will see specific configuration examples for endpoint elements, such as
router, transformer, service-activator, and so on. Most of these will support an input-channel attribute and
many will support an output-channel attribute. After being parsed, these endpoint elements produce an instance
of either the PollingConsumer
or the
EventDrivenConsumer
depending on the type of the input-channel that is
referenced: PollableChannel
or SubscribableChannel
respectively. When the channel is pollable, then the polling behavior is determined based on the endpoint
element's poller sub-element and its attributes.
Below you find a poller with all available configuration options:
<int:poller cron="" default="false" error-channel="" fixed-delay="" fixed-rate="" id="" max-messages-per-poll="" receive-timeout="" ref="" task-executor="" time-unit="MILLISECONDS" trigger=""> <int:advice-chain /> <int:transactional /> </int:poller>
Provides the ability to configure Pollers using Cron expressions.
The underlying implementation uses a
| |
By setting this attribute to true,
it is possible to define exactly one (1) global default
poller. An exception is raised if more than one default
poller is defined in the application context.
Any endpoints connected to a PollableChannel (PollingConsumer)
or any SourcePollingChannelAdapter that does not have any
explicitly configured poller will then use the global default
Poller.
Optional. Defaults to | |
Identifies the channel which error messages will be sent to if
a failure occurs in this poller's invocation. To completely
suppress Exceptions, provide a reference to the
| |
The fixed delay trigger uses a | |
The fixed rate trigger uses a | |
The Id referring to the Poller's underlying bean-definition,
which is of type
| |
Please see Section 3.3.1, “Configuring An Inbound Channel Adapter”
for more information. Optional. If
not specified the default values used depends on the context.
If a | |
Value is set on the underlying class | |
Bean reference to another top-level poller. The | |
Provides the ability to reference a custom task executor. Please see the section below titled TaskExecutor Support for further information. Optional. | |
This attribute specifies the
The minimal supported granularity for a
Basically this enum provides a convenience for SECONDS-based
interval trigger values. For hourly, daily, and monthly
settings, consider using a | |
Reference to any spring configured bean which implements
the | |
Allows to specify extra AOP Advices to handle additional cross cutting concerns. Please see the section below titled Transaction Support for further information. Optional. | |
Pollers can be made transactional. Please see the section below titled AOP Advice chains for further information. Optional. |
Examples
For example, a simple interval-based poller with a 1-second interval would be configured like this:
<int:transformer input-channel="pollable" ref="transformer" output-channel="output"> <int:poller fixed-rate="1000"/> </int:transformer>
As an alternative to fixed-rate you can also use the fixed-delay attribute.
For a poller based on a Cron expression, use the cron attribute instead:
<int:transformer input-channel="pollable" ref="transformer" output-channel="output"> <int:poller cron="*/10 * * * * MON-FRI"/> </int:transformer>
If the input channel is a PollableChannel
, then the poller configuration is
required. Specifically, as mentioned above, the trigger is a required property of the PollingConsumer class.
Therefore, if you omit the poller sub-element for a Polling Consumer endpoint's configuration, an Exception
may be thrown. The exception will also be thrown if you attempt to configure a poller on the element that is
connected to a non-pollable channel.
It is also possible to create top-level pollers in which case only a ref is required:
<int:poller id="weekdayPoller" cron="*/10 * * * * MON-FRI"/> <int:transformer input-channel="pollable" ref="transformer" output-channel="output"> <int:poller ref="weekdayPoller"/> </int:transformer>
Note | |
---|---|
The ref attribute is only allowed on the inner-poller definitions. Defining this attribute on a top-level poller will result in a configuration exception thrown during initialization of the Application Context. |
Global Default Pollers
In fact, to simplify the configuration even further, you can define a global default poller. A single top-level poller within
an ApplicationContext may have the default
attribute with a value of true. In that case, any
endpoint with a PollableChannel for its input-channel that is defined within the same ApplicationContext and has
no explicitly configured poller sub-element will use that default.
<int:poller id="defaultPoller" default="true" max-messages-per-poll="5" fixed-rate="3000"/> <!-- No <poller/> sub-element is necessary since there is a default --> <int:transformer input-channel="pollable" ref="transformer" output-channel="output"/>
Transaction Support
Spring Integration also provides transaction support for the pollers so that each receive-and-forward operation can be performed as an atomic unit-of-work. To configure transactions for a poller, simply add the <transactional/> sub-element. The attributes for this element should be familiar to anyone who has experience with Spring's Transaction management:
<int:poller fixed-delay="1000"> <int:transactional transaction-manager="txManager" propagation="REQUIRED" isolation="REPEATABLE_READ" timeout="10000" read-only="false"/> </int:poller>
For more information please refer to Section B.1.1, “Poller Transaction Support”.
AOP Advice chains
Since Spring transaction support depends on the Proxy mechanism with TransactionInterceptor
(AOP Advice) handling transactional
behavior of the message flow initiated by the poller, some times there is a need to provide extra Advice(s) to handle other
cross cutting behavior associated with the poller. For that poller defines an advice-chain element allowing you to add
more advices - class that implements MethodInterceptor
interface..
<int:service-activator id="advicedSa" input-channel="goodInputWithAdvice" ref="testBean" method="good" output-channel="output"> <int:poller max-messages-per-poll="1" fixed-rate="10000"> <int:transactional transaction-manager="txManager" /> <int:advice-chain> <ref bean="adviceA" /> <beans:bean class="org.bar.SampleAdvice"/> </int:advice-chain> </int:poller> </int:service-activator>
For more information on how to implement MethodInterceptor please refer to AOP sections of Spring reference manual (section 8 and 9). Advice chain can also be applied on the poller that does not have any transaction configuration essentially allowing you to enhance the behavior of the message flow initiated by the poller.
TaskExecutor Support
The polling threads may be executed by any instance of Spring's TaskExecutor
abstraction. This enables concurrency for an endpoint or group of endpoints. As of Spring 3.0, there is a task
namespace in the core Spring Framework, and its <executor/> element supports the creation of a simple thread
pool executor. That element accepts attributes for common concurrency settings such as pool-size and queue-capacity.
Configuring a thread-pooling executor can make a substantial difference in how the endpoint performs under load. These
settings are available per-endpoint since the performance of an endpoint is one of the major factors to consider
(the other major factor being the expected volume on the channel to which the endpoint subscribes). To enable
concurrency for a polling endpoint that is configured with the XML namespace support, provide the task-executor
reference on its <poller/> element and then provide one or more of the properties shown below:
<int:poller task-executor="pool" fixed-rate="1000"/> <task:executor id="pool" pool-size="5-25" queue-capacity="20" keep-alive="120"/>
If no task-executor is provided, the consumer's handler will be invoked in the caller's thread. Note that the
caller is usually the default TaskScheduler
(see Section E.3, “Configuring the Task Scheduler”). Also, keep in mind that the task-executor attribute can
provide a reference to any implementation of Spring's TaskExecutor
interface by
specifying the bean name. The executor element above is simply provided for convenience.
As mentioned in the background section for Polling Consumers above, you can also configure a Polling Consumer in such a way as to emulate event-driven behavior. With a long receive-timeout and a short interval-trigger, you can ensure a very timely reaction to arriving messages even on a polled message source. Note that this will only apply to sources that have a blocking wait call with a timeout. For example, the File poller does not block, each receive() call returns immediately and either contains new files or not. Therefore, even if a poller contains a long receive-timeout, that value would never be usable in such a scenario. On the other hand when using Spring Integration's own queue-based channels, the timeout value does have a chance to participate. The following example demonstrates how a Polling Consumer will receive Messages nearly instantaneously.
<int:service-activator input-channel="someQueueChannel" output-channel="output"> <int:poller receive-timeout="30000" fixed-rate="10"/> </int:service-activator>
Using this approach does not carry much overhead since internally it is nothing more then a timed-wait thread which does not require nearly as much CPU resource usage as a thrashing, infinite while loop for example.
When configuring Pollers with a fixed-delay
or
fixed-rate
attribute, the default implementation will use
a PeriodicTrigger
instance. The
PeriodicTrigger
is part of the Core Spring Framework
and it accepts the interval as a constructor
argument, only. Therefore it cannot be changed at runtime.
However, you can define your own implementation of the
org.springframework.scheduling.Trigger
interface. You could even use the PeriodicTrigger as a starting point.
Then, you can add a setter for the interval (period), or you could even
embed your own throttling logic within the trigger itself if desired.
The period property will be used with each call to
nextExecutionTime to schedule the next poll.
To use this custom trigger within pollers, declare the bean defintion of
the custom Trigger in your application context and inject the dependency
into your Poller configuration using the trigger
attribute,
which references the custom Trigger bean instance.
You can now obtain a reference to the Trigger bean and the polling
interval can be changed between polls.
For an example, please see the Spring Integration Samples project. It contains a sample called dynamic-poller, which uses a custom Trigger and demonstrates the ability to change the polling interval at runtime.
The sample provides a custom Trigger which implements the org.springframework.scheduling.Trigger interface. The sample's Trigger is based on Spring's PeriodicTrigger implementation. However, the fields of the custom trigger are not final and the properties have explicit getters and setters, allowing to dynamically change the polling period at runtime.
Note | |
---|---|
It is important to note, though, that because the Trigger method is nextExecutionTime(), any changes to a dynamic trigger will not take effect until the next poll, based on the existing configuration. It is not possible to force a trigger to fire before it's currently configured next execution time. |
Throughout the reference manual, you will also see specific configuration and implementation examples of various endpoints
which can accept a Message or any arbitrary Object as an input parameter. In the case of an Object, such a parameter will
be mapped to a Message payload or part of the payload or header (when using the Spring Expression Language). However there
are times when the type of input parameter of the endpoint method does not match the type of the payload or its part.
In this scenario we need to perform type conversion. Spring Integration provides a convenient way for registering type
converters (using the Spring 3.x ConversionService) within its own instance of a conversion service bean named integrationConversionService.
That bean is automatically created as soon as the first converter is defined using the Spring Integration namespace support.
To register a Converter all you need is to implement
org.springframework.core.convert.converter.Converter
and define it via
convenient namespace support:
<int:converter ref="sampleConverter"/> <bean id="sampleConverter" class="foo.bar.TestConverter"/>
or as an inner bean:
<int:converter> <bean class="org.springframework.integration.config.xml.ConverterParserTests$TestConverter3"/> </int:converter>
If you want the polling to be asynchronous, a Poller can optionally specify a task-executor attribute
pointing to an existing instance of any TaskExecutor
bean
(Spring 3.0 provides a convenient namespace configuration via the task
namespace). However, there are certain things
you must understand when configuring a Poller with a TaskExecutor.
The problem is that there are two configurations in place. The Poller and the TaskExecutor, and they both have to be in tune with each other otherwise you might end up creating an artificial memory leak.
Let's look at the following configuration provided by one of the users on the Spring Integration forum (http://forum.springsource.org/showthread.php?t=94519):
<int:service-activator input-channel="publishChannel" ref="myService"> <int:poller receive-timeout="5000" task-executor="taskExecutor" fixed-rate="50"/> </int:service-activator> <task:executor id="taskExecutor" pool-size="20" queue-capacity="20"/>
The above configuration demonstrates one of those out of tune configurations.
The poller keeps scheduling new tasks even though all the threads are blocked waiting for either a new message to arrive, or the timeout to expire. Given that there are 20 threads executing tasks with a 5 second timeout, they will be executed at a rate of 4 per second (5000/20 = 250ms). But, new tasks are being scheduled at a rate of 20 per second, so the internal queue in the task executor will grow at a rate of 16 per second (while the process is idle), so we essentially have a memory leak.
One of the ways to handle this is to set the queue-capacity
attribute of the Task Executor to 0. You can also
manage it by specifying what to do with messages that can not be queued by setting the rejection-policy
attribute
of the Task Executor (e.g., DISCARD). In other words there are certain details you must understand with regard to configuring
the TaskExecutor. Please refer to - Section 25 - Task Execution and Scheduling of the Spring reference manual
for more detail on the subject.
The primary purpose of a Gateway is to hide the messaging API provided by Spring Integration. It allows your application's business logic to be completely unaware of the Spring Integration API and using a generic Gateway, your code interacts instead with a simple interface, only.
As mentioned above, it would be great to have no dependency on the Spring
Integration API at all - including the gateway class. For that reason, Spring
Integration provides the GatewayProxyFactoryBean
that
generates a proxy for any interface and internally invokes the gateway
methods shown below. Using dependency injection you can then expose the interface
to your business methods.
Here is an example of an interface that can be used to interact with Spring Integration:
package org.cafeteria; public interface Cafe { void placeOrder(Order order); }
Namespace support is also provided which allows you to configure such an interface as a service as demonstrated by the following example.
<int:gateway id="cafeService" service-interface="org.cafeteria.Cafe" default-request-channel="requestChannel" default-reply-channel="replyChannel"/>
With this configuration defined, the "cafeService" can now be injected into other beans, and the code that invokes the methods on that proxied instance of the Cafe interface has no awareness of the Spring Integration API. The general approach is similar to that of Spring Remoting (RMI, HttpInvoker, etc.). See the "Samples" Appendix for an example that uses this "gateway" element (in the Cafe demo).
Typically you don't have to specify the default-reply-channel
,
since a Gateway will auto-create a temporary, anonymous reply channel,
where it will listen for the reply. However, there are some cases which
may prompt you to define a default-reply-channel
(or reply-channel
with adapter gateways such as HTTP, JMS, etc.).
For some background, we'll quickly discuss some of the inner-workings of the Gateway.
A Gateway will create a temporary point-to-point reply channel which is anonymous and is added
to the Message Headers with the name replyChannel
.
When providing an explicit default-reply-channel
(reply-channel
with remote adapter gateways),
you have the option to point to a publish-subscribe channel, which is so named because you can add more than one subscriber to it.
Internally Spring Integration will create a Bridge between the temporary replyChannel
and the explicitly defined
default-reply-channel
.
So let's say you want your reply to go not only to the gateway, but also to some other consumer. In this case you
would want two things: a) a named channel you can subscribe to and b) that channel is a publish-subscribe-channel.
The default strategy used by the gateway will not satisfy those needs, because the reply channel added to the header is anonymous and
point-to-point. This means that no other subscriber can get a handle to it and even if it could, the channel
has point-to-point behavior such that only one subscriber would get the Message. So by defining a default-reply-channel
you can point to a channel of your choosing, which in this case would be a publish-subscribe-channel
.
The Gateway would create a bridge from it to the temporary, anonymous reply channel that is stored in the header.
Another case where you might want to provide a reply channel explicitly is for monitoring or auditing via an interceptor (e.g., wiretap). You need a named channel in order to configure a Channel Interceptor.
The reason that the attributes on the 'gateway' element are named 'default-request-channel' and
'default-reply-channel' is that you may also provide per-method channel references by using the
@Gateway
annotation.
public interface Cafe { @Gateway(requestChannel="orders") void placeOrder(Order order); }
You may alternatively provide such content in method
sub-elements if you prefer XML configuration (see the next paragraph).
It is also possible to pass values to be interpreted as Message headers on the Message that is created and sent to the request channel by using the @Header annotation:
public interface FileWriter { @Gateway(requestChannel="filesOut") void write(byte[] content, @Header(FileHeaders.FILENAME) String filename); }
If you prefer the XML approach of configuring Gateway methods, you can provide method sub-elements to the gateway configuration.
<int:gateway id="myGateway" service-interface="org.foo.bar.TestGateway" default-request-channel="inputC"> <int:method name="echo" request-channel="inputA" reply-timeout="2" request-timeout="200"/> <int:method name="echoUpperCase" request-channel="inputB"/> <int:method name="echoViaDefault"/> </int:gateway>
You can also provide individual headers per method invocation via XML.
This could be very useful if the headers you want to set are static in nature and you don't want
to embed them in the gateway's method signature via @Header
annotations.
For example, in the Loan Broker example we want to influence how aggregation of the Loan quotes
will be done based on what type of request was initiated (single quote or all quotes). Determining the
type of the request by evaluating what gateway method was invoked, although possible, would
violate the separation of concerns paradigm (the method is a java artifact), but expressing your
intention (meta information) via Message headers is natural in a Messaging architecture.
<int:gateway id="loanBrokerGateway" service-interface="org.springframework.integration.loanbroker.LoanBrokerGateway"> <int:method name="getLoanQuote" request-channel="loanBrokerPreProcessingChannel"> <int:header name="RESPONSE_TYPE" value="BEST"/> </int:method> <int:method name="getAllLoanQuotes" request-channel="loanBrokerPreProcessingChannel"> <int:header name="RESPONSE_TYPE" value="ALL"/> </int:method> </int:gateway>
In the above case you can clearly see how a different value will be set for the 'RESPONSE_TYPE' header based on the gateway's method.
When invoking methods on a Gateway interface that do not have any arguments,
the default behavior is to receive a Message
from a
PollableChannel
.
At times however, you may want to trigger no-argument methods so that you can in fact interact with other components downstream that do not require user-provided parameters, e.g. triggering no-argument SQL calls or Stored Procedures.
In order to achieve send-and-receive semantics, you must provide a payload.
In order to generate a payload, method parameters on the interface are
not necessary. You can either use the @Payload
annotation
or the payload-expression
attribute in XML on the method
sub-element. Below please find a few examples of what the payloads could be:
Here is an example using the @Payload
annotation:
public interface Cafe {
@Payload("new java.util.Date()")
List<Order> retrieveOpenOrders();
}
If a method has no argument and no return value, but does contain a payload expression, it will be treated as a send-only operation.
Of course, the Gateway invocation might result in errors.
By default any error that has occurred downstream will be re-thrown as a
MessagingException
(RuntimeException)
upon the Gateway's method invocation. However there are times when you may want
to simply log the error rather than propagating it, or you may want to treat an
Exception as a valid reply, by mapping it to a Message that will conform to some
"error message" contract that the caller understands. To accomplish this, our
Gateway provides support for a Message Channel dedicated to the errors via the
error-channel attribute. In the example below, you can see
that a 'transformer' is used to create a reply Message from the Exception.
<int:gateway id="sampleGateway" default-request-channel="gatewayChannel" service-interface="foo.bar.SimpleGateway" error-channel="exceptionTransformationChannel"/> <int:transformer input-channel="exceptionTransformationChannel" ref="exceptionTransformer" method="createErrorResponse"/>
The exceptionTransformer could be a simple POJO that knows how to create the expected error response objects. That would then be the payload that is sent back to the caller. Obviously, you could do many more elaborate things in such an "error flow" if necessary. It might involve routers (including Spring Integration's ErrorMessageExceptionTypeRouter), filters, and so on. Most of the time, a simple 'transformer' should be sufficient, however.
Alternatively, you might want to only log the Exception (or send it somewhere asynchronously). If you provide a one-way flow, then nothing would be sent back to the caller. In the case that you want to completely suppress Exceptions, you can provide a reference to the global "nullChannel" (essentially a /dev/null approach). Finally, as mentioned above, if no "error-channel" is defined at all, then the Exceptions will propagate as usual.
Important | |
---|---|
Exposing the messaging system via simple POJI Gateways obviously provides benefits, but "hiding" the reality of the underlying messaging system does come at a price so there are certain things you should consider. We want our Java method to return as quickly as possible and not hang for an indefinite amount of time while the caller is waiting on it to return (void, return value, or a thrown Exception). When regular methods are used as a proxies in front of the Messaging system, we have to take into account the potentially asynchronous nature of the underlying messaging. This means that there might be a chance that a Message that was initiated by a Gateway could be dropped by a Filter, thus never reaching a component that is responsible for producing a reply. Some Service Activator method might result in an Exception, thus providing no reply (as we don't generate Null messages). So as you can see there are multiple scenarios where a reply message might not be coming. That is perfectly natural in messaging systems. However think about the implication on the gateway method. The Gateway's method input arguments were incorporated into a Message and sent downstream. The reply Message would be converted to a return value of the Gateway's method. So you might want to ensure that for each Gateway call there will always be a reply Message. Otherwise, your Gateway method might never return and will hang indefinitely. One of the ways of handling this situation is via an Asynchronous Gateway (explained later in this section). Another way of handling it is to explicitly set the reply-timeout attribute. That way, the gateway will not hang any longer than the time specified by the reply-timeout and will return 'null' if that timeout does elapse. Finally, you might want to consider setting downstream flags such as 'requires-reply' on a service-activator or 'throw-exceptions-on-rejection' on a filter. These options will be discussed in more detail in the final section of this chapter. |
As a pattern the Messaging Gateway is a very nice way to hide messaging-specific code while still exposing the full capabilities of the
messaging system. As you've seen, the GatewayProxyFactoryBean
provides a convenient way to expose a Proxy over a service-interface
thus giving you POJO-based access to a messaging system (based on objects in your own domain, or primitives/Strings, etc). But when a
gateway is exposed via simple POJO methods which return values it does imply that for each Request message (generated when the method is invoked)
there must be a Reply message (generated when the method has returned). Since Messaging systems naturally are asynchronous you may not always be
able to guarantee the contract where "for each request there will always be be a reply".
With Spring Integration 2.0 we are introducing support for an Asynchronous Gateway which is a convenient way to initiate
flows where you may not know if a reply is expected or how long will it take for replies to arrive.
A natural way to handle these types of scenarios in Java would be relying upon java.util.concurrent.Future instances, and that is exactly what Spring Integration uses to support an Asynchronous Gateway.
From the XML configuration, there is nothing different and you still define Asynchronous Gateway the same way as a regular Gateway.
<int:gateway id="mathService" service-interface="org.springframework.integration.sample.gateway.futures.MathServiceGateway" default-request-channel="requestChannel"/>
However the Gateway Interface (service-interface) is a bit different.
public interface MathServiceGateway { Future<Integer> multiplyByTwo(int i); }
As you can see from the example above the return type for the gateway method is a Future
. When
GatewayProxyFactoryBean
sees that the
return type of the gateway method is a Future
, it immediately switches to the async mode by utilizing
an AsyncTaskExecutor
. That is all. The call to such a method always returns immediately with a Future
instance.
Then, you can interact with the Future
at your own pace to get the result, cancel, etc. And, as with
any other use of Future instances, calling get() may reveal a timeout, an execution exception, and so on.
MathServiceGateway mathService = ac.getBean("mathService", MathServiceGateway.class); Future<Integer> result = mathService.multiplyByTwo(number); // do something else here since the reply might take a moment int finalResult = result.get(1000, TimeUnit.SECONDS);
For a more detailed example, please refer to the async-gateway sample distributed within the Spring Integration samples.
Asynchronous Gateway and AsyncTaskExecutor
By default GatewayProxyFactoryBean
uses org.springframework.core.task.SimpleAsyncTaskExecutor
when submitting internal AsyncInvocationTask
instances for any gateway method whose
return type is Future.class
. However the async-executor
attribute in the
<gateway/>
element's configuration allows you to provide a reference to any implementation of
java.util.concurrent.Executor
available within the Spring application context.
As it was explained earlier, the Gateway provides a convenient way of interacting with a Messaging system via POJO method invocations, but realizing that a typical method invocation, which is generally expected to always return (even with an Exception), might not always map one-to-one to message exchanges (e.g., a reply message might not arrive - which is equivalent to a method not returning). It is important to go over several scenarios especially in the Sync Gateway case and understand the default behavior of the Gateway and how to deal with these scenarios to make the Sync Gateway behavior more predictable regardless of the outcome of the message flow that was initialed from such Gateway.
There are certain attributes that could be configured to make Sync Gateway behavior more predictable, but some of them might not always work as you might have expected. One of them is reply-timeout. So, lets look at the reply-timeout attribute and see how it can/can't influence the behavior of the Sync Gateway in various scenarios. We will look at single-threaded scenario (all components downstream are connected via Direct Channel) and multi-threaded scenarios (e.g., somewhere downstream you may have Pollable or Executor Channel which breaks single-thread boundary)
Long running process downstream
Sync Gateway - single-threaded.
If a component downstream is still running (e.g., infinite loop or a very slow service), then setting a reply-timeout
has no effect and the Gateway method call will not return until such downstream service exits (via return or exception).
Sync Gateway - multi-threaded.
If a component downstream is still running (e.g., infinite loop or a very slow service), in a multi-threaded message
flow setting the reply-timeout will have an effect by allowing gateway method invocation to
return once the timeout has been reached, since the GatewayProxyFactoryBean
will simply
poll on the reply channel waiting for a message until the timeout expires. However it could result in a 'null' return
from the Gateway method if the timeout has been reached before the actual reply was produced. It is also important to understand that
the reply message (if produced) will be sent to a reply channel after the Gateway method invocation might have returned, so you must be aware of that and design your flow with this in mind.
Downstream component returns 'null'
Sync Gateway - single-threaded. If a component downstream returns 'null' and no reply-timeout has been configured, the Gateway method call will hang indefinitely unless: a) a reply-timeout has been configured or b) the requires-reply attribute has been set on the downstream component (e.g., service-activator) that might return 'null'. In this case, an Exception would be thrown and propagated to the Gateway. Sync Gateway - multi-threaded. Behavior is the same as above.
Downstream component return signature is 'void' while Gateway method signature is non-void
Sync Gateway - single-threaded. If a component downstream returns 'void' and no reply-timeout has been configured, the Gateway method call will hang indefinitely unless a reply-timeout has been configured Sync Gateway - multi-threaded Behavior is the same as above.
Downstream component results in Runtime Exception (regardless of the method signature)
Sync Gateway - single-threaded. If a component downstream throws a Runtime Exception, such exception will be propagated via an Error Message back to the gateway and re-thrown. Sync Gateway - multi-threaded Behavior is the same as above.
Important | |||
---|---|---|---|
It is also important to understand that by default reply-timeout is unbounded* which means that
if not explicitly set there are several scenarios (described above) where your Gateway method invocation might
hang indefinitely. So, make sure you analyze your flow and if there is even a remote possibility of one of these
scenarios to occur, set the reply-timeout attribute to a 'safe' value or, even better,
set the requires-reply attribute of the downstream component to 'true' to ensure a timely response
as produced by the throwing of an Exception as soon as that downstream component does return null internally.
But also, realize that there are some scenarios (see the very first one)
where reply-timeout will not help. That means it is also important to analyze your message
flow and decide when to use a Sync Gateway vs an Async Gateway. As you've seen the latter case is simply a matter of
defining Gateway methods that return Future instances. Then, you are guaranteed to receive that return value, and
you will have more granular control over the results of the invocation.
Also, when dealing with a Router you should remember that setting the resolution-required attribute to 'true' will result in an Exception thrown by the router if it can not resolve a particular channel. Likewise, when dealing with a Filter, you can set the throw-exception-on-rejection attribute. In both of these cases, the resulting flow will behave like that containing a service-activator with the 'requires-reply' attribute. In other words, it will help to ensure a timely response from the Gateway method invocation.
|
The Service Activator is the endpoint type for connecting any Spring-managed Object to an input channel so that it may play the role of a service. If the service produces output, it may also be connected to an output channel. Alternatively, an output producing service may be located at the end of a processing pipeline or message flow in which case, the inbound Message's "replyChannel" header can be used. This is the default behavior if no output channel is defined, and as with most of the configuration options you'll see here, the same behavior actually applies for most of the other components we have seen.
To create a Service Activator, use the 'service-activator' element with the 'input-channel' and 'ref' attributes:
<int:service-activator input-channel="exampleChannel" ref="exampleHandler"/>
The configuration above assumes that "exampleHandler" either contains a single method annotated with the @ServiceActivator annotation or that it contains only one public method at all. To delegate to an explicitly defined method of any object, simply add the "method" attribute.
<int:service-activator input-channel="exampleChannel" ref="somePojo" method="someMethod"/>
In either case, when the service method returns a non-null value, the endpoint will attempt to send the reply message to an appropriate reply channel. To determine the reply channel, it will first check if an "output-channel" was provided in the endpoint configuration:
<int:service-activator input-channel="exampleChannel" output-channel="replyChannel" ref="somePojo" method="someMethod"/>
If no "output-channel" is available, it will then check the Message's replyChannel
header
value. If that value is available, it will then check its type. If it is a
MessageChannel
, the reply message will be sent to that channel. If it is a
String
, then the endpoint will attempt to resolve the channel name to a channel instance.
If the channel cannot be resolved, then a ChannelResolutionException
will be thrown.
It it can be resolved, the Message will be sent there. This is the technique used for Request Reply messaging
in Spring Integration, and it is also an example of the Return Address pattern.
The argument in the service method could be either a Message or an arbitrary type. If the latter, then it will be assumed that it is a Message payload, which will be extracted from the message and injected into such service method. This is generally the recommended approach as it follows and promotes a POJO model when working with Spring Integration. Arguments may also have @Header or @Headers annotations as described in Section E.5, “Annotation Support”
Note | |
---|---|
The service method is not required to have any arguments at all, which means you can implement event-style Service Activators, where all you care about is an invocation of the service method, not worrying about the contents of the message. Think of it as a NULL JMS message. An example use-case for such an implementation could be a simple counter/monitor of messages deposited on the input channel. |
Using a "ref" attribute is generally recommended if the custom Service Activator handler implementation can be reused
in other <service-activator>
definitions. However if the custom Service Activator handler implementation
is only used within a single definition of the <service-activator>
, you can provide an inner bean definition:
<int:service-activator id="exampleServiceActivator" input-channel="inChannel" output-channel = "outChannel" method="foo"> <beans:bean class="org.foo.ExampleServiceActivator"/> </int:service-activator>
Note | |
---|---|
Using both the "ref" attribute and an inner handler definition in the same |
Service Activators and the Spring Expression Language (SpEL)
Since Spring Integration 2.0, Service Activators can also benefit from SpEL (http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/expressions.html).
For example, you may now invoke any bean method without pointing to the bean via a ref
attribute or including it as an
inner bean definition. For example:
<int:service-activator input-channel="in" output-channel="out" expression="@accountService.processAccount(payload, headers.accountId)"/> <bean id="accountService" class="foo.bar.Account"/>
In the above configuration instead of injecting 'accountService' using a ref
or as an inner bean,
we are simply using SpEL's @beanId
notation and invoking a method which takes a type compatible with Message payload. We
are also passing a header value. As you can see, any valid SpEL expression can be evaluated against any content in the Message.
For simple scenarios your Service Activators do not even have to reference a bean if all logic can be encapsulated
by such an expression.
<int:service-activator input-channel="in" output-channel="out" expression="payload * 2"/>
In the above configuration our service logic is to simply multiply the payload value by 2, and SpEL lets us handle it relatively easy.
A Delayer is a simple endpoint that allows a Message flow to be delayed by a certain interval. When
a Message is delayed, the original sender will not block. Instead, the delayed Messages will be
scheduled with an instance of org.springframework.scheduling.TaskScheduler
to be sent to the output channel after the delay has passed. This approach is scalable even for
rather long delays, since it does not result in a large number of blocked sender Threads. On the
contrary, in the typical case a thread pool will be used for the actual execution of releasing the
Messages. Below you will find several examples of configuring a Delayer.
The <delayer>
element is used to delay the Message flow between two Message Channels.
As with the other endpoints, you can provide the 'input-channel' and 'output-channel' attributes,
but the delayer also has 'default-delay' and 'delay-header-name' attributes that are used to
determine the number of milliseconds
that each Message should be delayed. The following delays all messages by 3 seconds:
<int:delayer id="delayer" input-channel="input" default-delay="3000" output-channel="output"/>
If you need per-Message determination of the delay, then you can also provide the name of a header using the 'delay-header-name' attribute:
<int:delayer id="delayer" input-channel="input" output-channel="output" default-delay="3000" delay-header-name="delay"/>
In the example above the 3 second delay would only apply in the case that the header value is not present for a given inbound Message. If you only want to apply a delay to Messages that have an explicit header value, then you can set the 'default-delay' to 0 or don't use it at all (by default it is 0). For any Message that has a delay of 0 (or less), the Message will be sent directly. In fact, if there is not a positive delay value for a Message, it will be sent to the output channel on the calling Thread.
Tip | |
---|---|
The delay handler supports header values that represent an interval in milliseconds (any
Object whose toString() method produces a value that can be parsed into a
Long) as well as java.util.Date instances representing an absolute time.
In the first case, the milliseconds will be counted from the current time (e.g. a value of 5000
would delay the Message for at least 5 seconds from the time it is received by the Delayer).
With a Date instance, the Message will not be released until that Date
occurs. In either case, a value that equates to a non-positive delay, or a Date in the past, will
not result in any delay. Instead, it will be sent directly to the output channel on the original
sender's Thread. If the header is not a Date, and can not be parsed as a Long, the default
delay (if any) will be applied.
|
The delayer delegates to an instance of Spring's TaskScheduler
abstraction.
The default scheduler used by the delayer is the ThreadPoolTaskScheduler
instance
provided by Spring Integration on startup: Section E.3, “Configuring the Task Scheduler”.
If you want to delegate to a different scheduler, you can provide a reference through the delayer element's
'scheduler' attribute:
<int:delayer id="delayer" input-channel="input" output-channel="output" delay-header-name="delay" scheduler="exampleTaskScheduler"/> <task:scheduler id="exampleTaskScheduler" pool-size="3"/>
Tip | |
---|---|
If you configure an external ThreadPoolTaskScheduler
you can set on this scheduler property waitForTasksToCompleteOnShutdown = true .
It allows successful completion of 'delay' tasks, which already in the execution state (releasing the Message),
when the application is shutdown. Before Spring Integration 2.2 this property was available on
the <delayer> element, because DelayHandler could create its own
scheduler on the background. Since 2.2 delayer requires an external scheduler
instance and waitForTasksToCompleteOnShutdown was deleted; you should use the scheduler's own configuration.
|
Tip | |
---|---|
Also keep in mind ThreadPoolTaskScheduler has a property errorHandler which
can be injected with some implementation of org.springframework.util.ErrorHandler .
This handler allows to process an Exception from the thread of the scheduled task sending
the delayed message.
By default it uses an org.springframework.scheduling.support.TaskUtils$LoggingErrorHandler
and you will see a stack trace in the logs. You might want to consider using an
org.springframework.integration.channel.MessagePublishingErrorHandler ,
which sends an ErrorMessage into an error-channel , either from the failed Message's header or
into the default error-channel .
|
The DelayHandler
persists delayed Messages into the Message Group in the provided
MessageStore
. (The 'groupId' is based on required 'id' attribute of <delayer>
element.)
A delayed message is removed from the MessageStore
by the scheduled task just before
the DelayHandler
sends the Message to the output-channel
. If the provided
MessageStore
is persistent (e.g. JdbcMessageStore
) it provides
the ability to not lose Messages on the application shutdown. After application startup, the
DelayHandler
reads Messages from its Message Group in the MessageStore
and reschedules them with a delay based on the original arrival time of the Message (if the delay is numeric). For messages
where the delay header was a Date
, that is used when rescheduling.
If a delayed Message remained in the MessageStore
more
than its 'delay', it will be sent immediately after startup.
The <delayer>
can be enriched with mutually exclusive sub-elements <transactional>
or <advice-chain>
. The List of these AOP Advices is applied to the proxied internal
DelayHandler.ReleaseMessageHandler
, which has the responsibility to release the Message, after the delay,
on a Thread
of the scheduled task. It might be used, for example, when the downstream message flow throws an
Exception and the ReleaseMessageHandler
's transaction will be rolled back. In this case the delayed
Message will remain in the persistent MessageStore
. You can use any custom
org.aopalliance.aop.Advice
implementation within the <advice-chain>
.
A sample configuration of the <delayer>
may look like this:
<int:delayer id="delayer" input-channel="input" output-channel="output" delay-header-name="delay" message-store="jdbcMessageStore"> <int:advice-chain> <beans:ref bean="customAdviceBean"/> <tx:advice> <tx:attributes> <tx:method name="*" read-only="true"/> </tx:attributes> </tx:advice> </int:advice-chain> </int:delayer>
The DelayHandler
can be exported as a JMX MBean
with managed operations getDelayedMessageCount
and reschedulePersistedMessages
,
which allows the rescheduling of delayed persisted Messages at runtime, for example, if the
TaskScheduler
has previously been stopped.
These operations can be invoked via a Control Bus
command:
Message<String> delayerReschedulingMessage =
MessageBuilder.withPayload("@'delayer.handler'.reschedulePersistedMessages()").build();
controlBusChannel.send(delayerReschedulingMessage);
Note | |
---|---|
For more information regarding the Message Store, JMX and the Control Bus, please read Chapter 8, System Management. |
With Spring Integration 2.1 we've added support for the JSR223 Scripting for Java specification, introduced in Java version 6. This allows you to use scripts written in any supported language including Ruby/JRuby, Javascript and Groovy to provide the logic for various integration components similar to the way the Spring Expression Language (SpEL) is used in Spring Integration. For more information about JSR223 please refer to the documentation
Important | |
---|---|
Note that this feature requires Java 6 or higher. Sun developed a JSR223 reference implementation which works with Java 5 but it is not officially supported and we have not tested it with Spring Integration. |
In order to use a JVM scripting language, a JSR223 implementation for that language must be included in your class path. Java 6 natively supports Javascript. The Groovy and JRuby projects provide JSR233 support in their standard distribution. Other language implementations may be available or under development. Please refer to the appropriate project website for more information.
Important | |
---|---|
Various JSR223 language implementations have been developed by third parties. A particular implementation's compatibility with Spring Integration depends on how well it conforms to the specification and/or the implementer's interpretation of the specification. |
Tip | |
---|---|
If you plan to use Groovy as your scripting language, we recommended you use Section 7.6, “Groovy support” as it offers additional features specific to Groovy. However you will find this section relevant as well. |
Depending on the complexity of your integration requirements
scripts may be provided inline as CDATA in XML configuration or as a
reference to a Spring resource containing the script. To enable scripting support
Spring Integration defines a
ScriptExecutingMessageProcessor
which will
bind the Message Payload to a
variable named payload
and the Message Headers to a headers
variable, both accessible within the script execution context. All that is left for you to do is
write a script that uses these variables. Below are a couple of sample configurations:
Filter
<int:filter input-channel="referencedScriptInput"> <int-script:script lang="ruby" location="some/path/to/ruby/script/RubyFilterTests.rb"/> </int:filter> <int:filter input-channel="inlineScriptInput"> <int-script:script lang="groovy"><![CDATA[ return payload == 'good' ]]></int-script:script> </int:filter>
Here, you see that the script can be included inline
or can reference a resource location via the location
attribute. Additionally the lang
attribute
corresponds to the language name (or JSR223 alias)
Other Spring Integration endpoint elements which support scripting include router, service-activator, transformer, and splitter. The scripting configuration in each case would be identical to the above (besides the endpoint element).
Another useful feature of Scripting support is the ability to update (reload) scripts without
having to restart the Application Context. To accomplish this, specify the refresh-check-delay
attribute on the script element:
<int-script:script location="..." refresh-check-delay="5000"/>
In the above example, the script location will be checked for updates every 5 seconds. If the script is updated, any invocation that occurs later than 5 seconds since the update will result in execution of the new script.
<int-script:script location="..." refresh-check-delay="0"/>
In the above example the context will be updated with any script modifications as soon as such modification occurs, providing a simple mechanism for 'real-time' configuration. Any negative number value means the script will not be reloaded after initialization of the application context. This is the default behavior.
Important | |
---|---|
Inline scripts can not be reloaded. |
<int-script:script location="..." refresh-check-delay="-1"/>
Script variable bindings
Variable bindings are required to enable the script to reference variables externally provided to the script's execution context.
As we have seen, payload
and headers
are used as binding variables by default. You can bind additional variables
to a script via <variable>
sub-elements:
<script:script lang="js" location="foo/bar/MyScript.js"> <script:variable name="foo" value="foo"/> <script:variable name="bar" value="bar"/> <script:variable name="date" ref="date"/> </script:script>
As shown in the above example, you can bind a script variable either to a scalar value or a Spring bean reference. Note that
payload
and headers
will still be included as binding variables.
If you need more control over how variables are generated, you can implement your own Java class
using the ScriptVariableGenerator
strategy:
public interface ScriptVariableGenerator { Map<String, Object> generateScriptVariables(Message<?> message); }
This interface requires you to implement the method generateScriptVariables(Message)
. The Message
argument allows you to access any data available in the Message payload and headers and the return value is
the Map of bound variables. This method will be called every time the script is executed for a Message. All you need to do is
provide an implementation of ScriptVariableGenerator
and reference it with the script-variable-generator
attribute:
<int-script:script location="foo/bar/MyScript.groovy" script-variable-generator="variableGenerator"/> <bean id="variableGenerator" class="foo.bar.MyScriptVariableGenerator"/>
Important | |
---|---|
You cannot provide both the script-variable-generator attribute and <variable> sub-element(s)
as they are mutually exclusive. Also, custom variable bindings cannot be used with an inline script.
|
In Spring Integration 2.0 we added Groovy support allowing you to use the Groovy scripting language to provide the logic for various integration components similar to the way the Spring Expression Language (SpEL) is supported for routing, transformation and other integration concerns. For more information about Groovy please refer to the Groovy documentation which you can find on the project website
With Spring Integration 2.1, Groovy Support's configuration namespace
is an extension of Spring Integration's Scripting Support and shares the core configuration
and behavior described in detail in the
Section 7.5, “Scripting support” section. Even though Groovy scripts are
well supported by generic Scripting Support, Groovy Support provides the
Groovy configuration namespace which is backed by the Spring Framework's
org.springframework.scripting.groovy.GroovyScriptFactory
and related components,
offering extended capabilities for using Groovy. Below are a couple of sample configurations:
Filter
<int:filter input-channel="referencedScriptInput"> <int-groovy:script location="some/path/to/groovy/file/GroovyFilterTests.groovy"/> </int:filter> <int:filter input-channel="inlineScriptInput"> <int-groovy:script><![CDATA[ return payload == 'good' ]]></int-groovy:script> </int:filter>
As the above examples show, the configuration looks identical to the general Scripting Support configuration. The only
difference is the use of the Groovy namespace as indicated in the examples by the int-groovy namespace prefix.
Also note that the lang
attribute on the <script>
tag is not valid in this namespace.
Groovy object customization
If you need to customize the Groovy object itself, beyond setting variables, you can reference
a bean that implements org.springframework.scripting.groovy.GroovyObjectCustomizer
via the
customizer
attribute. For example, this might be useful if you want to implement a domain-specific
language (DSL) by modifying the MetaClass and registering functions to be available within the script:
<int:service-activator input-channel="groovyChannel"> <int-groovy:script location="foo/SomeScript.groovy" customizer="groovyCustomizer"/> </int:service-activator> <beans:bean id="groovyCustomizer" class="org.foo.MyGroovyObjectCustomizer"/>
Setting a custom GroovyObjectCustomizer is not mutually exclusive with <variable>
sub-elements or
the script-variable-generator
attribute. It can also be provided when defining an inline script.
As described in (EIP), the idea behind the Control Bus is that the same messaging system can be used for monitoring and managing the components within the framework as is used for "application-level" messaging. In Spring Integration we build upon the adapters described above so that it's possible to send Messages as a means of invoking exposed operations. One option for those operations is Groovy scripts.
<int-groovy:control-bus input-channel="operationChannel"/>
The Control Bus has an input channel that can be accessed for invoking operations on the beans in the application context.
The Groovy Control Bus executes messages on the input channel as
Groovy scripts. It takes a message, compiles the body to a Script,
customizes it with a GroovyObjectCustomizer
, and then executes it. The
Control Bus' MessageProcessor
exposes all beans in the application context
that are annotated with @ManagedResource
, implement Spring's
Lifecycle
interface or extend Spring's CustomizableThreadCreator
base class
(e.g. several of the TaskExecutor
and TaskScheduler
implementations).
Important | |
---|---|
Be careful about using managed beans with custom scopes (e.g. 'request') in the Control Bus' command scripts, especially
inside an async message flow. If The Control Bus' MessageProcessor
can't expose a bean from the application context, you may end up with some BeansException
during command script's executing. For example, if a custom scope's context is not established,
the attempt to get a bean within that scope will trigger a BeanCreationException .
|
If you need to further customize the Groovy objects, you can also provide a reference to a bean
that implements org.springframework.scripting.groovy.GroovyObjectCustomizer
via
the customizer
attribute.
<int-groovy:control-bus input-channel="input" output-channel="output" customizer="groovyCustomizer"/> <beans:bean id="groovyCustomizer" class="org.foo.MyGroovyObjectCustomizer"/>
Prior to Spring Integration 2.2, you could add behavior to an entire Integration flow by adding an AOP Advice to a poller's <advice-chain /> element. However, let's say you want to retry, say, just a ReST Web Service call, and not any downstream endpoints.
For example, consider the following flow:
inbound-adapter->poller->http-gateway1->http-gateway2->jdbc-outbound-adapter
If you configure some retry-logic into an advice chain on the poller, and, the call to http-gateway2 failed because of a network glitch, the retry would cause both http-gateway1 and http-gateway2 to be called a second time. Similarly, after a transient failure in the jdbc-outbound-adapter, both http-gateways would be called a second time before again calling the jdbc-outbound-adapter.
Spring Integration 2.2 adds the ability to add behavior to individual endpoints. This is achieved by the addition of the <request-handler-advice-chain /> element to many endpoints. For example:
<int-http:outbound-gateway id="withAdvice" url-expression="'http://localhost/test1'" request-channel="requests" reply-channel="nextChannel"> <int:request-handler-advice-chain> <ref bean="myRetryAdvice" /> </request-handler-advice-chain> </int-http:outbound-gateway>
In this case, myRetryAdvice will only be applied locally to this gateway and will not apply to further actions taken downstream after the reply is sent to the nextChannel. The scope of the advice is limited to the endpoint itself.
Important | |
---|---|
At this time, you cannot advise an entire <chain/> of endpoints. The schema does not allow a <request-handler-advice-chain/> as a child element of the chain itself. However, a <request-handler-advice-chain/> can be added to individual reply-producing endpoints within a <chain/> element. An exception is that, in a chain that produces no reply, because the last element in the chain is an outbound-channel-adapter, that last element cannot be advised. If you need to advise such an element, it must be moved outside of the chain (with the output-channel of the chain being the input-channel of the adapter. The adapter can then be advised as normal. For chains that produce a reply, every child element can be advised. |
In addition to providing the general mechanism to apply AOP Advice classes in this way, three standard Advices are provided:
These are each described in detail in the following sections.
The retry advice (org.springframework.integration.handler.advice.RequestHandlerRetryAdvice
)
leverages the rich retry mechanisms provided by the
spring-retry project. The core component
of spring-retry is the RetryTemplate
, which allows configuration
of sophisticated retry scenarios, including RetryPolicy
and BackoffPolicy
strategies, with a number of implementations,
as well as a RecoveryCallback
strategy to determine the action to take when retries
are exhausted.
Stateless Retry
Stateless retry is the case where the retry activity is handled entirely within the advice, where the thread pauses (if so configured) and retries the action.
Stateful Retry
Stateful retry is the case where the retry state is managed within the advice, but where an exception is thrown and the caller resubmits the request. An example for stateful retry is when we want the message originator (e.g. JMS) to be responsible for resubmitting, rather than performing it on the current thread. Stateful retry needs some mechanism to detect a retried submission.
Further Information
For more information on spring-retry, refer to the project's javadocs, as well as the reference documentation for Spring Batch, where spring-retry originated.
Caution | |
---|---|
The default back off behavior is no back off - retries are attempted immediately. Using a back off policy that causes threads to pause between attempts may cause performance issues, including excessive memory use and thread starvation. In high volume environments, back off policies should be used with caution. |
The following examples use a simple <service-activator />> that always throws an exception:
public class FailingService { public void service(String message) { throw new RuntimeException("foo"); } }
Simple Stateless Retry
This example uses the default RetryTemplate which has a SimpleRetryPolicy which tries 3 times. There is no BackoffPolicy so the 3 attempts are made back-to-back-to-back with no delay between attempts. There is no RecoveryCallback so, the result is to throw the exception to the caller after the final failed retry occurs. In a Spring Integration environment, this final exception might be handled using an error-channel on the inbound endpoint.
<int:service-activator input-channel="input" ref="failer" method="service"> <int:request-handler-advice-chain> <bean class="org.springframework.integration.handler.advice.RequestHandlerRetryAdvice" /> </request-handler-advice-chain> </int:service-activator> DEBUG [task-scheduler-2]preSend on channel 'input', message: [Payload=...] DEBUG [task-scheduler-2]Retry: count=0 DEBUG [task-scheduler-2]Checking for rethrow: count=1 DEBUG [task-scheduler-2]Retry: count=1 DEBUG [task-scheduler-2]Checking for rethrow: count=2 DEBUG [task-scheduler-2]Retry: count=2 DEBUG [task-scheduler-2]Checking for rethrow: count=3 DEBUG [task-scheduler-2]Retry failed last attempt: count=3
Simple Stateless Retry with Recovery
This example adds a RecoveryCallback
to the
above example; it uses a ErrorMessageSendingRecoverer
to send an ErrorMessage to a channel.
<int:service-activator input-channel="input" ref="failer" method="service"> <int:request-handler-advice-chain> <bean class="org.springframework.integration.handler.advice.RequestHandlerRetryAdvice"> <property name="recoveryCallback"> <bean class="org.springframework.integration.handler.advice.ErrorMessageSendingRecoverer"> <constructor-arg ref="myErrorChannel" /> </bean> </property> </bean> </request-handler-advice-chain> </int:int:service-activator> DEBUG [task-scheduler-2]preSend on channel 'input', message: [Payload=...] DEBUG [task-scheduler-2]Retry: count=0 DEBUG [task-scheduler-2]Checking for rethrow: count=1 DEBUG [task-scheduler-2]Retry: count=1 DEBUG [task-scheduler-2]Checking for rethrow: count=2 DEBUG [task-scheduler-2]Retry: count=2 DEBUG [task-scheduler-2]Checking for rethrow: count=3 DEBUG [task-scheduler-2]Retry failed last attempt: count=3 DEBUG [task-scheduler-2]Sending ErrorMessage :failedMessage:[Payload=...]
Stateless Retry with Customized Policies, and Recovery
For more sophistication, we can provide the advice with a customized RetryTemplate.
This example continues to use the SimpleRetryPolicy
but it
increases the attempts to 4. It also adds an ExponentialBackoffPolicy
where the first retry waits 1 second, the second waits 5 seconds and the third waits 25 (for 4
attempts in all).
<int:service-activator input-channel="input" ref="failer" method="service"> <int:request-handler-advice-chain> <bean class="org.springframework.integration.handler.advice.RequestHandlerRetryAdvice"> <property name="recoveryCallback"> <bean class="org.springframework.integration.handler.advice.ErrorMessageSendingRecoverer"> <constructor-arg ref="myErrorChannel" /> </bean> </property> <property name="retryTemplate" ref="retryTemplate" /> </bean> </request-handler-advice-chain> </int:service-activator> <bean id="retryTemplate" class="org.springframework.retry.support.RetryTemplate"> <property name="retryPolicy"> <bean class="org.springframework.retry.policy.SimpleRetryPolicy"> <property name="maxAttempts" value="4" /> </bean> </property> <property name="backOffPolicy"> <bean class="org.springframework.retry.backoff.ExponentialBackOffPolicy"> <property name="initialInterval" value="1000" /> <property name="multiplier" value="5" /> </bean> </property> </bean> 27.058 DEBUG [task-scheduler-1]preSend on channel 'input', message: [Payload=...] 27.071 DEBUG [task-scheduler-1]Retry: count=0 27.080 DEBUG [task-scheduler-1]Sleeping for 1000 28.081 DEBUG [task-scheduler-1]Checking for rethrow: count=1 28.081 DEBUG [task-scheduler-1]Retry: count=1 28.081 DEBUG [task-scheduler-1]Sleeping for 5000 33.082 DEBUG [task-scheduler-1]Checking for rethrow: count=2 33.082 DEBUG [task-scheduler-1]Retry: count=2 33.083 DEBUG [task-scheduler-1]Sleeping for 25000 58.083 DEBUG [task-scheduler-1]Checking for rethrow: count=3 58.083 DEBUG [task-scheduler-1]Retry: count=3 58.084 DEBUG [task-scheduler-1]Checking for rethrow: count=4 58.084 DEBUG [task-scheduler-1]Retry failed last attempt: count=4 58.086 DEBUG [task-scheduler-1]Sending ErrorMessage :failedMessage:[Payload=...]
Simple Stateful Retry with Recovery
To make retry stateful, we need to provide the Advice with a RetryStateGenerator
implementation. This class is used to identify a message as being a resubmission
so that the RetryTemplate can determine the current state of retry
for this message. The framework provides a SpelExpressionRetryStateGenerator
which determines the message identifier using a SpEL expression.
This is shown below; this example again uses the default policies (3 attempts with no back off); of
course, as with stateless retry, these policies can be customized.
<int:service-activator input-channel="input" ref="failer" method="service"> <int:request-handler-advice-chain> <bean class="org.springframework.integration.handler.advice.RequestHandlerRetryAdvice"> <property name="retryStateGenerator"> <bean class="org.springframework.integration.handler.advice.SpelExpressionRetryStateGenerator"> <constructor-arg value="headers['jms_messageId']" /> </bean> </property> <property name="recoveryCallback"> <bean class="org.springframework.integration.handler.advice.ErrorMessageSendingRecoverer"> <constructor-arg ref="myErrorChannel" /> </bean> </property> </bean> </int:request-handler-advice-chain> </int:service-activator> 24.351 DEBUG [Container#0-1]preSend on channel 'input', message: [Payload=...] 24.368 DEBUG [Container#0-1]Retry: count=0 24.387 DEBUG [Container#0-1]Checking for rethrow: count=1 24.387 DEBUG [Container#0-1]Rethrow in retry for policy: count=1 24.387 WARN [Container#0-1]failure occurred in gateway sendAndReceive org.springframework.integration.MessagingException: Failed to invoke handler ... Caused by: java.lang.RuntimeException: foo ... 24.391 DEBUG [Container#0-1]Initiating transaction rollback on application exception ... 25.412 DEBUG [Container#0-1]preSend on channel 'input', message: [Payload=...] 25.412 DEBUG [Container#0-1]Retry: count=1 25.413 DEBUG [Container#0-1]Checking for rethrow: count=2 25.413 DEBUG [Container#0-1]Rethrow in retry for policy: count=2 25.413 WARN [Container#0-1]failure occurred in gateway sendAndReceive org.springframework.integration.MessagingException: Failed to invoke handler ... Caused by: java.lang.RuntimeException: foo ... 25.414 DEBUG [Container#0-1]Initiating transaction rollback on application exception ... 26.418 DEBUG [Container#0-1]preSend on channel 'input', message: [Payload=...] 26.418 DEBUG [Container#0-1]Retry: count=2 26.419 DEBUG [Container#0-1]Checking for rethrow: count=3 26.419 DEBUG [Container#0-1]Rethrow in retry for policy: count=3 26.419 WARN [Container#0-1]failure occurred in gateway sendAndReceive org.springframework.integration.MessagingException: Failed to invoke handler ... Caused by: java.lang.RuntimeException: foo ... 26.420 DEBUG [Container#0-1]Initiating transaction rollback on application exception ... 27.425 DEBUG [Container#0-1]preSend on channel 'input', message: [Payload=...] 27.426 DEBUG [Container#0-1]Retry failed last attempt: count=3 27.426 DEBUG [Container#0-1]Sending ErrorMessage :failedMessage:[Payload=...]
Comparing with the stateless examples, you can see that with stateful retry, the exception is thrown to the caller on each failure.
The general idea of the Circuit Breaker Pattern is that, if a service is not currently available, then
don't waste time (and resources) trying to use it. The
org.springframework.integration.handler.advice.RequestHandlerCircuitBreakerAdvice
implements this pattern. When the circuit breaker is in the closed state,
the endpoint will attempt to invoke the
service. The circuit breaker goes to the open state
if a certain number of consecutive attempts fail; when it is in the open state, new requests will
"fail fast" and no attempt will be made to invoke the service until some time has expired.
When that time has expired, the circuit breaker is set to the half-open state. When in this state, if even a single attempt fails, the breaker will immediately go to the open state; if the attempt succeeds, the breaker will go to the closed state, in which case, it won't go to the open state again until the configured number of consecutive failures again occur. Any successful attempt resets the state to zero failures for the purpose of determining when the breaker might go to the open state again.
Typically, this Advice might be used for external services, where it might take some time to fail (such as a timeout attempting to make a network connection).
The RequestHandlerCircuitBreakerAdvice
has two properties:
threshold
and halfOpenAfter
. The threshold
property represents the number of consecutive failures that need to occur before the breaker goes
open. It defaults to 5. The halfOpenAfter property represents
the time after the last failure that the breaker will wait before attempting another request. Default is
1000 milliseconds.
Example:
<int:service-activator input-channel="input" ref="failer" method="service"> <int:request-handler-advice-chain> <bean class="org.springframework.integration.handler.advice.RequestHandlerCircuitBreakerAdvice"> <property name="threshold" value="2" /> <property name="halfOpenAfter" value="12000" /> </bean> </int:request-handler-advice-chain> </int:service-activator> 05.617 DEBUG [task-scheduler-1]preSend on channel 'input', message: [Payload=...] 05.638 ERROR [task-scheduler-1]org.springframework.integration.MessageHandlingException: java.lang.RuntimeException: foo ... 10.598 DEBUG [task-scheduler-2]preSend on channel 'input', message: [Payload=...] 10.600 ERROR [task-scheduler-2]org.springframework.integration.MessageHandlingException: java.lang.RuntimeException: foo ... 15.598 DEBUG [task-scheduler-3]preSend on channel 'input', message: [Payload=...] 15.599 ERROR [task-scheduler-3]org.springframework.integration.MessagingException: Circuit Breaker is Open for ServiceActivator ... 20.598 DEBUG [task-scheduler-2]preSend on channel 'input', message: [Payload=...] 20.598 ERROR [task-scheduler-2]org.springframework.integration.MessagingException: Circuit Breaker is Open for ServiceActivator ... 25.598 DEBUG [task-scheduler-5]preSend on channel 'input', message: [Payload=...] 25.601 ERROR [task-scheduler-5]org.springframework.integration.MessageHandlingException: java.lang.RuntimeException: foo ... 30.598 DEBUG [task-scheduler-1]preSend on channel 'input', message: [Payload=foo...] 30.599 ERROR [task-scheduler-1]org.springframework.integration.MessagingException: Circuit Breaker is Open for ServiceActivator
In the above example, the threshold is set to 2 and halfOpenAfter is set to 12 seconds; a new request arrives every 5 seconds. You can see that the first two attempts invoked the service; the third and fourth failed with an exception indicating the circuit breaker is open. The fifth request was attempted because the request was 15 seconds after the last failure; the sixth attempt fails immediately because the breaker immediately went to open.
The final supplied advice class is the
org.springframework.integration.handler.advice.ExpressionEvaluatingRequestHandlerAdvice
.
This advice is more general than the other two advices. It provides a mechanism to evaluate an expression on the
original inbound message sent to the endpoint. Separate expressions are available to be evaluated, either after
success, or failure. Optionally, a message containing the evaluation result, together with the input message,
can be sent to a message channel.
A typical use case for this advice might be with an <ftp:outbound-channel-adapter />, perhaps to move the file to one directory if the transfer was successful, or to another directory if it fails:
The Advice has properties to set an expression when successful, an expression for failures,
and corresponding channels for each. For the successful case, the message sent to the
successChannel is an AdviceMessage
, with
the payload being the result of the expression evaluation, and an additional property
inputMessage
which contains the original message sent to the handler. A message
sent to the failureChannel (when the handler throws an excecption)
is an ErrorMessage with a payload of MessageHandlingExpressionEvaluatingAdviceException
.
Like all MessagingException
s, this payload has failedMessage
and cause
properties, as well as an additional property evaluationResult
,
containing the result of the expression evaluation.
In addition to the provided Advice classes above, you can implement your own Advice classes. While you can
provide any implementation of org.aopalliance.aop.Advice
, it is generally recommended
that you subclass org.springframework.integration.handler.advice.AbstractRequestHandlerAdvice
.
This has the benefit of avoiding writing low-level Aspect Oriented Programming code as well
as providing a starting point that is specifically tailored for use in this environment.
Subclasses need to implement the doInvoke() method:
/** * Subclasses implement this method to apply behavior to the {@link MessageHandler} callback.execute() * invokes the handler method and returns its result, or null). * @param callback Subclasses invoke the execute() method on this interface to invoke the handler method. * @param target The target handler. * @param message The message that will be sent to the handler. * @return the result after invoking the {@link MessageHandler}. * @throws Exception */ protected abstract Object doInvoke(ExecutionCallback callback, Object target, Message<?> message) throws Exception;
The callback parameter is simply a convenience to avoid subclasses dealing with AOP directly; invoking the
callback.execute()
method invokes the message handler.
The target parameter is provided for those subclasses that need to maintain state for a
specific handler, perhaps by maintaining that state in a Map
, keyed by the target.
This allows the same advice to be applied to multiple handlers. The
RequestHandlerCircuitBreakerAdvice
uses this to
keep circuit breaker state for each handler.
The message parameter is the message that will be sent to the handler. While the advice cannot modify the message before invoking the handler, it can modify the payload (if it has mutable properties). Typically, an advice would use the message for logging and/or to send a copy of the message somewhere before or after invoking the handler.
The return value would normally be the value returned by callback.execute()
;
but the advice does have the
ability to modify the return value. Note that only AbstractReplyProducingMessageHandler
s
return a value.
public class MyAdvice extends AbstractRequestHandlerAdvice { @Override protected Object doInvoke(ExecutionCallback callback, Object target, Message<?> message) throws Exception { // add code before the invocation Object result = callback.execute(); // add code after the invocation return result; } }
Note | |
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In addition to the For more information, see the ReflectiveMethodInvocation JavaDocs. |