Most applications will need to deal with input and output concerns at some point. Spring Boot provides utilities and integrations with a range of technologies to help when you need IO capabilities. This section covers standard IO features such as caching and validation as well as more advanced topics such as scheduling and distributed transactions. We will also cover calling remote REST or SOAP services and sending email.

1. Caching

The Spring Framework provides support for transparently adding caching to an application. At its core, the abstraction applies caching to methods, thus reducing the number of executions based on the information available in the cache. The caching logic is applied transparently, without any interference to the invoker. Spring Boot auto-configures the cache infrastructure as long as caching support is enabled by using the @EnableCaching annotation.

Check the relevant section of the Spring Framework reference for more details.

In a nutshell, to add caching to an operation of your service add the relevant annotation to its method, as shown in the following example:

import org.springframework.cache.annotation.Cacheable;
import org.springframework.stereotype.Component;

@Component
public class MyMathService {

    @Cacheable("piDecimals")
    public int computePiDecimal(int precision) {
        ...
    }

}

This example demonstrates the use of caching on a potentially costly operation. Before invoking computePiDecimal, the abstraction looks for an entry in the piDecimals cache that matches the i argument. If an entry is found, the content in the cache is immediately returned to the caller, and the method is not invoked. Otherwise, the method is invoked, and the cache is updated before returning the value.

You can also use the standard JSR-107 (JCache) annotations (such as @CacheResult) transparently. However, we strongly advise you to not mix and match the Spring Cache and JCache annotations.

If you do not add any specific cache library, Spring Boot auto-configures a simple provider that uses concurrent maps in memory. When a cache is required (such as piDecimals in the preceding example), this provider creates it for you. The simple provider is not really recommended for production usage, but it is great for getting started and making sure that you understand the features. When you have made up your mind about the cache provider to use, please make sure to read its documentation to figure out how to configure the caches that your application uses. Nearly all providers require you to explicitly configure every cache that you use in the application. Some offer a way to customize the default caches defined by the spring.cache.cache-names property.

It is also possible to transparently update or evict data from the cache.

1.1. Supported Cache Providers

The cache abstraction does not provide an actual store and relies on abstraction materialized by the org.springframework.cache.Cache and org.springframework.cache.CacheManager interfaces.

If you have not defined a bean of type CacheManager or a CacheResolver named cacheResolver (see CachingConfigurer), Spring Boot tries to detect the following providers (in the indicated order):

  1. Generic

  2. JCache (JSR-107) (EhCache 3, Hazelcast, Infinispan, and others)

  3. EhCache 2.x

  4. Hazelcast

  5. Infinispan

  6. Couchbase

  7. Redis

  8. Caffeine

  9. Simple

Additionally, Spring Boot for Apache Geode provides auto-configuration for using Apache Geode as a cache provider.

It is also possible to force a particular cache provider by setting the spring.cache.type property. Use this property if you need to disable caching altogether in certain environments (such as tests).
Use the spring-boot-starter-cache “Starter” to quickly add basic caching dependencies. The starter brings in spring-context-support. If you add dependencies manually, you must include spring-context-support in order to use the JCache, EhCache 2.x, or Caffeine support.

If the CacheManager is auto-configured by Spring Boot, you can further tune its configuration before it is fully initialized by exposing a bean that implements the CacheManagerCustomizer interface. The following example sets a flag to say that null values should not be passed down to the underlying map:

import org.springframework.boot.autoconfigure.cache.CacheManagerCustomizer;
import org.springframework.cache.concurrent.ConcurrentMapCacheManager;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration(proxyBeanMethods = false)
public class MyCacheManagerConfiguration {

    @Bean
    public CacheManagerCustomizer<ConcurrentMapCacheManager> cacheManagerCustomizer() {
        return (cacheManager) -> cacheManager.setAllowNullValues(false);
    }

}
In the preceding example, an auto-configured ConcurrentMapCacheManager is expected. If that is not the case (either you provided your own config or a different cache provider was auto-configured), the customizer is not invoked at all. You can have as many customizers as you want, and you can also order them by using @Order or Ordered.

1.1.1. Generic

Generic caching is used if the context defines at least one org.springframework.cache.Cache bean. A CacheManager wrapping all beans of that type is created.

1.1.2. JCache (JSR-107)

JCache is bootstrapped through the presence of a javax.cache.spi.CachingProvider on the classpath (that is, a JSR-107 compliant caching library exists on the classpath), and the JCacheCacheManager is provided by the spring-boot-starter-cache “Starter”. Various compliant libraries are available, and Spring Boot provides dependency management for Ehcache 3, Hazelcast, and Infinispan. Any other compliant library can be added as well.

It might happen that more than one provider is present, in which case the provider must be explicitly specified. Even if the JSR-107 standard does not enforce a standardized way to define the location of the configuration file, Spring Boot does its best to accommodate setting a cache with implementation details, as shown in the following example:

Properties
# Only necessary if more than one provider is present
spring.cache.jcache.provider=com.example.MyCachingProvider
spring.cache.jcache.config=classpath:example.xml
Yaml
# Only necessary if more than one provider is present
spring:
  cache:
    jcache:
      provider: "com.example.MyCachingProvider"
      config: "classpath:example.xml"
When a cache library offers both a native implementation and JSR-107 support, Spring Boot prefers the JSR-107 support, so that the same features are available if you switch to a different JSR-107 implementation.
Spring Boot has general support for Hazelcast. If a single HazelcastInstance is available, it is automatically reused for the CacheManager as well, unless the spring.cache.jcache.config property is specified.

There are two ways to customize the underlying javax.cache.cacheManager:

  • Caches can be created on startup by setting the spring.cache.cache-names property. If a custom javax.cache.configuration.Configuration bean is defined, it is used to customize them.

  • org.springframework.boot.autoconfigure.cache.JCacheManagerCustomizer beans are invoked with the reference of the CacheManager for full customization.

If a standard javax.cache.CacheManager bean is defined, it is wrapped automatically in an org.springframework.cache.CacheManager implementation that the abstraction expects. No further customization is applied to it.

1.1.3. EhCache 2.x

EhCache 2.x is used if a file named ehcache.xml can be found at the root of the classpath. If EhCache 2.x is found, the EhCacheCacheManager provided by the spring-boot-starter-cache “Starter” is used to bootstrap the cache manager. An alternate configuration file can be provided as well, as shown in the following example:

Properties
spring.cache.ehcache.config=classpath:config/another-config.xml
Yaml
spring:
  cache:
    ehcache:
      config: "classpath:config/another-config.xml"

1.1.4. Hazelcast

Spring Boot has general support for Hazelcast. If a HazelcastInstance has been auto-configured, it is automatically wrapped in a CacheManager.

1.1.5. Infinispan

Infinispan has no default configuration file location, so it must be specified explicitly. Otherwise, the default bootstrap is used.

Properties
spring.cache.infinispan.config=infinispan.xml
Yaml
spring:
  cache:
    infinispan:
      config: "infinispan.xml"

Caches can be created on startup by setting the spring.cache.cache-names property. If a custom ConfigurationBuilder bean is defined, it is used to customize the caches.

The support of Infinispan in Spring Boot is restricted to the embedded mode and is quite basic. If you want more options, you should use the official Infinispan Spring Boot starter instead. See Infinispan’s documentation for more details.

1.1.6. Couchbase

If Spring Data Couchbase is available and Couchbase is configured, a CouchbaseCacheManager is auto-configured. It is possible to create additional caches on startup by setting the spring.cache.cache-names property and cache defaults can be configured by using spring.cache.couchbase.* properties. For instance, the following configuration creates cache1 and cache2 caches with an entry expiration of 10 minutes:

Properties
spring.cache.cache-names=cache1,cache2
spring.cache.couchbase.expiration=10m
Yaml
spring:
  cache:
    cache-names: "cache1,cache2"
    couchbase:
      expiration: "10m"

If you need more control over the configuration, consider registering a CouchbaseCacheManagerBuilderCustomizer bean. The following example shows a customizer that configures a specific entry expiration for cache1 and cache2:

import java.time.Duration;

import org.springframework.boot.autoconfigure.cache.CouchbaseCacheManagerBuilderCustomizer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.data.couchbase.cache.CouchbaseCacheConfiguration;

@Configuration(proxyBeanMethods = false)
public class MyCouchbaseCacheManagerConfiguration {

    @Bean
    public CouchbaseCacheManagerBuilderCustomizer myCouchbaseCacheManagerBuilderCustomizer() {
        return (builder) -> builder
                .withCacheConfiguration("cache1", CouchbaseCacheConfiguration
                        .defaultCacheConfig().entryExpiry(Duration.ofSeconds(10)))
                .withCacheConfiguration("cache2", CouchbaseCacheConfiguration
                        .defaultCacheConfig().entryExpiry(Duration.ofMinutes(1)));

    }

}

1.1.7. Redis

If Redis is available and configured, a RedisCacheManager is auto-configured. It is possible to create additional caches on startup by setting the spring.cache.cache-names property and cache defaults can be configured by using spring.cache.redis.* properties. For instance, the following configuration creates cache1 and cache2 caches with a time to live of 10 minutes:

Properties
spring.cache.cache-names=cache1,cache2
spring.cache.redis.time-to-live=10m
Yaml
spring:
  cache:
    cache-names: "cache1,cache2"
    redis:
      time-to-live: "10m"
By default, a key prefix is added so that, if two separate caches use the same key, Redis does not have overlapping keys and cannot return invalid values. We strongly recommend keeping this setting enabled if you create your own RedisCacheManager.
You can take full control of the default configuration by adding a RedisCacheConfiguration @Bean of your own. This can be useful if you need to customize the default serialization strategy.

If you need more control over the configuration, consider registering a RedisCacheManagerBuilderCustomizer bean. The following example shows a customizer that configures a specific time to live for cache1 and cache2:

import java.time.Duration;

import org.springframework.boot.autoconfigure.cache.RedisCacheManagerBuilderCustomizer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.data.redis.cache.RedisCacheConfiguration;

@Configuration(proxyBeanMethods = false)
public class MyRedisCacheManagerConfiguration {

    @Bean
    public RedisCacheManagerBuilderCustomizer myRedisCacheManagerBuilderCustomizer() {
        return (builder) -> builder
                .withCacheConfiguration("cache1", RedisCacheConfiguration
                        .defaultCacheConfig().entryTtl(Duration.ofSeconds(10)))
                .withCacheConfiguration("cache2", RedisCacheConfiguration
                        .defaultCacheConfig().entryTtl(Duration.ofMinutes(1)));

    }

}

1.1.8. Caffeine

Caffeine is a Java 8 rewrite of Guava’s cache that supersedes support for Guava. If Caffeine is present, a CaffeineCacheManager (provided by the spring-boot-starter-cache “Starter”) is auto-configured. Caches can be created on startup by setting the spring.cache.cache-names property and can be customized by one of the following (in the indicated order):

  1. A cache spec defined by spring.cache.caffeine.spec

  2. A com.github.benmanes.caffeine.cache.CaffeineSpec bean is defined

  3. A com.github.benmanes.caffeine.cache.Caffeine bean is defined

For instance, the following configuration creates cache1 and cache2 caches with a maximum size of 500 and a time to live of 10 minutes

Properties
spring.cache.cache-names=cache1,cache2
spring.cache.caffeine.spec=maximumSize=500,expireAfterAccess=600s
Yaml
spring:
  cache:
    cache-names: "cache1,cache2"
    caffeine:
      spec: "maximumSize=500,expireAfterAccess=600s"

If a com.github.benmanes.caffeine.cache.CacheLoader bean is defined, it is automatically associated to the CaffeineCacheManager. Since the CacheLoader is going to be associated with all caches managed by the cache manager, it must be defined as CacheLoader<Object, Object>. The auto-configuration ignores any other generic type.

1.1.9. Simple

If none of the other providers can be found, a simple implementation using a ConcurrentHashMap as the cache store is configured. This is the default if no caching library is present in your application. By default, caches are created as needed, but you can restrict the list of available caches by setting the cache-names property. For instance, if you want only cache1 and cache2 caches, set the cache-names property as follows:

Properties
spring.cache.cache-names=cache1,cache2
Yaml
spring:
  cache:
    cache-names: "cache1,cache2"

If you do so and your application uses a cache not listed, then it fails at runtime when the cache is needed, but not on startup. This is similar to the way the "real" cache providers behave if you use an undeclared cache.

1.1.10. None

When @EnableCaching is present in your configuration, a suitable cache configuration is expected as well. If you need to disable caching altogether in certain environments, force the cache type to none to use a no-op implementation, as shown in the following example:

Properties
spring.cache.type=none
Yaml
spring:
  cache:
    type: "none"

2. Hazelcast

If Hazelcast is on the classpath and a suitable configuration is found, Spring Boot auto-configures a HazelcastInstance that you can inject in your application.

Spring Boot first attempts to create a client by checking the following configuration options:

  • The presence of a com.hazelcast.client.config.ClientConfig bean.

  • A configuration file defined by the spring.hazelcast.config property.

  • The presence of the hazelcast.client.config system property.

  • A hazelcast-client.xml in the working directory or at the root of the classpath.

  • A hazelcast-client.yaml (or hazelcast-client.yml) in the working directory or at the root of the classpath.

Spring Boot supports both Hazelcast 4 and Hazelcast 3. If you downgrade to Hazelcast 3, hazelcast-client should be added to the classpath to configure a client.

If a client can not be created, Spring Boot attempts to configure an embedded server. If you define a com.hazelcast.config.Config bean, Spring Boot uses that. If your configuration defines an instance name, Spring Boot tries to locate an existing instance rather than creating a new one.

You could also specify the Hazelcast configuration file to use through configuration, as shown in the following example:

Properties
spring.hazelcast.config=classpath:config/my-hazelcast.xml
Yaml
spring:
  hazelcast:
    config: "classpath:config/my-hazelcast.xml"

Otherwise, Spring Boot tries to find the Hazelcast configuration from the default locations: hazelcast.xml in the working directory or at the root of the classpath, or a .yaml/.yml counterpart in the same locations. We also check if the hazelcast.config system property is set. See the Hazelcast documentation for more details.

Spring Boot also has explicit caching support for Hazelcast. If caching is enabled, the HazelcastInstance is automatically wrapped in a CacheManager implementation.

3. Quartz Scheduler

Spring Boot offers several conveniences for working with the Quartz scheduler, including the spring-boot-starter-quartz “Starter”. If Quartz is available, a Scheduler is auto-configured (through the SchedulerFactoryBean abstraction).

Beans of the following types are automatically picked up and associated with the Scheduler:

  • JobDetail: defines a particular Job. JobDetail instances can be built with the JobBuilder API.

  • Calendar.

  • Trigger: defines when a particular job is triggered.

By default, an in-memory JobStore is used. However, it is possible to configure a JDBC-based store if a DataSource bean is available in your application and if the spring.quartz.job-store-type property is configured accordingly, as shown in the following example:

Properties
spring.quartz.job-store-type=jdbc
Yaml
spring:
  quartz:
    job-store-type: "jdbc"

When the JDBC store is used, the schema can be initialized on startup, as shown in the following example:

Properties
spring.quartz.jdbc.initialize-schema=always
Yaml
spring:
  quartz:
    jdbc:
      initialize-schema: "always"
By default, the database is detected and initialized by using the standard scripts provided with the Quartz library. These scripts drop existing tables, deleting all triggers on every restart. It is also possible to provide a custom script by setting the spring.quartz.jdbc.schema property.

To have Quartz use a DataSource other than the application’s main DataSource, declare a DataSource bean, annotating its @Bean method with @QuartzDataSource. Doing so ensures that the Quartz-specific DataSource is used by both the SchedulerFactoryBean and for schema initialization. Similarly, to have Quartz use a TransactionManager other than the application’s main TransactionManager declare a TransactionManager bean, annotating its @Bean method with @QuartzTransactionManager.

By default, jobs created by configuration will not overwrite already registered jobs that have been read from a persistent job store. To enable overwriting existing job definitions set the spring.quartz.overwrite-existing-jobs property.

Quartz Scheduler configuration can be customized using spring.quartz properties and SchedulerFactoryBeanCustomizer beans, which allow programmatic SchedulerFactoryBean customization. Advanced Quartz configuration properties can be customized using spring.quartz.properties.*.

In particular, an Executor bean is not associated with the scheduler as Quartz offers a way to configure the scheduler through spring.quartz.properties. If you need to customize the task executor, consider implementing SchedulerFactoryBeanCustomizer.

Jobs can define setters to inject data map properties. Regular beans can also be injected in a similar manner, as shown in the following example:

import org.quartz.JobExecutionContext;
import org.quartz.JobExecutionException;

import org.springframework.scheduling.quartz.QuartzJobBean;

public class MySampleJob extends QuartzJobBean {

    // fields ...

    private MyService myService;

    private String name;

    // Inject "MyService" bean
    public void setMyService(MyService myService) {
        this.myService = myService;
    }

    // Inject the "name" job data property
    public void setName(String name) {
        this.name = name;
    }

    @Override
    protected void executeInternal(JobExecutionContext context) throws JobExecutionException {
        this.myService.someMethod(context.getFireTime(), this.name);
    }

}

4. Sending Email

The Spring Framework provides an abstraction for sending email by using the JavaMailSender interface, and Spring Boot provides auto-configuration for it as well as a starter module.

See the reference documentation for a detailed explanation of how you can use JavaMailSender.

If spring.mail.host and the relevant libraries (as defined by spring-boot-starter-mail) are available, a default JavaMailSender is created if none exists. The sender can be further customized by configuration items from the spring.mail namespace. See MailProperties for more details.

In particular, certain default timeout values are infinite, and you may want to change that to avoid having a thread blocked by an unresponsive mail server, as shown in the following example:

Properties
spring.mail.properties[mail.smtp.connectiontimeout]=5000
spring.mail.properties[mail.smtp.timeout]=3000
spring.mail.properties[mail.smtp.writetimeout]=5000
Yaml
spring:
  mail:
    properties:
      "[mail.smtp.connectiontimeout]": 5000
      "[mail.smtp.timeout]": 3000
      "[mail.smtp.writetimeout]": 5000

It is also possible to configure a JavaMailSender with an existing Session from JNDI:

Properties
spring.mail.jndi-name=mail/Session
Yaml
spring:
  mail:
    jndi-name: "mail/Session"

When a jndi-name is set, it takes precedence over all other Session-related settings.

5. Validation

The method validation feature supported by Bean Validation 1.1 is automatically enabled as long as a JSR-303 implementation (such as Hibernate validator) is on the classpath. This lets bean methods be annotated with javax.validation constraints on their parameters and/or on their return value. Target classes with such annotated methods need to be annotated with the @Validated annotation at the type level for their methods to be searched for inline constraint annotations.

For instance, the following service triggers the validation of the first argument, making sure its size is between 8 and 10:

import javax.validation.constraints.Size;

import org.springframework.stereotype.Service;
import org.springframework.validation.annotation.Validated;

@Service
@Validated
public class MyBean {

    public Archive findByCodeAndAuthor(@Size(min = 8, max = 10) String code, Author author) {
        return ...
    }

}

The application’s MessageSource is used when resolving {parameters} in constraint messages. This allows you to use your application’s messages.properties files for Bean Validation messages. Once the parameters have been resolved, message interpolation is completed using Bean Validation’s default interpolator.

6. Calling REST Services

If your application calls remote REST services, Spring Boot makes that very convenient using a RestTemplate or a WebClient.

6.1. RestTemplate

If you need to call remote REST services from your application, you can use the Spring Framework’s RestTemplate class. Since RestTemplate instances often need to be customized before being used, Spring Boot does not provide any single auto-configured RestTemplate bean. It does, however, auto-configure a RestTemplateBuilder, which can be used to create RestTemplate instances when needed. The auto-configured RestTemplateBuilder ensures that sensible HttpMessageConverters are applied to RestTemplate instances.

The following code shows a typical example:

import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;

@Service
public class MyService {

    private final RestTemplate restTemplate;

    public MyService(RestTemplateBuilder restTemplateBuilder) {
        this.restTemplate = restTemplateBuilder.build();
    }

    public Details someRestCall(String name) {
        return this.restTemplate.getForObject("/{name}/details", Details.class, name);
    }

}
RestTemplateBuilder includes a number of useful methods that can be used to quickly configure a RestTemplate. For example, to add BASIC auth support, you can use builder.basicAuthentication("user", "password").build().

6.1.1. RestTemplate Customization

There are three main approaches to RestTemplate customization, depending on how broadly you want the customizations to apply.

To make the scope of any customizations as narrow as possible, inject the auto-configured RestTemplateBuilder and then call its methods as required. Each method call returns a new RestTemplateBuilder instance, so the customizations only affect this use of the builder.

To make an application-wide, additive customization, use a RestTemplateCustomizer bean. All such beans are automatically registered with the auto-configured RestTemplateBuilder and are applied to any templates that are built with it.

The following example shows a customizer that configures the use of a proxy for all hosts except 192.168.0.5:

import org.apache.http.HttpException;
import org.apache.http.HttpHost;
import org.apache.http.HttpRequest;
import org.apache.http.client.HttpClient;
import org.apache.http.conn.routing.HttpRoutePlanner;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.impl.conn.DefaultProxyRoutePlanner;
import org.apache.http.protocol.HttpContext;

import org.springframework.boot.web.client.RestTemplateCustomizer;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
import org.springframework.web.client.RestTemplate;

public class MyRestTemplateCustomizer implements RestTemplateCustomizer {

    @Override
    public void customize(RestTemplate restTemplate) {
        HttpRoutePlanner routePlanner = new CustomRoutePlanner(new HttpHost("proxy.example.com"));
        HttpClient httpClient = HttpClientBuilder.create().setRoutePlanner(routePlanner).build();
        restTemplate.setRequestFactory(new HttpComponentsClientHttpRequestFactory(httpClient));
    }

    static class CustomRoutePlanner extends DefaultProxyRoutePlanner {

        CustomRoutePlanner(HttpHost proxy) {
            super(proxy);
        }

        @Override
        public HttpHost determineProxy(HttpHost target, HttpRequest request, HttpContext context) throws HttpException {
            if (target.getHostName().equals("192.168.0.5")) {
                return null;
            }
            return super.determineProxy(target, request, context);
        }

    }

}

Finally, you can define your own RestTemplateBuilder bean. Doing so will replace the auto-configured builder. If you want any RestTemplateCustomizer beans to be applied to your custom builder, as the auto-configuration would have done, configure it using a RestTemplateBuilderConfigurer. The following example exposes a RestTemplateBuilder that matches what Spring Boot’s auto-configuration would have done, except that custom connect and read timeouts are also specified:

import java.time.Duration;

import org.springframework.boot.autoconfigure.web.client.RestTemplateBuilderConfigurer;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration(proxyBeanMethods = false)
public class MyRestTemplateBuilderConfiguration {

    @Bean
    public RestTemplateBuilder restTemplateBuilder(RestTemplateBuilderConfigurer configurer) {
        return configurer.configure(new RestTemplateBuilder()).setConnectTimeout(Duration.ofSeconds(5))
                .setReadTimeout(Duration.ofSeconds(2));
    }

}

The most extreme (and rarely used) option is to create your own RestTemplateBuilder bean without using a configurer. In addition to replacing the auto-configured builder, this also prevents any RestTemplateCustomizer beans from being used.

6.2. WebClient

If you have Spring WebFlux on your classpath, you can also choose to use WebClient to call remote REST services. Compared to RestTemplate, this client has a more functional feel and is fully reactive. You can learn more about the WebClient in the dedicated section in the Spring Framework docs.

Spring Boot creates and pre-configures a WebClient.Builder for you. It is strongly advised to inject it in your components and use it to create WebClient instances. Spring Boot is configuring that builder to share HTTP resources, reflect codecs setup in the same fashion as the server ones (see WebFlux HTTP codecs auto-configuration), and more.

The following code shows a typical example:

import org.neo4j.cypherdsl.core.Relationship.Details;
import reactor.core.publisher.Mono;

import org.springframework.stereotype.Service;
import org.springframework.web.reactive.function.client.WebClient;

@Service
public class MyService {

    private final WebClient webClient;

    public MyService(WebClient.Builder webClientBuilder) {
        this.webClient = webClientBuilder.baseUrl("https://example.org").build();
    }

    public Mono<Details> someRestCall(String name) {
        return this.webClient.get().uri("/{name}/details", name).retrieve().bodyToMono(Details.class);
    }

}

6.2.1. WebClient Runtime

Spring Boot will auto-detect which ClientHttpConnector to use to drive WebClient, depending on the libraries available on the application classpath. For now, Reactor Netty, Jetty RS client and Apache HttpClient are supported.

The spring-boot-starter-webflux starter depends on io.projectreactor.netty:reactor-netty by default, which brings both server and client implementations. If you choose to use Jetty as a reactive server instead, you should add a dependency on the Jetty Reactive HTTP client library, org.eclipse.jetty:jetty-reactive-httpclient. Using the same technology for server and client has its advantages, as it will automatically share HTTP resources between client and server.

Developers can override the resource configuration for Jetty and Reactor Netty by providing a custom ReactorResourceFactory or JettyResourceFactory bean - this will be applied to both clients and servers.

If you wish to override that choice for the client, you can define your own ClientHttpConnector bean and have full control over the client configuration.

You can learn more about the WebClient configuration options in the Spring Framework reference documentation.

6.2.2. WebClient Customization

There are three main approaches to WebClient customization, depending on how broadly you want the customizations to apply.

To make the scope of any customizations as narrow as possible, inject the auto-configured WebClient.Builder and then call its methods as required. WebClient.Builder instances are stateful: Any change on the builder is reflected in all clients subsequently created with it. If you want to create several clients with the same builder, you can also consider cloning the builder with WebClient.Builder other = builder.clone();.

To make an application-wide, additive customization to all WebClient.Builder instances, you can declare WebClientCustomizer beans and change the WebClient.Builder locally at the point of injection.

Finally, you can fall back to the original API and use WebClient.create(). In that case, no auto-configuration or WebClientCustomizer is applied.

7. Web Services

Spring Boot provides Web Services auto-configuration so that all you must do is define your Endpoints.

The Spring Web Services features can be easily accessed with the spring-boot-starter-webservices module.

SimpleWsdl11Definition and SimpleXsdSchema beans can be automatically created for your WSDLs and XSDs respectively. To do so, configure their location, as shown in the following example:

Properties
spring.webservices.wsdl-locations=classpath:/wsdl
Yaml
spring:
  webservices:
    wsdl-locations: "classpath:/wsdl"

7.1. Calling Web Services with WebServiceTemplate

If you need to call remote Web services from your application, you can use the WebServiceTemplate class. Since WebServiceTemplate instances often need to be customized before being used, Spring Boot does not provide any single auto-configured WebServiceTemplate bean. It does, however, auto-configure a WebServiceTemplateBuilder, which can be used to create WebServiceTemplate instances when needed.

The following code shows a typical example:

import org.springframework.boot.webservices.client.WebServiceTemplateBuilder;
import org.springframework.stereotype.Service;
import org.springframework.ws.client.core.WebServiceTemplate;
import org.springframework.ws.soap.client.core.SoapActionCallback;

@Service
public class MyService {

    private final WebServiceTemplate webServiceTemplate;

    public MyService(WebServiceTemplateBuilder webServiceTemplateBuilder) {
        this.webServiceTemplate = webServiceTemplateBuilder.build();
    }

    public SomeResponse someWsCall(SomeRequest detailsReq) {
        return (SomeResponse) this.webServiceTemplate.marshalSendAndReceive(detailsReq,
                new SoapActionCallback("https://ws.example.com/action"));
    }

}

By default, WebServiceTemplateBuilder detects a suitable HTTP-based WebServiceMessageSender using the available HTTP client libraries on the classpath. You can also customize read and connection timeouts as follows:

import java.time.Duration;

import org.springframework.boot.webservices.client.HttpWebServiceMessageSenderBuilder;
import org.springframework.boot.webservices.client.WebServiceTemplateBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.ws.client.core.WebServiceTemplate;
import org.springframework.ws.transport.WebServiceMessageSender;

@Configuration(proxyBeanMethods = false)
public class MyWebServiceTemplateConfiguration {

    @Bean
    public WebServiceTemplate webServiceTemplate(WebServiceTemplateBuilder builder) {
        WebServiceMessageSender sender = new HttpWebServiceMessageSenderBuilder()
                .setConnectTimeout(Duration.ofSeconds(5))
                .setReadTimeout(Duration.ofSeconds(2))
                .build();
        return builder.messageSenders(sender).build();
    }

}

8. Distributed Transactions With JTA

Spring Boot supports distributed JTA transactions across multiple XA resources by using an Atomikos embedded transaction manager. JTA transactions are also supported when deploying to a suitable Java EE Application Server.

When a JTA environment is detected, Spring’s JtaTransactionManager is used to manage transactions. Auto-configured JMS, DataSource, and JPA beans are upgraded to support XA transactions. You can use standard Spring idioms, such as @Transactional, to participate in a distributed transaction. If you are within a JTA environment and still want to use local transactions, you can set the spring.jta.enabled property to false to disable the JTA auto-configuration.

8.1. Using an Atomikos Transaction Manager

Atomikos is a popular open source transaction manager which can be embedded into your Spring Boot application. You can use the spring-boot-starter-jta-atomikos starter to pull in the appropriate Atomikos libraries. Spring Boot auto-configures Atomikos and ensures that appropriate depends-on settings are applied to your Spring beans for correct startup and shutdown ordering.

By default, Atomikos transaction logs are written to a transaction-logs directory in your application’s home directory (the directory in which your application jar file resides). You can customize the location of this directory by setting a spring.jta.log-dir property in your application.properties file. Properties starting with spring.jta.atomikos.properties can also be used to customize the Atomikos UserTransactionServiceImp. See the AtomikosProperties Javadoc for complete details.

To ensure that multiple transaction managers can safely coordinate the same resource managers, each Atomikos instance must be configured with a unique ID. By default, this ID is the IP address of the machine on which Atomikos is running. To ensure uniqueness in production, you should configure the spring.jta.transaction-manager-id property with a different value for each instance of your application.

8.2. Using a Java EE Managed Transaction Manager

If you package your Spring Boot application as a war or ear file and deploy it to a Java EE application server, you can use your application server’s built-in transaction manager. Spring Boot tries to auto-configure a transaction manager by looking at common JNDI locations (java:comp/UserTransaction, java:comp/TransactionManager, and so on). If you use a transaction service provided by your application server, you generally also want to ensure that all resources are managed by the server and exposed over JNDI. Spring Boot tries to auto-configure JMS by looking for a ConnectionFactory at the JNDI path (java:/JmsXA or java:/XAConnectionFactory), and you can use the spring.datasource.jndi-name property to configure your DataSource.

8.3. Mixing XA and Non-XA JMS Connections

When using JTA, the primary JMS ConnectionFactory bean is XA-aware and participates in distributed transactions. You can inject into your bean without needing to use any @Qualifier:

public MyBean(ConnectionFactory connectionFactory) {
    // ...
}

In some situations, you might want to process certain JMS messages by using a non-XA ConnectionFactory. For example, your JMS processing logic might take longer than the XA timeout.

If you want to use a non-XA ConnectionFactory, you can the nonXaJmsConnectionFactory bean:

public MyBean(@Qualifier("nonXaJmsConnectionFactory") ConnectionFactory connectionFactory) {
    // ...
}

For consistency, the jmsConnectionFactory bean is also provided by using the bean alias xaJmsConnectionFactory:

public MyBean(@Qualifier("xaJmsConnectionFactory") ConnectionFactory connectionFactory) {
    // ...
}

8.4. Supporting an Alternative Embedded Transaction Manager

The XAConnectionFactoryWrapper and XADataSourceWrapper interfaces can be used to support alternative embedded transaction managers. The interfaces are responsible for wrapping XAConnectionFactory and XADataSource beans and exposing them as regular ConnectionFactory and DataSource beans, which transparently enroll in the distributed transaction. DataSource and JMS auto-configuration use JTA variants, provided you have a JtaTransactionManager bean and appropriate XA wrapper beans registered within your ApplicationContext.

The AtomikosXAConnectionFactoryWrapper and AtomikosXADataSourceWrapper provide good examples of how to write XA wrappers.

9. What to Read Next

You should now have a good understanding of Spring Boot’s core features and the various technologies that Spring Boot provides support for via auto-configuration.

The next few sections go into detail about deploying applications to cloud platforms. You can read about building container images in the next section or skip to the production-ready features section.