If your application calls remote REST services, Spring Boot makes that very convenient using a RestTemplate or a WebClient.
If you need to call remote REST services from your application, you can use the Spring Framework’s {spring-framework-api}/web/client/RestTemplate.html[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:
link:{docs-java}/io/restclient/resttemplate/MyService.java[role=include]|
Tip
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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().
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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:
link:{docs-java}/io/restclient/resttemplate/customization/MyRestTemplateCustomizer.java[role=include]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:
link:{docs-java}/io/restclient/resttemplate/customization/MyRestTemplateBuilderConfiguration.java[role=include]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.
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 {spring-framework-docs}/web-reactive.html#webflux-client[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:
link:{docs-java}/io/restclient/webclient/MyService.java[role=include]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 {spring-framework-docs}/web-reactive.html#webflux-client-builder[WebClient configuration options in the Spring Framework reference documentation].
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.