[[io.rest-client]]
= Calling REST Services

Spring Boot provides various convenient ways to call remote REST services.
If you are developing a non-blocking reactive application and you're using Spring WebFlux, then you can use `WebClient`.
If you prefer blocking APIs then you can use `RestClient` or `RestTemplate`.



[[io.rest-client.webclient]]
== WebClient

If you have Spring WebFlux on your classpath we recommend that you use `WebClient` to call remote REST services.
The `WebClient` interface provides a functional style API and is fully reactive.
You can learn more about the `WebClient` in the dedicated {url-spring-framework-docs}/web/webflux-webclient.html[section in the Spring Framework docs].

TIP: If you are not writing a reactive Spring WebFlux application you can use the xref:io/rest-client.adoc#io.rest-client.restclient[`RestClient`] instead of a `WebClient`.
This provides a similar functional API, but is blocking rather than reactive.

Spring Boot creates and pre-configures a prototype `WebClient.Builder` bean 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 and reflect codecs setup in the same fashion as the server ones (see xref:web/reactive.adoc#web.reactive.webflux.httpcodecs[WebFlux HTTP codecs auto-configuration]), and more.

The following code shows a typical example:

include-code::MyService[]



[[io.rest-client.webclient.runtime]]
=== WebClient Runtime

Spring Boot will auto-detect which `ClientHttpConnector` to use to drive `WebClient` depending on the libraries available on the application classpath.
In order of preference, the following clients are supported:

. Reactor Netty
. Jetty RS client
. Apache HttpClient
. JDK HttpClient

If multiple clients are available on the classpath, the most preferred client will be used.

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 {url-spring-framework-docs}/web/webflux-webclient/client-builder.html[`WebClient` configuration options in the Spring Framework reference documentation].



[[io.rest-client.webclient.customization]]
=== 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.



[[io.rest-client.webclient.ssl]]
=== WebClient SSL Support

If you need custom SSL configuration on the `ClientHttpConnector` used by the `WebClient`, you can inject a `WebClientSsl` instance that can be used with the builder's `apply` method.

The `WebClientSsl` interface provides access to any xref:features/ssl.adoc#features.ssl.bundles[SSL bundles] that you have defined in your `application.properties` or `application.yaml` file.

The following code shows a typical example:

include-code::MyService[]



[[io.rest-client.restclient]]
== RestClient

If you are not using Spring WebFlux or Project Reactor in your application we recommend that you use `RestClient` to call remote REST services.

The `RestClient` interface provides a functional style blocking API.

Spring Boot creates and pre-configures a prototype `RestClient.Builder` bean for you.
It is strongly advised to inject it in your components and use it to create `RestClient` instances.
Spring Boot is configuring that builder with `HttpMessageConverters` and an appropriate `ClientHttpRequestFactory`.

The following code shows a typical example:

include-code::MyService[]



[[io.rest-client.restclient.customization]]
=== RestClient Customization

There are three main approaches to `RestClient` 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 `RestClient.Builder` and then call its methods as required.
`RestClient.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 `RestClient.Builder other = builder.clone();`.

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

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



[[io.rest-client.restclient.ssl]]
=== RestClient SSL Support

If you need custom SSL configuration on the `ClientHttpRequestFactory` used by the `RestClient`, you can inject a `RestClientSsl` instance that can be used with the builder's `apply` method.

The `RestClientSsl` interface provides access to any xref:features/ssl.adoc#features.ssl.bundles[SSL bundles] that you have defined in your `application.properties` or `application.yaml` file.

The following code shows a typical example:

include-code::MyService[]

If you need to apply other customization in addition to an SSL bundle, you can use the `ClientHttpRequestFactorySettings` class with `ClientHttpRequestFactories`:

include-code::settings/MyService[]



[[io.rest-client.resttemplate]]
== RestTemplate

Spring Framework's {url-spring-framework-javadoc}/org/springframework/web/client/RestTemplate.html[`RestTemplate`] class predates `RestClient` and is the classic way that many applications use to call remote REST services.
You might choose to use `RestTemplate` when you have existing code that you don't want to migrate to `RestClient`, or because you're already familiar with the `RestTemplate` API.

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` and an appropriate `ClientHttpRequestFactory` are applied to `RestTemplate` instances.

The following code shows a typical example:

include-code::MyService[]

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



[[io.rest-client.resttemplate.customization]]
=== 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`:

include-code::MyRestTemplateCustomizer[]

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:

include-code::MyRestTemplateBuilderConfiguration[]

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.



[[io.rest-client.resttemplate.ssl]]
=== RestTemplate SSL Support

If you need custom SSL configuration on the `RestTemplate`, you can apply an xref:features/ssl.adoc#features.ssl.bundles[SSL bundle] to the `RestTemplateBuilder` as shown in this example:

include-code::MyService[]



[[io.rest-client.clienthttprequestfactory]]
== HTTP Client Detection for RestClient and RestTemplate

Spring Boot will auto-detect which HTTP client to use with `RestClient` and `RestTemplate` depending on the libraries available on the application classpath.
In order of preference, the following clients are supported:

. Apache HttpClient
. Jetty HttpClient
. OkHttp (deprecated)
. Simple JDK client (`HttpURLConnection`)

If multiple clients are available on the classpath, the most preferred client will be used.
