Calling REST services

If you need to call remote REST services from your application, you can use 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 will ensure that sensible HttpMessageConverters are applied to RestTemplate instances.

Here’s a typical example:

@Service
public class MyBean {

	private final RestTemplate restTemplate;

	public MyBean(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.basicAuthorization("user", "password").build().

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 will only affect this use of the builder.

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

Here’s an example of a customizer that configures the use of a proxy for all hosts except 192.168.0.5:

static class ProxyCustomizer implements RestTemplateCustomizer {

	@Override
	public void customize(RestTemplate restTemplate) {
		HttpHost proxy = new HttpHost("proxy.example.com");
		HttpClient httpClient = HttpClientBuilder.create()
				.setRoutePlanner(new DefaultProxyRoutePlanner(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);
					}

				}).build();
		restTemplate.setRequestFactory(
				new HttpComponentsClientHttpRequestFactory(httpClient));
	}

}

Lastly, the most extreme (and rarely used) option is to create your own RestTemplateBuilder bean. This will switch off the auto-configuration of a RestTemplateBuilder and will prevent any RestTemplateCustomizer beans from being used.