Invoking RESTful services in Java is typically done using a helper class such as Jakarta Commons HttpClient. For common REST operations this approach is too low level as shown below.

String uri = "http://example.com/hotels/1/bookings";

PostMethod post = new PostMethod(uri);
String request = // create booking request content
post.setRequestEntity(new StringRequestEntity(request));

httpClient.executeMethod(post);

if (HttpStatus.SC_CREATED == post.getStatusCode()) {
  Header location = post.getRequestHeader("Location");
  if (location != null) {
    System.out.println("Created new booking at :" + location.getValue());
  }
}

RestTemplate provides higher level methods that correspond to each of the six main HTTP methods that make invoking many RESTful services a one-liner and enforce REST best practices.

The names of RestTemplate methods follow a naming convention, the first part indicates what HTTP method is being invoked and the second part indicates what is returned. For example, the method getForObject will perform a GET, convert the HTTP response into an object type of your choice and return that object. The method postForLocation will do a POST, converting the given object into a HTTP request and return the response HTTP Location header where the newly created object can be found. In case of an exception processing the HTTP request, an exception of the type RestClientException will be thrown, this behavior can be changed by plugging in another ResponseErrorHandler implementation into the RestTemplate.

Objects passed to and returned from these methods are converted to and from HTTP messages by HttpMessageConverter instances. Converters for the main mime types are registered by default, but you can also write your own converter and register it via the messageConverters bean property. The default converter instances registered with the template are ByteArrayHttpMessageConverter, StringHttpMessageConverter, FormHttpMessageConverter and SourceHttpMessageConverter. You can override these defaults using the messageConverters bean property as would be required if using the MarshallingHttpMessageConverter.

Each method takes URI template arguments in two forms, either as a String variable length argument or a Map<String,String>. For example,

String result = restTemplate.getForObject("http://example.com/hotels/{hotel}/bookings/{booking}",
                                          String.class,"42", "21");

using variable length arguments and

Map<String, String> vars = Collections.singletonMap("hotel", "42");
String result =
  restTemplate.getForObject("http://example.com/hotels/{hotel}/rooms/{hotel}", String.class, vars);

using a Map<String,String>.

To create an instance of RestTemplate you can simply call the default constructor. This will use standard Java classes from the java.net package as the underlying implementation to create HTTP requests. This can be overridden by specifying an implementation of ClientHttpRequestFactory. Spring provides the implementation CommonsClientHttpRequestFactory that uses the Jakarta Commons HttpClient to create requests. CommonsClientHttpRequestFactory is configured using an instance of org.apache.commons.httpclient.HttpClient which can in turn be configured with credentials information or connection pooling functionality.

The previous example using Jakarta Commons HttpClient directly rewritten to use the RestTemplate is shown below

uri = "http://example.com/hotels/{id}/bookings";

RestTemplate template = new RestTemplate();

Booking booking = // create booking object

URI location = template.postForLocation(uri, booking, "1");

The general callback interface is RequestCallback and is called when the execute method is invoked.

public <T> T execute(String url, HttpMethod method, RequestCallback requestCallback,
                     ResponseExtractor<T> responseExtractor,
                     String... urlVariables)


// also has an overload with urlVariables as a Map<String, String>.

The RequestCallback interface is defined as

public interface RequestCallback {
 void doWithRequest(ClientHttpRequest request) throws IOException;
}

and allows you to manipulate the request headers and write to the request body. When using the execute method you do not have to worry about any resource management, the template will always close the request and handle any errors. Refer to the API documentation for more information on using the execute method and the meaning of its other method arguments.

Objects passed to and returned from the methods getForObject, postForLocation, and put are converted to HTTP requests and from HTTP responses by HttpMessageConverters. The HttpMessageConverter interface is shown below to give you a better feel for its functionality

public interface HttpMessageConverter<T> {

      // Indicate whether the given class is supported by this converter.
      boolean supports(Class<? extends T> clazz);

      // Return the list of MediaType objects supported by this converter.
      List<MediaType> getSupportedMediaTypes();

      // Read an object of the given type form the given input message, and returns it.
      T read(Class<T> clazz, HttpInputMessage inputMessage) throws IOException,
                                                                   HttpMessageNotReadableException;

      // Write an given object to the given output message.
      void write(T t, HttpOutputMessage outputMessage) throws IOException,
                                                              HttpMessageNotWritableException;

    }

Concrete implementations for the main media (mime) types are provided in the framework and are registered by default with the RestTemplate on the client-side and with AnnotationMethodHandlerAdapter on the server-side.

The implementations of HttpMessageConverters are described in the following sections. For all converters a default media type is used but can be overridden by setting the supportedMediaTypes bean property