HTTP Outbound Components

This section describes Spring Integration’s HTTP outbound components.

Using HttpRequestExecutingMessageHandler

To configure the HttpRequestExecutingMessageHandler, write a bean definition similar to the following:

<bean id="httpOutbound"
  class="org.springframework.integration.http.outbound.HttpRequestExecutingMessageHandler">
  <constructor-arg value="http://localhost:8080/example" />
  <property name="outputChannel" ref="responseChannel" />
</bean>

This bean definition runs HTTP requests by delegating to a RestTemplate. That template, in turn, delegates to a list of HttpMessageConverter instances to generate the HTTP request body from the Message payload. You can configure those converters as well as the ClientHttpRequestFactory instance to use, as the following example shows:

<bean id="httpOutbound"
  class="org.springframework.integration.http.outbound.HttpRequestExecutingMessageHandler">
  <constructor-arg value="http://localhost:8080/example" />
  <property name="outputChannel" ref="responseChannel" />
  <property name="messageConverters" ref="messageConverterList" />
  <property name="requestFactory" ref="customRequestFactory" />
</bean>

By default, the HTTP request is generated by using an instance of SimpleClientHttpRequestFactory, which uses the JDK HttpURLConnection. Use of the Apache Commons HTTP Client is also supported through CommonsClientHttpRequestFactory, which you can inject (as shown earlier).

For the outbound gateway, the reply message produced by the gateway contains all the message headers that are present in the request message.

Using Cookies

Basic cookie support is provided by the transfer-cookies attribute on the outbound gateway. When set to true (the default is false), a Set-Cookie header received from the server in a response is converted to Cookie in the reply message. This header is then used on subsequent sends. This enables simple stateful interactions, such as the following:

…​→logonGateway→…​→doWorkGateway→…​→logoffGateway→…​

If transfer-cookies is false, any Set-Cookie header received remains as Set-Cookie in the reply message and is dropped on subsequent sends.

Empty Response Bodies

HTTP is a request-response protocol. However, the response may not have a body, only headers. In this case, the HttpRequestExecutingMessageHandler produces a reply Message with the payload being an org.springframework.http.ResponseEntity, regardless of any provided expected-response-type. According to the HTTP RFC Status Code Definitions, there are many statuses that mandate that a response must not contain a message-body (for example, 204 No Content). There are also cases where calls to the same URL might or might not return a response body. For example, the first request to an HTTP resource returns content, but the second does not (returning a 304 Not Modified). In all cases, however, the http_statusCode message header is populated. This can be used in some routing logic after the HTTP outbound gateway. You could also use a`<payload-type-router/>` to route messages with a ResponseEntity to a different flow than that used for responses with a body.
expected-response-type

Further to the preceding note about empty response bodies, if a response does contain a body, you must provide an appropriate expected-response-type attribute or, again, you receive a ResponseEntity with no body. The expected-response-type must be compatible with the (configured or default) HttpMessageConverter instances and the Content-Type header in the response. This can be an abstract class or even an interface (such as java.io.Serializable when you use Java serialization and Content-Type: application/x-java-serialized-object).

Starting with version 5.5, the HttpRequestExecutingMessageHandler exposes an extractResponseBody flag (which is true by default) to return just the response body, or to return the whole ResponseEntity as the reply message payload, independently of the provided expectedResponseType. If a body is not present in the ResponseEntity, this flag is ignored and the whole ResponseEntity is returned.