Gateways provide a convenient way to expose a Proxy over a service-interface thus giving you POJO-based access to a messaging system (based on objects in your own domain, or primitives/Strings, etc). However, when you invoke a method, you expect the method to return. A gateway's method call represents a contract with the messaging system, which states that for each request, there will always be a reply. Therefore you must always guarantee that your message flow is in compliance with such a contract.
But what about the cases where you can't (e.g, message was filtered out and discarded or routed into a unidirectional sub-flow)?
Starting with Spring Integration 2.0, we introduced support for an Asynchronous Gateway, which is a convenient way to initiate flows, where you may not know, if a reply is expected or how long will it take for it to arrive. A natural way to handle these types of scenarios in Java would be to rely upon java.util.concurrent.Future instances. That is exactly what Spring Integration uses to support Asynchronous Gateways.
This example demonstrates how you can apply an Asynchronous Gateway based on the following simple use case:
We are sending a request to a MathService to multiply random numbers by 2. As you can see from the configuration there is a filter that discards any request for the number that is less then a 100. This means that there will be no replies coming for the requests with numbers less then 100. Typically, when using the regular Gateway, the gateway method would lock until a timeout occurs. In this example, however, the responses are coming back right away as Java Futures which we evaluate.
To run this sample, simply execute org.springframework.integration.samples.async.gateway.AsyncGatewayTest.
You should see the following output:
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Result of multiplication of 107 by 2 is 214
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Result of multiplication of 146 by 2 is 292
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Result of multiplication of 189 by 2 is 378
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Result of multiplication of 130 by 2 is 260
. . . . .
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Multiplication of 38 by 2 is can not be accomplished in 20 seconds
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Multiplication of 39 by 2 is can not be accomplished in 20 seconds
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Multiplication of 36 by 2 is can not be accomplished in 20 seconds
INFO : org.springframework.integration.samples.async.gateway.AsyncGatewayTest - Multiplication of 37 by 2 is can not be accomplished in 20 seconds
Spring Integration 4.0 provided the capability to more easily configure Messaging Gateways with Java configuration.
Spring Integration 4.1 added support for ListenableFuture and Promise (from project reactor) return types.
The ListenableFutureTest and PromiseTest test classes replicate the above test case, using those return types, and showing the use of @MessagingGateway java configuration.