This section covers the various channel adapters and messaging gateways provided by Spring Integration to support message-based communication with external systems.
Each system, from AMQP to Zookeeper, has its own integration requirements, and this section covers them.
Endpoint Quick Reference Table
As discussed in the earlier sections, Spring Integration provides a number of endpoints used to interface with external systems, file systems, and others.
For transparent dependency management Spring Integration provides a bill-of-materials POM to be imported into the Maven configuration:
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.integration</groupId>
<artifactId>spring-integration-bom</artifactId>
<version>5.3.7.RELEASE</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>To recap:
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Inbound channel adapters are used for one-way integration to bring data into the messaging application.
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Outbound channel adapters are used for one-way integration to send data out of the messaging application.
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Inbound gateways are used for a bidirectional integration flow, where some other system invokes the messaging application and receives a reply.
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Outbound Gateways are used for a bidirectional integration flow, where the messaging application invokes some external service or entity and expects a result.
The following table summarizes the various endpoints with quick links to the appropriate chapter.
In addition, as discussed in Core Messaging, Spring Integration provides endpoints for interfacing with Plain Old Java Objects (POJOs).
As discussed in Channel Adapter, the <int:inbound-channel-adapter> element lets you poll a Java method for data.
The <int:outbound-channel-adapter> element lets you send data to a void method.
As discussed in Messaging Gateways, the <int:gateway> element lets any Java program invoke a messaging flow.
Each of these works without requiring any source-level dependencies on Spring Integration.
The equivalent of an outbound gateway in this context is using a service activator (see Service Activator) to invoke a method that returns an Object of some kind.
Starting with version 5.2.2, all the inbound gateways can be configured with an errorOnTimeout boolean flag to throw a MessageTimeoutException when the downstream flow doesn’t return a reply during the reply timeout.
The timer is not started until the thread returns control to the gateway, so usually it is only useful when the downstream flow is asynchronous or it stops because of a null return from some handler, e.g. filter.
Such an exception can be handled on the errorChannel flow, e.g. producing a compensation reply for requesting client.