Message Channels

除了带有 EIP 方法的 IntegrationFlowBuilder 之外，Java DSL 还提供了一个流畅的 API 来配置 MessageChannel 实例。为此，提供了 MessageChannels 生成器工厂。以下示例显示了如何使用它：

In addition to the IntegrationFlowBuilder with EIP methods, the Java DSL provides a fluent API to configure MessageChannel instances. For this purpose the MessageChannels builder factory is provided. The following example shows how to use it:

@Bean
public PriorityChannelSpec priorityChannel() {
    return MessageChannels.priority(this.mongoDbChannelMessageStore, "priorityGroup")
                        .interceptor(wireTap());
}

相同的 MessageChannels`生成器工厂可用于 `channel()`EIP 方法从 `IntegrationFlowBuilder`布线端点，类似于在 XML 配置中布线 `input-channel/output-channel`对。默认情况下，端点使用 `DirectChannel`实例进行布线，其中 bean 名称基于以下模式：[IntegrationFlow.beanName].channel#。该规则也适用于通过内联 `MessageChannels`生成器工厂用法生成的未命名通道。但是，所有 `MessageChannels`方法都有一个变体，它知道您可以用来设置 `MessageChannel`实例的 bean 名称的 `channelId。`MessageChannel`引用和 `beanName`可用作 bean 方法调用。以下示例显示了使用 `channel()`EIP 方法的可能方式：

The same MessageChannels builder factory can be used in the channel() EIP method from IntegrationFlowBuilder to wire endpoints, similar to wiring an input-channel/output-channel pair in the XML configuration. By default, endpoints are wired with DirectChannel instances where the bean name is based on the following pattern: [IntegrationFlow.beanName].channel#[channelNameIndex]. This rule is also applied for unnamed channels produced by inline MessageChannels builder factory usage. However, all MessageChannels methods have a variant that is aware of the channelId that you can use to set the bean names for MessageChannel instances. The MessageChannel references and beanName can be used as bean-method invocations. The following example shows the possible ways to use the channel() EIP method:

@Bean
public QueueChannelSpec queueChannel() {
    return MessageChannels.queue();
}

@Bean
public PublishSubscribeChannelSpec<?> publishSubscribe() {
    return MessageChannels.publishSubscribe();
}

@Bean
public IntegrationFlow channelFlow() {
    return IntegrationFlow.from("input")
                .fixedSubscriberChannel()
                .channel("queueChannel")
                .channel(publishSubscribe())
                .channel(MessageChannels.executor("executorChannel", this.taskExecutor))
                .channel("output")
                .get();
}

from("input") means "'find and use the MessageChannel with the "input" id, or create one'".
fixedSubscriberChannel() produces an instance of FixedSubscriberChannel and registers it with a name of channelFlow.channel#0.
channel("queueChannel") works the same way but uses an existing queueChannel bean.
channel(publishSubscribe()) is the bean-method reference.
channel(MessageChannels.executor("executorChannel", this.taskExecutor)) is the IntegrationFlowBuilder that exposes IntegrationComponentSpec to the ExecutorChannel and registers it as executorChannel.
channel("output") registers the DirectChannel bean with output as its name, as long as no beans with this name already exist.

注意：前面的 IntegrationFlow 定义有效，并且其所有通道都使用 BridgeHandler 实例应用到端点。

Note: The preceding IntegrationFlow definition is valid, and all of its channels are applied to endpoints with BridgeHandler instances.

请务必通过 MessageChannels 工厂从不同的 IntegrationFlow 实例使用相同的内联通道定义。即使 DSL 解析器将不存在的对象注册为 Bean，它也无法从不同的 IntegrationFlow 容器确定相同对象 (MessageChannel)。以下示例有误：

Be careful to use the same inline channel definition through MessageChannels factory from different IntegrationFlow instances. Even if the DSL parser registers non-existent objects as beans, it cannot determine the same object (MessageChannel) from different IntegrationFlow containers. The following example is wrong:

@Bean
public IntegrationFlow startFlow() {
    return IntegrationFlow.from("input")
                .transform(...)
                .channel(MessageChannels.queue("queueChannel"))
                .get();
}

@Bean
public IntegrationFlow endFlow() {
    return IntegrationFlow.from(MessageChannels.queue("queueChannel"))
                .handle(...)
                .get();
}

该错误示例的结果是以下异常：

The result of that bad example is the following exception:

Caused by: java.lang.IllegalStateException:
Could not register object [queueChannel] under bean name 'queueChannel':
     there is already object [queueChannel] bound
	    at o.s.b.f.s.DefaultSingletonBeanRegistry.registerSingleton(DefaultSingletonBeanRegistry.java:129)

为了让它正常工作，您需要为该通道声明 @Bean，并从不同的 IntegrationFlow 实例使用其 bean 方法。

To make it work, you need to declare @Bean for that channel and use its bean method from different IntegrationFlow instances.