Messaging Bridge

消息桥接器是一个相对微不足道的端点，连接两个消息通道或通道适配器。例如，您可能希望将 PollableChannel 连接到 SubscribableChannel，以便订阅端点不必担心任何轮询配置。相反，消息桥接器提供轮询配置。

A messaging bridge is a relatively trivial endpoint that connects two message channels or channel adapters. For example, you may want to connect a PollableChannel to a SubscribableChannel so that the subscribing endpoints do not have to worry about any polling configuration. Instead, the messaging bridge provides the polling configuration.

通过在两个通道之间提供一个中介轮询器，您可以使用消息桥接器来限制入站消息。轮询器的触发器决定了消息到达第二个通道的速率，而轮询器的 maxMessagesPerPoll 属性对吞吐量执行限制。

By providing an intermediary poller between two channels, you can use a messaging bridge to throttle inbound messages. The poller’s trigger determines the rate at which messages arrive at the second channel, and the poller’s maxMessagesPerPoll property enforces a limit on the throughput.

消息桥接器的另一个有效用途是连接两个不同的系统。在这种情况下，Spring 集成的作用仅限于在这些系统之间建立连接和管理轮询器（如果需要）。在两个系统之间至少有一个转换器（转换它们的格式）可能是更为常见的。在这种情况下，通道可以作为转换器端点的“输入通道”和“输出通道”提供。如果不需要数据格式转换，则消息桥接器实际上就足够了。

Another valid use for a messaging bridge is to connect two different systems. In such a scenario, Spring Integration’s role is limited to making the connection between these systems and managing a poller, if necessary. It is probably more common to have at least a transformer between the two systems, to translate between their formats. In that case, the channels can be provided as the 'input-channel' and 'output-channel' of a transformer endpoint. If data format translation is not required, the messaging bridge may indeed be sufficient.

Configuring a Bridge with XML

可以使用 <bridge> 元素在两个消息管道或管道适配器之间创建一个消息桥。要进行此操作，请提供 input-channel 和 output-channel 属性，如以下示例所示：

You can use the <bridge> element is used to create a messaging bridge between two message channels or channel adapters. To do so, provide the input-channel and output-channel attributes, as the following example shows:

<int:bridge input-channel="input" output-channel="output"/>

如上所述，消息桥的一个常见用例是将 PollableChannel 连接到 SubscribableChannel。在执行此角色时，消息桥还可以用作限流器：

As mentioned above, a common use case for the messaging bridge is to connect a PollableChannel to a SubscribableChannel. When performing this role, the messaging bridge may also serve as a throttler:

<int:bridge input-channel="pollable" output-channel="subscribable">
     <int:poller max-messages-per-poll="10" fixed-rate="5000"/>
 </int:bridge>

可以使用类似的机制连接管道适配器。以下示例展示了 Spring Integration 的 stream 命名空间中 stdin 和 stdout 适配器之间的简单“echo”：

You can use a similar mechanism to connecting channel adapters. The following example shows a simple “echo” between the stdin and stdout adapters from Spring Integration’s stream namespace:

<int-stream:stdin-channel-adapter id="stdin"/>

 <int-stream:stdout-channel-adapter id="stdout"/>

 <int:bridge id="echo" input-channel="stdin" output-channel="stdout"/>

类似的配置适用于其他（可能更有用的）管道适配器桥，例如文件到 JMS 或邮件到文件。即将到来的章节将介绍各种管道适配器。

Similar configurations work for other (potentially more useful) Channel Adapter bridges, such as file-to-JMS or mail-to-file. Upcoming chapters cover the various channel adapters.

如果桥接上未定义“output-channel”，则将使用输入消息提供的响应通道（如果可用）。如果既没有输出信道也没有响应信道可用，则会引发异常。

If no 'output-channel' is defined on a bridge, the reply channel provided by the inbound message is used, if available. If neither an output nor a reply channel is available, an exception is thrown.

Configuring a Bridge with Java Configuration

以下示例展示了如何通过使用 @BridgeFrom 注解在 Java 中配置一个桥：

The following example shows how to configure a bridge in Java by using the @BridgeFrom annotation:

@Bean
public PollableChannel polled() {
    return new QueueChannel();
}

@Bean
@BridgeFrom(value = "polled", poller = @Poller(fixedDelay = "5000", maxMessagesPerPoll = "10"))
public SubscribableChannel direct() {
    return new DirectChannel();
}

以下示例演示了如何通过使用 @BridgeTo 注解在 Java 中配置桥：

The following example shows how to configure a bridge in Java by using the @BridgeTo annotation:

@Bean
@BridgeTo(value = "direct", poller = @Poller(fixedDelay = "5000", maxMessagesPerPoll = "10"))
public PollableChannel polled() {
    return new QueueChannel();
}

@Bean
public SubscribableChannel direct() {
    return new DirectChannel();
}

或者，可以使用 BridgeHandler，如下例所示：

Alternately, you can use a BridgeHandler, as the following example shows:

@Bean
@ServiceActivator(inputChannel = "polled",
        poller = @Poller(fixedRate = "5000", maxMessagesPerPoll = "10"))
public BridgeHandler bridge() {
    BridgeHandler bridge = new BridgeHandler();
    bridge.setOutputChannelName("direct");
    return bridge;
}

Configuring a Bridge with the Java DSL

可以使用 Java 领域特定语言 (DSL) 来配置桥，如下例所示：

You can use the Java Domain Specific Language (DSL) to configure a bridge, as the following example shows:

@Bean
public IntegrationFlow bridgeFlow() {
    return IntegrationFlow.from("polled")
            .bridge(e -> e.poller(Pollers.fixedDelay(5000).maxMessagesPerPoll(10)))
            .channel("direct")
            .get();
}