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.spring-doc.cn

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.spring-doc.cn

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.spring-doc.cn

Configuring a Bridge with XML

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:spring-doc.cn

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

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:spring-doc.cn

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

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:spring-doc.cn

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

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

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

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.spring-doc.cn

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.
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

The following example shows how to configure a bridge in Java by using the @BridgeFrom annotation:spring-doc.cn

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

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

The following example shows how to configure a bridge in Java by using the @BridgeTo annotation:spring-doc.cn

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

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

Alternately, you can use a BridgeHandler, as the following example shows:spring-doc.cn

@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

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

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