A Messaging Bridge is a relatively trivial endpoint that simply 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.

By providing an intermediary poller between two channels, a Messaging Bridge can be used to throttle inbound Messages. The poller's trigger will determine the rate at which messages arrive on the second channel, and the poller's "maxMessagesPerPoll" property will enforce a limit on the throughput.

Another valid use for a Messaging Bridge is to connect two different systems. In such a scenario, Spring Integration's role would be 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, and in that case, the channels would 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.

The <bridge> element is used to create a Messaging Bridge between two Message Channels or Channel Adapters. Simply provide the "input-channel" and "output-channel" attributes:

 <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, and when performing this role, the Messaging Bridge may also serve as a throttler:

 <bridge input-channel="pollable" output-channel="subscribable">
     <poller max-messages-per-poll="10">
         <interval-trigger interval="5" time-unit="SECONDS"/>
     </poller>
 </bridge>

Connecting Channel Adapters is just as easy. Here is a simple echo example between the "stdin" and "stdout" adapters from Spring Integration's "stream" namespace.

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

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

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

Of course, the configuration would be similar for other (potentially more useful) Channel Adapter bridges, such as File to JMS, or Mail to File. The various Channel Adapters will be discussed in upcoming chapters.

	Note
	If no 'output-channel' is defined on a bridge, the reply channel provided by the inbound Message will be used, if available. If neither output or reply channel is available, an Exception will be thrown.