MessageHandlerChain is an implementation of
MessageHandler that can be configured as a single Message Endpoint while
actually delegating to a chain of other handlers, such as Filters, Transformers, Splitters, and so on.
This can lead to a much simpler configuration when several handlers need to be connected in a fixed, linear
progression. For example, it is fairly common to provide a Transformer before other components. Similarly, when
providing a Filter before some other component in a chain, you are essentially creating a
Selective Consumer. In either case, the
chain only requires a single input-channel and a single output-channel as opposed to the configuration of
channels for each individual component.
|Spring Integration's Filter provides a boolean property 'throwExceptionOnRejection'. When providing multiple Selective Consumers on the same point-to-point channel with different acceptance criteria, this value should be set to 'true' (the default is false) so that the dispatcher will know that the Message was rejected and as a result will attempt to pass the Message on to other subscribers. If the Exception were not thrown, then it would appear to the dispatcher as if the Message had been passed on successfully even though the Filter had dropped the Message to prevent further processing.|
The handler chain simplifies configuration while internally maintaining the same degree of loose coupling between components, and it is trivial to modify the configuration if at some point a non-linear arrangement is required.
Internally, the chain will be expanded into a linear setup of the listed endpoints, separated by direct channels.
The reply channel header will not be taken into account within the chain: only after the last handler is invoked
will the resulting message be forwarded on to the reply channel or the chain's output channel. Because of this
setup all handlers except the last require a
setOutputChannel implementation. The last
handler only needs an output channel if the outputChannel on the MessageHandlerChain is set.
As with other endpoints, the output-channel is optional. If there is a reply Message at the end of the chain, the output-channel takes precedence, but if not available, the chain handler will check for a reply channel header on the inbound Message.
In most cases there is no need to implement MessageHandlers yourself. The next section will focus on namespace
support for the chain element. Most Spring Integration endpoints, like Service Activators and Transformers, are
suitable for use within a
The <chain> element provides an 'input-channel' attribute, and if the last element in the chain is capable of producing reply messages (optional), it also supports an 'output-channel' attribute. The sub-elements are then filters, transformers, splitters, and service-activators. The last element may also be a router.
<chain input-channel="input" output-channel="output"> <filter ref="someSelector" throw-exception-on-rejection="true"/> <header-enricher error-channel="customErrorChannel"> <header name="foo" value="bar"/> </header-enricher> <service-activator ref="someService" method="someMethod"/> </chain>
The <header-enricher> element used in the above example will set a message header with name "foo" and value "bar" on the message. A header enricher is a specialization of Transformer that touches only header values. You could obtain the same result by implementing a MessageHandler that did the header modifications and wiring that as a bean.
Some time you need to make a nested call to another chain from within the chain and then come back and continue execution within the original chain. To accomplish this you can utilize Messaging Gateway by including light-configuration via <gateway> element. For example:
<si:chain id="main-chain" input-channel="inputA" output-channel="inputB"> <si:header-enricher> <si:header name="name" value="Many" /> </si:header-enricher> <si:service-activator> <bean class="org.foo.SampleService" /> </si:service-activator> <si:gateway request-channel="inputC"/> </si:chain> <si:chain id="nested-chain-a" input-channel="inputC"> <si:header-enricher> <si:header name="name" value="Moe" /> </si:header-enricher> <si:gateway request-channel="inputD"/> <si:service-activator> <bean class="org.foo.SampleService" /> </si:service-activator> </si:chain> <si:chain id="nested-chain-b" input-channel="inputD"> <si:header-enricher> <si:header name="name" value="Jack" /> </si:header-enricher> <si:service-activator> <bean class="org.foo.SampleService" /> </si:service-activator> </si:chain>
In the above example the nested-chain-a will be called at the end of main-chain processing by the 'gateway' element
configured there. While in nested-chain-a a call to a nested-chain-b will be made after header enrichment and then it will
come back to finish execution in nested-chain-b finally getting back to the main-chain.
When light version of <gateway> element is defined in the chain SI will construct an instance
(no need to provide 'service-interface' configuration) which will take the message in its current state and will place it on the channel defined via 'request-channel' attribute.
Message will be returned to the gateway and continue its journey within the current chain.