Spring Integration: Filter

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Message Transformer

A message transformer is responsible for converting a message’s content or structure and returning the modified message. Probably the most common type of transformer is one that converts the payload of the message from one format to another (such as from XML to java.lang.String). Similarly, a transformer can add, remove, or modify the message’s header values.

Message Filter

A message filter determines whether a message should be passed to an output channel at all. This simply requires a boolean test method that may check for a particular payload content type, a property value, the presence of a header, or other conditions. If the message is accepted, it is sent to the output channel. If not, it is dropped (or, for a more severe implementation, an Exception could be thrown). Message filters are often used in conjunction with a publish-subscribe channel, where multiple consumers may receive the same message and use the criteria of the filter to narrow down the set of messages to be processed.

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Message Router

A message router is responsible for deciding what channel or channels (if any) should receive the message next. Typically, the decision is based upon the message’s content or the metadata available in the message headers. A message router is often used as a dynamic alternative to a statically configured output channel on a service activator or other endpoint capable of sending reply messages. Likewise, a message router provides a proactive alternative to the reactive message filters used by multiple subscribers, as described earlier.

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Aggregator

Basically a mirror-image of the splitter, the aggregator is a type of message endpoint that receives multiple messages and combines them into a single message. In fact, aggregators are often downstream consumers in a pipeline that includes a splitter. Technically, the aggregator is more complex than a splitter, because it is required to maintain state (the messages to be aggregated), to decide when the complete group of messages is available, and to timeout if necessary. Furthermore, in case of a timeout, the aggregator needs to know whether to send the partial results, discard them, or send them to a separate channel. Spring Integration provides a CorrelationStrategy, a ReleaseStrategy, and configurable settings for timeout, whether to send partial results upon timeout, and a discard channel.