The Aggregation API consists of a number of classes:

The AbstractMessageAggregator is a MessageConsumer implementation, encapsulating the common functionalities of an Aggregator, which are: storing messages until the message sequence to aggregate is complete (and grouping them according to their CORRELATION_ID), and implementing the timeout functionality. The responsibility of deciding whether the message sequence is complete is delegated to a CompletionStrategy instance.

A brief highlight of the base AbstractMessageAggregator (the responsibility of implementing the aggregateMessages method is left to the developer):

public abstract class AbstractMessageAggregator 
              extends AbstractMessageBarrierConsumer {

  private volatile CompletionStrategy completionStrategy
                            = new SequenceSizeCompletionStrategy();
  ....

  protected abstract Message<?> aggregateMessages(List<Message<?>> messages);

}

For implementing a specific aggregator object for an application, a developer can extend AbstractMessageAggregator and implement the aggregateMessages method. However, there are better suited (which reads, less coupled to the API) solutions for implementing the aggregation logic, which can be configured easily either through XML or through annotations.

In general, any ordinary Java class (i.e. POJO) can implement the aggregation algorithm. For doing so, it must provide a method that accepts as an argument a single java.util.List (parametrized lists are supported as well). This method will be invoked for aggregating messages, as follows:

	Note
	In the interest of code simplicity, and promoting best practices such as low coupling, testability, etc., the preferred way of implementing the aggregation logic is through a POJO, and using the XML or annotation support for setting it up in the application.

The CompletionStrategy interface is defined as follows:

public interface CompletionStrategy {

  boolean isComplete(List<Message<?>> messages);

}

In general, any ordinary Java class (i.e. POJO) can implement the completion decision mechanism. For doing so, it must provide a method that accepts as an argument a single java.util.List (parametrized lists are supported as well), and returns a boolean value. This method will be invoked after the arrival of a new message, to decide whether the group is complete or not, as follows:

Spring Integration provides an out-of-the box implementation for CompletionStrategy, the SequenceSizeCompletionStrategy This implementation uses the SEQUENCE_NUMBER and SEQUENCE_SIZE of the arriving messages for deciding when a message group is complete and ready to be aggregated.