Service Activator

See Core J2EE Patterns, 2nd Edition for full description of this pattern and its strategies.

Problem

You want to invoke services asynchronously.

Forces

• You want to invoke business services, POJOs, or EJB components in an asynchronous manner.
• You want to integrate publish/subscribe and point-to-point messaging to enable asynchronous processing services.
• You want to perform a business task that is logically composed of several business tasks.

Solution

Use a Service Activator to receive asynchronous requests and invoke one or more business services.

Class Diagram

Sequence Diagram

Strategies

• POJO Service Activator Strategy
• MDB Service Activator Strategy
• Service Activator Aggregator Strategy
• Response Strategies
  • Database Response Strategy
  • Email Response Strategy
  • JMS Message Response Strategy

Consequences

• Integrates JMS into enterprise applications
• Provides asynchronous processing for any business-tier component
• Enables standalone JMS listener

Related Patterns

• Session Façade
  The Session Façade encapsulates the complexity of the system and provides coarse-grained access to business objects. A Service Activator can access a Session Façade as a business service to invoke business processing.
• Application Services
  An Application Service can also be a kind of business service that a Service Activator invokes to process the request.
• Business Delegate
  The Service Activator typically uses a Business Delegate to access a Session Façade. This results in simpler code for the Service Activator and results in Business Delegate reuse across different tiers.
• Service Locator
  The client can use the Service Locator pattern to look up and create JMS-related service objects. The Service Activator can use the Service Locator pattern to look up and create enterprise bean components.
• Half-Sync/Half-Async [POSA2]
  The Service Activator is related to the Half-Sync/Half-Async pattern. The pattern describes architectural decoupling by suggesting different layers for synchronous and asynchronous processing, and an intermediate queuing layer in between.
• Aggregator [EIP]
  The Aggregator pattern discusses the problem of converting a request into several asynchronous tasks and aggregating the results. The Service Activator Aggregator strategy is based on similar concepts.

Excerpted from Core J2EE Patterns, 2nd Edition. See the book for full description of the pattern including class diagrams, sequence diagrams and sample code.