Core J2EE Patterns

You want to minimize business logic in service facades.
You have business logic acting on multiple Business Objects or services.
You want to provide a coarser-grained service API over existing business-tier components and services.
You want to encapsulate use case-specific logic outside of individual Business Objects.

Solution

Use an Application Service to centralize and aggregate behavior to provide a uniform service layer.

Class Diagram

Sequence Diagram

Strategies

Application Service Command Strategy
GoF Strategy for Application Service Strategy
Application Service Layer Strategy

Consequences

Centralizes reusable business and workflow logic
Improves reusability of business logic
Avoids duplication of code
Simplifies facade implementations
Introduces additional layer in the business tier

Related Patterns

Session Façade
Application Services provide the background infrastructure for Session Façades, which become simpler to implement and contain less code because they can delegate the business processing to Application Services.
Business Object
In applications that use Business Objects, Application Services encapsulate cross-Business Objects logic and interact with several Business Objects.
Data Access Object
In some applications, an Application Service can use a Data Access Object directly to access data in a data store.
Service Layer [PEAA]
Application Service is similar to Service Layer pattern in that both aim to promote a service layer in your application. Service Layer explains how a set of services can be used to create a boundary layer for your application.
Transaction Script [PEAA]
When an Application Service is used without Business Objects (374), it becomes a service object where you can implement your procedural logic. Transaction Script describes the use of procedural approach to implementing business logic in your application.

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