Core J2EE Patterns

You want to use the JNDI API to look up and use business components, such as enterprise beans and JMS components, and services such as data sources.
You want to centralize and reuse the implementation of lookup mechanisms for J2EE application clients.
You want to encapsulate vendor dependencies for registry implementations, and hide the dependency and complexity from the clients.
You want to avoid performance overhead related to initial context creation and service lookups.
You want to reestablish a connection to a previously accessed enterprise bean instance, using its Handle object.

Solution

Use a Service Locator to implement and encapsulate service and component lookup. A Service Locator hides the implementation details of the lookup mechanism and encapsulates related dependencies.

Class Diagram

Sequence Diagram

Strategies

EJB Service Locator Strategy
JDBC DataSource Service Locator Strategy
JMS Service Locator Strategies
- JMS Queue Service Locator Strategy
- JMS Topic Service Locator Strategy
Web Service Locator Strategy

Consequences

Abstracts complexity
Provides uniform service access to clients
Facilitates adding EJB business components
Improves network performance
Improves client performance by caching

Related Patterns

Business Delegate
Business Delegate uses a Service Locator to locate and obtain references to the business service objects, such as EJB objects, JMS topics, and JMS queues. This separates the complexity of service location from the Business Delegate, leading to loose coupling and increased manageability.
Session Façade
Session Façade uses a Service Locator to locate and obtain home and remote references to the session beans and entity beans, as well as to locate a data source.
Transfer Object Assembler
Transfer Object Assembler uses a Service Locator to locate references to session beans and entity beans that it needs to access data and build a composite transfer object.
Data Access Object
A Data Access Object uses a Service Locator to look up and obtain a reference to a data source.

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