J2EE Design Patterns

Name

Lazy Load

Goal

To avoid delays created by excessive and unnecessary database access (Figure B-5).

Figure B-5. Lazy load sequence

Participants

Client

Presentation tier or other system component.

Business object

Business delegate, session façade or other business logic related object.

DAO

Provides access to the persistence layer.

DTOs

Objects for exchange of data between participants.

Interactions

The client calls methods on a business object, requesting specific data. The business object calls a DAO method to retrieve only the data required for the current method. If the client needs additional data, the business object performs an additional request to the DAO.

Notes

Implementing the Lazy Load pattern intelligently requires a bit of analysis of your application. If your application can retrieve several kinds of data with a single database query, it will generally be more efficient to do that then to retrieve each piece separately. If multiple queries would be required anyway, and different use cases require different data, the Lazy Load pattern can improve performance by limiting queries to only those strictly necessary.