The Java EE 7 Tutorial
43 Creating Fetch Plans with Entity Graphs
This chapter explains how to use entity graphs to create fetch plans for Java Persistence API operations and queries.
Entity graphs are templates for a particular Persistence query or operation. They are used when creating fetch plans, or groups of persistent fields that are retrieved at the same time. Application developers use fetch plans to group together related persistent fields to improve runtime performance.
By default, entity fields or properties are fetched lazily. Developers specify fields or properties as part of a fetch plan, and the persistence provider will fetch them eagerly.
For example, an email application that stores messages as
EmailMessage entities prioritizes fetching some fields over others. The sender, subject, and date will be viewed the most often, in mailbox views and when the message is displayed. The
EmailMessage entity has a collection of related
EmailAttachment entities. For performance reasons the attachments should not be fetched until they are needed, but the file names of the attachment are important. A developer working on this appication might make a fetch plan that eagerly fetches the important fields from
EmailAttachment while fetching the lower priority data lazily.
The following topics are addressed here: