Revision as of 12:02, 17 June 2010

Fetch Type

By default, EclipseLink persistence provider uses a fetch type of javax.persitence.FetchType.EAGER: this is a requirement on the persistence provider runtime that data must be eagerly fetched.If the default is inappropriate for your application or a particular persistent field, set fetch to FetchType.LAZY: this is a hint to the persistence provider that data should be fetched lazily when it is first accessed (if possible). We recommend using the FetchType.LAZY on all relationships.

You are not required to provide value for this attribute.

How to Configure Default Fetch Group Behavior

You can optionally designate at most one fetch group as the default fetch group for a descriptor's reference class.

If you execute a ReadObjectQuery or ReadAllQuery without specifying a fetch group, EclipseLink will use the default fetch group unless you configure the query otherwise, as this example shows.

Configuring Default Fetch Group Behavior

 // at the descriptor level
 FetchGroup group = new FetchGroup("nameOnly");
 group.addAttribute("firstName");
 group.addAttribute("lastName");
 employeeDescriptor.getFetchGroupManager().addFetchGroup(group);
 // set the default fetch group'''
 employeeDescriptor.getFetchGroupManager().setDefaultFetchGroup(group);
 
 // when query1 is executed, the default fetch group applies
 ReadAllQuery query1 = new ReadAllQuery(Employee.class);
 
 // when query2 is executed, the default fetch group does not apply
 ReadAllQuery query2 = new ReadAllQuery(Employee.class);
 query2.setShouldUsedefaultFetchGroup(false);

How to Query with a Static Fetch Group

Configuring a Query with a FetchGroup Using the FetchGroupManager shows how to configure a ReadObjectQuery for the Employee class with a FetchGroup named nameOnly previously stored in the FetchGroupManager owned by the Employee class's descriptor.

' Configuring a Query with a FetchGroup Using the FetchGroupManager

In this example, only the Employee attributes firstName and lastName are fetched. If you call the Employee method get for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that get method will return the value directly from the object.

 
 // create static fetch group at the descriptor level
 FetchGroup group = new FetchGroup("nameOnly");
 group.addAttribute("firstName");
 group.addAttribute("lastName");
 descriptor.getFetchGroupManager().addFetchGroup(group);
 
 // use static fetch group at query level
 ReadAllQuery query = new ReadAllQuery(Employee.class);
 query.setFetchGroupName("nameOnly");

How to Query with a Dynamic Fetch Group

Configuring a Query with a FetchGroup Dynamically shows how to create a FetchGroup instance dynamically, at the time you create and execute a query, and configure the query with that FetchGroup directly.

In this example, only the firstName, lastName, and salary attributes are fetched. If you call the Employee method get for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that get method will return the value directly from the object.

Configuring a Query with a FetchGroup Dynamically

 // dynamic fetch group query
 ReadAllQuery query = new ReadAllQuery(Employee.class);
 FetchGroup group = new FetchGroup("nameAndSalary");
 group.addAttribute("firstName");
 group.addAttribute("lastName");
 group.addAttribute("salary");
 query. setFetchGroup(group);

Common Relationship Configurations	Common Relationship Configurations	Collection Mappings
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