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JPA Performance settings Explained

Lets consider this simple Entity, Ignore @SecondaryTables and other annotations if you are not familiar with just focus on @Basic, @OneToOne, @OneToMany annotations and get ready to answer these questions.

@Entity
@SecondaryTables({
    @SecondaryTable(name="Employee_Comments")
})
public class Employee implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    private Long id;
    @Column(name = "name")
    private String name;
    @Basic(fetch = FetchType.LAZY)
    @Column(name = "comments", table = "Employee_Comments")
    private String comments;
    @OneToMany
    private List tags = new ArrayList();

    @OneToOne(fetch=FetchType.LAZY)
    private Address address;
..
}

What happens when we load the above entity using em.find or by executing some select query ?

  1.   "comments" is Eagerly loaded
  2.   "comments" is Lazily loaded
  3.   "address" is Eagerly loaded
  4.   "address is Lazily loaded
  5.   "tags" is Eagerly loaded
  6.   "tags" is Lazily loaded
You might have guessed 2, 4 and 6 or 1, 3, 6 though neither of these are correct.

Since

@Basic(fetch = FetchType.LAZY)
@OneToOne(fetch=FetchType.LAZY)
@OneToMany(fetch=FetchType.LAZY)


 Are just "hints" to the JPA provider and its not mandatory for JPA provider to implement them, So their is no quarantee that these objects when loaded will be lazily loaded.

Conclusion : Entities heavily using above annotations and also if we load and them in bulk could seriously cause performance bottlenecks, however there are nice ways of loading entities with trim data using "Contructor" Queries. which will be topic of my next thread.

Reference JPA Specs.

http://jcp.org/aboutJava/communityprocess/final/jsr317/index.html
JPA 2.0 Specification document Page 364

"FetchType fetch (Optional) Whether the value of the field or property
should be lazily loaded or must be eagerly fetched. The
EAGER strategy is a requirement on the persistence provider
runtime that the value must be eagerly fetched. The
LAZY strategy is a hint to the persistence provider runtime.
EAGER "

@OneToOne(fetch=FetchType.LAZY)
JPA 2.0 Specification document Page 402
Also have the same comment 


@OneToMany(fetch=FetchType.LAZY)
JPA 2.0 Specification document Page 400Also have the same comment 

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