JPA

JPA

Lazy loading #

The default is to load all basic mappings eagerly

Example:

 @Basic(fetch=FetchType.LAZY)
 @Column(name="COMM")
 private String comments;

Large Objects #

Annotation: @Lob

Two types:

  • CLOB - character large objects (char[], Character[], and String)
  • BLOB - binary large objects (byte[], Byte[] and Serializable types)

Example:

@Entity
public class Employee {
 @Id
 private int id;
 @Basic(fetch=FetchType.LAZY)
 @Lob @Column(name="PIC")
 private byte[] picture;
 // ...
}

Enumerated Types #

By default Enumerated types are represented as number - an index of value in the enum class. It is possible to change this behaviour by adding @Enumerated annotation with value:

  • EnumType.ORDINAL - enum represented as enum value index
  • EnumType.STRING - enum represented as String

Example:

@Entity
public class Employee {
 @Id
 private int id;
 @Enumerated(EnumType.STRING)
 private EmployeeType type;
 // ...
}

Temporal Types #

Temporal types are the set of time-based types that can be used in persistent state mappings. Supported formats:

  • java.sql.Date
  • java.sql.Time
  • java.sql.Timestamp

Supported formats that needs extra annotation explaination:

  • java.util.Date
  • java.util.Calendar

An annotation @Temporal needs to explain what specific kind of value it stores:

  • TemporalType.DATE
  • TemporalType.TIME

Transient State #

Attributes that are part of a persistent entity but not intended to be persistent can either be modified with the transient modifier in Java or be annotated with the @Transient annotation.

Relations #

OneToOne #

Unidirectional #

The owning side is definied by @OneToOne annotation.

Employee owning side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;

    @OneToOne
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;

}

JEE

Department owning side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;
    @OneToOne
    Employee employee;

}

JEE

Bidirectional #

The inverse side is definied with mappedBy parameter in @OneToOne annotation.

No owning side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;

    @OneToOne
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;

    @OneToOne
    Employee employee;
}

JEE

Employee owning side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;

    @OneToOne
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;

    @OneToOne(mappedBy = "department")
    Employee employee;
}

JEE

Department owning side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;

    @OneToOne(mappedBy = "employee")
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;

    @OneToOne
    Employee employee;
}

JEE

ManyToOne and OneToMany #

The one-to-many association is almost always bidirectional and never the owning side. The only difference between the many-to-one and this one is that we are now implementing the inverse side of the relationship.

There are two important points to remember when defining bidirectional one-to-many (or many-to-one) relationships:

  • The many-to-one side is the owning side, so the join column is defined on that side.
  • The one-to-many mapping is the inverse side, so the mappedBy element must be used.

Failing to specify the mappedBy element in the @OneToMany annotation will cause the provider to treat it as a unidirectional one-to-many relationship that is defined to use a join table (described later). This is an easy mistake to make and should be the first thing you look for if you see a missing table error with a name that has two entity names concatenated together.

Unidirectional #

Employee owning side with annotation on Employee side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;
    @ManyToOne
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;

}

JEE

Employee owning side with annotation on Department side (wrong) #

@Entity
public class Employee {

    @Id
    Long id;
    String name;
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;
    @OneToMany
    List<Employee> employee;

}

JEE

Bidirectional #

Employee owning side without mappingBy attribute (wrong) #

@Entity
public class Employee {

    @Id
    Long id;
    String name;
    @ManyToOne
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;
    @OneToMany
    List<Employee> employee;

}

JEE

Employee owning side with mappingBy attribute #

@Entity
public class Employee {

    @Id
    Long id;
    String name;
    @ManyToOne
    Department department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;
    @OneToMany(mappedBy = "department")
    List<Employee> employee;

}

JEE

ManyToMany #

Bidirectional #

Employee owning side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;

    @ManyToMany
    List<Department> department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;

    @ManyToMany(mappedBy = "department")
    List<Employee> employee;
}

JEE

Department owning side #

@Entity
public class Employee {

    @Id
    Long id;
    String name;

    @ManyToMany
    List<Department> department;

}

@Entity
public class Department {

    @Id
    Long id;
    String name;

    @ManyToMany(mappedBy = "department")
    List<Employee> employee;
}

JEE

Collection Mapping #

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