Hibernate JPA Annotations with Explanation

             In this post, we will learn about some of the most important Hibernate JPA annotations. The following annotations are commonly used in Hibernate applications.

For Spring annotations, refer to the Spring Annotations article. 


 1) @Entity

     Annotate all your entity beans with @Entity. This contains in the javax.persistence package. 

@Entity
public class Employee implements Serializable {
  //properties with setters and getters
}

2) @Table
      The @Table annotation allows you to specify the details of the table that will be used to persist the entity in the database.

       The @Table annotation provides four attributes, allowing you to override the name of the table,  its catalog,  and its schema, and enforce unique constraints on columns in the table.

@Entity
@Table(name = "employee")
public class Employee implements Serializable {
     //properties with setters and getters
}

3) @Column

        The @Column annotation is used to specify the details of the column to which a field or property will be mapped.

      You can use column annotation with the following most commonly used attributes,

• name - attribute permits the name of the column to be explicitly specified.
• length - attribute permits the size of the column used to map a value particularly for a String value
• nullable - attribute permits the column to be marked NOT NULL when the schema is generated
• unique - attribute permits the column to be marked as containing only unique values.

@Entity
@Table(name = "employee")
public class Employee implements Serializable {
 
     @Column(name = "emp_name")
     private String name;
   
...
}

4) @Id

             This annotation specifies that a field is mapped to a primary key column in the table.
Since the column emp_id is a primary key,  we have to use this annotation as follows,

@Entity
@Table(name = "employee")
public class Employee implements Serializable {
 
     @Id
     @Column(name = "emp_id")
     private int empId;
   
...
}

5) @GeneratedValue

           If the values of the primary column are auto-increment, we need to use this annotation to tell Hibernate knows, along with one of the following strategy types: AUTO, IDENTITY, SEQUENCE, and TABLE. 
           In below example, strategy AUTO implies that the generated values are unique at database level.

@Entity
@Table(name = "employee")
public class Employee implements Serializable {
 
     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     @Column(name = "emp_id", updatable = false, nullable = false)
   
...
}

6)  @OrderBy


   The @OrderBy orders the column values and put into the list as ordered list data. By Default @OrderBy orders the element in ascending order. We need to define property name on the basis of which values will be ordered.

@Entity
@Table(name = "employee")
public class Employee implements Serializable {
 
       @OneToMany(cascade=CascadeType.ALL)
       @JoinColumn(name="address_id")
       @OrderBy("name")
       private List<Address> address;
   
...
}

Hibernate Association Mapping Annotations

7)  @OneToOne

      The @OneToOne annotation is using for mapping between two tables that should be One To One mapping. 
      The below example illustrates the OneToOne Mapping between Employee and Department Entity.

Employee.java

@Entity
@Table(name = "employee")
public class Employee implements Serializable {
   
     @Id
     @Column(name = "emp_id")
     @GeneratedValue
     private int id;
   
     @Column(name="emp_name")
     private String name;

     @OneToOne(cascade=CascadeType.ALL)  
     @JoinColumn(name="dept_id")
     private Department department;
   
     // setters and getters
}
 

Department.java

@Entity
@Table(name = "department")
public class Department implements Serializable {
   
     @Id
     @Column(name = "dept_id")
     @GeneratedValue
     private int id;
   
     @Column(name="dept_name")
     private String name;

     @OneToOne(mappedBy="department")  
     private Employee employee;
   
     // setters and getters
}
 

8) @ManyToOne

    The below example illustrates the  ManyToOne Mapping between Employee and Address Entity.  One Employee can have multiple addresses.

Address.java

@Entity
@Table(name = "address")
public class Address implements Serializable {
   
     @Id
     @Column(name = "address_id")
     @GeneratedValue
     private int id;
   
     @Column(name="add_type")
     private String type;

     @ManyToOne
     @JoinColumn(name="emp_id") 
     private Employee employee;
   
     // setters and getters
}
 

9) @OneToMany

     It's opposite to @ManyToOne. The previous example, One employee can have multiple address so mapping should be OneToMany as below.

Employee.java

@Entity
@Table(name = "employee")
public class Employee implements Serializable {
   
     @Id
     @Column(name = "emp_id")
     @GeneratedValue
     private int id;
   
     @Column(name="emp_name")
     private String name;

     @OneToMany(mappedBy="employee") 
     private Set<Address> address;
   
     // setters and getters
}
 

10) @ManyToMany

It's example of Student and Department association.  Many students mapped with many departments using std_id and dept_id.

Student.java

@Entity
@Table(name = "student")
public class Student {
    
     @Id 
     @GeneratedValue
          (strategy = GenerationType.IDENTITY)
     @Column(name="std_id")
     private int id;

     @Column(name"std_name")
     private String name;

     @ManyToMany 
     @JoinTable(name="student_dept",  
      joinColumns = @JoinColumn(name="std_id"),
      inverseJoinColumns = @JoinColumn(name="dept_id"))  
     private Collection<Department> departments;

    // Setters and getters
    ....
}

Department.java

@Entity
@Table (name="student_dept")
public class Department {
     
     @Id 
     @GeneratedValue (strategy=GenerationType.IDENTITY)
     @Column(name="dept_id")
     private int id;
     
     @Column(name="dept_name")
     private String name;

     @ManyToMany(mappedBy="departments")
     private Collection<Student> students;

     // Setters and getters
     ...
}

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3) Spring MVC with Hibernate CRUD Example