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What are the basic hibernate persistent annotations?


This page talks about Hibernate mapping with JPA (Java Persistence Annotations). JPA entities are plain POJOs. Actually, they are Hibernate persistent entities. Their mappings are defined through JDK 5.0 annotations instead of hbm.xml files. JPA annotations are in the javax.persistence.* package.

@Entity - Marking a POJO as persistent entity

Every persistent POJO class is an entity and is declared using the @Entity annotation at the class level as shown below:

@Entity example code
import javax.persistence.Entity;

@Entity
public class Employee implements Serializable {

	@Id
	private Long empId;

@Entity declares the class as an entity i.e. a persistent POJO class.

@Id - Marking primary key of an entity

@Id declares the identifier property of the given entity. The mapped column for the primary key of the entity is assumed to be the primary key of the primary table.

@Table - Defining the table

@Table is set at the class level; it allows you to define the table, catalog, and schema names for your entity mapping. If no @Table is defined the default values are used: the unqualified class name of the entity.

@Entity
@Table(name="EMPLOYEES")
public class Employee implements Serializable {
...
}

The @Table element contains a schema and catalog attributes, if they need to be defined. You can also define unique constraints to the table using the @UniqueConstraint annotation in conjunction with @Table for a unique constraint bound to a single column, it is recommended to use the @Column.unique approach.

@Entity
@Table(name="EMPLOYEES",
    uniqueConstraints = {@UniqueConstraint(columnNames={"email", "emp_code"})})
public class Employee implements Serializable {
...
}

@Column - Declaring column attributes

The column(s) used for a property mapping can be defined using the @Column annotation. You can use this annotation at the property level.

@Entity
@Table(name="EMPLOYEES")
public class Employee implements Serializable {

	@Id
	@Column(name="EMP_ID")
	private Long empId;

	private String name;

	private String department;

	private Long salary;

	@Column(name="JOINED_ON")
	private Date joinedOn;

    ...
}

In the above example, EMP_ID table column will be mapped to empId property. Incase if the table column names are exactly matching with property names, you dont need to specify @Column annotation. By default hibernate maps them.

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Hibernate Examples

  1. Hibernate hello world (initial setup) example.
  2. What is hibernate.cfg.xml configuration?
  3. What are the basic hibernate persistent annotations?
  4. What is SessionFactory in Hibernate?
  5. What is Session object in Hibernate?
  6. List Hibernate Session interface methods.
  7. What is Hibernate Query object?
  8. Basic Hibernate CRUD operations example.
  9. Hibernate Bidirectional One-to-One mapping using @OneToOne annotation.
  10. Hibernate Unidirectional One-to-One mapping using @OneToOne annotation.
  11. Hibernate Eager vs Lazy Fetch Type
  12. Hibernate Unidirectional One-to-Many mapping using @OneToMany annotation.
  13. Hibernate Bidirectional One-to-Many mapping using @OneToMany annotation.
  14. Hibernate Many-to-Many mapping example using @ManyToMany annotation.
  15. How to enable logging (log4j) in Hibernate?
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String Vs StringBuffer
We know that String is immutable object. We can not change the value of a String object once it is initiated. If we try to change the value of the existing String object then it creates new object rather than changing the value of the existing object. So incase, we are going to do more modificatios on String, then use StringBuffer. StringBuffer updates the existing objects value, rather creating new object.
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About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java™ Platform Standard Ed. 8 - API Specification | Java is registered trademark of Oracle.
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