Retrieving or finding an Entity using JPA

Retrieving or Finding an Entity

• Once an entity in the database, Student student = entintyManager.find(Student.class, 1L); this line will retrieve student entity where identifier value is 1. This is equal to query SELECT * FROM STUDENT WHERE ID=1;
• what happens if the object has been removed or we supplied wrong id by accident? then the results will be simply null.
• It’s just one line, but what we need to understand really is, Persistence transition  or State of Student entity.

Note:
To understand JPA entity life cycle completely read JPA Entity lifecycle

Persistence transition or state change when EntityManager’s find() called

• On successful call entintyManager.find(Student.class, 1L); student entity state become persistent in the persistence context.
• Persistent state instances are always associated with persistence-context and transnational.
• In this state if you change any value of student object using student reference, it will be reflected in persistence context.

Technologies Used in example :

• JPA 2.1
• Hibernate 5.2.6
• MySql 8.0
• Maven 3
• Spring Tool Suite (STS) 3.9.8
• Java 1.8

Mapping Studen.java class

@Entity(name="STUDENT")
public class Student {
	
	@Id
	@GeneratedValue(strategy=GenerationType.AUTO, generator="native")
	@GenericGenerator(name = "native", strategy = "native")
	@Column(name = "ID")
	private Long studentId;
	
	@Column(name = "FNAME")
	private String firstName;
	
	@Column(name = "LNAME")
	private String lastName;
	
	@Column(name = "CONTACT_NO")
	private String contactNo;

       // setters and getters

Finding an Entity Code Example 1 :

public static void main( String[] args )
    {
        EntityManagerFactory emf = null;
        EntityManager entityManager = null;
        EntityTransaction transaction = null;
        
        try{
            emf = Persistence.createEntityManagerFactory("jbd-pu");
            entityManager = emf.createEntityManager();
            transaction = entityManager.getTransaction();
            transaction.begin();
            
            /* When ever EntityManager's find() calls the state of Student instace will be "Persitent" */
            Student student = entityManager.find(Student.class, 2L);
            System.err.println("Student First Name: "+student.getFirstName());
            
            transaction.commit();
        }catch(Exception e){
            transaction.rollback();
        }finally{
            entityManager.close();
            emf.close();
        }
    }

Output :

INFO - HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select student0_.ID as ID1_0_0_, student0_.CONTACT_NO as CONTACT_2_0_0_, student0_.FNAME as FNAME3_0_0_, student0_.LNAME as LNAME4_0_0_ from STUDENT student0_ where student0_.ID=?
Student First Name: Arnold
INFO - HHH000030: Cleaning up connection pool [jdbc:mysql://localhost:3306/jpa_JBD]

Finding an Entity Code Example 2 :

In the following example once you retrieve entity change the value of first name, you will notice that value will be reflected in persistence context.

public class App 
{
    public static void main( String[] args )
    {
    	EntityManagerFactory emf = null;
		EntityManager entityManager = null;
		EntityTransaction transaction = null;
		
		try{
			emf = Persistence.createEntityManagerFactory("jbd-pu");
			entityManager = emf.createEntityManager();
			transaction = entityManager.getTransaction();
			transaction.begin();
			
			/* When ever EntityManager's find() calls the state of Student instace will be "Persitent" */
			Student student = entityManager.find(Student.class, 2L);
			System.out.println("Student First Name: "+student.getFirstName());
			student.setFirstName("Peter"); // value reflects in persistence context
			
			student = entityManager.find(Student.class, 2L);
			System.out.println("Student First Name: "+student.getFirstName());
	
			transaction.commit();
		}catch(Exception e){
			transaction.rollback();
		}finally{
			entityManager.close();
			emf.close();
		}
    }
}

Output :

You will see like following output in console.

Hibernate: select student0_.ID as ID1_0_0_, student0_.CONTACT_NO as CONTACT_2_0_0_, student0_.FNAME as FNAME3_0_0_, student0_.LNAME as LNAME4_0_0_ from STUDENT student0_ where student0_.ID=?
Student First Name: Arnold
Student First Name: Peter
Hibernate: update STUDENT set CONTACT_NO=?, FNAME=?, LNAME=? where ID=?

Download Application – JPA-Entity-Retrieve.zip (10 KB)