Detachment Triggered Lazy Loading : Lazy Eager « JPA « Java Tutorial

1. Java Tutorial
2. JPA
3. Lazy Eager

File: Main.java

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

public class Main {
  static EntityManagerFactory emf = Persistence.createEntityManagerFactory("JPAService");

  static EntityManager em = emf.createEntityManager();

  public static void main(String[] a) throws Exception {
    em.getTransaction().begin();

    Student emp1 = new Student();
    emp1.setId(1);

    em.persist(emp1);

    Query query = em.createQuery("SELECT e FROM Student e");
    List<Student> list = (List<Student>) query.getResultList();
    System.out.println(list);

    List<Student> emps = em.createQuery("SELECT e FROM Student e").getResultList();
    for (Student emp : emps) {
      if (emp.getDepartment() != null) {
        emp.getDepartment().getName();
      }
    }

    em.getTransaction().commit();
    em.close();
    emf.close();

    Helper.checkData();
  }
}

File: Student.java

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Student {
    @Id
    private int id;
    private String name;
    private long salary;
    
    @ManyToOne(fetch=FetchType.LAZY)
    private Department department;
    

    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }


    public long getSalary() {
        return salary;
    }

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department dept) {
        this.department = dept;
    }
    
    public String toString() {
        return "Student " + getId() + 
               ": name: " + getName() +
               ", salary: " + getSalary();
    }
}

File: Department.java

import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Department {
    @Id
    private int id;
    private String name;
    @OneToMany(mappedBy="department")
    private Collection<Student> students;

    public Department() {
        students = new ArrayList<Student>();
    }
    
    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    
    public Collection<Student> getStudents() {
        return students;
    }

    public String toString() {
        return "Department no: " + getId() + 
               ", name: " + getName();
    }
}

File: Helper.java

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;

public class Helper {
  public static void checkData() throws Exception {
    Class.forName("org.hsqldb.jdbcDriver");
    Connection conn = DriverManager.getConnection("jdbc:hsqldb:data/tutorial", "sa", "");
    Statement st = conn.createStatement();

    ResultSet mrs = conn.getMetaData().getTables(null, null, null, new String[] { "TABLE" });
    while (mrs.next()) {
      String tableName = mrs.getString(3);
      System.out.println("\n\n\n\nTable Name: "+ tableName);

      ResultSet rs = st.executeQuery("select * from " + tableName);
      ResultSetMetaData metadata = rs.getMetaData();
      while (rs.next()) {
        System.out.println(" Row:");
        for (int i = 0; i < metadata.getColumnCount(); i++) {
          System.out.println("    Column Name: "+ metadata.getColumnLabel(i + 1)+ ",  ");
          System.out.println("    Column Type: "+ metadata.getColumnTypeName(i + 1)+ ":  ");
          Object value = rs.getObject(i + 1);
          System.out.println("    Column Value: "+value+"\n");
        }
      }
    }
  }
}

File: persistence.xml

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence" version="1.0">
  <persistence-unit name="JPAService" transaction-type="RESOURCE_LOCAL">
    <properties>
      <property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/>
      <property name="hibernate.hbm2ddl.auto" value="update"/>
      <property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver"/>
      <property name="hibernate.connection.username" value="sa"/>
      <property name="hibernate.connection.password" value=""/>
      <property name="hibernate.connection.url" value="jdbc:hsqldb:data/tutorial"/>
    </properties>
  </persistence-unit>
</persistence>

22.13.Lazy Eager
	22.13.1.	Detachment Triggered Lazy Loading
	22.13.2.	Detachment With Eager Loading