Hibernate迫切连接和普通连接的区别实例详解

Hibernate 迫切连接和普通连接的区别

相关的介绍和解释在代码中已注释，大家可以参考。

package com.baidu.test;
import java.util.ArrayList;
import java.util.linkedHashSet;
import java.util.List;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import com.baidu.leftJoin.Department;
import com.baidu.leftJoin.Employee;
public class TestHQL_LeftJoin {
	private SessionFactory sessionFactory;
	private Session session;
	private Transaction transaction;
	@Before
	  public void init(){
		Configuration configuration = new Configuration().configure();
		ServiceRegistry serviceRegistry = new ServiceRegistryBuilder()
		      .applySettings(configuration.getProperties())
		      .buildServiceRegistry();
		sessionFactory = configuration.buildSessionFactory(serviceRegistry);
		session = sessionFactory.openSession();
		transaction = session.beginTransaction();
	}
	@After
	  public void destroy(){
		transaction.commit();
		session.close();
		sessionFactory.close();
	}
	// ~~~~~~~~~~~~~~~~~~~~~~~~~~下面的例子是 从 1 对 多  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	
	@Test
	  public void testLeftJoinFetch(){
		//    String hql = "SELECt DISTINCT d FROM Department d LEFT JOIN FETCH d.emps ";
		//    Query query = session.createQuery(hql);
		//    
		//    List depts = query.list();
		//    System.out.println(depts.size());
		//    
		String hql = "FROM Department d LEFT JOIN FETCH d.emps ";
		Query query = session.createQuery(hql);
		List depts = query.list();
		System.out.println(depts.size());
		depts = new ArrayList<>(new linkedHashSet(depts));
		System.out.println(depts.size());
		for (Department dept:depts){
			System.out.println(dept.getName() + "--" + dept.getEmps().size() );
		}
	}
	
	@Test
	  public void testLeftJoin(){
		String hql = "SELECt DISTINCT d FROM Department d LEFT JOIN d.emps";
		Query query = session.createQuery(hql);
		List depts = query.list();
		System.out.println(depts.size());
		for (Department dept:depts){
			System.out.println(dept.getName() + dept.getEmps().size());
		}
	}
	
	@Test
	  public void testInnerJoinFetch(){
		//String hql = "SELECt DISTINCT d FROM Department d LEFT JOIN FETCH d.emps ";
		String hql = "FROM Department d INNER JOIN FETCH d.emps ";
		Query query = session.createQuery(hql);
		List depts = query.list();
		depts = new ArrayList<>(new linkedHashSet(depts));
		System.out.println(depts.size());
		for (Department dept:depts){
			System.out.println(dept.getName() + "--" + dept.getEmps().size() );
		}
	}
	// ~~~~~~~~~~~~~~~~~~~~~~~~~~下面的例子是 从多 对 1  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	@Test
	  public void testLeftJoinFetch2(){
		String hql = "FROM Employee e LEFT JOIN FETCH e.dept";
		Query query = session.createQuery(hql);
		List emps = query.list();
		System.out.println(emps.size());
		for (Employee emp:emps){
			System.out.println(emp + " -- " + emp.getDept());
		}
	}
}

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