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  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	/**
   *
   * 迫切左外连接: 特点是：如果左表有不满足条件的，也返回左表不满足条件
   *    1. left join fetch 关键字表示迫切左外连接检索策略.
   *    2. list() 方法返回的集合中存放实体对象的引用, 每个 department 对象关联的 employee 集合都被初始化,
   *       存放所有关联的 employee 的实体对象.
   *    3. 查询结果中可能会包含重复元素, 可以通过一个 hashset 来过滤重复元素
   *
   *     去重：
   *       方法一:使用 distinct
   *         string hql = "select distinct d from department d left join fetch d.emps ";
   *        query query = session.createquery(hql);
   *
   *        list<department> depts = query.list();
   *        system.out.println(depts.size());
   *       
   *       方法二
   *         string hql = "from department d left join fetch d.emps ";
   *        query query = session.createquery(hql);
   *
   *        list<department> 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() );
   *        }
   *
   *
   */
	@test
	  public void testleftjoinfetch(){
		//    string hql = "select distinct d from department d left join fetch d.emps ";
		//    query query = session.createquery(hql);
		//    
		//    list<department> depts = query.list();
		//    system.out.println(depts.size());
		//    
		string hql = "from department d left join fetch d.emps ";
		query query = session.createquery(hql);
		list<department> 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() );
		}
	}
	/**
   * 左外连接:
   *    1. left join 关键字表示左外连接查询.
   *    2. list() 方法返回的集合中存放的是对象数组类型
   *    3. 根据配置文件来决定 employee 集合的检索策略.
   *    4. 如果希望 list() 方法返回的集合中仅包含 department 对象,
   *      可以在hql 查询语句中使用 select 关键字
   *    
   *    这样的语句查询的结果有重复：
   *      string hql = "from department d left join d.emps";
   *      query query = session.createquery(hql);
   *    
   *      list<object[]> results = query.list();
   *      system.out.println(results.size());
   *  
   *     去重：
   *       仅能使用 distinct 的方法去除重复
   *   
   *       string hql = "select distinct d from department d left join d.emps";
   *       query query = session.createquery(hql);
   *
   *       list<department> depts = query.list();
   *       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<department> depts = query.list();
		system.out.println(depts.size());
		for (department dept:depts){
			system.out.println(dept.getname() + dept.getemps().size());
		}
	}
	/**
   * 迫切内连接: 特点是：不返回左表不满足条件
   *    inner join fetch 关键字表示迫切内连接, 也可以省略 inner 关键字
   *    list() 方法返回的集合中存放 department 对象的引用, 每个 department
   *        对象的 employee 集合都被初始化, 存放所有关联的 employee 对象
   *
   * 内连接:
   *    inner join 关键字表示内连接, 也可以省略 inner 关键字
   *    list() 方法的集合中存放的每个元素对应查询结果的一条记录, 每个元素都是对象数组类型
   *    如果希望 list() 方法的返回的集合仅包含 department 对象, 可以在 hql 查询语句中使用 select 关键字
   *
   *
   *
   */
	@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<department> 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<employee> emps = query.list();
		system.out.println(emps.size());
		for (employee emp:emps){
			system.out.println(emp + " -- " + emp.getdept());
		}
	}
}

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