hibernate>lazy

hibernate lazy策略

hibernate lazy策略可以使用在：

 *<class>标签上，可以取值：true/false

 <property>标签上，可以取值：true/false，指定实例变量第一次被访问时，这个属性是否延迟抓取，需要运行时字节码增强。

* <set><list>标签上，可以取值：true/false/extra

 *<one-to-one><many-to-one>单端关联上，可以取值：false/proxy/noproxy

 
lazy概念：只有真正使用该对象时，才会创建，对于hibernate而言，正真使用的时候才会发出sql

hibernate支持lazy策略只有在session打开状态下有效

<class>标签上的lazy特性只对普通属性起作用,不影响list，set类型

一、class上的lazy

1、设置<class>标签上的lazy=true，也就是默认配置

package com.ncepu.hibernate;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 运行本单元测试的条件：
 * 	
 * 设置<class>标签上的lazy=true，也就是默认配置
 *  
 * @author Administrator
 *
 */
public class ClassLazyTest extends TestCase {

	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//不会发出sql
			Group group = (Group)session.load(Group.class, 1);
			
			//不会发出sql
			System.out.println("group.id=" + group.getId());
			
			//会发出sql
			System.out.println("group.name=" + group.getName());
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	

	public void testLoad2() {
		Session session = null;
		Group group = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			group = (Group)session.load(Group.class, 1);
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
		
		//不能正确输出，抛出LazyInitializationException 异常，因为session已经关闭
		//hibernate支持lazy策略只有在session打开状态下有效
		System.out.println("group.name=" + group.getName());
	}	
	
}

2、设置<class>标签上的lazy=false

package com.ncepu.hibernate;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import junit.framework.TestCase;

import org.hibernate.Session;

/**
 * 设置<class>标签上的lazy=false
 * @author Administrator
 *
 */
public class CellectionlazyTest2 extends TestCase {


	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//会发出sql
			Classes classes = (Classes)session.load(Classes.class, 1);
			
			//不会发出sql
			System.out.println("classes.name=" + classes.getName());
			
			//不会发出sql
			Set students = classes.getStudents();
			
			//会发出sql
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println("student.name=" + student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
}

二、集合上的Lazy

hibernate在集合上的lazy策略，可以取值：true/false/extra

设置成extra，比如查询size时，会发比较智能的sql语句，利用count，而不是全部查出来。

<class>标签上的lazy不会影响到集合上的lazy特性，

1、默认为true时

package com.ncepu.hibernate;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import junit.framework.TestCase;

import org.hibernate.Session;

/**
 * 保持lazy默认
 * @author Administrator
 *
 */
public class CellectionlazyTest1 extends TestCase {


	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//不会发出sql
			Classes classes = (Classes)session.load(Classes.class, 1);
			
			//会发出sql
			System.out.println("classes.name=" + classes.getName());
			
			//不会发出sql
			Set students = classes.getStudents();
			
			//会发出sql
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println("student.name=" + student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	public void testLoad2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//不会发出sql
			Classes classes = (Classes)session.load(Classes.class, 1);
			
			//会发出sql
			System.out.println("classes.name=" + classes.getName());
			
			//不会发出sql
			Set students = classes.getStudents();
			
			//会发出sql,发出查询全部数据的sql
			System.out.println("student.count=" + students.size());
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
}

2、集合上lazy为false时

package com.ncepu.hibernate;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import junit.framework.TestCase;

import org.hibernate.Session;

/**
 * 设置集合上的lazy=false,其它默认
 * @author Administrator
 *
 */
public class CellectionlazyTest3 extends TestCase {


	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//不会发出sql
			Classes classes = (Classes)session.load(Classes.class, 1);
			
			//会发出sql,会发出两条sql分别加载Classes和Student
			System.out.println("classes.name=" + classes.getName());
			
			//不会发出sql
			Set students = classes.getStudents();
			
			//不会发出sql
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println("student.name=" + student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	public void testLoad2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//不会发出sql
			Classes classes = (Classes)session.load(Classes.class, 1);
			
			//会发出sql,会发出两条sql分别加载Classes和Student
			System.out.println("classes.name=" + classes.getName());
			
			//不会发出sql
			Set students = classes.getStudents();
			
			//不会发出sql
			System.out.println("student.count=" + students.size());
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
}

3、集合上lazy为extra时

package com.ncepu.hibernate;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import junit.framework.TestCase;

import org.hibernate.Session;

/**
 * 设置<class>标签上的lazy=false
 * @author Administrator
 *
 */
public class CellectionlazyTest2 extends TestCase {


 public void testLoad1() {
  Session session = null;
  try {
   session = HibernateUtils.getSession();
   session.beginTransaction();
   
   //会发出sql
   Classes classes = (Classes)session.load(Classes.class, 1);
   
   //不会发出sql
   System.out.println("classes.name=" + classes.getName());
   
   //不会发出sql
   Set students = classes.getStudents();
   
   //会发出sql
   for (Iterator iter=students.iterator(); iter.hasNext();) {
    Student student = (Student)iter.next();
    System.out.println("student.name=" + student.getName());
   }
   session.getTransaction().commit();
  }catch(Exception e) {
   e.printStackTrace();
   session.getTransaction().rollback();
  }finally {
   HibernateUtils.closeSession(session);
  }
 } 
}

三、单端关联上lazy

package com.ncepu.hibernate;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 将<many-to-one>中的lazy设置为false,其它默认
 * @author Administrator
 *
 */
public class SingleEndTest2 extends TestCase {
	
	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//不会发出sql
			User user = (User)session.load(User.class, 1);
			
			//会发出sql，发出两条sql分别加载User和Group
			System.out.println("user.name=" + user.getName());
			
			//不会发出sql
			Group group = user.getGroup();
			
			//不会发出sql
			System.out.println("group.name=" + group.getName());
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
}