hibernate 集合上的lazy策略

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

<class>标签上的lazy不会影响到集合上的lazy特性

建议将集合上的lazy设置成extra

1.实体类

Java代码

package com.yx.zzg.bo;
import java.util.Set;
public class Classes {
private int id;
private String name;
private Set students;
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 Set getStudents() {
return students;
}
public void setStudents(Set students) {
this.students = students;
}
}

Java代码

package com.yx.zzg.bo;
public class Student {
private int id;
private String name;
private Classes classes;
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 Classes getClasses() {
return classes;
}
public void setClasses(Classes classes) {
this.classes = classes;
}
}

2.实体类映射文件

Java代码

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="com.yx.zzg.bo.Student" table="t_student">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<many-to-one name="classes" column="classesid"/>
</class>
</hibernate-mapping>

Java代码

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.yx.zzg.bo">
<class name="Classes" table="t_classes">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students" inverse="true" cascade="all">
<key column="classesid"/>
<one-to-many class="Student"/>
</set>
</class>
</hibernate-mapping>

3.测试类1：CellectionlazyTest1.java

Java代码

package com.yx.zzg.test;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import com.yx.zzg.bo.*;
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);
}
}
}

测试类2：CellectionlazyTest2.java

Java代码

package com.yx.zzg.test;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import com.yx.zzg.bo.*;
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);
}
}
}

测试类3：CellectionlazyTest3.java

Java代码

package com.yx.zzg.bo;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import com.yx.zzg.bo.*
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);
}
}
}

测试类4：CellectionlazyTest4.java

Java代码

package com.yx.zzg.test;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import com.yx.zzg.bo.*;
import junit.framework.TestCase;
import org.hibernate.Session;
/**
* 设置集合上的lazy=extra,其它默认
* @author Administrator
*
*/
public class CellectionlazyTest4 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,发出一条比较智能的sql
System.out.println("student.count=" + students.size());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
}

原文：http://skyuck.iteye.com/blog/484561 未验证。。