hibernate实例简析-hibernate_lazy_for_collection

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
===
<hibernate-mapping package="com.bjsxt.hibernate">
<class name="Classes" table="t_classes">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students" inverse="true" cascade="all" lazy="extra">
<key column="classesid"/>
<one-to-many class="Student"/>
</set>
</class>
</hibernate-mapping>

===
<hibernate-mapping>
<class name="com.bjsxt.hibernate.Student" table="t_student">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<many-to-one name="classes" column="classesid"/>
</class>
</hibernate-mapping>

===
/**
* 保持lazy默认**/
//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());
}
---
//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());
===
/**
* 设置<class>标签上的lazy=false**/
//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());
}
===
/**
* 设置集合上的lazy=false,其它默认**/
//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());
}
---
//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());
===
/**
* 设置集合上的lazy=extra,其它默认**/
//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());
}
---
//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());
===
Classes classes = new Classes();
classes.setName("尚学堂");
session.save(classes);

Student student1 = new Student();
student1.setName("10");
student1.setClasses(classes);
session.save(student1);

Student student2 = new Student();
student2.setName("祖儿");
student2.setClasses(classes);
session.save(student2);