Hibernate HQL语言查询实体对象实例。
hql实体对象查询：
N + 1问题，在默认情况下，使用query.iterate查询，有可以能出现N+1问题，所谓的N+1是在查询的时候发出了N+1条sql语句：
1：首先发出一条查询对象id列表的sql
N：根据id列表到缓存中查询，如果缓存中不存在与之匹配的数据，那么会根据id发出相应的sql语句
² list和iterate的区别？
list每次都会发出sql语句，list会向缓存中放入数据，而不利用缓存中的数据。
iterate：在默认情况下iterate利用缓存数据，但如果缓存中不存在数据有可以能出现N+1问题。
hql语言实体对象查询例子1代码如下：
/**
* 实体对象查询
*/
public class SimpleObjectQueryTest1 extends TestCase {
public void testQuery1() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//返回Student对象的集合
//可以忽略select
List students = session.createQuery
(“from Student”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testQuery2() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//返回Student对象的集合
//可以忽略select,表可以使用别名
List students = session.createQuery
(“from Student s”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testQuery3() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//返回Student对象的集合
//可以忽略select,表可以使用as命名别名
List students = session.createQuery
(“from Student as s”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testQuery4() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//返回Student对象的集合
//使用select查询实体对象，必须采用别名
List students = session.createQuery
(“select s from Student as s”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testQuery5() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//不支持select * from …..这样的查询语句
List students = session.createQuery
(“select * from Student”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
}
例子2代码如下：
/**
* 实体对象查询
*/
public class SimpleObjectQueryTest2 extends TestCase {
public void testQuery1() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
/**
* 采用list查询发出一条查询语句，取得Student对象数据、
*
* Hibernate: select student0_.id as id1_,
student0_.name as name1_,
* student0_.createTime as createTime1_,
student0_.classesid as classesid1_
* from t_student student0_
*
*/
List students = session.createQuery
(“from Student”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testQuery2() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
/**
* 出现N+1问题
* 1：发出查询id列表的sql
* Hibernate: select student0_.id as col_0_0_ from
t_student student0_
*
* N：在依次发出根据id查询Student对象的sql
* Hibernate: select student0_.id as id1_0_,
student0_.name as name1_0_,
* student0_.createTime as createTime1_0_,
student0_.classesid as classesid1_0_
* from t_student student0_ where student0_.id=?
*/
Iterator iter = session.createQuery
(“from Student”).iterate();
while(iter.hasNext()) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testQuery3() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
List students = session.createQuery
(“from Student”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
System.out.println(“———————-”);
/**
* 不会出现N+1问题
* 因为list操作已经将Student对象放到了一级缓存中，所以再次使用
iterate操作的时候
* 它首先发出一条查询id列表的sql，在根据id到缓存中去数据，只有
在缓存中找不到相应的
* 数据时，才会发出sql到数据库中查询
*/
Iterator iter = session.createQuery
(“from Student”).iterate();
while(iter.hasNext()) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testQuery4() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
List students = session.createQuery
(“from Student”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
System.out.println(“——————-”);
/**
* 再次发出查询sql
* 在默认情况下list每次都会向数据库发出查询对象的sql，除非配置
查询缓存，所以下面的list操作
* 虽然在一级缓存中已经有了对象数据，但list默认情况下不会利用缓
存，而再次发出sql
* 默认情况下，list会向缓存中放入数据，但不会利用数据
*/
students = session.createQuery(“from Student”).list();
for (Iterator iter=students.iterator();
iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
}