Hibernate初识4——hibernate查询及优化策略
一、HQL查询
1、单表查询
1）全表查询
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("from User",User.class);
List list = query.list();
for(User user:list){
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 2）别名查询
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("select u from User u",User.class);
List list = query.list();
for(User user:list){
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 3）条件查询1
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("from User where id=6",User.class);
List list = query.list();
for(User user:list){
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 条件查询2
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("from User where id=?",User.class);
query.setParameter(0, 6L);
List list = query.list();
for(User user:list){
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 4）具名查询
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("from User where id=:id",User.class);
query.setParameter("id", 6L);
List list = query.list();
for(User user:list){
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 5）分页查询
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("from User",User.class);
query.setFirstResult(3);
query.setMaxResults(3);
List list = query.list();
for(User user:list){
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 6）查询排序
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("from User order by id asc",User.class);
List list = query.list();
for(User user:list){
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 7）聚合查询
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("select count(*) from User ",Long.class);
Long result = query.uniqueResult();
LOG.info(result);
bt.commit();
session.close();
HibernateUtil.close();
} 8）投影查询1
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("select name,id from User ",Object[].class);
List list = query.list();
for (Object[] objects : list) {
LOG.info(Arrays.toString(objects));
}
bt.commit();
session.close();
HibernateUtil.close();
} 投影查询2：注意创建对应的两个参数的构造方法
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("select new com.starli.pojo.User(id,name) from User ",User.class);
List list = query.list();
for (User user : list) {
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 2、多表查询
1）内连接查询： inner join
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("select u.name,r.name from User u inner join u.roles r",Object[].class);
List list = query.list();
for (Object[] o : list) {
LOG.info(Arrays.toString(o));
}
bt.commit();
session.close();
HibernateUtil.close();
} 2）左连接查询： left join
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("select u.name,r.name from User u left join u.roles r",Object[].class);
List list = query.list();
for (Object[] o : list) {
LOG.info(Arrays.toString(o));
}
bt.commit();
session.close();
HibernateUtil.close();
} 3）右连接查询：right join
@Test
public void query(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Query query = session.createQuery("select u.name,r.name from User u right join u.roles r",Object[].class);
List list = query.list();
for (Object[] o : list) {
LOG.info(Arrays.toString(o));
}
bt.commit();
session.close();
HibernateUtil.close();
} ps：内连接直接显示非null的，左连接以join左边的非null为主，右连接以右边的非null为主
二、QBC查询（使用Criteria）
1）全表查询
@Test
public void query2(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Criteria c = session.createCriteria(User.class);
List list = c.list();
for (User user : list) {
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 2）条件查询1
@Test
public void query2(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Criteria c = session.createCriteria(User.class);
c.add(Restrictions.eq("id", 1L));
User result = (User) c.uniqueResult();
LOG.info(result);
bt.commit();
session.close();
HibernateUtil.close();
}@Test
public void query2(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Criteria c = session.createCriteria(User.class);
c.add(Restrictions.and(Restrictions.eq("id", 1L),Restrictions.like("name", "%小%")));
User result = (User) c.uniqueResult();
LOG.info(result);
bt.commit();
session.close();
HibernateUtil.close();
} 3）分页查询
@Test
public void query2(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Criteria c = session.createCriteria(User.class);
c.add(Restrictions.like("name", "%小%"));
c.setFirstResult(0);
c.setMaxResults(3);
List list = c.list();
for (User user : list) {
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 4）查询排序
@Test
public void query2(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Criteria c = session.createCriteria(User.class);
c.addOrder(org.hibernate.criterion.Order.desc("id"));
List list = c.list();
for (User user : list) {
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 5）聚合查询
@Test
public void query2(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Criteria c = session.createCriteria(User.class);
c.setProjection(Projections.count("id"));
Long result = (Long) c.uniqueResult();
LOG.info(result);
bt.commit();
session.close();
HibernateUtil.close();
} 6）投影查询
@Test
public void query2(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
Criteria c = session.createCriteria(User.class);
ProjectionList list = Projections.projectionList();
list.add(Property.forName("id"));
list.add(Property.forName("name"));
c.setProjection(list);
List list2 = c.list();
for (Object[] objects : list2) {
LOG.info(Arrays.toString(objects));
}
bt.commit();
session.close();
HibernateUtil.close();
}
@Test
public void query3(){
Session session = HibernateUtil.openSession();
Transaction bt = session.beginTransaction();getClass();
NativeQuery query = session.createNativeQuery("select * from user",User.class);
List list = query.list();
for (User user : list) {
LOG.info(user);
}
bt.commit();
session.close();
HibernateUtil.close();
} 延迟加载是为了减少程序和数据库的访问次数，提供程序的执行性能。
延迟加载的执行机制：
1）在查询一个对象的时候，不会到数据库查询对象的属性或者其关联的数据
2）在需要使用到对象的属性或关联数据的才会去查询数据库！
1、类级别的延迟加载 @Test
public void test1(){
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
//get()：get方法不支持类级别的延迟加载
User cust = session.get(User.class, 1L);//debug：此行查询了数据库
LOG.info(333);
System.out.println(cust.getName());
tx.commit();
session.close();
}@Test
public void test1(){
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
//load()：load方法支持类级别的延迟加载
User cust = session.load(User.class, 1L);//debug：此行没有查询数据库
LOG.info(333);
System.out.println(cust.getName());//debug：此行查询了数据库
tx.commit();
session.close();
}load()：只有 load 方法才支持类级别的延迟加载
get()：get 方法不支持类级别的延迟加载
使用 load() 方法的默认延迟加载策略是延迟加载，可以在配置文件中修改延迟加载策略
@Test
public void test1(){
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
User cust = session.get(User.class, 1L);//debug：此行没有查询数据库
LOG.info(333);
System.out.println(cust.getRoles().size());//debug：此行查询了数据库
tx.commit();
session.close();
}<set name="orders" cascade="all" inverse="true" lazy="false">五、抓取策略
抓取策略，是为了改变 SQL 语句查询的方式，从而提高 SQL 语句查询的效率（优 化 SQL 语句）
可以设置以下三个值：fetch="select(默认值)|join|subselect"
1、查询一方单条记录在Customer.hbm.xml中配置 fetch="select"会执行两条sql
@Test
public void test1(){
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
User cust = session.get(User.class, 1L);
LOG.info(333);
System.out.println(cust.getRoles().size());
tx.commit();
session.close();
}
2、查询一方多条记录
需求：在查询多个一方（客户列表）的数据，关联查询多方（订单）的数据
如果fetch的配置是select或join的时候，一共发出n+1条sql语句
fetch="subselect"： 使用子查询进行关联查询
3、查询多方记录
fetch="select" : 默认情况，执行两条sql语句（支持延迟加载）
fetch="join"： 把两条sql合并成左外连接查询（效率更高）
注意：如果配置了join，那么延迟加载就会失效！
六、整合连接池 1、maven项目中添加依赖，导入 c3p0 的整合相关包
org.hibernate
hibernate-c3p0
5.2.10.Final
com.mchange
c3p0
0.9.5.2
org.hibernate.c3p0.internal.C3P0ConnectionProvider
10
20 @Test
public void test(){
Session session = HibernateUtil.openSession();
session.doWork(new Work(){
@Override
public void execute(Connection arg0) throws SQLException {
LOG.info(arg0);
}
});
session.close();
} Hibernate 的一级缓存：就是 Session 对象的缓存，而 Session 对象在每次操作之 后都会关闭，那么一级缓存就丢失！
结论 ：一级缓存只用于一次业务操作内的缓存。
Hibernate 的二级缓存：就是 SessionFactory 的缓存，二级缓存和 SessionFactory 对象的生命周期是一致的，SessionFactory 不消耗，那么二级缓存的数据就不会 丢失！
结论：二级缓存可以用于多次业务操作的缓存。
注意的问题：
1）Hibernate 一级缓存默认是开启的，而且无法关闭。
2）Hibernate 二级缓存默认是关闭的，如果使用需要开启，而且需要引入第三 方的缓存工具，例如 EhCache 等。
1. 添加二级缓存需要实现jar包
jar包位置: hibernate解压目录下 lib/optional/ehcache下找到相关包!
maven项目,pom文件添加
org.hibernate
hibernate-ehcache
5.2.10.Final
2. 配置二级缓存
在hibernate.cfg.xml中配置以下节点:property 节点 要放在 mapping 节点的上方
true
org.hibernate.cache.ehcache.EhCacheRegionFactory
class-cache节点 要放在 mapping节点的下方
true
org.hibernate.cache.ehcache.EhCacheRegionFactory
package com.qfedu.hibernate.test;
import org.hibernate.Session;
import org.junit.Test;
import com.qfedu.hibernate.pojo.Customer;
import com.qfedu.hibernate.utils.HibernateUtil;
public class CacheLevelTwoTest {
/**
* 演示二级缓存
*/
@Test
public void test1(){
Session session = HibernateUtil.openSession();
//第1次操作
Customer cust = session.get(Customer.class, 1L);
System.out.println(cust.getName());
//关闭session
session.close();
//第2次操作
session = HibernateUtil.openSession();
cust = session.get(Customer.class, 1L);
System.out.println(cust.getName());
session.close();
}
}