面试：Hibernate效率很低，如何优化？
缓存怎么弄的，语句怎么优化？
聊聊一级缓存，聊聊抓取策略和延迟加载,聊聊批量抓取
延迟加载：
一般不单独使用，和延迟加载一起使用
延迟加载：lazy(懒加载)
执行到该行代码的时候不会发送语句，真正使用这个对象的属性的时候才会发送sql语句进行查询。
- 类级别延迟加载：指的是是通过load方法查询某个对象的时候是否采用延迟，通过class标签上的lazy来配置。
- 让类级别延迟加载失效：1 lazy设为false 2 final修饰 3 调用Hibernate.initialize
- 关联级别的延迟加载：指的是查询某个对象的关联对象的时候，是否采用延迟加载。session.get(Customer.class,1l) , customer.getLinkMans();
- 抓取策略往往和关联级别的延迟加载一起使用，优化语句
类级别延迟加载配置：默认为true
关联级别的延迟加载配置：在映射文件被关联的对象上配置
抓取策略：
通过一个对象抓取到关联对象需要发送sql语句，sql语句如何发送，发送成什么格式通过策略进行配置
- 通过<set> 或者发送一条子查询上通过fetch属性进行设置
- fetch和这些标签上的lazy如何设置优化发送的语句
<set>上的fetch和lazy
fetch：抓取策略，控制sql语句格式
- select： 默认值，发送普通select语句，查询关联对象，至少交互两次
- join: 发送迫切左外连接查询关联，与数据库交互一次，发一条数据
- subselect：发送一条子查询查询关联对象
lazy:延迟加载，控制查询关联对象的时候是否采用延迟
- true：默认值，查询关联对象默认采用延迟加载
- false：查询关联对象不采用延迟加载
- extra ：极其懒惰。
在实际开发中，一般都采用默认值。如果有特殊需求，可能配置join
各种配置的区别测试：
/** * 在<set>上的fetch和lazy */public class HibernateDemo4 { ???@Test ???/** ????* 默认情况 ????*/ ???public void demo1() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????// 查询1号客户 ???????Customer customer = session.get(Customer.class, 1l);// 发送一条查询客户的sql ???????System.out.println(customer.getCust_name()); ???????// 查看1号客户每个联系人的信息 ???????for (LinkMan linkMan : customer.getLinkMans()) {// 发送一条根据客户查询联系人的sql ???????????System.out.println(linkMan.getLkm_name()); ???????} ???????transaction.commit(); ???} ???@Test ???/** ????* 设置fetch=select,lazy = true ????*/ ???public void demo2() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????// 查询1号客户 ???????Customer customer = session.get(Customer.class, 1l);// 发送一条查询客户的sql ???????System.out.println(customer.getCust_name()); ???????// 查看1号客户每个联系人的信息 ???????for (LinkMan linkMan : customer.getLinkMans()) {// 发送一条根据客户查询联系人的sql ???????????System.out.println(linkMan.getLkm_name()); ???????} ???????transaction.commit(); ???} ???@Test ???/** ????* 设置fetch=select,lazy =false ????*/ ???public void demo3() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????// 查询1号客户 ???????Customer customer = session.get(Customer.class, 1l);// 不延迟，直接发送两条语句，查询客户的sql，根据客户查询联系人的sql ???????System.out.println(customer.getCust_name()); ???????System.out.println(customer.getLinkMans().size());// 发送一条select ???????????????????????????????????????????????????????????// count(lkm_id) ???????????????????????????????????????????????????????????// from ???????transaction.commit(); ???} ???@Test ???/** ????* 设置fetch=select,lazy =extra ????*/ ???public void demo4() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????// 查询1号客户 ???????Customer customer = session.get(Customer.class, 1l);// 发送一条查询客户语句 ???????System.out.println(customer.getCust_name()); ???????System.out.println(customer.getLinkMans().size());// 发送一条select ???????????????????????????????????????????????????????????// count(lkm_id) ???????????????????????????????????????????????????????????// from ???????transaction.commit(); ???} ???@Test ???/** ????* 设置fetch=join,lazy =失效 ????*/ ???public void demo5() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????// 查询1号客户 ???????Customer customer = session.get(Customer.class, 1l);// 发送一条迫切左外连接语句，两表全查 ???????System.out.println(customer.getCust_name()); ???????System.out.println(customer.getLinkMans().size());// 不发语句 ???????transaction.commit(); ???} ???@Test ???/** ????* 设置fetch=subselect,lazy =true ????*/ ???public void demo6() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????// 查询1号客户 ???????List<Customer> list = session.createQuery("from Customer").list();// 发送查询所有客户的sql语句 ???????for (Customer customer : list) { ???????????System.out.println(customer.getCust_name()); ???????????System.out.println(customer.getLinkMans().size());// 发送一条子查询 ???????} ???????transaction.commit(); ???} ???@Test ???/** ????* 设置fetch=subselect,lazy =false ????*/ ???public void demo7() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????// 查询1号客户 ???????List<Customer> list = session.createQuery("from Customer").list();// 发送查询所有客户的sql语句,发送一条子查询 ???????for (Customer customer : list) { ???????????System.out.println(customer.getCust_name()); ???????????System.out.println(customer.getLinkMans().size()); ???????} ???????transaction.commit(); ???}}<many-to-one>上的fetch和lazy
fetch：抓取策略
- select：默认值，发送普通sql查询关联对象
- join:发送迫切左外连接
lazy：延迟加载，控制查询关联对象的时候是否采用延迟
- proxy:默认值，具体的取值和另一端<class>上lazy的值相同
- false：查询关联对象，不采用延迟
- no-proxy(不会使用)
一般使用默认值
各种配置的区别测试：
/** * 在<many-to-one>上的fetch和lazy */public class HibernateDemo5 { ???@Test ???/** ????* 默认情况 ????*/ ???public void demo1() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????LinkMan linkMan = session.get(LinkMan.class, 1l);// 发送一条查询联系人的语句 ???????System.out.println(linkMan.getLkm_name()); ???????System.out.println(linkMan.getCustomer());// 发送一条select查询联系人所关联的客户的语句 ???????transaction.commit(); ???} ???@Test ???/** ????* fetch = "select" lazy = "proxy",默认值 ????*/ ???public void demo2() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????LinkMan linkMan = session.get(LinkMan.class, 1l);// 发送一条查询联系人的语句 ???????System.out.println(linkMan.getLkm_name()); ???????System.out.println(linkMan.getCustomer());// 发送一条select查询联系人所关联的客户的语句 ???????transaction.commit(); ???} ???@Test ???/** ????* fetch = "select" lazy = "false" ????*/ ???public void demo3() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????LinkMan linkMan = session.get(LinkMan.class, 1l);// 不延迟，发送一条查询联系人的语句,发送一条select查询联系人所关联的客户的语句 ???????System.out.println(linkMan.getLkm_name()); ???????System.out.println(linkMan.getCustomer()); ???????transaction.commit(); ???} ???@Test ???/** ????* fetch = "join" lazy = 失效 ????*/ ???public void demo4() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????LinkMan linkMan = session.get(LinkMan.class, 1l);// 发送一条迫切左外连接，两表全查 ???????System.out.println(linkMan.getLkm_name()); ???????System.out.println(linkMan.getCustomer()); ???????transaction.commit(); ???}}批量抓取：
批量抓取：一批关联对象一起抓取，batch-size,
/** * Hibernate批量抓取 */public class HibernateDemo6 { ???@Test ???/** ????* 获取客户的时候批量抓取联系人，在Customer.hbm.xml中设置set标签batch-size="4" ????*/ ???public void demo1() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????List<Customer> list = session.createQuery("from Customer").list(); ???????for (Customer customer : list) { ???????????System.out.println(customer.getCust_name()); ???????????for (LinkMan linkMan : customer.getLinkMans()) { ???????????????System.out.println(linkMan.getLkm_name()); ???????????} ???????} ???????transaction.commit(); ???} ???@Test ???/** ????* 获取联系的时候批量抓取客户，在Customer.hbm.xml中设置class标签batch-size="4" ????*/ ???public void demo2() { ???????Session session = HibernateUtils.getCurrentSession(); ???????Transaction transaction = session.beginTransaction(); ???????List<LinkMan> list = session.createQuery("from LinkMan").list(); ???????for (LinkMan linkMan : list) { ???????????System.out.println(linkMan.getLkm_name()); ???????????System.out.println(linkMan.getCustomer().getCust_name()); ???????} ???????transaction.commit(); ???}}八 ?Hibernate延迟加载&抓取策略（优化）
原文地址：https://www.cnblogs.com/ltfxy/p/10313664.html