hibernate学习笔记第19讲-一级缓存

 

hibernate一级缓存   

一级缓存很短和session的生命周期一致，一级缓存也叫session级的缓存或事务级缓存

 

那些方法支持一级缓存：

     * get()

     * load()

     * iterate（查询实体对象）

    

如何管理一级缓存：

     * session.clear(),session.evict()

    

如何避免一次性大量的实体数据入库导致内存溢出

     * 先flush，再clear

    

如果数据量特别大，考虑采用jdbc实现，如果jdbc也不能满足要求可以考虑采用数据本身的特定导入工具      

     /**

      * 在同一个session中发出两次load查询

      */

session.beginTransaction();

                       Student student = (Student)session.load(Student.class, 1);

                       System.out.println("student.name=" + student.getName());

                       //不会发出sql，因为load使用缓存

                       student = (Student)session.load(Student.class, 1);

                       System.out.println("student.name=" + student.getName());

                       session.getTransaction().commit();

/**

      * 在同一个session中发出两次get查询

      */

session.beginTransaction();

                       Student student = (Student)session.get(Student.class, 1);

                       System.out.println("student.name=" + student.getName());

                       //不会发出sql，因为get使用缓存

                       student = (Student)session.get(Student.class, 1);

                       System.out.println("student.name=" + student.getName());

                       session.getTransaction().commit();

/**

      * 在同一个session中发出两次iterate查询实体对象

      */

                        session.beginTransaction();

                       Student student = (Student)session.createQuery("from Student s where s.id=1").iterate().next();

                       System.out.println("student.name=" + student.getName());

                       //会发出查询id的sql，不会发出查询实体对象的sql，因为iterate使用缓存

                       student = (Student)session.createQuery("from Student s where s.id=1").iterate().next();

                       System.out.println("student.name=" + student.getName());

                      

                       session.getTransaction().commit();

     /**

      * 在同一个session中发出两次iterate查询普通属性

      */

session.beginTransaction();

                       String name = (String)session.createQuery("select s.name from Student s where s.id=1").iterate().next();

                       System.out.println("student.name=" + name);

                       //iterate查询普通属性，一级缓存不会缓存，所以发出sql

                       //一级缓存是缓存实体对象的

                       name = (String)session.createQuery("select s.name from Student s where s.id=1").iterate().next();

                       System.out.println("student.name=" + name);

                       session.getTransaction().commit();

     /**

      * 开启两个session中发出load查询

      */

//会发出查询语句，session间不能共享一级缓存的数据

//因为它会伴随session的生命周期存在和消亡

     /**

      * 在同一个session中先save，在发出load查询save过的数据

      */

session.beginTransaction();

                       Student stu = new Student();

                       stu.setName("王五");

                       Serializable id = session.save(stu);

                       //不会发出sql，因为save是使用缓存的

                       Student student = (Student)session.load(Student.class, id);

                       System.out.println("student.name=" + student.getName());

                       session.getTransaction().commit();

     /**

      * 向数据库中批量加入1000条数据

      */

session.beginTransaction();

                       for (int i=0; i<1000; i++) {

                                 Student student = new Student();

                                 student.setName("s_" + i);

                                 session.save(student);

                                 //每20条数据就强制session将数据持久化

                                 //同时清除缓存，避免大量数据造成内存溢出

                                 if ( i % 20 == 0) {

                                          session.flush();

                                          session.clear();

                                 }

                       }

                       session.getTransaction().commit();