hibernate缓存机制

  在hibernate的使用中，大家多数时间都在讨论一级缓存和二级缓存，而往往忽略了查询缓存。其实hibernate的查询缓存在使用过程中也起着同样重要的作用。hibernate的查询缓存是主要是针对普通属性结果集的缓存， 而对于实体对象的结果集只缓存id。在一级缓存,二级缓存和查询缓存都打开的情况下作查询操作时这样的：查询普通属性，会先到查询缓存中取，如果没有，则查询数据库；查询实体，会先到查询缓存中取id，如果有，则根据id到缓存(一级/二级)中取实体，如果缓存中取不到实体，再查询数据库。

 

    和一级/二级缓存不同，查询缓存的生命周期 ，是不确定的，当前关联的表发生改变时，查询缓存的生命周期结束。

 

     查询缓存的配置和使用也是很简单的：
         1>查询缓存的启用不但要在配置文件中进行配置
             <property name="hibernate.cache.use_query_cache">true</property>
         2>还要在程序中显示的进行启用
             query.setCacheable(true);

 

1.实体类：

  Student.jsva

public class Student {
  private Integer id;
  private String name;
  //一系列的setter.getter方法
}

 

2.映射文件

  Student.hbm.xml

  <class name="com.sxt.hibernate.cache.entity.Student"table="sxt_hibernate_student">
        
    <!-- 指定本类的对象使用二级缓存(这也可以放在hibernate.cfg.xml中统一指定) -->
    <!--
    <cache usage="read-only"/>
     -->
    <id name="id" length="4">
      <generatorclass="native"></generator>
    </id>
    <propertyname="name"length="10"></property>
  </class>

 

3.hibernate配置文件：

  hibernate.cfg.xml

<hibernate-configuration>
  <session-factory>
    <propertyname="hibernate.connection.url">jdbc:oracle:thin:@localhost:1521:ORCL10</property>
    <propertyname="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
    <propertyname="hibernate.connection.username">scott</property>
    <propertyname="hibernate.connection.password">yf123</property>
    <propertyname="hibernate.dialect">org.hibernate.dialect.Oracle9Dialect</property>
    <propertyname="hibernate.show_sql">true</property>
    
    <!-- 开启二级缓存,其实hibernate默认就是开启的,这里显示的指定一下 -->
    <propertyname="hibernate.cache.use_second_level_cache">true</property>
    <!-- 指定二级缓存产品的提供商 -->
    <propertyname="hibernate.cache.provider_class">org.hibernate.cache.EhCacheProvider</property>
    
    <!-- 启用查询缓存 -->
    <propertyname="hibernate.cache.use_query_cache">true</property>
    
    <mappingresource="com/sxt/hibernate/cache/entity/Student.hbm.xml"/>
    
    <!-- 指定那些类使用二级缓存 -->
    <class-cacheusage="read-only"class="com.sxt.hibernate.cache.entity.Student"/>
  </session-factory>
</hibernate-configuration>

 

4.测试方法：

  public staticvoid main(String[] args) {
    Session session = null;
    Transaction t = null;

    *//**
     * 开启查询缓存,关闭二级缓存, 开启一个session,分别调用query.list
     */
  //如果不用查询缓存的话,那两个都发出查询语句,这也是默认的情况.
  /*
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s.name from Student s");
      //启用查询缓存    
      query.setCacheable(true);
      List<String> names = query.list();
      for (Iterator<String> it = names.iterator(); it.hasNext();) {
        String name = it.next();
        System.out.println(name);
      }
      System.out.println("================================");
      query = session.createQuery("select s.name from Student s");
      //启用查询缓存
      query.setCacheable(true);
      //没有发出查询语句,因为这里使用的查询缓存
      names = query.list();
      for (Iterator<String> it = names.iterator(); it.hasNext();) {
        String name = it.next();
        System.out.println(name);
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
  }*/
    
/*  @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    Session session = null;
    Transaction t = null;

    *//**
     * 开启查询缓存,关闭二级缓存, 开启两个session,分别调用query.list
     *//*
    //如果不用查询缓存的话,那两个都发出查询语句,这也是默认的情况.
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s.name from Student s");
      //启用查询缓存    
      //query.setCacheable(true);
      List<String> names = query.list();
      for (Iterator<String> it = names.iterator(); it.hasNext();) {
        String name = it.next();
        System.out.println(name);
      }

      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
    
    System.out.println("================================");
    
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s.name from Student s");
      //启用查询缓存    
      //query.setCacheable(true);
      //不会发出查询语句,因为查询缓存和session无关.
      List<String> names = query.list();
      for (Iterator<String> it = names.iterator(); it.hasNext();) {
        String name = it.next();
        System.out.println(name);
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
  }*/
    
/*  @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    Session session = null;
    Transaction t = null;

    *//**
     * 开启查询缓存,关闭二级缓存, 开启两个session,分别调用query.iterate
     *//*
    //如果不用查询缓存的话,那两个都发出查询语句,这也是默认的情况.
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s.name from Student s");
      //启用查询缓存    
      query.setCacheable(true);
      for (Iterator<String> it = query.iterate(); it.hasNext();) {
        String name = it.next();
        System.out.println(name);
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
    
    System.out.println("================================");
    
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s.name from Student s");
      //启用查询缓存    
      query.setCacheable(true);
      //会发出查询语句,因为query.iterate不使用查询缓存
      for (Iterator<String> it = query.iterate(); it.hasNext();) {
        String name = it.next();
        System.out.println(name);
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
  }*/
    
/*    @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    Session session = null;
    Transaction t = null;

    *//**
     * 关闭查询缓存,关闭二级缓存, 开启两个session,分别调用query.list查询实体对象
     *//*
    //如果不用查询缓存的话,那两个都发出查询语句,这也是默认的情况.
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s from Student s");
      //启用查询缓存    
      //query.setCacheable(true);
      List<Student> students = query.list();
      for (Iterator<Student> it = students.iterator(); it.hasNext();) {
        Student s = it.next();
        System.out.println(s.getName());
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
    
    System.out.println("================================");
    
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s from Student s");
      //启用查询缓存    
      //query.setCacheable(true);
      //会发出查询语句,因为list默认每次都会发出sql语句
      List<Student> students = query.list();
      for (Iterator<Student> it = students.iterator(); it.hasNext();) {
        Student s = it.next();
        System.out.println(s.getName());
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
  }*/
    
/*  @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    Session session = null;
    Transaction t = null;

    *//**
     * 开启查询缓存,关闭二级缓存, 开启两个session,分别调用query.list查询实体对象
     *//*
    //如果不用查询缓存的话,那两个都发出查询语句,这也是默认的情况.
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s from Student s");
      //启用查询缓存    
      query.setCacheable(true);
      List<Student> students = query.list();
      for (Iterator<Student> it = students.iterator(); it.hasNext();) {
        Student s = it.next();
        System.out.println(s.getName());
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
    
    System.out.println("================================");
    
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s from Student s");
      //启用查询缓存    
      query.setCacheable(true);
      //会发出根据id查询实体的n条查询语句,因为这种情况下,查询过程是这样的：
      // 在第一次执行list时,会把查询对象的id缓存到查询缓存里
      // 第二次执行list时, 会遍历查询缓存里的id到缓存里去找实体对象,由于这里没找到实体对象,
      //所以就发出n条查询语句到数据库中查询.
      List<Student> students = query.list();
      for (Iterator<Student> it = students.iterator(); it.hasNext();) {
        Student s = it.next();
        System.out.println(s.getName());
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
  }*/
    
  @SuppressWarnings("unchecked")
  public staticvoid main(String[] args) {
    Session session = null;
    Transaction t = null;

    /**
     * 开启查询缓存,开启二级缓存, 开启两个session,分别调用query.list查询实体对象
     */
    //如果不用查询缓存的话,那两个都发出查询语句,这也是默认的情况.
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s from Student s");
      //启用查询缓存    
      query.setCacheable(true);
      List<Student> students = query.list();
      for (Iterator<Student> it = students.iterator(); it.hasNext();) {
        Student s = it.next();
        System.out.println(s.getName());
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
    
    System.out.println("================================");
    
    try {
      session = HibernateUtils.getSession();
      t = session.beginTransaction();
      Query query = session.createQuery("select s from Student s");
      //启用查询缓存    
      query.setCacheable(true);
      //不会发出查询语句,因为这种情况下,查询过程是这样的：
      // 在第一次执行list时,会把查询对象的id缓存到查询缓存里
      // 第二次执行list时, 会遍历查询缓存里的id到缓存里去找实体对象,由于这里开启了二级缓存,可以找到目标实体对象,
      //所以就不会再发出n条查询语句.
      List<Student> students = query.list();
      for (Iterator<Student> it = students.iterator(); it.hasNext();) {
        Student s = it.next();
        System.out.println(s.getName());
      }
      t.commit();
    } catch (Exception e) {
      e.printStackTrace();
      t.rollback();
    } finally {
      HibernateUtils.closeSession(session);
    }
  }