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MyBatis缓存机制之一级缓存
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MyBatis缓存分为一级缓存和二级缓存。

public class MyBatisUtil {
    ...
    public static void closeSession() {
        SqlSession session = local.get();
        if (session != null) {
            session.close();
            local.set(null);
        }
    }
    ...
}

public interface MemberDao {
    ...
    public Member searchById(int id);
    public int modifyAge(@Param("id") int id, @Param("age") int age);
    ...
}

<mapper namespace="cn.tedu.dynamic.dao.MemberDao">
    ...
    <select id="searchById" resultType="Member">
        select id, nick, gender, age, city from t_member
        where id = #{id}
    </select>
    <update id="modifyAge">
        update t_member set age = #{age} where id = #{id}
    </update>
    ...
</mapper>

public class MemberDaoTest {
    ...
    @Test
    public void testSearchById() {
        SqlSession session = MyBatisUtil.getSession();
        MemberDao dao = session.getMapper(MemberDao.class);

        System.out.println("\n第一次查询...");
        System.out.println(dao.searchById(1));

        System.out.println("\n第二次查询...");
        System.out.println(dao.searchById(1));

        session.clearCache();

        System.out.println("\n第三次查询...");
        System.out.println(dao.searchById(1));

        System.out.println("\n第四次查询...");
        System.out.println(dao.searchById(1));

        MyBatisUtil.closeSession();
        session = MyBatisUtil.getSession();
        dao = session.getMapper(MemberDao.class);

        System.out.println("\n第五次查询...");
        System.out.println(dao.searchById(1));

        Member member = dao.searchById(1);
        member.setAge(member.getAge() + 1);

        System.out.println("\n第六次查询...");
        System.out.println(dao.searchById(1));

        session.clearCache();

        System.out.println("\n第七次查询...");
        System.out.println(dao.searchById(1));

        member = dao.searchById(1);
        dao.modifyAge(member.getId(), member.getAge() + 1);

        System.out.println("\n第八次查询...");
        System.out.println(dao.searchById(1));

        SqlSession s = MyBatisUtil.getFactory().openSession();
        MemberDao d = s.getMapper(MemberDao.class);
        Member m = d.searchById(1);
        d.modifyAge(m.getId(), m.getAge() + 1);
        s.commit();

        System.out.println("\n第九次查询...");
        System.out.println(dao.searchById(1));
    }
    ...
}

运行测试用例：

第一次查询...
[main] DEBUG - ==>  Preparing: select id, nick, gender, age, city from t_member where id = ? sg :==>  Preparing: select id, nick, gender, age, city from t_member where id = ?
[main] DEBUG - ==> Parameters: 1(Integer)sg :==> Parameters: 1(Integer)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Member(id=1, nick=john, gender=男, age=20, city=北京)

第二次查询...
Member(id=1, nick=john, gender=男, age=20, city=北京)

第三次查询...
[main] DEBUG - ==>  Preparing: select id, nick, gender, age, city from t_member where id = ? sg :==>  Preparing: select id, nick, gender, age, city from t_member where id = ?
[main] DEBUG - ==> Parameters: 1(Integer)sg :==> Parameters: 1(Integer)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Member(id=1, nick=john, gender=男, age=20, city=北京)

第四次查询...
Member(id=1, nick=john, gender=男, age=20, city=北京)

第五次查询...
[main] DEBUG - ==>  Preparing: select id, nick, gender, age, city from t_member where id = ? sg :==>  Preparing: select id, nick, gender, age, city from t_member where id = ?
[main] DEBUG - ==> Parameters: 1(Integer)sg :==> Parameters: 1(Integer)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Member(id=1, nick=john, gender=男, age=20, city=北京)

第六次查询...
Member(id=1, nick=john, gender=男, age=21, city=北京)
                                  <库中20>
第七次查询...
[main] DEBUG - ==>  Preparing: select id, nick, gender, age, city from t_member where id = ? sg :==>  Preparing: select id, nick, gender, age, city from t_member where id = ?
[main] DEBUG - ==> Parameters: 1(Integer)sg :==> Parameters: 1(Integer)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Member(id=1, nick=john, gender=男, age=20, city=北京)

第八次查询...
[main] DEBUG - ==>  Preparing: select id, nick, gender, age, city from t_member where id = ? sg :==>  Preparing: select id, nick, gender, age, city from t_member where id = ?
[main] DEBUG - ==> Parameters: 1(Integer)sg :==> Parameters: 1(Integer)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Member(id=1, nick=john, gender=男, age=21, city=北京)
                                  <库中21>
第九次查询...
Member(id=1, nick=john, gender=男, age=21, city=北京)
                                  <库中22>

例程：Dynamic

一级缓存具有如下特性：

- 一级缓存也叫会话(SqlSession)级缓存，无需手动开启即可直接使用
- 每个SqlSession对象都有独立的缓存区，相互之间不共享
- 若欲通过缓存提高多次查询的性能，必须使用同一个SqlSession对象
- 缓存中存放的是查询所获得的实体对象，修改该对象实际是在修改缓存
- 及时清空缓存(clearCache)有利于保持与数据库的一致，但会牺牲性能
- 在会话内更新数据库，会导致缓存失效，保证缓存与数据库一致
- 在会话外修改数据库，不会使缓存失效，有可能导致数据不一致