【MyBatis】mybatis执行流程与缓存机制分析

1. 编程式使用mybatis

首先，我们直观地看一下怎样编程式（非配置式）使用mybatis作为ORM框架实现数据库的基本操作。

    private static void testId() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
        SqlSession sqlSession = factory.openSession();

        // 获取代理 -> MapperProxy
        StudentDao studentDao = sqlSession.getMapper(StudentDao.class);

        Student student = new Student();
        student.setStudentName("AutoIncrementId");
        student.setStudentNumber("2014216003");
        // sqlSession.insert("insertEntry", student);
        studentDao.insertEntry(student);
        System.out.println(student.getId());// 3

        // commit & close
        sqlSession.commit();
        sqlSession.close();
    }

可以清晰地看到一共分为三个步骤：第一步，使用mybatis配置文件创建SqlSessionFactory，并打开一个sqlSession；第二步，获取代理MapperProxy类；第三步，执行数据库基本操作。

2. 第一步，获取sqlSession

图片来源： 深入浅出Mybatis系列（十）---SQL执行流程分析（源码篇）

3. 第二步，获取代理MapperProxy

3.0 
sqlSession.getMapper(StudentDao. class );

3.1  DefaultSqlSession

	public <T> T getMapper(Class<T> type)

		configuration.<T>getMapper(type, this);

3.2 Configuration

	public <T> T getMapper(Class<T> type, SqlSession sqlSession)

		mapperRegistry.getMapper(type, sqlSession);

3.3 MapperRegistry

	public <T> T getMapper(Class<T> type, SqlSession sqlSession)

		mapperProxyFactory.newInstance(sqlSession);

3.4 MapperProxyFactory

	public T newInstance(SqlSession sqlSession)

		(T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);

3.5 Proxy

	public static Object newProxyInstance(ClassLoader loader,
                                      Class[] interfaces,
                                      InvocationHandler h)

图片来源： 深入浅出Mybatis系列（十）---SQL执行流程分析（源码篇）

4. 第三步，执行数据库操作

4.1 MapperProxy

	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable

		mapperMethod.execute(sqlSession, args);

4.2 MapperMethod

	public Object execute(SqlSession sqlSession, Object[] args)

		sqlSession.insert(command.getName(), param)

4.3 DefaultSqlSession

	public int insert(String statement, Object parameter)
		update(statement, parameter);

public int update(String statement, Object parameter)
executor .update(ms, wrapCollection(parameter));
4.4 CachingExcutor

public int update(MappedStatement ms, Object parameterObject) throws SQLException
delegate .update(ms, parameterObject);
4.5 BaseExcutor

public int update(MappedStatement ms, Object parameter) throws SQLException
doUpdate(ms, parameter);

4.6 SimpleExcutor

public int doUpdate(MappedStatement ms, Object parameter)
handler.update(stmt);
4.7 PreparedStatementHandler

  public int update(Statement statement) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    int rows = ps.getUpdateCount();
    Object parameterObject = boundSql.getParameterObject();
    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
    keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
    return rows;
  }

图片来源： 深入浅出Mybatis系列（十）---SQL执行流程分析（源码篇）

5. MyBatis缓存机制分析

    private static void testCache() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config2.xml");
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
        SqlSession sqlSession = factory.openSession();

        long id = 1L;
        Student student = new Student();
        student.setStudentName("Ethan");
        student.setStudentNumber("2014216001");
        sqlSession.insert("insertEntry", student);
        System.out.println(student.getId());// null

        // query from db
        // 加入一级缓存和二级transactional缓存，由于sqlsession未commit，所以二级transactional缓存的代理未真正缓存
        student = sqlSession.selectOne("selectByKey", id);
        System.out.println(student.getId());
        System.out.println(student.getStudentName());

        // local cache hit
        // 由于二级缓存未真正缓存，只能从一级缓存那里命中
        student = sqlSession.selectOne("selectByKey", id);
        System.out.println(student.getId());
        System.out.println(student.getStudentName());

        // commit
        sqlSession.commit();

        // sqlsession在commit之后，transactional缓存的代理才真正缓存起来，二级缓存命中
        student = sqlSession.selectOne("selectByKey", id);
        System.out.println(student.getId());
        System.out.println(student.getStudentName());

        // update 会clear未commit的transactional缓存中的内容，同时设置标志位clearOnCommit为true，会clear掉local cache
        student.setStudentName("Hunt");
        int update = sqlSession.update("updateByKey", student);
        System.out.println("update: " + update);

        // select again
        // 在二级缓存中找到了缓存，但是由于clearOnCommit标志位为true，所以返回null
        student = sqlSession.selectOne("selectByKey", id);
        System.out.println(student.getId());
        System.out.println(student.getStudentName());

        // commit & close
        sqlSession.commit();
        sqlSession.close();
    }

其中，二级缓存默认为：

6. References

深入浅出Mybatis系列（十）---SQL执行流程分析（源码篇）

MyBatis 缓存

MyBatis 缓存机制深度解剖 / 自定义二级缓存

mybatis的缓存机制(一级缓存二级缓存和刷新缓存)和mybatis整合ehcache