Mybatis--SQL执行流程

1. SqlSessionFactory 与 SqlSession

我们首先来看一张图片，如下：

（1）首先，SqlSessionFactoryBuilder去读取mybatis的配置文件，然后build一个DefaultSqlSessionFactory。源码如下： 

/**
   * 一系列的构造方法最终都会调用本方法（配置文件为Reader时会调用本方法，还有一个InputStream方法与此对应）
   * @param reader
   * @param environment
   * @param properties
   * @return
   */
  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      //通过XMLConfigBuilder解析配置文件，解析的配置相关信息都会封装为一个Configuration对象
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      //这儿创建DefaultSessionFactory对象
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

 （2）当我们获取到SqlSessionFactory之后，就可以通过SqlSessionFactory去获取SqlSession对象。源码如下：

    @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

    private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      //通过Confuguration对象去获取Mybatis相关配置信息, Environment对象包含了数据源和事务的配置
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //从表面上来看，咱们是用sqlSession在执行sql语句， 实际呢，其实是通过excutor执行， excutor是对于Statement的封装
      final Executor executor = configuration.newExecutor(tx, execType);
      //关键看这儿，创建了一个DefaultSqlSession对象
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

经过上面一系列过程，可以得到SqlSession，然后可以利用SqlSession做CRUD操作，接下来我们在看看Mybatis如何加载映射文件。

2. MapperProxy

我们还是先看一张图片，如下：

mybatis中，通过MapperProxy动态代理咱们的dao， 也就是说， 当咱们执行自己写的dao里面的方法的时候，其实是对应的MapperProxy在代理。 我们接下来看看怎么获取MapperProxy，源码如下：

DefaultSqlSession.java
//SqlSession什么都没做，直接通过 Configuration获取
@Override
  public  T getMapper(Class type) {
    //
    return configuration.getMapper(type, this);
  }
// Configuration也没做什么处理，也丢给MapperRegistry
 public  T getMapper(Class type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

MapperRegistry.java
//
  public  T getMapper(Class type, SqlSession sqlSession) {
    final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      //关键在这儿
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

@SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy mapperProxy) {
    //动态代理我们写的dao接口
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
  
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

经过上面一系列过程，我们拿到了MapperProxy，下面我们再看看通过MapperProxy怎么执行SQL语句。

3. Excutor

还和上面一样，我们首先看张图片，如下：

invoke(Object proxy, Method method, Object[] args)

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //交给MapperMethod执行
    return mapperMethod.execute(sqlSession, args);
  }

 MapperMethod

//根据类型去选择到底执行sqlSession中的哪个方法 
 public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    if (SqlCommandType.INSERT == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
    } else if (SqlCommandType.SELECT == command.getType()) {
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else {
      throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

我们选择selectList方法看看，源码如下：

public  List selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      //CRUD实际上是交给Excetor去处理
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

然后，通过一层一层的调用，最终会来到doQuery方法， 这儿咱们就随便找个SimpleExecutor看看：

public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      //StatementHandler封装了Statement, 让 StatementHandler 去处理
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

接下来，咱们看看StatementHandler 的一个实现类 PreparedStatementHandler看看它使怎么去处理的：

public  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
     
	// 调用preparedStatemnt。execute()方法，然后将resultSet交给ResultSetHandler处理  
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    //使用ResultHandler来处理ResultSet
    return resultSetHandler. handleResultSets(ps);
  }

/**  
*ResultSetHandler类的handleResultSets()方法实现
*
*/
public List