Mybatis的SqlSession理解（一）

SqlSession是Mybatis最重要的构建之一，可以认为Mybatis一系列的配置目的是生成类似JDBC生成的Connection对象的statement对象，这样才能与数据库开启“沟通”，通过SqlSession可以实现增删改查（当然现在更加推荐是使用Mapper接口形式）

1 .sqlsession的创建：

SqlSessionFactoryBuilder创建SqlSessionFactory  openSession,sqlSession 执行增删改查
用了注解是通过org.mybatis.spring.SqlSessionFactoryBean该类创建sqlsession的，而mapper里面的每一个方法称为statement。

    public void deleteUserTest() throws IOException {
        // mybatis配置文件
        String resource = "SqlMapConfig.xml";
        // 得到配置文件流
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 创建会话工厂，传入mybatis的配置文件信息
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
                .build(inputStream);
        // 通过工厂得到SqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 传入id删除 用户
        sqlSession.delete("test.deleteUser", 39);
        //更新
        sqlSession.update("test.updateUser", user);
        //插入
        sqlSession.insert("test.insertUser", user);
        //查询
        List<User> list = sqlSession.selectOne("test.findUserByName", "小明");
        // 提交事务 增删改 需要commit，查询无需commit
        sqlSession.commit();
        // 关闭会话
        sqlSession.close();

    }

2.SqlSession原理

SqlSession提供select/insert/update/delete方法，在旧版本中使用使用SqlSession接口的这些方法，但是新版的Mybatis中就会建议使用Mapper接口的方法。

映射器其实就是一个动态代理对象，进入到MapperMethod的execute方法就能简单找到SqlSession的删除、更新、查询、选择方法，从底层实现来说：通过动态代理技术，让接口跑起来，之后采用命令模式，最后还是采用了SqlSession的接口方法（getMapper()方法等到Mapper）执行SQL查询（也就是说Mapper接口方法的实现底层还是采用SqlSession接口方法实现的）。

3.SqlSession重要的四个对象

1）Execute：调度执行StatementHandler、ParmmeterHandler、ResultHandler执行相应的SQL语句；

2）StatementHandler：使用数据库中Statement（PrepareStatement）执行操作，即底层是封装好了的prepareStatement；

3）ParammeterHandler：处理SQL参数；

4）ResultHandler：结果集ResultSet封装处理返回。

1 Execute执行器

execute接口有以下方法

执行器起到至关重要的作用，它是真正执行Java与数据库交互的东西，参与了整个SQL查询执行过程中。

1）主要有三种执行器：简易执行器SIMPLE（不配置就是默认执行器）、REUSE是一种重用预处理语句、BATCH批量更新、批量专用处理器

2）执行器作用：Executor会先调用StatementHandler的prepare()方法预编译SQL语句，同时设置一些基本的运行参数，然后调用StatementHandler的parameterize()方法（实际上是启用了ParameterHandler设置参数）设置参数，resultHandler再组装查询结果返回调用者完成一次查询完成预编译，简单总结起来就是即先预编译SQL语句，之后设置参数（跟JDBC的prepareStatement过程类似）最后如果有查询结果就会组装返回。

StatementHandler 接口方法

public interface StatementHandler {
　　//预编译SQL语句
  Statement prepare(Connection connection)
      throws SQLException;
  //设置一些基本的运行参数
  void parameterize(Statement statement)
      throws SQLException;

  void batch(Statement statement)
      throws SQLException;

  int update(Statement statement)
      throws SQLException;

  <E> List<E> query(Statement statement, ResultHandler resultHandler)
      throws SQLException;

  BoundSql getBoundSql();

  ParameterHandler getParameterHandler();

}

第一：Executor通过Configuration对象中newExecutor()方法中选择相应的执行器生成

//Configuration 类
 public Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor, autoCommit);
    }
     //最后在可以在调用真正的Executor前可以修改插件代码,执行拦截器
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

//InterceptorChain 类中pluginAll 方法：　　  

  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

第二：在执行器中StatementHandler是根据Configuration构建的

public class SimpleExecutor extends BaseExecutor {

  public SimpleExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }

  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      //获取StatementHandler 对象
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      //调到最后一个方法查看详细内容
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

  public <E> List<E> 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(this, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
    return Collections.emptyList();
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection);
    //设置执行参数
    handler.parameterize(stmt);
    return stmt;
  }

}

第三：Executor会执行StatementHandler的prepare()方法进行预编译---->填入connection对象等参数---->再调用parameterize()方法设置参数---->完成预编译

   //SimpleExecutor类
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    // 执行handler.prepare()预编译sql
    stmt = handler.prepare(connection);
    //设置执行参数
    handler.parameterize(stmt);
    return stmt;
  }

2 StatementHanlder数据库会话器

1）作用：简单来说就是专门处理数据库会话。详细来说就是进行预编译并且调用ParameterHandler的setParameters()方法设置参数。

2）数据库会话器主要有三种：SimpleStatementHandler、PrepareStatementHandler、CallableStatementHandler，分别对应Executor的三种执行器（SIMPLE、REUSE、BATCH）

Executor的prepareStatement()方法中调用了StatementHandler的parameterize()

第一：StatementHandler的生成是由Configuration方法中newStatementHandler()方法生成的，但是正在创建的是实现了StatementHandler接口的RoutingStatementHandler对象

//Configuration
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

第二：RoutingStatementHandler的通过适配器模式找到对应（根据上下文）的StatementHandler执行的，并且有SimpleStatementHandler、PrepareStatementHandler、CallableStatementHandler，分别对应Executor的三种执行器（SIMPLE、REUSE、BATCH）

//RoutingStatementHandler
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

PreparedStatementHandler , public class PreparedStatementHandler extends BaseStatementHandler  , BaseStatementHandler implements StatementHandler
第三：在BaseStatementHandler中重写prepare()方法，instantiateStatement()方法完成预编译，之后设置一些基础配置（获取最大行数，超时）

//BaseStatementHandler
 public Statement prepare(Connection connection) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      statement = instantiateStatement(connection);
      setStatementTimeout(statement);
      setFetchSize(statement);
      return statement;
    } catch (SQLException e) {
      closeStatement(statement);
      throw e;
    } catch (Exception e) {
      closeStatement(statement);
      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
    }
  }

第四：instantiateStatement()预编译实际上也是使用了JDBC的prepareStatement()完成预编译

  protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      if (keyColumnNames == null) {
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
        return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() != null) {
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      return connection.prepareStatement(sql);
    }
  }

第五：在prepareStatement中重写parameterize()方法。prepare()预编译完成之后，Executor会调用parameterize()方法

 public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
  }

（3）ParameterHandler参数处理器

作用：对预编译中参数进行设置，如果有配置typeHandler，自然会对注册的typeHandler对参数进行处理