Mybaits源码解析4——SimpleExecutor、ReuseExecutor、BatchExecutor、CachingExecutor

上一篇对Executor接口进行了大致的分析，这边来针对它的实现进行分析。

现在我们知道，mybatis默认情况下是使用simpleExecutor的，如果你需要修改，有两种方式，一是在setting中配置defualtExecutorType，另一种方式是通过SqlSessionManager指定executorType并创建sqlsession。下面以doUpdate和doQuery方法为例，比较这几种executor的差异，因为executor提供sqlSession的就是这两个核心基础方法。

doUpdate

simpleExecutor：

  @Override
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());//创建statement
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);//关闭statement，意味着下一次使用需要重新开启statement
    }
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());//创建statement
    handler.parameterize(stmt);//将参数存入statement
    return stmt;
  }

ReuseExecutor：

将statement存入map中：

private final Map statementMap = new HashMap();

  @Override
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.update(stmt);//没有关闭statement
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    BoundSql boundSql = handler.getBoundSql();
    String sql = boundSql.getSql();
    if (hasStatementFor(sql)) {
      stmt = getStatement(sql);//如果已缓存statement则取出
      applyTransactionTimeout(stmt);
    } else {
      Connection connection = getConnection(statementLog);
      stmt = handler.prepare(connection, transaction.getTimeout());
      putStatement(sql, stmt);//否则创建statement并存入
    }
    handler.parameterize(stmt);
    return stmt;
  }

看看它是怎么映射判断已缓存的statement的：

  private boolean hasStatementFor(String sql) {
    try {
      return statementMap.keySet().contains(sql) && !statementMap.get(sql).getConnection().isClosed();//sql语句就是key，statement是值
    } catch (SQLException e) {
      return false;
    }
  }

BatchExecutor：

  public static final int BATCH_UPDATE_RETURN_VALUE = Integer.MIN_VALUE + 1002;//为何？防无限循环

  private final List statementList = new ArrayList();//使用list缓存statement
  private final List batchResultList = new ArrayList();
  private String currentSql;
  private MappedStatement currentStatement;

 @Override
  public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
    final Configuration configuration = ms.getConfiguration();
    final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
    final BoundSql boundSql = handler.getBoundSql();
    final String sql = boundSql.getSql();
    final Statement stmt;
    if (sql.equals(currentSql) && ms.equals(currentStatement)) {//判断当前使用sql和statement是否是上一次的statement和sql
      int last = statementList.size() - 1;
      stmt = statementList.get(last);//如果是则取出
      applyTransactionTimeout(stmt);
     handler.parameterize(stmt);//fix Issues 322
      BatchResult batchResult = batchResultList.get(last);
      batchResult.addParameterObject(parameterObject);
    } else {
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection, transaction.getTimeout());
      handler.parameterize(stmt);    //fix Issues 322
      currentSql = sql;
      currentStatement = ms;
      statementList.add(stmt);//否则创建statement并存入
      batchResultList.add(new BatchResult(ms, sql, parameterObject));
    }
  // handler.parameterize(stmt);
    handler.batch(stmt);//仅仅是存入批处理参数而不执行
    return BATCH_UPDATE_RETURN_VALUE;//始终返回一个常量
  }

在BatchExecutor中的doupdate并不会想前面两者那样执行返回行数，而是每次执行将statement预存到有序集合，官方说明这个executor是用于执行存储过程的和批量操作的，因此这个方法是循环或者多次执行构建一个存储过程或批处理过程。

doQuery

simpleExecutor：

没有什么特别，常规操作

  @Override
  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());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);//也是关闭statement
    }
  }

ReuseExecutor：

  @Override
  public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);//不关闭，对缓存的statement进行操作
  }

BatchExecutor：

  @Override
  public  List doQuery(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException {
    Statement stmt = null;
    try {
      flushStatements();//刷新statement
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection, transaction.getTimeout());
      handler.parameterize(stmt);
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);//关闭statement
    }
  }

可以看到，batch的执行方式也有所不同，每次执行查询都会执行刷新statement，为何？如下：

@Override
  public List doFlushStatements(boolean isRollback) throws SQLException {
    try {
      List results = new ArrayList();
      if (isRollback) {
        return Collections.emptyList();
      }
      for (int i = 0, n = statementList.size(); i < n; i++) {//批量执行有序集合中的statement
        Statement stmt = statementList.get(i);
        applyTransactionTimeout(stmt);
        BatchResult batchResult = batchResultList.get(i);
        try {
          batchResult.setUpdateCounts(stmt.executeBatch());//执行批处理或存储过程
          MappedStatement ms = batchResult.getMappedStatement();
          List