Mybatis之StatementHandler

克虏伯

修改于 2019-10-22 10:27:18

6720

修改于 2019-10-22 10:27:18

文章被收录于专栏：软件开发-青出于蓝软件开发-青出于蓝

mybatis-3.4.6.release.

图1

StatementHandler是对CallableStatement、PreparedStatement、Statement的统一封装，在JDK中CallableStatement继承PreparedStatement，PreparedStatement继承Statement。

这几个StatementHandler之间有什么区别呢，我们来看下，RoutingStatementHandler很特殊，使用了委托模式，如List-1

List-1

public class RoutingStatementHandler implements StatementHandler {
  private final StatementHandler delegate;

  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());
    }
  }

如List-1，根据MappedStatement.getStatementType()的值选择 SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler。 CallableStatementHandler是处理存储过程的，而SimpleStatementHandler、PreparedStatementHandler有什么区别呢?区别是SimpleStatementHandler处理sql中不含有参数的，即sql中不含有参数，而PreparedStatementHandler则处理sql中有?的需要预编译的sql:

List-2是PreparedStatementHandler的，通过ParameterHandler来设置参数，List-3是SimpleStatementHandler的，实现为空。

List-2

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

List-3

@Override
public void parameterize(Statement statement) throws SQLException {
   // N/A
}

单独看StatementHandler，看起来它的方法之间联系不起来，我们需要结合Executor来看，以SimpleExecutor为例，List-4所示，看doQuery方法:

List-4

public class SimpleExecutor extends BaseExecutor {
  ...
  @Override
  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(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
  ...
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }
}

假设使用的SimpleExecutor、PreparedStatementHandler，SimpleExecutor.doQuery->SimpleExecutor.prepareStatement，

调用PreparedStatementHandler.prepare方法，得到jdbc的prepareStatement
PreparedStatementHandler.parameterize方法，见List-2，对prepareStatement进行sql参数设置

之后在doQuery方法中，调用PreparedStatementHandler.query()，如List-5，query方法中再通过ResultSetHandler.handleResultSets来封装成对象。

List-5

@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
}

Executor->StatementHandler->parameterHandler/ResultSetHandler

针对接口编程，用模板模式，用抽象来降低使用者与具体实现类之间的耦合。