mybatis四大接口之 StatementHandler

1. 继承结构

　　　　

 

2. BaseStatementHandler 

　　三个子类都 继承 prepare() 方法，并没有重写该方法。该方法中调用了方法 instantiateStatement ();

　　 instantiateStatement 是一个抽象方法，根据一个Connection 返回一个Statement 对象；

　　三个子类都实现了该方法，分别返回了 Statement，PrepareStaement 和 CallableStatement 对象。

  @Override
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      statement = instantiateStatement(connection);
      setStatementTimeout(statement, transactionTimeout);
      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);
    }
  }

  protected abstract Statement instantiateStatement(Connection connection) throws SQLException;

 3. 抽象类的子类实现的接口方法

　　三个子类都实现了 update() ，batch() ，query()。

　　在对应的方法中将传入的 Statement 对象转型为 具体的 Statement 类型进行相应的操作。

　　以 PrepareStatement 示例：

  @Override
  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;
  }

  @Override
  public void batch(Statement statement) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.addBatch();
  }

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

4. RoutingStatementHandler

　　该类采用了代理模式（静态代理）；

　　该类直接实现了 StatmentHandler 接口，实现了接口的全部方法。

　　该类持有一个 StatmentHandler  的实例，并在创建该类时根据传入的 MappedStatement 的 type 创建不同的 Statement 实例，然后通过这个具体的实例去实现相应的操作。

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());
    }

  }

  @Override
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    return delegate.prepare(connection, transactionTimeout);
  }

  @Override
  public void parameterize(Statement statement) throws SQLException {
    delegate.parameterize(statement);
  }

  @Override
  public void batch(Statement statement) throws SQLException {
    delegate.batch(statement);
  }

    // 其他方法同理
    ...
}