MyBatis之StatementHandler简析

StatementHandler

StatementHandler就是Statement的处理器，Executor是委托给StatementHandler处理数据库的CRUD操作。例如Executor的query方法，其实调用的handler.query方法，调用前会调用handler取到Java中的Statement实例。

public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {

Statement stmt = null;

Configuration configuration = ms.getConfiguration();

StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);

stmt = prepareStatement(handler, ms.getStatementLog());

return handler.query(stmt, resultHandler);

}

先看下接口StatementHandler的方法，一共八个方法，前几个方法都会调用Statement的对应方法。

StatementHandler

再看下StatementHandler的实现类，RoutingStatementHandler，SimpleStatementHandler，PreparedStatementHandler和CallableStatementHandler，默认是PreparedStatementHandler。

StatementHandler实现类

看Executor中生成Statementhandler的地方会发现使用的是RoutingStatementHandler。然而RoutingStatementHandler又会根据StatementType创建真正的Statementhandler。

StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);

/** 创建一个StatementHandler */

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;

}

为了验证StatementHandler会调用Statement的相应方法，我们看看PreparedStatementHandler中的update方法，代码如下：

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;

}

下文简析ResultSetHandler......