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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。
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原文链接:https://my.oschina.net/u/2518341/blog/3119843