Mybatis中StatementHandler的作用是什么

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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

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

List-3

@Overridepublic 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  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);    }  }  ...  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，

1. 调用PreparedStatementHandler.prepare方法，得到jdbc的prepareStatement

2. 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

@Overridepublic  List query(Statement statement, ResultHandler resultHandler) throws SQLException {    PreparedStatement ps = (PreparedStatement) statement;    ps.execute();    return resultSetHandler. 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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