MyBatis中StatementHandler java

StatementHandler负责操作Statement对象和数据库进行交流,其中还会用到ParameterHandler和ResultSetHandler进行参数的赋值和结果集的映射
1.StatementHandler中主要的方法

Statement prepare(Connection connection)throws SQLException; void parameterize(Statement statement)throws SQLException; int update(Statement statement) throws SQLException; List query(Statement statement,ResultHandler resultHandler)throws SQLException;

prepare: 用于创建一个Statement实例
parametersize: 用于初始化Statement对象以及对sql的占位符进行赋值
update: 用于通知Statement对象将 insert、update、delete 操作推送到数据库
query: 用于通知Statement对象将 select 操作推送数据库并返回对应的查询结果
2.StatementHandler的继承机构

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StatementHandler的两个实现类
BaseStatementHandler:抽象类,提供了一些公共的实现,简化StatementHandler接口的实现难度
RoutingStatementHandler:StatementHandler接口的默认实现, 根据StatementType获取到真正的StatementHandler,然后赋值给属性delegate

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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()); } }

BaseStatementHandler的三个实现类:
SimpleStatementHandler:处理不需要预编译的sql,不需要处理参数的sql
PreparedStatementHandler:处理需要预编译的sql
CallableStatementHandler:调用存储过程

在查询和更新时才会去获取StatementHandler对象,因此它是由Executor管理和创建的。以SimpleExecutor为例分析生成StatementHandler和Statement的源码 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); //获取Statement对象 ** stmt = prepareStatement(handler, ms.getStatementLog()); ** return handler.query(stmt, resultHandler); } finally { closeStatement(stmt); } }

【MyBatis中StatementHandler】获取Statement实例源码分析

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Statement stmt; Connection connection = getConnection(statementLog); //获取Statement实例 stmt = handler.prepare(connection, transaction.getTimeout()); //stmt进行参数处理 //SimpleStatementHandler中parameterize为空方法,因为它只处理简单sql,有参数的sql由 PreparedStatementHandler处理 handler.parameterize(stmt); return stmt; }

//调用RoutingStatementHandler中的方法,实质是调用了属性delegate的prepare方法 //上面提到了delegate的赋值 public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException { return delegate.prepare(connection, transactionTimeout); } //调用BaseStatementHandler中的prepare public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException { ErrorContext.instance().sql(boundSql.getSql()); Statement statement = null; try { //由子类提供实现 statement = instantiateStatement(connection); setStatementTimeout(statement, transactionTimeout); setFetchSize(statement); return statement; } //SimpleStatementHandler提供的实现 protected Statement instantiateStatement(Connection connection) throws SQLException { if (mappedStatement.getResultSetType() = null) { return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY); } else { return connection.createStatement(); } }