上文mybatis源码简书我们讲到sqlsession中通过executor来执行sql，我们接着往下看
update方法点进去，我们进到baseexecutor
这里我们看到 clearLocalCache 方法，可见每次更新都会清除缓存
我们再看到doUpdate
1 public int doUpdate(MappedStatement ms, Object parameter) throws SQLException { 2 Statement stmt = null; 3 try { 4 Configuration configuration = ms.getConfiguration(); 5 StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); 6 stmt = prepareStatement(handler, ms.getStatementLog()); 7 return handler.update(stmt); 8 } finally { 9 closeStatement(stmt);10 }11 } 先看第5行，生成一个 StatementHandler，StatementHandler是mybatis4大对象之一，负责处理Mybatis与JDBC之间Statement的交互。
我们再看下newStatementHandler，
这里又看 RoutingStatementHandler，先不管，再点进去，会发现会有不同类型的StatementHandler
SimpleStatementHandler，这个很简单了，就是对应我们JDBC中常用的Statement接口，用于简单SQL的处理；PreparedStatementHandler，这个对应JDBC中的PreparedStatement，预编译SQL的接口；CallableStatementHandler，这个对应JDBC中CallableStatement，用于执行存储过程相关的接口；RoutingStatementHandler，这个接口是以上三个接口的路由，没有实际操作，只是负责上面三个StatementHandler的创建及调用。
这几个StatementHandler他有共同的构造器，都在BaseStatementHandler
1 protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 2 this.configuration = mappedStatement.getConfiguration(); 3 this.executor = executor; 4 this.mappedStatement = mappedStatement; 5 this.rowBounds = rowBounds; 6 7 this.typeHandlerRegistry = configuration.getTypeHandlerRegistry(); 8 this.objectFactory = configuration.getObjectFactory(); 9 10 if (boundSql == null) { // issue #435, get the key before calculating the statement11 generateKeys(parameterObject);12 boundSql = mappedStatement.getBoundSql(parameterObject);13 }14 15 this.boundSql = boundSql;16 17 this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);18 this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);19 } 这里顺便创建了parameterHandler和 resultSetHandler
parameterHandler的作用就是进行参数预编译设置，resultSetHandler封装我们得到的结果。
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException { 2 Statement stmt = null; 3 try { 4 Configuration configuration = ms.getConfiguration(); 5 StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); 6 stmt = prepareStatement(handler, ms.getStatementLog()); 7 return handler.update(stmt); 8 } finally { 9 closeStatement(stmt);10 }11 } 再看第六行，我们在生成StatementHandler，开始执行prepareStatement
1 private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {2 Statement stmt;3 Connection connection = getConnection(statementLog);4 //调用StatementHandler.prepare5 stmt = handler.prepare(connection);6 //调用StatementHandler.parameterize7 handler.parameterize(stmt);8 return stmt;9 } 我们看到prepare方法，生成statement,用过jdbc的都知道，Statement对象主要用于将 SQL 语句发送到数据库中，执行对数据库的数据的检索或者更新
生成好statement后，我们再看下第7行
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException { 2 Statement stmt = null; 3 try { 4 Configuration configuration = ms.getConfiguration(); 5 StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); 6 stmt = prepareStatement(handler, ms.getStatementLog()); 7 return handler.update(stmt); 8 } finally { 9 closeStatement(stmt);10 }11 } 这里就是真正的执行sql的地方。
