从之前的文章，我们知道了其实mapper真正执行的方法就下面的最后两行。（以下所有的分析都基于一次mybatis的一次select查询。

MapperProxy类中的invoke函数

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);} else if (isDefaultMethod(method)) {return invokeDefaultMethod(proxy, method, args);}} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}//获取MapperMethod final MapperMethod mapperMethod = cachedMapperMethod(method);//执行查询并且返回结果return mapperMethod.execute(sqlSession, args);}

MapperMthod

 private MapperMethod cachedMapperMethod(Method method) {//methodCache其实就是一个缓存，将方法与mapperMethod作为一组键值对进行缓存MapperMethod mapperMethod = methodCache.get(method);//若缓存中没找到则生成一个，在把生成的mapperMethod加入缓存if (mapperMethod == null) {//生成mapperMethod的方法mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());methodCache.put(method, mapperMethod);}return mapperMethod;}

这里解释一下mapperMethod有什么作用，首先看一下MapperMethod类的具体参数

public class MapperMethod {//记录方法是什么类型的方法（UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH;）private final SqlCommand command;//记录方法的具体情况（比如说返回一个还是多个，方法返回类型，方法的参数是啥等信息）private final MethodSignature method;//MapperMethod的构造函数public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {this.command = new SqlCommand(config, mapperInterface, method);this.method = new MethodSignature(config, mapperInterface, method);}}//执行方法public Object execute(SqlSession sqlSession, Object[] args) {Object result;//通过command的类型去找实际需要执行的方法，不一个个分析了，只以select的executeForMany为例子//其他都是差不了多少的switch (command.getType()) {//若是插入类型走这里case INSERT: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));break;}//若是更新类型走这里case UPDATE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break;}//若是删除类型走这里case DELETE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break;}//若是选择类型走这里case SELECT:if (method.returnsVoid() && method.hasResultHandler()) {executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {result = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {result = executeForCursor(sqlSession, args);} else {Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);}break;case FLUSH:result = sqlSession.flushStatements();break;default:throw new BindingException("Unknown execution method for: " + command.getName());}if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {throw new BindingException("Mapper method '" + command.getName() + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");}return result;}//查询方法private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {List<E> result;//Object param = method.convertArgsToSqlCommandParam(args);//是否需要分页if (method.hasRowBounds()) {//拿到rowBoundsRowBounds rowBounds = method.extractRowBounds(args);//查询result = sqlSession.<E>selectList(command.getName(), param, rowBounds);} else {//查询，进入这条result = sqlSession.<E>selectList(command.getName(), param);}// issue #510 Collections & arrays supportif (!method.getReturnType().isAssignableFrom(result.getClass())) {if (method.getReturnType().isArray()) {return convertToArray(result);} else {return convertToDeclaredCollection(sqlSession.getConfiguration(), result);}}return result;}//实际还是走的有RowBounds 的语句，只是给了默认值，默认值是拿到Integer的最大值@Overridepublic <E> List<E> selectList(String statement, Object parameter) {return this.selectList(statement, parameter, RowBounds.DEFAULT);}@Overridepublic <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {//从configuration获取MappedStatement （在初始化的时候就已经放到缓存中了，这里只是获取一下）MappedStatement ms = configuration.getMappedStatement(statement);//通过executor来查询return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);} catch (Exception e) {throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);} finally {ErrorContext.instance().reset();}}

因为mybatis默认开始Cache，所以我们的Executor是CachingExecutor。但是我们没在mapper.xml中配置Cache的属性，所以最终是没缓存功能的。通过装饰器模式来增加了Executor的功能

Executor

//这两个方法是CachingExecutor里面的@Overridepublic <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {//生成BoundSql，里面存放着sql语句BoundSql boundSql = ms.getBoundSql(parameterObject);//生成缓存用的keyCacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}@Overridepublic <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)throws SQLException {//因为mapper中没有配置cache，所以这里的cahce是没有的Cache cache = ms.getCache();//cache为空，所以不会走这里面的逻辑if (cache != null) {flushCacheIfRequired(ms);if (ms.isUseCache() && resultHandler == null) {ensureNoOutParams(ms, parameterObject, boundSql);@SuppressWarnings("unchecked")List<E> list = (List<E>) tcm.getObject(cache, key);if (list == null) {list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);tcm.putObject(cache, key, list); // issue #578 and #116}return list;}}//到来这里，通过delegate来执行query（因为CachingExecutor是一个装饰类，delegate是原始类（在这里是SimpleExecutor））return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}

  @SuppressWarnings("unchecked")@Overridepublic <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());if (closed) {throw new ExecutorException("Executor was closed.");}if (queryStack == 0 && ms.isFlushCacheRequired()) {clearLocalCache();}List<E> list;try {queryStack++;//这里的localCache是一级缓存，是在BaseExecutor中的list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);} else {//如果缓存中没有找到的话，则取数据库中查找list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);}} finally {queryStack--;}if (queryStack == 0) {for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load();}// issue #601deferredLoads.clear();if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}}return list;}private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {List<E> list;//先从key中放个占位的值localCache.putObject(key, EXECUTION_PLACEHOLDER);try {//查询list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {//移除值localCache.removeObject(key);}//把查询结果缓存localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);}return list;}

 @Overridepublic <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {Statement stmt = null;try {Configuration configuration = ms.getConfiguration();//生成一个StatementHandler，对Statement进行处理StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);//生成statementstmt = prepareStatement(handler, ms.getStatementLog());//进行访问return handler.<E>query(stmt, resultHandler);} finally {closeStatement(stmt);}}//这里是prepareStatement函数，用来生成访问数据库需要的Statement对象private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {Statement stmt;//如果是debug状态则对sqlSession进行代理，因为要打印logConnection connection = getConnection(statementLog);//stmt = handler.prepare(connection, transaction.getTimeout());//对参数进行处理handler.parameterize(stmt);return stmt;}

我们进handler的prepare方法看看

  @Overridepublic Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {ErrorContext.instance().sql(boundSql.getSql());Statement statement = null;try {//初始化statement statement = instantiateStatement(connection);//设置超时setStatementTimeout(statement, transactionTimeout);//设置fetchSize(mysql不起作用，不支持这个)setFetchSize(statement);return statement;} catch (SQLException e) {closeStatement(statement);throw e;} catch (Exception e) {closeStatement(statement);throw new ExecutorException("Error preparing statement.  Cause: " + e, e);}}

之后得到了Statement对象后，要去访问数据库了。（上面的return handler.query(stmt, resultHandler);）调用的是下面的query方法

  @Overridepublic <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {return delegate.<E>query(statement, resultHandler);}1234@Overridepublic <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {//将Statement强转成PreparedStatementPreparedStatement ps = (PreparedStatement) statement;//对数据库进行查询（对数据库进行查询是jdbc做的事情，mybatis也只是对jdbc进行了包装）ps.execute();//对返回结果进行处理return resultSetHandler.<E> handleResultSets(ps);}

到这里，我们就进行了一次对数据库的访问，并拿到了数据。数据此时在Statement的ResultSet里面，
如果你熟悉jdbc的话，一定不陌生下面的代码，我们拿到resultSet后的处理方式如下。

        ResultSet resultSet = preparedStatement.executeQuery();while(resultSet.next()) {String string = resultSet.getString(1);}