这边博客衔接上一篇mybatis的xml解析的博客，在xml解析完成之后，首先会解析成一个Configuration对象，然后创建一个DefaultSqlSessionFactory的session工厂。在这一切的准备过程完成之后，就可以开始对数据库的操作了。
首先看openSession()方法
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); final Executor executor = configuration.newExecutor(tx, execType); return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }首先，根据configuration中取出的environment ，然后获取一个TransactionFactory，接着通过事务工厂新建一个事务对象，其实在这一个步骤，并没有对数据库进行操作newTransaction方法仅仅是返回了一个Transaction对象，这个对象包含了Datasource, level,autocommit这几个属性，并没有做其他操作。（这里我xml中配置了JDBC事务，具体看这个事务，而不是第三方的事务）。
接下来创建一个Executor对象
public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; if (ExecutorType.BATCH == executorType) { executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) { executor = new ReuseExecutor(this, transaction); } else { executor = new SimpleExecutor(this, transaction); } if (cacheEnabled) { executor = new CachingExecutor(executor); } executor = (Executor) interceptorChain.pluginAll(executor); return executor; }当前肯定是创建了一个默认的Executor，就是SimpleExecutor，然后往下，判断是否配置了开启缓存，是的话则通过装饰器模式创建一个CachingExecutor，接着调用interceptorChain.pluginAll方法返回一个被层层代理的对象，这部分在上一篇博客中分析过。返回executor对象，再接下来，new了一个DefaultSqlSession再返回，至此openSession方法就执行结束了。接下来我们就可以调用DefaultSqlSession的select或者update等方法操作数据库了，不过还是看比较主流的方法。
BlogMapper mapper = session.getMapper(BlogMapper.class); Blog blog = mapper.selectBlog(101);
首先看getMapper方法，调用的是configuration对象中mapRegister的getMapper方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) { final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MapperRegistry."); } try { return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } }所以在上一篇博客中看到哪个addMapper方法，存放的是一个MapperProxyFactory工厂，就是因为这里每次getMapper会从对应的工厂中创建代理，这里是Proxy动态代理
@SuppressWarnings("unchecked") protected T newInstance(MapperProxy<T> mapperProxy) { return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); } public T newInstance(SqlSession sqlSession) { final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); }先返回，此时已经获取到了BlogMapper的代理对象，然后执行selectBlog方法，这时候会执行到之前的代理方法中，找到之前的
MapperProxy类的invoke方法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, args); } else { return cachedInvoker(method).invoke(proxy, method, args, sqlSession); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } }这里肯定不是Object类，所以执行cachedInvoker()
private MapperMethodInvoker cachedInvoker(Method method) throws Throwable { try { return methodCache.computeIfAbsent(method, m -> { if (m.isDefault()) { try { if (privateLookupInMethod == null) { return new DefaultMethodInvoker(getMethodHandleJava8(method)); } else { return new DefaultMethodInvoker(getMethodHandleJava9(method)); } } catch (IllegalAccessException | InstantiationException | InvocationTargetException | NoSuchMethodException e) { throw new RuntimeException(e); } } else { return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration())); } }); } catch (RuntimeException re) { Throwable cause = re.getCause(); throw cause == null ? re : cause; } }computeIfAbsent，这是jdk8的语法，大概就是看map中有没有这个key，没有就新建一个并返回新建的这个，有就直接返回，所以这里就是对方法会做一个缓存。现在是第一次执行，肯定是没有，所以会执行后面的创建方法。m.isDefault这些是兼容jdk8以上的接口的默认方法，实现是直接运行那个默认方法。
直接看PlainMethodInvoker，进入new MapperMethod
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) { this.command = new SqlCommand(config, mapperInterface, method); this.method = new MethodSignature(config, mapperInterface, method); }先看SqlCommand的创建
public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) { final String methodName = method.getName(); final Class<?> declaringClass = method.getDeclaringClass(); MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass, configuration); if (ms == null) { if (method.getAnnotation(Flush.class) != null) { name = null; type = SqlCommandType.FLUSH; } else { throw new BindingException("Invalid bound statement (not found): " + mapperInterface.getName() + "." + methodName); } } else { name = ms.getId(); type = ms.getSqlCommandType(); if (type == SqlCommandType.UNKNOWN) { throw new BindingException("Unknown execution method for: " + name); } } }首先，先从configuration中查找出对应的MappedStatement，查找的过程是这样的，先查看当前的类是否存在对应的MappedStatement，如果有直接返回，否则从父类中查找是否有对应的MappedStatement
private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName, Class<?> declaringClass, Configuration configuration) { String statementId = mapperInterface.getName() + "." + methodName; if (configuration.hasStatement(statementId)) { return configuration.getMappedStatement(statementId); } else if (mapperInterface.equals(declaringClass)) { return null; } for (Class<?> superInterface : mapperInterface.getInterfaces()) { if (declaringClass.isAssignableFrom(superInterface)) { MappedStatement ms = resolveMappedStatement(superInterface, methodName, declaringClass, configuration); if (ms != null) { return ms; } } } return null; } }SqlCommand创建完成之后，再看MethodSignature的创建
public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) { Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface); if (resolvedReturnType instanceof Class<?>) { this.returnType = (Class<?>) resolvedReturnType; } else if (resolvedReturnType instanceof ParameterizedType) { this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType(); } else { this.returnType = method.getReturnType(); } //返回类型是否是void this.returnsVoid = void.class.equals(this.returnType); //返回类型是否是集合 this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray(); //返回类型是否是游标 this.returnsCursor = Cursor.class.equals(this.returnType); //返回类型是否是Optional this.returnsOptional = Optional.class.equals(this.returnType); //如果有@Mapkey,返回mapKey this.mapKey = getMapKey(method); //是否是Map this.returnsMap = this.mapKey != null; //找到第几个参数是RowBounds this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class); //找到第几个参数是ResultHandler this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class); //参数解析器,解析@Param中的名称 this.paramNameResolver = new ParamNameResolver(configuration, method); }创建完成之后，返回，调用PlainMethodInvoker的invoke方法，方法中调用的是mapperMethod的execute
public Object execute(SqlSession sqlSession, Object[] args) { Object result; switch (command.getType()) { case INSERT: { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.insert(command.getName(), param)); break; } case UPDATE: { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.update(command.getName(), param)); break; } case DELETE: { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.delete(command.getName(), param)); break; } 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); if (method.returnsOptional() && (result == null || !method.getReturnType().equals(result.getClass()))) { result = Optional.ofNullable(result); } } 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; }这里看select方法
result = sqlSession.selectOne(command.getName(), param);
@Override public <T> T selectOne(String statement, Object parameter) { // Popular vote was to return null on 0 results and throw exception on too many. List<T> list = this.selectList(statement, parameter); if (list.size() == 1) { return list.get(0); } else if (list.size() > 1) { throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size()); } else { return null; } }public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { MappedStatement ms = configuration.getMappedStatement(statement); 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(); } }在这里调用executor的query方法，因为这里是个装饰者对象，所以看CachingExecutor的query方法
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { BoundSql boundSql = ms.getBoundSql(parameterObject); CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }@Override public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, boundSql); @SuppressWarnings("unchecked") List<E> list = (List<E>) tcm.getObject(cache, key); if (list == null) { list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116 } return list; } } return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }首先从MappedStatement中获取Cache，这个Cache在解析xml的时候就已经创建了，如果获取到的不是null，那么首先执行flushCacheIfRequired，这个是通过在解析xml的时候判断是不是select决定的，除了select语句都是true,执行清除缓存，接下来从缓存中获取，如果有缓存，直接返回，如果没有，就执行查询。
接着看委托类的实现，这个委托类的实现在BaseExecutor中
public <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++; list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } if (queryStack == 0) { for (DeferredLoad deferredLoad : deferredLoads) { deferredLoad.load(); } // issue #601 deferredLoads.clear(); if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { // issue #482 clearLocalCache(); } } return list; }这里又是一个Cache,不过这个Cache是mybatis内置的Cache，这就是常说的一级缓存，而这个一级缓存的清除，从代码上看，首先是配置了LocalCacheScope是STATEMENT的时候，默认是Session，然后就是当执行了close方法的时候。
再接着往下看，如果没有命中缓存，就会继续执行查询方法
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List<E> list; localCache.putObject(key, EXECUTION_PLACEHOLDER); try { list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { localCache.removeObject(key); } localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }@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.query(stmt, resultHandler); } finally { closeStatement(stmt); } }doQuery方法是实现类中的方法，当前是SimpleExecutor，先看StatementHandler 的获取
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql); statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler); return statementHandler; }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()); } }首先看RoutingStatementHandler，是通过不同的StatementType创建不同的Handler处理器，MappedStatement 新建默认是PREPARED,CALLABLE是存储过程，STATEMENT就不说了，所以正常情况下创建的都是PreparedStatementHandler，进入构造方法
public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql); }protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { this.configuration = mappedStatement.getConfiguration(); this.executor = executor; this.mappedStatement = mappedStatement; this.rowBounds = rowBounds; this.typeHandlerRegistry = configuration.getTypeHandlerRegistry(); this.objectFactory = configuration.getObjectFactory(); if (boundSql == null) { // issue #435, get the key before calculating the statement generateKeys(parameterObject); boundSql = mappedStatement.getBoundSql(parameterObject); } this.boundSql = boundSql; this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql); this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql); }重点是最后两行，参数处理器和结果处理器的创建
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql); parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler); return parameterHandler; }public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler, ResultHandler resultHandler, BoundSql boundSql) { ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds); resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler); return resultSetHandler; }创建之后，两者都有一个操作，就是使用interceptorChain.pluginAll进行了包装代理，
返回到newStatementHandler，interceptorChain.pluginAll对RoutingStatementHandler同样做了一个包装代理,继续返回
再往下看prepareStatement方法
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; }首先获取一个连接，接着执行handler的prepare方法，方法中调用的是委托类也就是PrepareStatementHandler的prepare方法，其是由父类实现的
@Override public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException { ErrorContext.instance().sql(boundSql.getSql()); Statement statement = null; try { statement = instantiateStatement(connection); setStatementTimeout(statement, transactionTimeout); 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); } }这就是创建了一个JDBC的statement,接下来返回继续执行parameterize方法
public void parameterize(Statement statement) throws SQLException { parameterHandler.setParameters((PreparedStatement) statement); }可以看到调用的是之前创建的ParameterHandler的setParameters方法，把参数设置到statement中，这里需要注意的是，虽然ParameterHandler被plugins代理比RoutingStatementHandler晚，但是实际上ParameterHandler方法的调用是在后面，所以拦截的顺序也在后面。
再往下看，接着会调用query方法
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { PreparedStatement ps = (PreparedStatement) statement; ps.execute(); return resultSetHandler.handleResultSets(ps); }这里就是JDBC的执行了，最后看ResultSetHandler执行的handleResultSets方法
public List<Object> handleResultSets(Statement stmt) throws SQLException { ErrorContext.instance().activity("handling results").object(mappedStatement.getId()); final List<Object> multipleResults = new ArrayList<>(); int resultSetCount = 0; ResultSetWrapper rsw = getFirstResultSet(stmt); List<ResultMap> resultMaps = mappedStatement.getResultMaps(); int resultMapCount = resultMaps.size(); validateResultMapsCount(rsw, resultMapCount); while (rsw != null && resultMapCount > resultSetCount) { ResultMap resultMap = resultMaps.get(resultSetCount); handleResultSet(rsw, resultMap, multipleResults, null); rsw = getNextResultSet(stmt); cleanUpAfterHandlingResultSet(); resultSetCount++; } String[] resultSets = mappedStatement.getResultSets(); if (resultSets != null) { while (rsw != null && resultSetCount < resultSets.length) { ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]); if (parentMapping != null) { String nestedResultMapId = parentMapping.getNestedResultMapId(); ResultMap resultMap = configuration.getResultMap(nestedResultMapId); handleResultSet(rsw, resultMap, null, parentMapping); } rsw = getNextResultSet(stmt); cleanUpAfterHandlingResultSet(); resultSetCount++; } } return collapseSingleResultList(multipleResults); }这段代码大概就是获取出resultsmap，然后对结果进行解析，所以重点是handeResultSet方法
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException { try { if (parentMapping != null) { handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping); } else { if (resultHandler == null) { DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory); handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null); multipleResults.add(defaultResultHandler.getResultList()); } else { handleRowValues(rsw, resultMap, resultHandler, rowBounds, null); } } } finally { // issue #228 (close resultsets) closeResultSet(rsw.getResultSet()); } }首先判断parentMapping 是否null，null就是最外层的resultMap, 然后判断是否有自定义的resultHandler，有的话用自定义的，没有就用默认的，解析完成之后返回，一路回到SimpleExecutor中，最后会执行closeStatement方法关闭连接。
返回结果，执行结束，mybatis的执行流程也就结束了。
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