可以看到，mapper是一个代理对象，它实现的接口就是传入的type，这就是为什么mapper对象可以通过接口直接访问 。同时还可以看到，创建mapper代理对象时传入了sqlsession对象，这样就把sqlsession也关联起来了 。我们进入Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy)方法，注意这个方法传入的参数mapperProxy
@CallerSensitivepublic static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)throws IllegalArgumentException{Objects.requireNonNull(h);//拿到mapper接口类final Class<?>[] intfs = interfaces.clone();final SecurityManager sm = System.getSecurityManager();if (sm != null) {//进行权限检查checkProxyAccess(Reflection.getCallerClass(), loader, intfs);}/** Look up or generate the designated proxy class.*///查找/生成代理类Class<?> cl = getProxyClass0(loader, intfs);/** Invoke its constructor with the designated invocation handler.*/try {if (sm != null) {checkNewProxyPermission(Reflection.getCallerClass(), cl);}//获取构造器final Constructor<?> cons = cl.getConstructor(constructorParams);final InvocationHandler ih = h;if (!Modifier.isPublic(cl.getModifiers())) {AccessController.doPrivileged(new PrivilegedAction<Void>() {public Void run() {cons.setAccessible(true);return null;}});}//将mapperProxy参数转为InvocationHandler 传入return cons.newInstance(new Object[]{h});} catch (IllegalAccessException|InstantiationException e) {throw new InternalError(e.toString(), e);} catch (InvocationTargetException e) {Throwable t = e.getCause();if (t instanceof RuntimeException) {throw (RuntimeException) t;} else {throw new InternalError(t.toString(), t);}} catch (NoSuchMethodException e) {throw new InternalError(e.toString(), e);}}mapperProxy是InvocationHandler的子类，再进入cons.newInstance(new Object[]{h})方法
public T newInstance(Object ... initargs)throws InstantiationException, IllegalAccessException,IllegalArgumentException, InvocationTargetException{if (!override) {if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {Class<?> caller = Reflection.getCallerClass();checkAccess(caller, clazz, null, modifiers);}}if ((clazz.getModifiers() & Modifier.ENUM) != 0)throw new IllegalArgumentException("Cannot reflectively create enum objects");ConstructorAccessor ca = constructorAccessor;// read volatileif (ca == null) {ca = acquireConstructorAccessor();}@SuppressWarnings("unchecked")T inst = (T) ca.newInstance(initargs);return inst;}结果是将mapperProxy作为构造参数返回了一个代理实现类，我们再看下mapperProxy这个类的主要方法 invoke 我们知道对被代理对象的方法的访问都会落实到代理者的invoke上来，MapperProxy的invoke如下：
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);}if (this.isDefaultMethod(method)) {return this.invokeDefaultMethod(proxy, method, args);}} catch (Throwable var5) {throw ExceptionUtil.unwrapThrowable(var5);}MapperMethod mapperMethod = this.cachedMapperMethod(method);return mapperMethod.execute(this.sqlSession, args);}可以看到invoke把执行权转交给了MapperMethod，我们来看看MapperMethod里又是怎么运作的：
public Object execute(SqlSession sqlSession, Object[] args) {Object result;Object param;switch(this.command.getType()) {case INSERT:param = this.method.convertArgsToSqlCommandParam(args);result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));break;case UPDATE:param = this.method.convertArgsToSqlCommandParam(args);result = this.rowCountResult(sqlSession.update(this.command.getName(), param));break;case DELETE:param = this.method.convertArgsToSqlCommandParam(args);result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));break;case SELECT:if (this.method.returnsVoid() && this.method.hasResultHandler()) {this.executeWithResultHandler(sqlSession, args);result = null;} else if (this.method.returnsMany()) {result = this.executeForMany(sqlSession, args);} else if (this.method.returnsMap()) {result = this.executeForMap(sqlSession, args);} else if (this.method.returnsCursor()) {result = this.executeForCursor(sqlSession, args);} else {param = this.method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(this.command.getName(), param);if (this.method.returnsOptional() && (result == null || !this.method.getReturnType().equals(result.getClass()))) {result = Optional.ofNullable(result);}}break;case FLUSH:result = sqlSession.flushStatements();break;default:throw new BindingException("Unknown execution method for: " + this.command.getName());}if (result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {throw new BindingException("Mapper method '" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");} else {return result;}}