适配器模式在Spring源码中的应用

在 Spring 的 Aop 中，适配器模式应用的非常广泛。Spring 使用 Advice（通知）来增强被代理类的功能，Advice 的类型主要有 BeforeAdvice、AfterReturningAdvice、ThrowsAdvice。每种 Advice 都有对应的拦截器，即 MethodBeforeAdviceInterceptor、AfterReturningAdviceInterceptor、ThrowsAdviceInterceptor。

各种不同类型的 Interceptor，通过适配器统一对外提供接口，如下类图所示：client ---> target ---> adapter ---> interceptor ---> advice。最终调用不同的 advice来实现被代理类的增强。


Spring AOP 的 AdvisorAdapter 类有 4 个实现类，即 SimpleBeforeAdviceAdapter、MethodBeforeAdviceAdapter、AfterReturningAdviceAdapter 和 ThrowsAdviceAdapter。先来看顶层接口 AdvisorAdapter。

public interface AdvisorAdapter {

    boolean supportsAdvice(Advice advice);
    MethodInterceptor getInterceptor(Advisor advisor);
}

再来看 MethodBeforeAdviceAdapter 类。

class MethodBeforeAdviceAdapter implements AdvisorAdapter, Serializable {

    @Override
    public boolean supportsAdvice(Advice advice) {
        return (advice instanceof MethodBeforeAdvice);
    }

    @Override
    public MethodInterceptor getInterceptor(Advisor advisor) {
        MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
        return new MethodBeforeAdviceInterceptor(advice);
    }
}

这里就不把其他两个类的代码贴出来了。Spring 会根据不同的 AOP 配置来确定使用对应的 Advice，与策略模式不同的是，一个方法可以同时拥有多个 Advice。

SpringMVC中的适配器模式

适配器模式在 SpringMVC 中的经典使用体现在它的核心方法 doDispatch 方法中，再来看一个 Spring MVC 中的 HandlerAdapter 类，它也有多个子类，类图如下。


其适配调用的关键代码还是在 DispatcherServlet 的 doDispatch() 方法中，代码如下。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

    try {
        ModelAndView mv = null;
        Exception dispatchException = null;

        try {
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // 此处通过HandlerMapping来映射Controller
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null || mappedHandler.getHandler() == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // 获取适配器
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // Process last-modified header, if supported by the handler.
            String method = request.getMethod();
            boolean isGet = "GET".equals(method);
            if (isGet || "HEAD".equals(method)) {
                long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                if (logger.isDebugEnabled()) {
                    logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                }
                if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                    return;
                }
            }

            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }

            // 通过适配器调用controller的方法并返回ModelAndView
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }

            applyDefaultViewName(processedRequest, mv);
            mappedHandler.applyPostHandle(processedRequest, response, mv);
        } catch (Exception ex) {
            dispatchException = ex;
        } catch (Throwable err) {
            // As of 4.3, we're processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    } catch (Exception ex) {
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    } catch (Throwable err) {
        triggerAfterCompletion(processedRequest, response, mappedHandler,
                new NestedServletException("Handler processing failed", err));
    } finally {
        if (asyncManager.isConcurrentHandlingStarted()) {
            // Instead of postHandle and afterCompletion
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        } else {
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }
}

在 doDispatch() 方法中调用了 getHandlerAdapter() 方法，代码如下。

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
    if (this.handlerAdapters != null) {
        Iterator var2 = this.handlerAdapters.iterator();
        while (var2.hasNext()) {
            HandlerAdapter ha = (HandlerAdapter) var2.next();
            if (this.logger.isTraceEnabled()) {
                this.logger.trace("Testing handler adapter [" + ha + "]");
            }
            if (ha.supports(handler)) {
                return ha;
            }
        }
    }
    throw new ServletException("No adapter for handler [" + handler + "]:The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");

}

在 getHandlerAdapter() 方法中循环调用 supports() 方法来判断是否兼容，循环迭代集合中的 Adapter 在初始化时早已被赋值。