一、什么是Spring中的处理程序执行链？

Spring中的处理程序执行链是一种由处理程序映射和处理程序拦截器(简单点说就是由谁来处理，处理之前和之后应该干点啥)组成的责任链设计模式。处理器映射器用于将当前请求与其专用的controller进行匹配。拦截器是用来在一些调度动作(如controller解析，视图渲染等)之前和之后所调用的对象。

我们所说的一个处理程序执行链是dispatcher servlet用来处理接收到的请求的一组元素。需要说的是，所有执行链调用都由dispatcher servlet类来进行。其实执行链只是一种容器(见源码):

定义处理程序映射和拦截器
定义在某些时刻应用所应该调度的方法(如处理程序适配器适配之后，controller的方法调用之后等等)

二、HandlerExecutionChain类

处理程序执行链由org.springframework.web.servlet.HandlerExecutionChain类表示。它的主要包含两个私有字段:Object handler 和 HandlerInterceptor[] interceptors,它们被用在请求的调度过程中。第一个包含用于查找处理程序适配器实例的处理程序对象。第二个是包含拦截器的数组，用来应用于处理过的请求(这里这么说是因为这是一条执行链，一个接一个来对这个请求进行处理)。

在DispatcherServlet类中，HandlerExecutionChain的查找通过protected HandlerExecutionChain getHandler(HttpServletRequest request)完成。它遍历所有可用的处理程序映射，并返回能够处理请求的第一个处理程序。

在DispatcherServlet与HandlerExecutionChain实例中要完成的第二件事是应用拦截器的前后调用。这是由DispatcherServlet的方法，如applyPreHandle，applyPostHandle，applyAfterConcurrentHandlingStarted和triggerAfterCompletion。

public class HandlerExecutionChain {

    private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);

    private final Object handler;

    @Nullable
    private HandlerInterceptor[] interceptors;

    @Nullable
    private List<HandlerInterceptor> interceptorList;

    private int interceptorIndex = -1;

  ...
    /**
     * Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
     * Will just invoke afterCompletion for all interceptors whose preHandle invocation
     * has successfully completed and returned true.
     */
    void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex)
            throws Exception {

        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = this.interceptorIndex; i >= 0; i--) {
                HandlerInterceptor interceptor = interceptors[i];
                try {
                    interceptor.afterCompletion(request, response, this.handler, ex);
                }
                catch (Throwable ex2) {
                    logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
                }
            }
        }
    }

    /**
     * Apply afterConcurrentHandlerStarted callback on mapped AsyncHandlerInterceptors.
     */
    void applyAfterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response) {
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = interceptors.length - 1; i >= 0; i--) {
                if (interceptors[i] instanceof AsyncHandlerInterceptor) {
                    try {
                        AsyncHandlerInterceptor asyncInterceptor = (AsyncHandlerInterceptor) interceptors[i];
                        asyncInterceptor.afterConcurrentHandlingStarted(request, response, this.handler);
                    }
                    catch (Throwable ex) {
                        logger.error("Interceptor [" + interceptors[i] + "] failed in afterConcurrentHandlingStarted", ex);
                    }
                }
            }
        }
    }

 /**
     * Apply afterConcurrentHandlerStarted callback on mapped AsyncHandlerInterceptors.
     */
    void applyAfterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response) {
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = interceptors.length - 1; i >= 0; i--) {
                if (interceptors[i] instanceof AsyncHandlerInterceptor) {
                    try {
                        AsyncHandlerInterceptor asyncInterceptor = (AsyncHandlerInterceptor) interceptors[i];
                        asyncInterceptor.afterConcurrentHandlingStarted(request, response, this.handler);
                    }
                    catch (Throwable ex) {
                        logger.error("Interceptor [" + interceptors[i] + "] failed in afterConcurrentHandlingStarted", ex);
                    }
                }
            }
        }
    }

org.springframework.web.servlet.DispatcherServlet

/**
     * Return the HandlerExecutionChain for this request.
     * <p>Tries all handler mappings in order.
     * @param request current HTTP request
     * @return the HandlerExecutionChain, or {@code null} if no handler could be found
     */
    @Nullable
    protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        if (this.handlerMappings != null) {
            for (HandlerMapping hm : this.handlerMappings) {
                if (logger.isTraceEnabled()) {
                    logger.trace(
                            "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
                }
                HandlerExecutionChain handler = hm.getHandler(request);
                if (handler != null) {
                    return handler;
                }
            }
        }
        return null;
    }

...

  protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        //HandlerExecutionChain定义出来，做成函数内局部变量可以做到逃逸管理，和request请求做到用完即毁，提        //高性能，防止内存泄漏
        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);

                // Determine handler for the current request.
                //拿到执行链
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                //通过处理器找到相应的适配器，其实就是个拓展代理，参考之前我写的Spring设计模式
                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;
                    }
                }
                //提前批预处理
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // Actually invoke the handler.
                //执行核心处理
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                //默认视图渲染
                applyDefaultViewName(processedRequest, mv);
                //秋后算账
                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
                //如果是异步的话，不需要解释了吧，看上面英文
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

三、自定义处理程序执行链

了说明处理程序执行链的使用，我们将从关于Spring DispatcherServlet生命周期的文章中拿到我们自定义的dispatcher servlet类，并向其添加一个自定义的处理程序执行链。但是，无须深挖并使用HandlerExecutionChain中的所有Spring的类，我们来创建一个新的对象(DumberHandlerExecutionChain)，添加两个方法来调用拦截器，并在DispatcherServlet的类中使用它。请看下面编写的代码:

// we start directly by doService method which handles incoming request
// retrieve execution chain and handler adapters adapted to received request
DumberHandlerExecutionChain executionChain = new DumberHandlerExecutionChain(getHandlerExecutionChain(request));
System.out.println("Working with following handler execution chain: "+executionChain);
HandlerAdapter adapter = getHandlerAdapter(executionChain.getHandler());

if (!executionChain.preHandle(request, response)) {
    throw new IllegalStateException("Some of defined interceptors weren't ivoked correctly.");
}
// handle the request and try to generate a ModelAndView instance
ModelAndView modelView = adapter.handle(request, response, executionChain.getHandler());
if (modelView == null) {
    throw new IllegalStateException("Handled ModelAndView can't be null (handled with adapter: "+adapter+")");
}
if (!modelView.isReference()) {
    throw new UnsupportedOperationException("Only view models defined as references can be used in this servlet");
}
executionChain.postHandle(request, response, modelView);

只需要改变3行代码。第一个是DumberHandlerExecutionChain实例的定义，替换掉HandlerExecutionChain。第二个更改是applyPreHandler和applyPostHandler方法。这段代码这样来看的话好理解吧。我们来看看DumberHandlerExecutionChain类的定义:

public class DumberHandlerExecutionChain extends HandlerExecutionChain {

    public DumberHandlerExecutionChain(HandlerExecutionChain chain) {
        super(chain);
        System.out.println("Overriden constructor DumberHandlerExecutionChain invoked");
    }

    public boolean preHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
        for (HandlerInterceptor interceptor : getInterceptors()) {
            System.out.println("Running pre handler for :"+interceptor);
            if (!interceptor.preHandle(request, response, this.getHandler())) {
                System.out.println("An error occured on calling handler for "+interceptor);
                return false;
            }
        }
        return true;
    }

    public void postHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView viewModel) throws Exception {
        for (HandlerInterceptor interceptor : getInterceptors()) {
            interceptor.postHandle(request, response, this.getHandler(), viewModel);
            System.out.println("Running post handler for :"+interceptor);
        }
    }

    @Override
    public String toString() {
        return "DumberHandlerExecutionChain {interceptors :"+Arrays.asList(this.getInterceptors())+", handler: "+this.getHandler()+"}";
    }
}

它们是前面提到的两种方法:preHandle和postHandle。两者很相似。他们首先迭代所有可用的拦截器。区别在于第一个调用拦截器的preHandle方法和第二个拦截器的postHandle方法。第二个区别是结果。如果所有拦截器正确完成了操作，preHandle将返回true。postHandle不返回任何东西(这里和HandlerExecutionChain源码内相应的方法实现大致一样，但是做了逻辑上的简单处理达到咱们想要的效果即可)。

但这两种方法并不是这个类的核心。它最重要的地方是调用这个父类的构造函数:

// 1. Invoked directly by super(chain) call
public HandlerExecutionChain(Object handler) {
        this(handler, null);
}

// 2. Called directly by previous constructor
public HandlerExecutionChain(Object handler, @Nullable HandlerInterceptor... interceptors) {
        if (handler instanceof HandlerExecutionChain) {
            HandlerExecutionChain originalChain = (HandlerExecutionChain) handler;
            this.handler = originalChain.getHandler();
            this.interceptorList = new ArrayList<>();
            CollectionUtils.mergeArrayIntoCollection(originalChain.getInterceptors(), this.interceptorList);
            CollectionUtils.mergeArrayIntoCollection(interceptors, this.interceptorList);
        }
        else {
            this.handler = handler;
            this.interceptors = interceptors;
        }
    }

由上可以看到，通过handler instanceof检查，我们可以在dispatcher servlet中本地生成HandlerExecutionChain。我们不需要查找产生HandlerExecutionChain实例的处理程序映射(例如:org.springframework.web.servlet.handler.AbstractHandlerMapping或org.springframework.web.servlet.handler.AbstractUrlHandlerMapping的实现类)并覆盖重写现有代码。而不是使用这些复杂的步骤，我们只需简单地将HandlerExecutionChain的实例传递给我们自定义的执行链类的构造函数即可。

完成上面的工作，接下来，你可以在日志中看到以下信息:

Overriden constructor DumberHandlerExecutionChain invoked
//tag1
Working with following handler execution chain: DumberHandlerExecutionChain {interceptors :[org.springframework.web.servlet.handler.ConversionServiceExposingInterceptor@77f6d2e3, com.migo.interceptors.LotteryInterceptor@6d8f729c], handler: public java.lang.String com.migo.controller.TestController.test(javax.servlet.http.HttpServletRequest)}
//pre
Running pre handler for :org.springframework.web.servlet.handler.ConversionServiceExposingInterceptor@77f6d2e3
Running pre handler for :com.migo.interceptors.LotteryInterceptor@6d8f729c
[LotteryInterceptor] preHandle
//excute Controller
Controller asks, are you a lottery winner ? true
Current locale is :org.springframework.web.servlet.DispatcherServlet$1@5cf346dc
Request attributes are :org.apache.catalina.connector.RequestFacade@7d9ccb73
//post
Running post handler for :org.springframework.web.servlet.handler.ConversionServiceExposingInterceptor@77f6d2e3
[LotteryInterceptor] postHandle
Running post handler for :com.migo.interceptors.LotteryInterceptor@6d8f729c

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