对HandlerExecutionChain类的理解分析

HandlerExecutionChain类比较简单，好理解。

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/*
 * 处理器执行链由处理器对象和拦截器组成。
 */
public class HandlerExecutionChain {

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下面是类的部分属性。

    private final Object handler;  //处理器对象。

    private HandlerInterceptor[] interceptors; //拦截器数组

    private List<HandlerInterceptor> interceptorList; //拦截器列表

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    /**
     * Apply preHandle methods of registered interceptors.
     * @return {@code true} if the execution chain should proceed with the
     * next interceptor or the handler itself. Else, DispatcherServlet assumes
     * that this interceptor has already dealt with the response itself.
     * 执行已经注册的拦截的 preHandle()方法。如果返回true,则执行链可以执行下一个拦截器的preHandle()方法或 handler 自身。
     * 否则，
     */
    boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = 0; i < interceptors.length; i++) {
                HandlerInterceptor interceptor = interceptors[i];
                if (!interceptor.preHandle(request, response, this.handler)) {
                    triggerAfterCompletion(request, response, null);
                    return false;
                }
                this.interceptorIndex = i;
            }
        }
        return true;
    }

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    /*
     * 执行已经注册的拦截器 postHandle()方法。
     */
    void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = interceptors.length - 1; i >= 0; i--) {
                HandlerInterceptor interceptor = interceptors[i];
                interceptor.postHandle(request, response, this.handler, mv);
            }
        }
    }

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    /**
     * 这个方法只会执行preHandle()方法已经成功执行并且返回true的拦截器中的postHandle()方法。
     */
    void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, 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);
                }
            }
        }
    }

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