责任链模式

责任链模式

理解:
1. 责任链类：内部持有链上处理类的集合,有特殊需求时还需要维护执行到链上第几个处理器的索引；
2. 处理接口：内部定义了处理类的规范，各个方法可以加上责任链类参数来保证是否需要完整的链式调用；
2. 链上处理类：实现处理接口，编写具体的处理方法，并根据实际的需求判断是否继续执行剩下的链上处理类；

具体应用场景：

1. Servlet的Filter：
   1.责任链类：FilterChain拦截器链,内部维护具体的拦截器集合;
   2.处理接口：Filter
   3.链上处理类：HttpFilter, CorsFilter等
2. Spring MVC的拦截器：
   1.责任链类：Spring MVC的DispatcherServlet中doDispatch中使用到HandlerExecutionChain这个类，它内部持有一个HandlerInterceptor集合和遍历handler并执行handler的一些方法，Spring MVC这段代码preHandle和postHandle的在责任链上的执行顺序分别是：preHanlde的执行是顺序遍历interceptor集合并执行，postHandle方法的执行是逆序遍历interceptor集合并执行;
   2.处理接口：HandlerInterceptor
   3.链上处理类：WebContentInterceptor，CorsInterceptor及自定义拦截器

   // 责任链类
   public class HandlerExecutionChain {
       private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);
       private final Object handler;
       @Nullable
       private HandlerInterceptor[] interceptors;
       @Nullable
       // 处理器拦截器集合
       private List<HandlerInterceptor> interceptorList;
       private int interceptorIndex;
   
       public HandlerExecutionChain(Object handler) {
           this(handler, (HandlerInterceptor[])null);
       }
   
       public HandlerExecutionChain(Object handler, @Nullable HandlerInterceptor... interceptors) {
           this.interceptorIndex = -1;
           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;
           }
   
       }
   
       boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
           HandlerInterceptor[] interceptors = this.getInterceptors();
           if (!ObjectUtils.isEmpty(interceptors)) {
           	// 顺序遍历执行interceptor的prehandle方法
               for(int i = 0; i < interceptors.length; this.interceptorIndex = i++) {
                   HandlerInterceptor interceptor = interceptors[i];
                   if (!interceptor.preHandle(request, response, this.handler)) {
                       this.triggerAfterCompletion(request, response, (Exception)null);
                       return false;
                   }
               }
           }
   
           return true;
       }
   
       void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv) throws Exception {
           HandlerInterceptor[] interceptors = this.getInterceptors();
           if (!ObjectUtils.isEmpty(interceptors)) {
           	// 逆序遍历执行interceptor的posthandle方法
               for(int i = interceptors.length - 1; i >= 0; --i) {
                   HandlerInterceptor interceptor = interceptors[i];
                   interceptor.postHandle(request, response, this.handler, mv);
               }
           }
   
       }
   
   }

// 处理接口
public interface HandlerInterceptor {
    default boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        return true;
    }

    default void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable ModelAndView modelAndView) throws Exception {
    }

    default void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable Exception ex) throws Exception {
    }
}

// 链上处理类
private class CorsInterceptor extends HandlerInterceptorAdapter implements CorsConfigurationSource {
    @Nullable
    private final CorsConfiguration config;

    public CorsInterceptor(@Nullable CorsConfiguration config) {
        this.config = config;
    }

    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        return asyncManager.hasConcurrentResult() ? true : AbstractHandlerMapping.this.corsProcessor.processRequest(this.config, request, response);
    }

    @Nullable
    public CorsConfiguration getCorsConfiguration(HttpServletRequest request) {
        return this.config;
    }
}

责任链模式使用

public class MyServlet {
    public static void main(String[] args) {
        Request request = new Request();
        request.str = "request, <script>,我是一名java工程师，我的梦想是做一名架构师";
        Response response = new Response();
        response.str = "response";

        FilterChain chain = new FilterChain();
        chain.add(new HTMLFilter()).add(new SensitiveFilter());
        chain.doFilter(request, response);
        System.out.println(request.str);
        System.out.println(response.str);

    }
}

class Request {
    String str;
}

class Response {
    String str;
}

interface Filter {
    void doFilter(Request request, Response response, FilterChain chain);
}

class HTMLFilter implements Filter {
    @Override
    public void doFilter(Request request, Response response, FilterChain chain) {
        request.str = request.str.replaceAll("<", "[").replaceAll(">", "]") + "HTMLFilter()";
        chain.doFilter(request, response);
        response.str += "--HTMLFilter()";

    }
}

class SensitiveFilter implements Filter {
    @Override
    public void doFilter(Request request, Response response, FilterChain chain) {
        request.str = request.str.replaceAll("java", "后端") + " SensitiveFilter()";
        chain.doFilter(request, response);
        response.str += "--SensitiveFilter()";

    }
}


class FilterChain {
    List<Filter> filters = new ArrayList<>();
    int index = 0;

    public FilterChain add(Filter f) {
        filters.add(f);
        return this;
    }

    public void doFilter(Request request, Response response) {
        if(index == filters.size()) return;
        Filter f = filters.get(index);
        index ++;

        f.doFilter(request, response, this);
    }
}