设计模式（十四）责任链模式

你的名字 2022-06-18 04:11 111阅读 0赞

前面讨论了创建型模式、结构型模式，今天我们来讨论一下行为型模式。责任链模式，就是我们讨论的第一种行为型模式。责任链模式具体不容易说出来，但是如果看一看责任链模式的实际例子就很容易明白了。最典型的例子就是Java EE Servlet中的过滤器，以链式方式对请求进行过滤了。当我们遇到类似需求的时候，也可以使用责任链模式来解决。

下面我们来模拟一下Java Servlet中的Filter。首先，需要请求和响应的接口和实现。这两个接口都很简单。

//请求
    public interface Request { 
        String getMessage();
    }
    
    class RequestImpl implements Request{ 
    private String msg;
    
        public RequestImpl(String msg) {
            this.msg = msg;
        }
    
        @Override
        public String getMessage() {
            return msg;
        }
    }
    
    //响应
    public interface Response { 
        String getMessage();
    }
    
    class ResponseImpl implements Response { 
        private String msg;
    
        public ResponseImpl(String msg) {
            this.msg = msg;
        }
    
        @Override
        public String getMessage() {
            return msg;
        }
    }

之后是过滤器的接口和实现。过滤器将会对请求和处理进行处理和过滤。过滤器1和2是正常的过滤器，过滤之后还会继续调用链上的其它过滤器。而过滤器3则会直接阻断链。

public interface Filter { 
        void doFilter(Request request, Response response, FilterChain chain);
    }
    
    class FilterImpl1 implements Filter { 
    
        @Override
        public void doFilter(Request request, Response response, FilterChain chain) {
            System.out.println("In filter1: " + request.getMessage() + ":" + response.getMessage());
            chain.doFilter(request, response);
        }
    }
    
    class FilterImpl2 implements Filter { 
    
        @Override
        public void doFilter(Request request, Response response, FilterChain chain) {
            System.out.println("In filter2: " + request.getMessage() + ":" + response.getMessage());
            chain.doFilter(request, response);
        }
    }
    
    class FilterImpl3 implements Filter { 
    
        @Override
        public void doFilter(Request request, Response response, FilterChain chain) {
            System.out.println("In filter3: " + request.getMessage() + ":" + response.getMessage());
        }
    }

下面来实现责任链。责任链其实和很简单，开始过滤之后，责任链就会不断的调用链上的过滤器，直到某个过滤器判断不需要继续过滤（例如过滤器3）或者链到头为止。为了让这个链能够复用，我还添加了当前调用链的标记current，标记调用到哪里了，再到头之后重新将current设置为0。

class FilterChain {
        private List<Filter> filters;
        private int current;
    
        public FilterChain(List<Filter> filters) {
            this.filters = filters;
            current = 0;
        }
    
        void doFilter(Request request, Response response) {
            if (current <= filters.size() - 1) {
                Filter currentFilter = filters.get(current);
                current++;
                currentFilter.doFilter(request, response, this);
            } else {
                current = 0;
            }
        }
    }

这样一个责任链就完成了。我们来调用一下。

public void run() {
            Filter filter1 = new FilterImpl1();
            Filter filter2 = new FilterImpl2();
            List<Filter> filters = new ArrayList<>();
            filters.add(filter1);
            filters.add(filter2);
            FilterChain chain = new FilterChain(filters);
    
            Request request = new RequestImpl("request1");
            Response response = new ResponseImpl("response1");
    
            chain.doFilter(request, response);
            System.out.println();
    
            filters.add(0,new FilterImpl3());
            chain.doFilter(request, response);
        }

结果应该类似这样。和我们的预期相同。

责任链模式
    In filter1: request1:response1
    In filter2: request1:response1
    
    In filter3: request1:response1