【设计模式】聊聊职责链模式

原理和实现

模板模式变化的是其中一个步骤，而责任链模式变化的是整个流程。
将请求的发送和接收解耦合，让多个接收对象有机会可以处理这个请求，形成一个链条。不同的处理器负责自己不同的职责。

定义接口

public interface Filter {/***  执行过滤方法* @param request* @param response* @param filterChain*/void doFilter (Request request, Response response,FilterChain filterChain);
}

定义职责链
通过内部一个链表 将职责链添加到对应的链表中

public class FilterChain implements Filter{private static List<Filter> filterList = new LinkedList<>();private Filter myFilter;private static int filterIndex = 0;/*** 添加一个职责工作链* @param filter*/public void addFilter (Filter filter) {filterList.add(filter);}public void addMyFilter (Filter filter) {myFilter = filter;}@Overridepublic void doFilter(Request request, Response response, FilterChain filterChain) {if (filterIndex < filterList.size()) {Filter filter = filterList.get(filterIndex);filterIndex++;filter.doFilter(request,response,filterChain);} else {myFilter.doFilter(request,response,filterChain);}}
}

public class HttpFilter implements Filter{@Overridepublic void doFilter(Request request, Response response, FilterChain filterChain) {System.out.println("httpFilter before...");filterChain.doFilter(request,response,filterChain);System.out.println("httpFilter after...");}
}

public class MyFilter implements Filter{@Overridepublic void doFilter(Request request, Response response, FilterChain filterChain) {System.out.println("MyFilter before...");System.out.println("hahaha 我是中间执行人");System.out.println("MyFilter after...");}
}

public class EncodingFilter implements Filter{@Overridepublic void doFilter(Request request, Response response, FilterChain filterChain) {System.out.println("EncodingFilter before...");filterChain.doFilter(request,response,filterChain);System.out.println("EncodingFilter after...");}
}

	public static void main(String[] args) {FilterChain filterChain = new FilterChain();Filter httpFilter = new HttpFilter();Filter encodingFilter = new EncodingFilter();Filter myFilter = new MyFilter();filterChain.addFilter(httpFilter);filterChain.addFilter(encodingFilter);filterChain.addMyFilter(myFilter);filterChain.doFilter(new Request(),new Response(),filterChain);}

其实不管是模板模式、策略模式、职责链模式 本质都是为了提高代码的拓展性、应对代码的复杂性，并且如果新增一个过滤器，那么只需要进行拓展、也满足开闭原则。

在实际的工作中，需要考虑什么场景下符合，就采用对应的设计模式，而不要乱用。

servlet中Filter就是职责链的体现。