设计模式—责任链模式

责任链将处理请求在链条的节点上传递，每到一个节点可被处理或者也可不被处理。

典型的是在servlet中过滤器filter的的处理方式。当request请求或者response请求时，请求经过过滤链上的节点后，又进入下一个节点，直到链尾。

下面以模仿servlet中filter的处理方式（马士兵老师设计模式讲解内容）：

1.过滤器接口

package com.bjsxt.dp.filter;
public interface Filter {
    void doFilter(Request request, Response response, FilterChain chain);
}

2.过滤器链

package com.bjsxt.dp.filter;
import java.util.ArrayList;
import java.util.List;

public class FilterChain implements Filter {
    List<Filter> filters = new ArrayList<Filter>();
    int index = 0;
    
    public FilterChain addFilter(Filter f) {
        this.filters.add(f);
        return this;
    }
    
    @Override
    public void doFilter(Request request, Response response, FilterChain chain) {
        if(index == filters.size()) return ;
        
        Filter f = filters.get(index);
        index ++;
        f.doFilter(request, response, chain);
    }
}

3.过滤链_1

package com.bjsxt.dp.filter;
public class HTMLFilter implements Filter {
  
    @Override
    public void doFilter(Request request, Response response, FilterChain chain) {
        //process the html tag <>
        request.requestStr = request.requestStr.replace('<', '[')
                   .replace('>', ']') + "---HTMLFilter()";
        chain.doFilter(request, response, chain);
        response.responseStr += "---HTMLFilter()";
    }
}

4.过滤器_2

package com.bjsxt.dp.filter;
public class SesitiveFilter implements Filter {

    @Override
    public void doFilter(Request request, Response response, FilterChain chain) {
        request.requestStr = request.requestStr.replace("被就业", "就业")
         .replace("敏感", "") + "---SesitiveFilter()";
        chain.doFilter(request, response, chain);
        response.responseStr += "---SesitiveFilter()";    
    }    
}

5.request请求

package com.bjsxt.dp.filter;
public class Request {
    String requestStr;
    public String getRequestStr() {
        return requestStr;
    }
    public void setRequestStr(String requestStr) {
        this.requestStr = requestStr;
    }
}

6.response请求

package com.bjsxt.dp.filter;
public class Response {
    String responseStr;
    public String getResponseStr() {
        return responseStr;
    }
    public void setResponseStr(String responseStr) {
        this.responseStr = responseStr;
    }
    
}

7.客户端请求

package com.bjsxt.dp.filter;
public class Main {
    /**
     * @param args
     */
    public static void main(String[] args) {
        String msg = "大家好:)，<script>，敏感，被就业，网络授课没感觉，因为看不见大家伙儿";
        Request request = new Request();
        request.setRequestStr(msg);
        Response response = new Response();
        response.setResponseStr("response");
        FilterChain fc = new FilterChain();
        fc.addFilter(new HTMLFilter())
          .addFilter(new SesitiveFilter())
          ;      
        fc.doFilter(request, response, fc);
        System.out.println(request.getRequestStr());
        System.out.println(response.getResponseStr());
    }

}

 FilterChain中创建了一个过滤链，记录了有多少个过滤器，并且递归调用doFilter，经过一个过滤器后就走下一个过滤器，直到链尾。在递归返回的时候接着对response处理，处理的顺序和request

的顺序相反。

过滤链处理之后结果：