设计模式之责任链

1. Chain  of responsibility.是一种比较简单的设计模式。个人理解，Servlet的filter和struts中的Interceptor都会采用这种模式。

2. 以filter为例，filterChain将filter的各个过滤步骤封装成了黑盒子，用户只需将request和response放进去，利用filterChain逐步进行过滤，拿到最终结果。

3. 以下是是一个例子，模拟Filter的执行步骤：其中request依次经过SpecialCharacterFilter和SensitiveWordsFilter的过滤，返回的response依次经过SensitiveWordsFilter和SpecialCharacterFilter的过滤。
   

• 定义Filter接口，这里定义过滤规则

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

   
• 定义分别定义Request和Response两个实体类，每个类中各有一个静态变量：requestStr和responseStr
• 分别定义SpecialCharacterFilter和SensitiveWordsFilter实现Filter接口，并实现doFilter()

  public class SpecialCharacterFilter implements Filter {
  	
  	/*
  	 * replace the '<' to '[', '>' to ']'
  	 * @see com.lisa.dp.filter.Filter#doFilter(java.lang.String)
  	 */
  	@Override
  	public void doFilter(Request request, Response response,FilterChain chain) {
  		String returnStr = null;
  		if(request.requestStr!= null){
  			request.requestStr = request.requestStr.replace('<', '[').replace('>', ']');
  		}
  		chain.doFilter(request, response,chain);
  		response.responseStr = response.responseStr +"-> SpecialCharacterFilter";
  	}
  } 

  public class SensitiveWordsFilter implements Filter {	
  	/*
  	 * delete the sensitive words
  	 * @see com.lisa.dp.filter.Filter#doFilter(java.lang.String)
  	 */
  	@Override
  	public void doFilter(Request request, Response response,FilterChain chain) {
  		if(request.requestStr != null){
  			request.requestStr = request.requestStr.replace("敏感","");
  		}
  		chain.doFilter(request, response,chain);
  		response.responseStr = response.responseStr +"-> SensitiveWordsFilter";
  	}
  
  }

   注意在这里，除了对request信息做完过滤后，交给chain继续执行，待对request操作完之后，便可实现倒序对response进行过滤
• 那么，这里还有一个FilterChain类，该类同时实现Filter接口，在该类中对Filter chain中的filter进行管理，实际操作filter类

  public class FilterChain implements Filter{
  	List<Filter> filters = new ArrayList<Filter>();
  	
  
  	private int index;
  	
  	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;
  		}else{
  			Filter filter = filters.get(index);
  			index ++;
  			filter.doFilter(request, response, chain);
  		}
  	}
  }

•  最后new Main类，进行测试

  public class Main {
  	/**
  	 * @param args
  	 */
  	public static void main(String[] args) {
  		String filterMsg = new String("大家好：)<script>,敏感，被就业！");
  		Request req = new Request();
  		req.setRequestStr(filterMsg);
  		Response response = new Response();
  		response.setResponseStr("response");
  		FilterChain fc = new FilterChain();
  		fc.addFilter(new SpecialCharacterFilter()).addFilter(new SensitiveWordsFilter());//add filter to filter chain
  		fc.doFilter(req, response,fc);//filter chain manage and execute the filters
  		System.out.println("after chain filter: request string = "+req.requestStr);//the final request result
  		System.out.println("after chain filter: response string = "+response.responseStr);//the final response result
  	}
  
  }

     执行结果：

  after chain filter: request string = 大家好：)[script],，被就业！
  after chain filter: response string = response-> SensitiveWordsFilter-> SpecialCharacterFilter

   可以发现，对fiterMsg依次进行了特殊字符过滤和敏感文字过滤。之后对返回的response进行了敏感文字和特殊字符过滤。