责任链模式

简介：javaweb中filter技术用的就是责任链模式。

例子：

• 假设请求和响应都是一个字符串。

public class Request {
    private String message;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

public class Response {
    private String message;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

• 定义一个处理器接口、具体的处理器实现类。

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

一系列处理器实现类

public class FilterOne implements Filter {
    @Override
    public void doFilter(Request request, Response response, FilterChain filterChain) {
        System.out.println("FilterOne:"+request.getMessage());
        filterChain.doFilter(request,response,filterChain);
        System.out.println("FilterOne:"+response.getMessage());
    }
}

public class FilterTwo implements Filter {
    @Override
    public void doFilter(Request request, Response response, FilterChain filterChain) {
        System.out.println("FilterTwo:"+request.getMessage());
        filterChain.doFilter(request,response,filterChain);
        System.out.println("FilterTwo:"+response.getMessage());
    }
}

......FilterThree、FilterFour、FilterFive

• 定义一个处理器链（实现Filter接口是为了实现嵌套用法，简单用法不需要实现）

public class FilterChain implements Filter{
　　//用于存储处理器
    private List<Filter> list = new ArrayList<>();
　　//索引
    private int index = 0;
　　//可以根据需求拓展处理器
    public FilterChain addList(Filter filter) {
        list.add(filter);
        return this;
    }

    public void doFilter(Request request, Response response, FilterChain filterChain) {
        if(list.size()<=index) return;
        Filter f = list.get(index);
        index++;
　　　　　//当FilterChain嵌套另一个FilterChain时需要传入this，完成FilterChain调用的转换
        f.doFilter(request,response,this);
    }
}

测试：

本例中FilterChain嵌套FilterChain，若难以理解，可以不嵌套（忽略-------以下代码），直接运行filterChain2.doFilter完成一个简单的责任链调用。

@Test
    public void testFilter(){
        //请求和响应都是一个字符串
        Request request = new Request();
        request.setMessage("requestMsg");
        Response response = new Response();
        response.setMessage("responseMsg");
        //将处理器one,two,three加入filterChain2
        FilterChain filterChain2 = new FilterChain();
        filterChain2.addList(new FilterOne()).addList(new FilterTwo()).addList(new FilterThree());
　　　　　　//---------------------------
        //将处理器four,five加入filterChain1,并将filterChain2也加入filterChain1,注意：这个责任链也实现了Filter接口,因此也可以加入责任链,且必须在所有处理器最后加入因为：
        //f.doFilter(request,response,this);filterChain调用最后一个处理器的时候也就是另一个filterChain的doFilter传入的this将是另一个filterChain的引用。
        FilterChain filterChain1 = new FilterChain();
        filterChain1.addList(new FilterFour()).addList(new FilterFive()).addList(filterChain2);

        filterChain1.doFilter(request,response,filterChain1);
        
    }

结果

FilterFour:requestMsg
FilterFive:requestMsg
FilterOne:requestMsg
FilterTwo:requestMsg
FilterThree:requestMsg
FilterThree:responseMsg
FilterTwo:responseMsg
FilterOne:responseMsg
FilterFive:responseMsg
FilterFour:responseMsg