设计模式（十四）责任链模式

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