责任链模式有几种写法

前言

本文结合设计模式中责任链模式的实现方式，看看 Android 源码和 OKHttp 等开源库中责任链模式的实现方式。

责任链模式

定义

责任链模式定义如下：Avoid coupling the sender of a request to its receiver by giving morethan one object a chance to handle the request.Chain the receivingobjects and pass the request along the chain until an object handles it.（使多个对象都有机会处理请求，从而避免了请求的发送者和接受者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该请求，直到有对象处理它为止。）

哪些场景需要责任链模式

实现

抽象处理者

public abstract class ResponseHandler {

    private ResponseHandler nextHandler;

    public abstract Level getLevel();

    public abstract Response process(Request request);

    public void setNextHandler(ResponseHandler next) {
        nextHandler = next;
    }

    public final Response handleRequest(Request request) {
        Response response = null;
        if (this.getLevel().equals(request.getRequestLevel())) {
            response = process(request);
        } else {
            if (nextHandler != null) {
                response = nextHandler.handleRequest(request);
            }
        }
        return response;
    }

    static class Level {
        public int weight;

        public Level(int weight) {
            this.weight = weight;
        }

        @Override
        public String toString() {
            return "{" +
                    "weight=" + weight +
                    '}';
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Level level = (Level) o;
            return weight == level.weight;
        }

        @Override
        public int hashCode() {
            return Objects.hash(weight);
        }
    }

    static class Request {
        public String input;
        public Level level;

        public Request(String input, Level level) {
            this.input = input;
            this.level = level;
        }


        public Level getRequestLevel() {
            return this.level;
        }
    }

    static class Response {
        public String content;

        public Response(String content) {
            this.content = content;
        }

        @Override
        public String toString() {
            return "Response{" +
                    "content='" + content + '\'' +
                    '}';
        }
    }
}

抽象的处理者实现三个职责：

一是定义一个请求的处理方法handleMessage，唯一对外开放的方法；
二是定义一个链的编排方法setNext，设置下一个处理者；
三是定义了具体的请求者必须实现的两个方法：定义自己能够处理的级别 getLevel 和具体的处理任务process。

具体的处理者

public class HandlerA extends ResponseHandler {
    @Override
    public Level getLevel() {
        return new Level(0);
    }

    @Override
    public Response process(Request request) {
        System.out.printf("from %s process request %s \n", this.getClass().getSimpleName(), request.input);

        String content = request.input + " level " + request.level;
        request.level.weight++;
        return new Response(content);
    }
}

public class HandlerB extends ResponseHandler {
    @Override
    public Level getLevel() {
        return new Level(1);
    }

    @Override
    public Response process(Request request) {
        System.out.printf("from %s process request %s \n", this.getClass().getSimpleName(), request.input);

        String content = request.input + " level " + request.level;
        request.level.weight++;
        return new Response(content);
    }
}

public class HandlerC extends ResponseHandler {
    @Override
    public Level getLevel() {
        return new Level(2);
    }

    @Override
    public Response process(Request request) {
        System.out.printf("from %s process request %s \n", this.getClass().getSimpleName(), request.input);

        String content = request.input + " level " + request.level;
        request.level.weight++;
        return new Response(content);
    }
}

这里定义了三个具体的处理者。

调用者实现

public class Client {
    public static void main(String[] args) {
        HandlerA handlerA = new HandlerA();
        HandlerB handlerB = new HandlerB();
        HandlerC handlerC = new HandlerC();

        handlerA.setNextHandler(handlerB);
        handlerB.setNextHandler(handlerC);

        ResponseHandler.Level level = new ResponseHandler.Level(2);
        ResponseHandler.Request request = new ResponseHandler.Request(" from client", level);

        ResponseHandler.Response response = handlerA.handleRequest(request);

        System.out.println(response);
    }
}

output

1 2	from HandlerC process request from client Response{content=' from client level {weight=2}'}

参考文档

责任链模式
《设计模式之禅》