责任链模式 Responsibility Chain Pattern

更新日期：2022-6-1

• 2022-6-1：更新格式

定义

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。
Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

每个处理者都必须判断请求，如果符合条件则进行处理，否则就交给下一个处理者。

优缺点

有点是将请求和处理分开。请求者可以不知道是谁处理的，处理者可以不知道请求的全貌。

缺点：性能可能会不好，每个请求都是逐次往后处理。调试不算方便。

处理者链中的节点需要控制，不能出现过长的情况。

代码示例

链式处理示例1

定义一个处理任务的抽象类Handler

public abstract class Handler {

    private Handler nextHandler;

    public final Response handleMessage(Request request) {
        Response response = new Response(Response.ERROR);
        if (this.getHandlerLevel().equals(request.getRequestLevel())) {
            response = this.deal(request);
        } else {
            if (null != this.nextHandler) {
                response = this.nextHandler.handleMessage(request);
            } else {
                System.out.println("WARNING: no handler to: " + request.toString());
            }
        }
        return response;
    }

    public void setNextHandler(Handler handler) {
        this.nextHandler = handler;
    }

    protected abstract Level getHandlerLevel();

    protected abstract Response deal(Request request);

}

定义了一个请求的处理方法handleMessage
setNextHandler是链的编排方法，设置下一个处理者
getHandlerLevel和deal是具体的请求者必须实现的方法

核心文件

ResponsibilityChain/
|-- define
|   |-- Handler.java        // 抽象的处理者
|   |-- Level.java          // 任务等级定义
|   |-- Request.java        // 请求
|   `-- Response.java       // 回应，处理结果
|-- handler                 // 具体的处理者
|   |-- Level1Manager.java
|   |-- Level2Manager.java
|   `-- Level3Manager.java
`-- TestMain.java           // 测试代码

define 包内的 Level， Request， Response 类

public class Level {

    private int intLevel;

    public Level(int levelInt) {
        this.intLevel = levelInt;
    }

    public int getIntLevel() {
        return intLevel;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (obj instanceof Level) {
            Level other = (Level) obj;
            return other.getIntLevel() == this.getIntLevel();
        }
        return false;
    }
}

public class Request {

    private String name;
    private Level requestLevel;

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

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

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return String.format(Locale.ENGLISH, "[%s, %d]", name, getRequestLevel().getIntLevel());
    }
}

public class Response {

    public static final String ERROR = "NO_RESPONSE";

    private String resStr = ERROR;

    public Response(String res) {
        this.resStr = res;
    }

    public String getResStr() {
        return resStr;
    }

    public void printRes() {
        System.out.println(resStr);
    }
}

具体的处理者类在handler包内

public class Level1Manager extends Handler {

    private Level level = new Level(1);

    @Override
    protected Level getHandlerLevel() {
        return level;
    }

    @Override
    protected Response deal(Request request) {
        return new Response(this.getClass().getSimpleName() + " handled this request");
    }

}

public class Level2Manager extends Handler {

    private Level level = new Level(2);

    @Override
    protected Level getHandlerLevel() {
        return this.level;
    }

    @Override
    protected Response deal(Request request) {
        return new Response(this.getClass().getSimpleName() + " handled " + request.toString());
    }
}

public class Level3Manager extends Handler {

    private Level level = new Level(3);

    @Override
    protected Level getHandlerLevel() {
        return this.level;
    }

    @Override
    protected Response deal(Request request) {
        return new Response(this.getClass().getSimpleName() + " handled " + request.toString());
    }
}

测试代码

先定义具体的处理者，并构建处理链
定义不同的请求，处理者处理这些请求

Level1Manager level1Manager = new Level1Manager();
Level2Manager level2Manager = new Level2Manager();
Level3Manager level3Manager = new Level3Manager();

level1Manager.setNextHandler(level2Manager);
level2Manager.setNextHandler(level3Manager);

Request request1 = new Request(new Level(1), "OA-1");
Request request2 = new Request(new Level(2), "OA-2");
Request request3 = new Request(new Level(3), "OA-3");
level1Manager.handleMessage(request2).printRes();
level2Manager.handleMessage(request1).printRes();
level3Manager.handleMessage(request3).printRes();

输出

Level2Manager handled [OA-2, 2]
WARNING: no handler to: [OA-1, 1]
NO_RESPONSE
Level3Manager handled [OA-3, 3]

参考：《设计模式之禅》 秦小波

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