设计模式之责任链模式

责任链模式

• 责任链模式（Chain of Responsibility Pattern）
• 责任链模式将链中每一个节点都看作一个对象，每个节点处理的请求均不同，且内部自动维护下一个节点对象。
• 当一个请求从链式的首端发出时，会沿着责任链预设的路径依次传递到每一个节点对象，直至被链中的某个对象处理为止

优点：职责链模式使多个对象都有机会处理请求，从而避免请求的发送者和接收者之间的耦合关系

1.2 两个角色

（1）抽象处理者（Handler）：定义处理请求的方法，并维护一个下一处理节点Handler对象的引用
（2）具体处理者（ConcreteHandler）：对请求进行处理，如果不感兴趣，则进行转发

2. 示例

用户的请求传递，直到可以解决这个请求为止

image.png

public abstract class AbstractHandler {

    protected AbstractHandler successor;
    /**
     * 设置继任者
     */
    public void setSuccessor(AbstractHandler successor) {
        this.successor = successor;
    }

    protected abstract void doHandle(String request);
}

public class ConcreteHandlerA extends AbstractHandler {
    @Override
    protected void doHandle(String request) {
        if (request.startsWith("handlerA")) {
            System.out.println("ConcreteHandlerA execute: " + request);
        } else {
            // 请求向下传递
            successor.doHandle(request);
        }
    }
}

public class ConcreteHandlerB extends AbstractHandler {
    @Override
    public void doHandle(String request) {
        if (request.startsWith("handlerB")) {
            System.out.println("ConcreteHandlerB execute: " + request);
        } else {
            successor.doHandle(request);
        }
    }
}

public class ConcreteHandlerC extends AbstractHandler {
    @Override
    protected void doHandle(String request) {
        if (request.startsWith("handlerC")) {
            System.out.println("ConcreteHandlerC execute: " + request);
        } else {
            successor.doHandle(request);
        }
    }
}

测试：

@Test
public void test() {
    AbstractHandler handlerA = new ConcreteHandlerA();
    AbstractHandler handlerB = new ConcreteHandlerB();
    AbstractHandler handlerC = new ConcreteHandlerC();
    handlerA.setSuccessor(handlerB);
    handlerB.setSuccessor(handlerC);

    List list = Arrays.asList("handlerA000", "handlerB111", "handlerC222");
    for (String str : list) {
        handlerA.doHandle(str);
    }
}
``
## 3. 其他
```java
public abstract class AbstractHandle {

    public static void main(String[] args) {
        AbstractHandle handle = Builder.create()
                .addHandler(new ValidateHandle())
                .addHandler(new LoginHandle())
                .addHandler(new AuthHandle())
                .build();
        handle.doHandle("111");
    }

    protected AbstractHandle chain;

    public void next(AbstractHandle handle) {
        this.chain = handle;
    }

    /**
     * 抽象方法
     * @param request
     */
    public abstract void doHandle(String request);

    public static class Builder {
        private AbstractHandle head;
        private AbstractHandle tail;

        public Builder addHandler(AbstractHandle handle) {
            if (this.head == null) {
                this.head = this.tail = handle;
                return this;
            }
            this.tail.next(handle);
            this.tail = handle;
            return this;
        }

        public AbstractHandle build() {
            return this.head;
        }

        public static Builder create() {
            return new Builder();
        }
    }
}

class ValidateHandle extends AbstractHandle {
    @Override
    public void doHandle(String request) {
        System.out.println("ValidateHandle start");
        if (chain != null) {
            chain.doHandle(request);
        }
    }
}

class LoginHandle extends AbstractHandle {
    @Override
    public void doHandle(String request) {
        System.out.println("LoginHandle start");
        if (chain != null) {
            chain.doHandle(request);
        }
    }
}

class AuthHandle extends AbstractHandle {
    @Override
    public void doHandle(String request) {
        System.out.println("AuthHandle start");
        // 若还有下一级，请求继续传递
        if (chain != null) {
            chain.doHandle(request);
        }
    }
}