Chain of Responsibility Pattern

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Chain of Responsibility Pattern (Object Behavioral Pattern)

Intent

Decouple the sender of a request from its receiver by giving multiple objects a chance to handle the request. Link these objects into a chain and pass the request along the chain until an object handles it.

Structure

责任链模式

Here’s the breakdown:

Handler: Defines an interface for handling requests; (Optional) implements the successor link.
ConcreteHandler: Handles requests it’s responsible for; can access its successor. If it can handle the request, it does; otherwise, it forwards it to the successor.
Client: Submits requests to a ConcreteHandler object in the chain.

Applicability

Apply the Chain of Responsibility pattern when:

Multiple objects can handle a request, and the specific handler is determined at runtime.
You want to issue a request to one of several objects without explicitly specifying the receiver.
The set of objects that can handle a request should be specifiable dynamically.

Example

A student asks for leave: first to the counsellor. If the counsellor can’t approve, it goes to the dean. If the dean can’t approve, it goes to the principal.

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public class ChainOfResponsibilityPattern {
    public static void main(String[] args) {
        // Instantiate objects
        Handler counsellor = new Counsellor();
        Handler president = new President(); // Represents the Dean
        Handler schoolmaster = new Schoolmaster(); // Represents the Principal

        // Set the next handler in the chain
        counsellor.next = president;
        president.next = schoolmaster;
        schoolmaster.next = null;

        // Request 7 days of leave from the counsellor
        counsellor.HandlerRequest(7);
    }
}

abstract class Handler{
    protected Handler next;

    public void setNext(Handler next){
        this.next = next;
    }

    // Student leave request
    public abstract void HandlerRequest(int request);
}

class Counsellor extends Handler{
    // Counsellor: Approves <= 7 days
    @Override
    public void HandlerRequest(int request){
        if (request <= 7) {
            System.out.println("Counsellor Agree!");
        }
        else {
            if(next != null){
                next.HandlerRequest(request);
            }
            else {
                System.out.println("Counsellor Refuse!");
            }
        }
    }
}

class President extends Handler{
    // Dean: Approves <= 15 days
    @Override
    public void HandlerRequest(int request){
        if (request <= 15) {
            System.out.println("President Agree!");
        }
        else {
            if(next != null){
                next.HandlerRequest(request);
            }
            else {
                System.out.println("President Refuse!");
            }
        }
    }
}

class Schoolmaster extends Handler{
    // Principal: Approves <= 30 days
    @Override
    public void HandlerRequest(int request){
        if (request <= 30) {
            System.out.println("Schoolmaster Agree!");
        }
        else {
            if(next != null){
                next.HandlerRequest(request);
            }
            else {
                System.out.println("Schoolmaster Refuse!");
            }
        }
    }
}