Chain of Responsibility Pattern责任链模式

In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain.
在面向对象的设计来讲，责任链模式是一种组装命令对象和多个处理对象的模式，每个处理对象定义了他能处理的对象的类型以及处理逻辑，否则将会交由之后的处理对象来进行处理。

说白了，实际责任链设计模式就是一种面向对象的if ... else if ... else if ...，只不过能够动态的进行组装。

在看责任链设计模式的时候，发现与Java中现在比较流行的流程化处理(Processor)的思想很相似。本质都是模块的动态组装。不过感觉较大的区别是，责任链模式是为了解决同一个问题来设计的，只不过每个模块能够处理的类型不相同，一个命令对象必定会落到最终的一个模块来处理，在处理过程中入参是并不改变的。而流程化处理则是每个流程处理不同的方面，比如参数校验模块->处理模块->....并且流程化处理主要需要上下文传递信息。

之后有时间专门对流程化的处理进行介绍。现在我们先来看责任链的例子。

责任链类图：

chain of responsibility pattern

例子，以wiki为例chain of responsibiliy pattern。每个模块只能处理在自己范围内的金额，否则会交由后继来处理。

abstract class PurchasePower {
    protected static final double BASE = 500;
    protected PurchasePower successor;

    abstract protected double getAllowable();
    abstract protected String getRole();

    public void setSuccessor(PurchasePower successor) {
        this.successor = successor;
    }

    public void processRequest(PurchaseRequest request) {
        if (request.getAmount() < this.getAllowable()) {
            System.out.println(this.getRole() + " will approve $" + request.getAmount());
        } else if (successor != null) {
            successor.processRequest(request);
        }
    }
}

class ManagerPPower extends PurchasePower {
    
    protected double getAllowable() {
        return BASE * 10;
    }

    protected String getRole() {
        return "Manager";
    }
}

class DirectorPPower extends PurchasePower {
    
    protected double getAllowable() {
        return BASE * 20;
    }

    protected String getRole() {
        return "Director";
    }
}

class VicePresidentPPower extends PurchasePower {
    
    protected double getAllowable() {
        return BASE * 40;
    }

    protected String getRole() {
        return "Vice President";
    }
}

class PresidentPPower extends PurchasePower {

    protected double getAllowable() {
        return BASE * 60;
    }

    protected String getRole() {
        return "President";
    }
}

class PurchaseRequest {

    private double amount;
    private String purpose;

    public PurchaseRequest(double amount, String purpose) {
        this.amount = amount;
        this.purpose = purpose;
    }

    public double getAmount() {
        return this.amount;
    }

    public void setAmount(double amount)  {
        this.amount = amount;
    }

    public String getPurpose() {
        return this.purpose;
    }
    public void setPurpose(String purpose) {
        this.purpose = purpose;
    }
}

class CheckAuthority {

    public static void main(String[] args) {
        ManagerPPower manager = new ManagerPPower();
        DirectorPPower director = new DirectorPPower();
        VicePresidentPPower vp = new VicePresidentPPower();
        PresidentPPower president = new PresidentPPower();
        manager.setSuccessor(director);
        director.setSuccessor(vp);
        vp.setSuccessor(president);

        // Press Ctrl+C to end.
        try {
            while (true) {
                System.out.println("Enter the amount to check who should approve your expenditure.");
                System.out.print(">");
                double d = Double.parseDouble(new BufferedReader(new InputStreamReader(System.in)).readLine());
                manager.processRequest(new PurchaseRequest(d, "General"));
            }
        }
        catch (Exception exc) {
            System.exit(1);
        }
    }
}