Chain of Responsibility — 責任鏈模式

Tue, 07 Aug 2018 22:37:08 +0800

1. 意圖

將請求沿著處理者鏈依序傳遞，每個處理者決定自己是否處理，或繼續往下傳。

2. UML 類圖

classDiagram
    class Handler {
        <>
        #next Handler
        +setNext(Handler)
        +handle(request)*
    }
    class ConcreteHandlerA {
        +handle(request)
    }
    class ConcreteHandlerB {
        +handle(request)
    }
    Handler <|-- ConcreteHandlerA
    Handler <|-- ConcreteHandlerB
    Handler --> Handler : next

3. 何時用 / 何時別用

用：

超過一個物件可能處理同一請求，且處理者在執行期才決定（審批流、過濾管道）
需要動態組裝或重排處理者順序
不想讓發送者知道具體由誰處理

別用：

每個請求必須保證被處理——責任鏈可能讓請求靜默消失
處理者固定且少（2-3 個），直接 if/else 更清楚
鏈很長時效能敏感的熱路徑

4. Java 範例

// 審批請求：金額超過閾值就往上傳
public abstract class Approver {
    private Approver next;

    public Approver setNext(Approver next) {
        this.next = next;
        return next;
    }

    public final void approve(long amount) {
        if (canApprove(amount)) {
            doApprove(amount);
        } else if (next != null) {
            next.approve(amount);
        } else {
            throw new IllegalStateException("No approver for amount: " + amount);
        }
    }

    protected abstract boolean canApprove(long amount);
    protected abstract void doApprove(long amount);
}

public class TeamLead extends Approver {
    @Override protected boolean canApprove(long amount) { return amount <= 1_000; }
    @Override protected void doApprove(long amount) {
        System.out.println("TeamLead approved: " + amount);
    }
}

public class Manager extends Approver {
    @Override protected boolean canApprove(long amount) { return amount <= 10_000; }
    @Override protected void doApprove(long amount) {
        System.out.println("Manager approved: " + amount);
    }
}

public class Director extends Approver {
    @Override protected boolean canApprove(long amount) { return true; }
    @Override protected void doApprove(long amount) {
        System.out.println("Director approved: " + amount);
    }
}

// 組裝鏈
Approver chain = new TeamLead();
chain.setNext(new Manager()).setNext(new Director());

chain.approve(500);     // TeamLead approved: 500
chain.approve(5_000);   // Manager approved: 5000
chain.approve(50_000);  // Director approved: 50000

5. JDK / Spring 真實案例

ClassLoader 親委派：ClassLoader.loadClass() 先委派給 parent，parent 找不到才自己載入，這就是責任鏈的「先傳再自處理」變體。

ChainOfResponsibility on Bill.Lin's Notes

Chain of Responsibility — 責任鏈模式

1. 意圖

2. UML 類圖

3. 何時用 / 何時別用

4. Java 範例

5. JDK / Spring 真實案例