Design Pattern - Chain of Responsibility Pattern

Overview - 概述

將鏈中的每個節點看作是一個對象，每個對象處理的請求均不相同，每個節點內部自動維護下一個節點。請求從鏈的首端出發，沿着鏈的路徑，直到有節點處理這個請求。它屬於行爲型設計模式。

【簡言之】
允許你將請求沿着處理者鏈進行發送。 收到請求後， 每個處理者均可對請求進行處理， 或將其傳遞給鏈上的下個處理者。

【WIKI】
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. A mechanism also exists for adding new processing objects to the end of this chain.

Structure - 結構

Applicability - 使用場景

• 需要使用不同的方式處理不同種類的請求，但是請求的類型和順序預先不知道。

這個模式讓幾個處理器連成鏈，一旦你收到請求，詢問每個處理器是否能夠處理它。每個處理器都有機會處理請求。

• 按順序執行多個處理者。

• 動態增加或者刪除處理器。

Pros and Cons - 優缺點

Code Demo - 代碼例子

@Data
@AllArgsConstructor
public class Member {

    private String loginName;

    private String loginPass;

    private String roleName;
}

Deprecated code like the following :

public class MemberService {

    public void login(String loginName, String loginPass) {
        if (StringUtils.isEmpty(loginName) || StringUtils.isEmpty(loginPass)) {
            System.out.println("用戶名或者密碼爲空");
            return;
        }
        Member member = checkExists(loginName, loginPass);
        if (member == null) {
            System.out.println("用戶不存在");
            return;
        }
        System.out.println("登陸成功");
        if (!"管理員".equals(member.getRoleName())) {
            System.out.println("您不是管理員，沒有操作權限 ");
            return;
        }
        System.out.println("允許操作");
    }

    private Member checkExists(String loginName, String loginPass) {
        return new Member(loginName, loginPass, "管理員");
    }
}

Now we can use the pattern to improve below code, and let us try the following code.

Handler

public abstract class Handler {

    @Setter
    protected Handler next;

    public Handler() {}

    public abstract void handle(Member member);
    
}

ValidateHandler

public class ValidateHandler extends Handler {

    @Override
    public void handle(Member member) {
        if (StringUtils.isEmpty(member.getLoginName()) || StringUtils.isEmpty(member.getLoginPass())) {
            System.out.println("用戶名或者密碼爲空");
            return;
        }
        if (super.next != null) {
            super.next.handle(member);
        }
    }
}

LoginHandler

public class LoginHandler extends Handler {

    @Override
    public void handle(Member member) {
        Member m = checkExists(member.getLoginName(), member.getLoginPass());
        if (m == null) {
            System.out.println("用戶不存在");
            return;
        }
        System.out.println("登陸成功");
        if (super.next != null) {
            super.next.handle(member);
        }
    }

    private Member checkExists(String loginName, String loginPass) {
        return new Member(loginName, loginPass, "管理員");
    }
}

AuthHandler

public class AuthHandler extends Handler {

    @Override
    public void handle(Member member) {
        if (!"管理員".equals(member.getRoleName())) {
            System.out.println("您不是管理員，沒有操作權限 ");
            return;
        }
        System.out.println("允許操作");
        if (super.next != null) {
            super.next.handle(member);
        }
    }
}

Client

public class Client {

    public static void main(String[] args) {
        Member member = new Member("Tom", "123", "管理員");
        Handler loginHandler = new LoginHandler();
        Handler validateHandler = new ValidateHandler();
        Handler authHandler = new AuthHandler();
        loginHandler.setNext(validateHandler);
        validateHandler.setNext(authHandler);
        loginHandler.handle(member);
    }
}

責任鏈模式與建造者模式 組合使用

Handler

public abstract class Handler {

    @Setter
    protected Handler next;

    public Handler() {}

    public abstract void handle(Member member);

    public static class Builder {

        private Handler head;
        private Handler tail;

        public Builder addHandler(Handler handler) {
            if (handler == null) {return null;}
            if (this.head == null) {
                this.tail = this.head = handler;
                return this;
            }
            this.tail.setNext(handler);
            this.tail = handler;
            return this;
        }

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

Client

public class Client {

    public static void main(String[] args) {
        Member member = new Member("Tom", "123", "管理員");
        Handler loginHandler = new LoginHandler();
        Handler validateHandler = new ValidateHandler();
        Handler authHandler = new AuthHandler();
        Handler.Builder builder = new Handler.Builder();
        builder.addHandler(loginHandler)
                .addHandler(validateHandler)
                .addHandler(authHandler)
                .build()
                .handle(member);
    }
}