Java 责任链模式讲解和代码示例

责任链是一种行为设计模式，允许你将请求沿着处理者链进行发送，直至其中一个处理者对其进行处理。

该模式允许多个对象来对请求进行处理，而无需让发送者类与具体接收者类相耦合。链可在运行时由遵循标准处理者接口的任意处理者动态生成。

进一步了解责任链模式

复杂度：

流行度：

使用示例： 责任链模式在 Java 程序中并不常见，因为它仅在代码与对象链打交道时才能发挥作用。

该模式最流行的使用案例之一是在 GUI 类中将事件向上传递给父组件。另一个值得注意的使用案例是依次访问过滤器。

下面是该模式在核心 Java 程序库中的一些示例：

识别方法： 该模式可通过一组对象的行为方法间接调用其他对象的相同方法来识别，而且所有对象都会遵循相同的接口。

过滤访问

本例展示了包含用户数据的请求如何依次通过处理者链来执行各种不同的行为（例如认证、授权与验证）。

本例与许多作者给出的该模式的标准版本有些不同。绝大多数模式示例都会寻找正确的处理者，并在处理后退出链。但在这里我们会执行每个处理者，直至某个处理者无法处理请求。请注意，尽管流程略有不同，但这仍是责任链模式。

middleware

middleware/Middleware.java: 基础验证接口

package refactoring_guru.chain_of_responsibility.example.middleware;

/**
 * Base middleware class.
 */
public abstract class Middleware {
    private Middleware next;

    /**
     * Builds chains of middleware objects.
     */
    public static Middleware link(Middleware first, Middleware... chain) {
        Middleware head = first;
        for (Middleware nextInChain: chain) {
            head.next = nextInChain;
            head = nextInChain;
        }
        return first;
    }

    /**
     * Subclasses will implement this method with concrete checks.
     */
    public abstract boolean check(String email, String password);

    /**
     * Runs check on the next object in chain or ends traversing if we're in
     * last object in chain.
     */
    protected boolean checkNext(String email, String password) {
        if (next == null) {
            return true;
        }
        return next.check(email, password);
    }
}

middleware/ThrottlingMiddleware.java: 检查请求数量限制

package refactoring_guru.chain_of_responsibility.example.middleware;

/**
 * ConcreteHandler. Checks whether there are too many failed login requests.
 */
public class ThrottlingMiddleware extends Middleware {
    private int requestPerMinute;
    private int request;
    private long currentTime;

    public ThrottlingMiddleware(int requestPerMinute) {
        this.requestPerMinute = requestPerMinute;
        this.currentTime = System.currentTimeMillis();
    }

    /**
     * Please, not that checkNext() call can be inserted both in the beginning
     * of this method and in the end.
     *
     * This gives much more flexibility than a simple loop over all middleware
     * objects. For instance, an element of a chain can change the order of
     * checks by running its check after all other checks.
     */
    public boolean check(String email, String password) {
        if (System.currentTimeMillis() > currentTime + 60_000) {
            request = 0;
            currentTime = System.currentTimeMillis();
        }

        request++;
        
        if (request > requestPerMinute) {
            System.out.println("Request limit exceeded!");
            Thread.currentThread().stop();
        }
        return checkNext(email, password);
    }
}

middleware/UserExistsMiddleware.java: 检查用户登录信息

package refactoring_guru.chain_of_responsibility.example.middleware;

import refactoring_guru.chain_of_responsibility.example.server.Server;

/**
 * ConcreteHandler. Checks whether a user with the given credentials exists.
 */
public class UserExistsMiddleware extends Middleware {
    private Server server;

    public UserExistsMiddleware(Server server) {
        this.server = server;
    }

    public boolean check(String email, String password) {
        if (!server.hasEmail(email)) {
            System.out.println("This email is not registered!");
            return false;
        }
        if (!server.isValidPassword(email, password)) {
            System.out.println("Wrong password!");
            return false;
        }
        return checkNext(email, password);
    }
}

middleware/RoleCheckMiddleware.java: 检查用户角色

package refactoring_guru.chain_of_responsibility.example.middleware;

/**
 * ConcreteHandler. Checks a user's role.
 */
public class RoleCheckMiddleware extends Middleware {
    public boolean check(String email, String password) {
        if (email.equals("admin@example.com")) {
            System.out.println("Hello, admin!");
            return true;
        }
        System.out.println("Hello, user!");
        return checkNext(email, password);
    }
}

server

server/Server.java: 授权目标

package refactoring_guru.chain_of_responsibility.example.server;

import refactoring_guru.chain_of_responsibility.example.middleware.Middleware;

import java.util.HashMap;
import java.util.Map;

/**
 * Server class.
 */
public class Server {
    private Map<String, String> users = new HashMap<>();
    private Middleware middleware;

    /**
     * Client passes a chain of object to server. This improves flexibility and
     * makes testing the server class easier.
     */
    public void setMiddleware(Middleware middleware) {
        this.middleware = middleware;
    }

    /**
     * Server gets email and password from client and sends the authorization
     * request to the chain.
     */
    public boolean logIn(String email, String password) {
        if (middleware.check(email, password)) {
            System.out.println("Authorization have been successful!");

            // Do something useful here for authorized users.

            return true;
        }
        return false;
    }

    public void register(String email, String password) {
        users.put(email, password);
    }

    public boolean hasEmail(String email) {
        return users.containsKey(email);
    }

    public boolean isValidPassword(String email, String password) {
        return users.get(email).equals(password);
    }
}

Demo.java: 客户端代码

package refactoring_guru.chain_of_responsibility.example;

import refactoring_guru.chain_of_responsibility.example.middleware.Middleware;
import refactoring_guru.chain_of_responsibility.example.middleware.RoleCheckMiddleware;
import refactoring_guru.chain_of_responsibility.example.middleware.ThrottlingMiddleware;
import refactoring_guru.chain_of_responsibility.example.middleware.UserExistsMiddleware;
import refactoring_guru.chain_of_responsibility.example.server.Server;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * Demo class. Everything comes together here.
 */
public class Demo {
    private static BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    private static Server server;

    private static void init() {
        server = new Server();
        server.register("admin@example.com", "admin_pass");
        server.register("user@example.com", "user_pass");

        // All checks are linked. Client can build various chains using the same
        // components.
        Middleware middleware = Middleware.link(
            new ThrottlingMiddleware(2),
            new UserExistsMiddleware(server),
            new RoleCheckMiddleware()
        );

        // Server gets a chain from client code.
        server.setMiddleware(middleware);
    }

    public static void main(String[] args) throws IOException {
        init();

        boolean success;
        do {
            System.out.print("Enter email: ");
            String email = reader.readLine();
            System.out.print("Input password: ");
            String password = reader.readLine();
            success = server.logIn(email, password);
        } while (!success);
    }
}

OutputDemo.txt: 执行结果

Enter email: admin@example.com
Input password: admin_pass
Hello, admin!
Authorization have been successful!


Enter email: wrong@example.com
Input password: wrong_pass
This email is not registered!
Enter email: wrong@example.com
Input password: wrong_pass
This email is not registered!
Enter email: wrong@example.com
Input password: wrong_pass
Request limit exceeded!