Java 状态机详解：三种实现方式消除 if-else 嵌套

又或者是这样的：

public class Order {
    private String state;
    public void process() {
        if ("NEW".equals(state)) {
            System.out.println("处理新订单");
            state = "PROCESSING";
        } else if ("PROCESSING".equals(state)) {
            System.out.println("订单处理中");
            state = "SHIPPED";
        } else if ("SHIPPED".equals(state)) {
            System.out.println("订单已发货");
            state = "DELIVERED";
        } else if ("DELIVERED".equals(state)) {
            System.out.println("订单已完成");
        } else {
            throw new IllegalStateException("无效订单状态：" + state);
        }
    }
}

无论是哪一种写法，都会存在以下问题： 当状态增多时，代码变得臃肿难以维护 违反开闭原则，添加新状态需要修改现有代码 状态转换逻辑分散在多个地方 难以跟踪状态转换规则

3. 用状态机模式改造

下面我们使用状态模式重构上述订单处理流程：

3.1 定义状态接口

public interface OrderState {
    void pay(OrderContext context);
    void ship(OrderContext context);
    void confirm(OrderContext context);
    void cancel(OrderContext context);
}

3.2 创建上下文类

public class OrderContext {
    private OrderState state;
    public OrderContext(OrderState state) {
        this.state = state;
    }
    public void setState(OrderState state) {
        this.state = state;
    }
    public void pay() { state.pay(this); }
    public void ship() { state.ship(this); }
    public void confirm() { state.confirm(this); }
    public void cancel() { state.cancel(this); }
}

3.3 定义具体状态类

// 待支付
public class PendingPayState implements OrderState {
    @Override
    public void pay(OrderContext context) {
        System.out.println("订单支付成功");
        context.setState(new PaidState());
    }
    @Override
    public void ship(OrderContext context) {
        System.out.println("请先支付订单");
    }
    @Override
    public void confirm(OrderContext context) {
        System.out.println("请先支付订单");
    }
    @Override
    public void cancel(OrderContext context) {
        System.out.println("订单已取消");
    }
}

// 已支付
public class PaidState implements OrderState {
    @Override
    public void pay(OrderContext context) {
        System.out.println("订单已支付，请勿重复支付");
    }
    @Override
    public void ship(OrderContext context) {
        System.out.println("订单已发货");
        context.setState(new ShippedState());
    }
    @Override
    public void confirm(OrderContext context) {
        System.out.println("订单还未发货");
    }
    @Override
    public void cancel(OrderContext context) {
        System.out.println("已支付订单不能取消");
    }
}

// 已发货
public class ShippedState implements OrderState {
    @Override
    public void pay(OrderContext context) {
        System.out.println("订单已支付");
    }
    @Override
    public void ship(OrderContext context) {
        System.out.println("订单已经发货");
    }
    @Override
    public void confirm(OrderContext context) {
        System.out.println("订单已完成");
        context.setState(new CompletedState());
    }
    @Override
    public void cancel(OrderContext context) {
        System.out.println("发货后不能取消订单");
    }
}

// 已完成
public class CompletedState implements OrderState {
    @Override
    public void pay(OrderContext context) {
        System.out.println("订单已完成，无法支付");
    }
    @Override
    public void ship(OrderContext context) {
        System.out.println("订单已完成，无法发货");
    }
    @Override
    public void confirm(OrderContext context) {
        System.out.println("订单已完成");
    }
    @Override
    public void cancel(OrderContext context) {
        System.out.println("订单已完成，无法取消");
    }
}

3.4 测试使用

public class StateMachineTest {
    public static void main(String[] args) {
        // 初始状态：待支付
        OrderContext order = new OrderContext(new PendingPayState());
        order.pay(); // 支付
        order.ship(); // 发货
        order.confirm(); // 确认收货
        //分别输出
        //订单支付成功
        //订单已发货
        //订单已完成
    }
}

4. 枚举 + Map 的轻量状态机实现

这里再升级一下使用枚举 + Map 的轻量状态机实现，这种写法相比经典状态模式，代码量更少，适合状态数不多、且状态逻辑相对简单的场景。

这种写法的核心思想是：

用枚举类定义所有状态 每个状态实现自己的行为逻辑 通过 Map 注册状态与处理逻辑的映射关系 直接通过当前状态对象来执行对应方法，避免 if-else

4.1 定义状态枚举

public enum OrderStateEnum {
    PENDING_PAY {
        @Override
        public OrderStateEnum pay() {
            System.out.println("订单支付成功");
            return PAID;
        }
        @Override
        public OrderStateEnum ship() {
            System.out.println("请先支付订单");
            return this;
        }
        @Override
        public OrderStateEnum confirm() {
            System.out.println("请先支付订单");
            return this;
        }
        @Override
        public OrderStateEnum cancel() {
            System.out.println("订单已取消");
            return this;
        }
    },
    PAID {
        @Override
        public OrderStateEnum pay() {
            System.out.println("订单已支付，请勿重复支付");
            return this;
        }
        @Override
        public OrderStateEnum ship() {
            System.out.println("订单已发货");
            return SHIPPED;
        }
        @Override
        public OrderStateEnum confirm() {
            System.out.println("订单还未发货");
            return this;
        }
        @Override
        public OrderStateEnum cancel() {
            System.out.println("已支付订单不能取消");
            return this;
        }
    },
    SHIPPED {
        @Override
        public OrderStateEnum pay() {
            System.out.println("订单已支付");
            return this;
        }
        @Override
        public OrderStateEnum ship() {
            System.out.println("订单已经发货");
            return this;
        }
        @Override
        public OrderStateEnum confirm() {
            System.out.println("订单已完成");
            return COMPLETED;
        }
        @Override
        public OrderStateEnum cancel() {
            System.out.println("发货后不能取消订单");
            return this;
        }
    },
    COMPLETED {
        @Override
        public OrderStateEnum pay() {
            System.out.println("订单已完成，无法支付");
            return this;
        }
        @Override
        public OrderStateEnum ship() {
            System.out.println("订单已完成，无法发货");
            return this;
        }
        @Override
        public OrderStateEnum confirm() {
            System.out.println("订单已完成");
            return this;
        }
        @Override
        public OrderStateEnum cancel() {
            System.out.println("订单已完成，无法取消");
            return this;
        }
    };

    public abstract OrderStateEnum pay();
    public abstract OrderStateEnum ship();
    public abstract OrderStateEnum confirm();
    public abstract OrderStateEnum cancel();
}

这里的设计是：

每个状态用一个枚举常量表示 每个枚举常量实现自己的状态切换逻辑 方法返回新的状态枚举，实现状态流转

4.2 配置上下文类

public class OrderContextEnum {
    private OrderStateEnum state;
    public OrderContextEnum(OrderStateEnum state) {
        this.state = state;
    }
    public void pay() { state = state.pay(); }
    public void ship() { state = state.ship(); }
    public void confirm() { state = state.confirm(); }
    public void cancel() { state = state.cancel(); }
}

4.3 测试使用

public class EnumStateMachineTest {
    public static void main(String[] args) {
        OrderContextEnum order = new OrderContextEnum(OrderStateEnum.PENDING_PAY);
        order.pay(); // 支付
        order.ship(); // 发货
        order.confirm(); // 确认收货
    }
}

5. 使用 Spring StateMachine 实现订单状态机

Spring StateMachine 是 Spring 官方提供的状态机框架，支持可配置状态流转、事件驱动、监听器回调等功能，特别适合业务流程复杂、状态多变的场景。

5.1 依赖引入

<dependency>
    <groupId>org.springframework.statemachine</groupId>
    <artifactId>spring-statemachine-core</artifactId>
    <version>3.2.1</version>
</dependency>

5.2 定义状态与事件枚举

// 订单状态
public enum OrderState {
    PENDING_PAY, // 待支付
    PAID, // 已支付
    SHIPPED, // 已发货
    COMPLETED // 已完成
}

// 触发事件
public enum OrderEvent {
    PAY, // 支付
    SHIP, // 发货
    CONFIRM, // 确认收货
    CANCEL // 取消订单
}

5.3 配置状态机

import org.springframework.context.annotation.Configuration;
import org.springframework.statemachine.config.EnableStateMachine;
import org.springframework.statemachine.config.EnumStateMachineConfigurerAdapter;
import org.springframework.statemachine.config.builders.StateMachineStateConfigurer;
import org.springframework.statemachine.config.builders.StateMachineTransitionConfigurer;
import org.springframework.statemachine.config.builders.StateMachineConfigurationConfigurer;
import org.springframework.statemachine.listener.StateMachineListenerAdapter;
import org.springframework.statemachine.state.State;
import java.util.EnumSet;

@Configuration
@EnableStateMachine
public class OrderStateMachineConfig extends EnumStateMachineConfigurerAdapter<OrderState, OrderEvent> {
    @Override
    public void configure(StateMachineStateConfigurer<OrderState, OrderEvent> states) throws Exception {
        states.withStates().initial(OrderState.PENDING_PAY).states(EnumSet.allOf(OrderState.class));
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<OrderState, OrderEvent> transitions) throws Exception {
        transitions
            .withExternal().source(OrderState.PENDING_PAY).target(OrderState.PAID).event(OrderEvent.PAY).and()
            .withExternal().source(OrderState.PAID).target(OrderState.SHIPPED).event(OrderEvent.SHIP).and()
            .withExternal().source(OrderState.SHIPPED).target(OrderState.COMPLETED).event(OrderEvent.CONFIRM).and()
            .withExternal().source(OrderState.PENDING_PAY).target(OrderState.PENDING_PAY).event(OrderEvent.CANCEL);
    }

    @Override
    public void configure(StateMachineConfigurationConfigurer<OrderState, OrderEvent> config) throws Exception {
        config.withConfiguration().listener(newStateMachineListenerAdapter<>() {
            @Override
            public void stateChanged(State<OrderState, OrderEvent> from, State<OrderState, OrderEvent> to) {
                System.out.println("状态切换：" + (from == null ? "无" : from.getId()) + " -> " + to.getId());
            }
        });
    }
}

5.4 测试代码

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.statemachine.StateMachine;
import org.springframework.stereotype.Component;

@Component
public class OrderStateMachineTest implements CommandLineRunner {
    @Autowired
    private StateMachine<OrderState, OrderEvent> stateMachine;

    @Override
    public void run(String... args) throws Exception {
        stateMachine.start();
        stateMachine.sendEvent(OrderEvent.PAY);
        stateMachine.sendEvent(OrderEvent.SHIP);
        stateMachine.sendEvent(OrderEvent.CONFIRM);
        stateMachine.stop();
    }
}

观察控制台会输出如下：

状态切换：无 -> PENDING_PAY 状态切换：PENDING_PAY -> PAID 状态切换：PAID -> SHIPPED 状态切换：SHIPPED -> COMPLETED

6. 对比三种方案

经典状态模式适合复杂业务流程，易于模块化管理 枚举 + Map 注册或枚举直接实现行为，适合小型项目或简单状态流转 在 Java 中，枚举天生是单例的，用它实现状态机既简洁又线程安全

7. 总结

本文详细介绍了使用状态模式消除 if-else，通过经典状态模式、枚举状态机、Spring StateMachine 三种方式，从纯手写模式 → 枚举模式 → 框架模式的完整对比，进行了相关代码演示。当发现自己在编写大量条件语句来处理对象状态时，考虑使用状态模式重构您的代码。