设计模式之备忘录模式

概念

备忘录（Memento）：在不破坏封装性的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态。这样以后就可将该对象恢复到原先保存的状态

优点

• 保留封装边界
• 创建者不需要直接管理版本历史或撤销功能，简化了创建者

什么时候使用备忘录模式

• 需要在Java中捕获对象的状态，然后在不暴露其内部结构的情况下恢复它这对于维护封装和简化对象状态的管理至关重要
• 直接获取状态的接口将暴露实现细节并破坏对象的封装

示例

星体随着时间发生变化

// 星体类型
public enum StarType {
  SUN("sun"),
  RED_GIANT("red giant"),
  WHITE_DWARF("white dwarf"),
  SUPERNOVA("supernova"),
  DEAD("dead star");
  // ...
}

// 星体备忘录和星体类
public interface StarMemento {
}

public class Star {

    private StarType type;
    private int ageYears;
    private int massTons;

    public Star(StarType startType, int startAge, int startMass) {
        this.type = startType;
        this.ageYears = startAge;
        this.massTons = startMass;
    }

    public void timePasses() {
        ageYears *= 2;
        massTons *= 8;
        switch (type) {
            case RED_GIANT -> type = StarType.WHITE_DWARF;
            case SUN -> type = StarType.RED_GIANT;
            case SUPERNOVA -> type = StarType.DEAD;
            case WHITE_DWARF -> type = StarType.SUPERNOVA;
            case DEAD -> {
                ageYears *= 2;
                massTons = 0;
            }
            default -> {
            }
        }
    }

    StarMemento getMemento() {
        var state = new StarMementoInternal();
        state.setAgeYears(ageYears);
        state.setMassTons(massTons);
        state.setType(type);
        return state;
    }

    void setMemento(StarMemento memento) {
        var state = (StarMementoInternal) memento;
        this.type = state.getType();
        this.ageYears = state.getAgeYears();
        this.massTons = state.getMassTons();
    }

    @Override
    public String toString() {
        return String.format("%s age: %d years mass: %d tons", type.toString(), ageYears, massTons);
    }

    private static class StarMementoInternal implements StarMemento {

        private StarType type;
        private int ageYears;
        private int massTons;

        // setters and getters ->
        // ...
    }
}

public static void main(String[] args) {
    var states = new Stack<StarMemento>();

    var star = new Star(StarType.SUN, 10000000, 500000);
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    while (!states.isEmpty()) {
        star.setMemento(states.pop());
        LOGGER.info(star.toString());
    }
}

14:09:15.878 [main] INFO com.iluwatar.memento.App -- sun age: 10000000 years mass: 500000 tons
14:09:15.880 [main] INFO com.iluwatar.memento.App -- red giant age: 20000000 years mass: 4000000 tons
14:09:15.880 [main] INFO com.iluwatar.memento.App -- white dwarf age: 40000000 years mass: 32000000 tons
14:09:15.880 [main] INFO com.iluwatar.memento.App -- supernova age: 80000000 years mass: 256000000 tons
14:09:15.880 [main] INFO com.iluwatar.memento.App -- dead star age: 160000000 years mass: 2048000000 tons
14:09:15.880 [main] INFO com.iluwatar.memento.App -- supernova age: 80000000 years mass: 256000000 tons
14:09:15.880 [main] INFO com.iluwatar.memento.App -- white dwarf age: 40000000 years mass: 32000000 tons
14:09:15.881 [main] INFO com.iluwatar.memento.App -- red giant age: 20000000 years mass: 4000000 tons
14:09:15.881 [main] INFO com.iluwatar.memento.App -- sun age: 10000000 years mass: 500000 tons

注意事项

• 如果保存了大量的状态，可能会消耗过多内存
• 必须小心管理备忘录的生命周期，以避免内存泄漏