How to utilize Java memo pattern to save and restore object state?

Introduction

Memento Pattern is a behavioral design pattern that allows the internal state of an object to be captured and saved without destroying encapsulation, and can be restored later. Object to the previous state. The core of this pattern is the Memento class, which stores the internal state of an object.

In Java, the Memento pattern can be implemented by defining a Memento class. Memento classes usually Contains one or more private fields to store the internal state of the object to be saved. The Memo class can also provide public methods to get and set these fields. The original object can use the Memo class to create a memo and save it to the history. In When the state of an object needs to be restored, the original object can obtain the memo from the history record and use the memo to restore its state.

In the memo mode, the following three roles are generally involved:

• Originator (initiator): It is the object whose state is to be saved. It provides an interface for creating memos and restoring memos, allowing other objects to obtain its internal state.

• Memento (Memo): It is an object used to store the internal state of Originator. Memento can save its internal state according to the needs of Originator, and can prevent other objects other than Originator from accessing the state.

• Caretaker (Manager): It is responsible for saving memos and providing them to Originator for recovery when needed. Caretaker can store multiple memos and support multiple undo operations.

When implementing the memo mode in Java, the above three roles are generally involved. Originator and Memento are required, and Caretaker can choose whether to implement it according to actual needs.

Implementation

Suppose there is a game character class Role, which has three attributes: attack (attack power), defense (defense power) and hp (health value). Now we want to implement a memo mode that can save the status of the character and update it when needed Restore the status of the character.

Memo Class

@Data
public class Memento {
    /**
     * 攻击力
     */
    private int attack;
    /**
     * 防御力
     */
    private int defense;
    /**
     * 生命值
     */
    private int hp;
    public Memento(int attack, int defense, int hp) {
        this.attack = attack;
        this.defense = defense;
        this.hp = hp;
    }
}

Initiating Human

@Data
public class Role {
    /**
     * 攻击力
     */
    private int attack;
    /**
     * 防御力
     */
    private int defense;
    /**
     * 生命值
     */
    private int hp;
    public Role(int attack, int defense, int hp) {
        this.attack = attack;
        this.defense = defense;
        this.hp = hp;
    }
    /**
     * 将当前对象储存值Memento中
     * @return
     */
    public Memento save(){
        return new Memento(attack,defense,hp);
    }
    /**
     * 从memento中获取状态;并恢复到当前状态
     * @param memento
     */
    public void restore(Memento memento){
        attack = memento.getAttack();
        defense = memento.getDefense();
        hp = memento.getHp();
    }
}

Manager Class

public class Caretaker {
    private List<Memento> mementos = new ArrayList<>();
    public void addMemento(Memento m){
        mementos.add(m);
    }
    public Memento getMemento(int index){
        return mementos.get(index);
    }
}

Test

public class Demo {
    public static void main(String[] args) {
        Role role = new Role(100,50,20);
        Caretaker caretaker = new Caretaker();
        Memento memento = role.save();
        caretaker.addMemento(memento);
        // 攻击力+10
        role.setAttack(role.getAttack()+10);
        System.out.println(JSON.toJSONString(role));
        // 恢复
        role.restore(caretaker.getMemento(0));
        System.out.println(JSON.toJSONString(role));
    }
}

This is a complete example of the memo mode. In this example, we create a memo class Memento, which saves the character's attack power, defense power and health. We also create a role class Role , which can save and restore the state of the character. Finally, we created a Caretaker class, which is used to save Memento objects. Through the combination of these classes, we implemented the memo mode.

Summary

Advantages

• It can save and restore the object state, making the changes in the object state more flexible and controllable.

• The mode is simple to implement, Easy to understand and use.

• The memo object and the original object are separated to ensure the encapsulation of the system.

Disadvantages

• If the amount of state data to be saved is relatively large, the memo object may occupy a large memory space.

• If the state that needs to be saved is relatively frequent, the memo object may Management may become more complicated.

Application Scenario

• Need to save the history of the object state so that it can be rolled back to a previous state. state, such as the undo operation in a text editor.

• Scenarios where the object state needs to be backed up and restored, such as transaction management in the database.

• Need to implement a snapshot of the object state, while performing subsequent analysis and statistics, such as in-game archiving and playback functions.

In short, the Java memo mode is suitable for objects that need to save and restore the state. Scenarios can help developers better manage changes in object status and improve the flexibility and controllability of the system.

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