备忘录模式主要是用于来防丢失、撤销、恢复等场景，定义是：在不违背封装原则的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态，以便之后恢复对象为先前的状态。

需要三个类， Memento 类提供保存对象和得到对象的方法， Originator 类利用 Memento 类提供保存和读取自身状态的方法，Caretaker 类来记录所有的备份。

把 维基百科 (opens new window) 的例子贴过来。

import java.util.List;
import java.util.ArrayList;
class Originator {
    private String state;
    // The class could also contain additional data that is not part of the
    // state saved in the memento..
 
    public void set(String state) {
        this.state = state;
        System.out.println("Originator: Setting state to " + state);
    }
 
    public Memento saveToMemento() {
        System.out.println("Originator: Saving to Memento.");
        return new Memento(this.state);
    }
 
    public void restoreFromMemento(Memento memento) {
        this.state = memento.getSavedState();
        System.out.println("Originator: State after restoring from Memento: " + state);
    }
 
    public static class Memento {
        private final String state;

        public Memento(String stateToSave) {
            state = stateToSave;
        }
 
        // accessible by outer class only
        private String getSavedState() {
            return state;
        }
    }
}
 
class Caretaker {
    public static void main(String[] args) {
        List<Originator.Memento> savedStates = new ArrayList<Originator.Memento>();
 
        Originator originator = new Originator();
        originator.set("State1");
        originator.set("State2");
        savedStates.add(originator.saveToMemento());
        originator.set("State3");
        // We can request multiple mementos, and choose which one to roll back to.
        savedStates.add(originator.saveToMemento());
        originator.set("State4");
 
        originator.restoreFromMemento(savedStates.get(1));   
    }
}

目前开发中还没有遇到过，此处留坑。