备忘录模式

备忘录模式

在对一个对象进行操作时，希望这个操作时可回退的。那么就要保存每一步操作时前一步对象的状态。

这个时候可以用备忘录模式。

组成部分

• 需要记录状态的类
• 保存对象当前状态和历史状态的类--备忘录类
• 管理备忘录类的类----提供对备忘录中的对象进行增删改查的方法

1.需要记录状态的类

@Data
public class BasePo {
    private String id;
}

@Data
@EqualsAndHashCode(callSuper=false)
public class Student extends BasePo{
    private String name;
    private int age;
    private String grade;
}

2.备忘录类--（保存历史状态时为了减少存储所需空间将对象序列化成了json）

public class Memento<T extends BasePo> {
    private T currentState;

    private LinkedList<String> stateJson;

    public T getCurrentState() {
        return currentState;
    }

    @SuppressWarnings("unchecked")
    public void setCurrentState(Object currentState) {
        this.currentState = (T) currentState;
    }

    public LinkedList<String> getStateJson() {
        return stateJson;
    }

    public void setStateJson(LinkedList<String> stateJson) {
        this.stateJson = stateJson;
    }

    @Override
    public String toString(){
        Field[] fields = currentState.getClass().getDeclaredFields();
        StringBuilder builder = new StringBuilder();
        for (Field field : fields) {
            field.setAccessible(true);
            String firstLetter = field.getName().substring(0,1).toUpperCase();
            String getMethodName = "get"+firstLetter+field.getName().substring(1);
            try {
                Method getMethod = currentState.getClass().getMethod(getMethodName, new Class[] {}); 
                builder.append(" 字段[");
                builder.append(field.getName());
                builder.append("] has value   ");
                builder.append(getMethod.invoke(currentState, new Object[] {})); 
            } catch (Exception e) {
                 
            }
            
        }
        return builder.toString();
    }
}

备忘录类的管理类

public class MementoManage<T extends BasePo> {
    private HashMap<String, Memento<T>> map = new HashMap<String, Memento<T>>();
    
    public Class<?> getTClass(T t) {
        return t.getClass();
    }
    
    public void add(T entity) throws IllegalAccessException, InvocationTargetException, InstantiationException{
        if (map.containsKey(entity.getId())) {
            update(entity);
        }else {
            Memento<T> memento = new Memento<T>();
            Object t = this.getTClass(entity).newInstance();
            BeanUtils.copyProperties(t, entity);
            memento.setCurrentState(t);
            memento.setStateJson(new LinkedList<String>());
            map.put(entity.getId(), memento);
            System.out.println(memento.toString());
        }
    }
    
    public void update(T entity) throws IllegalAccessException, InvocationTargetException, InstantiationException{
        Memento<T> memento = map.get(entity.getId());
        String json = new Gson().toJson(memento.getCurrentState());
        memento.getStateJson().addLast(json);
        Object t = this.getTClass(entity).newInstance();
        BeanUtils.copyProperties(t, entity);
        memento.setCurrentState(t);
        System.out.println(memento.toString());
    }
    
    public void cancel(T entity){
        Memento<T> memento = map.get(entity.getId());
        if (memento.getStateJson().size()==0) {
            System.out.println("沒有可回退的操作");
            System.out.println(memento.toString());
        }else {
            String json = memento.getStateJson().getLast().toString();
            memento.setCurrentState(new Gson().fromJson(json, Student.class));
            memento.getStateJson().removeLast();
            System.out.println(memento.toString());
        }
    }
    
    public void delete(T entity){
        if (map.containsKey(entity.getId())) {
            map.remove(entity.getId());
        }
    }
}

测试类

public static void main(String[] args) throws IllegalAccessException,
            InvocationTargetException, InstantiationException {
        Student student = new Student();
        student.setId(UUID.randomUUID().toString());
        student.setAge(19);
        student.setName("小红");
        student.setGrade("大一");
        MementoManage<Student> manage = new MementoManage<Student>();

        manage.add(student);

        student.setAge(22);
        manage.update(student);

        manage.cancel(student);

        student.setGrade("初三");
        manage.update(student);

        manage.cancel(student);
        manage.delete(student);
        
    }

测试结果

字段[name] has value   小红 字段[age] has value   19 字段[grade] has value   大一
字段[name] has value   小红 字段[age] has value   22 字段[grade] has value   大一
字段[name] has value   小红 字段[age] has value   19 字段[grade] has value   大一
字段[name] has value   小红 字段[age] has value   22 字段[grade] has value   初三
字段[name] has value   小红 字段[age] has value   19 字段[grade] has value   大一