备忘录模式(Memento Pattern)又称为快照模式(Snapshot Pattern)或者令牌模式(Token Pattern),是指在不破坏封装的前提下,捕获一个内部状态,并在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。

在软件系统中,备忘录模式为我们提供了一种“后悔药”的机制,它通过存储系统各个历史状态的快照,使得我们可以在任意时刻将系统回滚到某一个历史状态。

一、备忘录模式的应用场景

我们机会天天都在使用备忘录模式,比如使用Git、SVN提供一种代码版本撤回的功能。还有游戏的存档功能,通过将游戏当前进度存储到本地文件系统或数据库中,使得下次继续游戏时,玩家可以从之前的位置继续进行。

 

备忘录模式适用于以下两个场景:

  • 需要保存历史快照的场景;
  • 希望在对象之外保存状态,且除了自己其它类对象无法访问状态保存具体内容。

备忘录模式主要包含三种角色:

  • 发起人角色(Orgainator):负责创建一个备忘录,记录自身需要保存的状态,具备状态回滚功能;
  • 备忘录角色(Memento):用于存储发起人的内部状态,且可以防止发起人以外的对象进行访问;
  • 备忘录管理员(Caretaker):负责存储,提供管理备忘录,无法对备忘录内容进行操作和访问。

1.1 利用压栈管理落地备忘录模式

我们在网页上写文章或者博客都使用过富文本编辑器,它会附带草稿箱、撤销等这样的功能。

下面使用代码来实现这样的功能。假设我们需要发布一篇文章,这篇文章的编辑过程需要花很长的时间,编辑的过程中会不停的撤销,保存草稿、修改。首先创建发起人角色编辑器 Editor 类:

public class Editor {

    private String title;

    private String content;

    private String imgs;

    public Editor(String title, String content, String imgs) {
        this.title = title;
        this.content = content;
        this.imgs = imgs;
    }

    public ArticleMemento save2Memento() {
        ArticleMemento articleMemento =
                new ArticleMemento(this.title, this.content, this.imgs);
        return articleMemento;
    }

    public void undoFromMemento(ArticleMemento articleMemento) {
        this.title = articleMemento.getTitle();
        this.content = articleMemento.getContent();
        this.imgs = articleMemento.getImgs();
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public String getImgs() {
        return imgs;
    }

    public void setImgs(String imgs) {
        this.imgs = imgs;
    }

    @Override
    public String toString() {
        return "Editor{" +
                "title='" + title + '\'' +
                ", content='" + content + '\'' +
                ", imgs='" + imgs + '\'' +
                '}';
    }
}

然后创建备忘录角色 ArticleMemento 类:

public class ArticleMemento {

    private String title;

    private String content;

    private String imgs;

    public ArticleMemento(String title, String content, String imgs) {
        this.title = title;
        this.content = content;
        this.imgs = imgs;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public String getImgs() {
        return imgs;
    }

    public void setImgs(String imgs) {
        this.imgs = imgs;
    }

    @Override
    public String toString() {
        return "ArticleMemento{" +
                "title='" + title + '\'' +
                ", content='" + content + '\'' +
                ", imgs='" + imgs + '\'' +
                '}';
    }
}

创建备忘录管理角色草稿箱 DraftBox 类:

public class DraftBox {

    private final Stack<ArticleMemento> STACK = new Stack<>();

    public ArticleMemento getMemento() {
        ArticleMemento articleMemento = STACK.pop();
        return articleMemento;
    }

    public void addMemento(ArticleMemento articleMemento) {
        STACK.push(articleMemento);
    }

}

草稿箱的Stack类是Vector的一个子类,它实现了一个标准的后进先出的栈。

二、备忘录模式在源码中的体现

备忘录模式在框架源码中的应用还是比较少见的,主要还是结合具体的应用场景来使用。spring中的webfolw源码StateManageableMessageContext接口,我们来看它的源码:

public interface StateManageableMessageContext extends MessageContext {

    /**
     * Create a serializable memento, or token representing a snapshot of the internal state of this message context.
     * @return the messages memento
     */
    public Serializable createMessagesMemento();

    /**
     * Set the state of this context from the memento provided. After this call, the messages in this context will match
     * what is encapsulated inside the memento. Any previous state will be overridden.
     * @param messagesMemento the messages memento
     */
    public void restoreMessages(Serializable messagesMemento);

    /**
     * Configure the message source used to resolve messages added to this context. May be set at any time to change how
     * coded messages are resolved.
     * @param messageSource the message source
     * @see MessageContext#addMessage(MessageResolver)
     */
    public void setMessageSource(MessageSource messageSource);
}

createMessagesMemento()创建一个消息备忘录。可以看一下实现类:

public class DefaultMessageContext implements StateManageableMessageContext {

    private static final Log logger = LogFactory.getLog(DefaultMessageContext.class);

    private MessageSource messageSource;

    @SuppressWarnings("serial")
    private Map<Object, List<Message>> sourceMessages = new AbstractCachingMapDecorator<Object, List<Message>>(
            new LinkedHashMap<Object, List<Message>>()) {

        protected List<Message> create(Object source) {
            return new ArrayList<Message>();
        }
    };

    /**
     * Creates a new default message context. Defaults to a message source that simply resolves default text and cannot
     * resolve localized message codes.
     */
    public DefaultMessageContext() {
        init(null);
    }

    /**
     * Creates a new default message context.
     * @param messageSource the message source to resolve messages added to this context
     */
    public DefaultMessageContext(MessageSource messageSource) {
        init(messageSource);
    }

    public MessageSource getMessageSource() {
        return messageSource;
    }

    // implementing message context

    public Message[] getAllMessages() {
        List<Message> messages = new ArrayList<Message>();
        for (List<Message> list : sourceMessages.values()) {
            messages.addAll(list);
        }
        return messages.toArray(new Message[messages.size()]);
    }

    public Message[] getMessagesBySource(Object source) {
        List<Message> messages = sourceMessages.get(source);
        return messages.toArray(new Message[messages.size()]);
    }

    public Message[] getMessagesByCriteria(MessageCriteria criteria) {
        List<Message> messages = new ArrayList<Message>();
        for (List<Message> sourceMessages : this.sourceMessages.values()) {
            for (Message message : sourceMessages) {
                if (criteria.test(message)) {
                    messages.add(message);
                }
            }
        }
        return messages.toArray(new Message[messages.size()]);
    }

    public boolean hasErrorMessages() {
        for (List<Message> sourceMessages : this.sourceMessages.values()) {
            for (Message message : sourceMessages) {
                if (message.getSeverity() == Severity.ERROR) {
                    return true;
                }
            }
        }
        return false;
    }

    public void addMessage(MessageResolver messageResolver) {
        Locale currentLocale = LocaleContextHolder.getLocale();
        if (logger.isDebugEnabled()) {
            logger.debug("Resolving message using " + messageResolver);
        }
        Message message = messageResolver.resolveMessage(messageSource, currentLocale);
        List<Message> messages = sourceMessages.get(message.getSource());
        if (logger.isDebugEnabled()) {
            logger.debug("Adding resolved message " + message);
        }
        messages.add(message);
    }

    public void clearMessages() {
        sourceMessages.clear();
    }

    // implementing state manageable message context

    public Serializable createMessagesMemento() {
        return new LinkedHashMap<Object, List<Message>>(sourceMessages);
    }

    @SuppressWarnings("unchecked")
    public void restoreMessages(Serializable messagesMemento) {
        sourceMessages.putAll((Map<Object, List<Message>>) messagesMemento);
    }

    public void setMessageSource(MessageSource messageSource) {
        if (messageSource == null) {
            messageSource = new DefaultTextFallbackMessageSource();
        }
        this.messageSource = messageSource;
    }

    // internal helpers

    private void init(MessageSource messageSource) {
        setMessageSource(messageSource);
        // create the 'null' source message list eagerly to ensure global messages are indexed first
        this.sourceMessages.get(null);
    }

    public String toString() {
        return new ToStringCreator(this).append("sourceMessages", sourceMessages).toString();
    }

    private static class DefaultTextFallbackMessageSource extends AbstractMessageSource {
        protected MessageFormat resolveCode(String code, Locale locale) {
            return null;
        }
    }
}

主要逻辑就相当于是给Message留一个备份,以备恢复之用。

三、备忘录模式的优缺点

优点:

  • 简化发起人职责,隔离状态存储与获取,实现了信息的封装,客户端无需关心状态的保存细节;
  • 提供状态回滚功能。

缺点: 消耗资源:如果需要保存的状态过多时,每一次保存都会消耗很多内存。

转载于:https://www.cnblogs.com/1994july/p/12994279.html