23个设计模式学习笔记：06.装饰模式

“单一职责”模式

在软件组件的设计中，如果职责划分的不清晰，使用继承得到的结果往往是随着需求的变化，子类急剧膨胀，同时充斥着重复代码，这时候的关键是划清责任

典型模式

• 装饰模式(Decorator)
• 桥接模式(Bridge)

首先介绍装饰模式

动机(Motivation)

• 在某些情况下我们可能会“过度地使用继承来扩展对象的功能”，由于继承为类型引入的静态特质，使得这种扩展方式缺乏灵活性，并且随着子类的增多(扩展功能的增多)，各种子类的组合(扩展功能的组合)会导致更多子类的膨胀
• 如何使“对象功能的扩展”能够根据需求来动态地实现？同时避免“扩展功能的增多”带来的子类膨胀问题？从而使得任何“功能扩展变化”所导致的影响降为最低？

模式定义

动态(组合)地给一个对象增加一些额外的职责，就增加功能而言，装饰模式比生成子类(继承)更为灵活(消除重复代码&减少子类个数)

结构(Structure)

要点总结

• 通过采用组合而非继承的手法，装饰模式实现了在运行时动态扩展对象功能的能力，而且可以根据需要扩展多个功能，避免了使用继承带来的“灵活性差”和“多子类衍生问题”
• Decorator类在接口上表现为is-a Component的继承关系，即Decorator类继承了Component类所具有的接口，但在实现上又表现为has-a Component的组合关系，即Decorator类又使用了另外一个Component类
• 装饰模式的目的并非解决“多子类衍生的多继承问题”，装饰模式应用的要点在于解决“主体类在多个方向上的扩展功能”——是为“装饰”的含义

代码示例

Mybatis中缓存模块的实现应用到了装饰模式，我们把相关代码提取出来做一个简单的分析，首先我们把缓存模块的代码结构和类继承结构图展示如下，便于理解：

• Mybatis缓存模块代码结构图
  
• Mybatis缓存模块类继承结构图
  

public interface Cache {

  /**
   * @return The identifier of this cache
   */
  String getId();

  /**
   * @param key Can be any object but usually it is a {@link CacheKey}
   * @param value The result of a select.
   */
  void putObject(Object key, Object value);

  /**
   * @param key The key
   * @return The object stored in the cache.
   */
  Object getObject(Object key);

  /**
   * As of 3.3.0 this method is only called during a rollback
   * for any previous value that was missing in the cache.
   * This lets any blocking cache to release the lock that
   * may have previously put on the key.
   * A blocking cache puts a lock when a value is null
   * and releases it when the value is back again.
   * This way other threads will wait for the value to be
   * available instead of hitting the database.
   *
   *
   * @param key The key
   * @return Not used
   */
  Object removeObject(Object key);

  /**
   * Clears this cache instance.
   */
  void clear();

  /**
   * Optional. This method is not called by the core.
   *
   * @return The number of elements stored in the cache (not its capacity).
   */
  int getSize();

  /**
   * Optional. As of 3.2.6 this method is no longer called by the core.
   * 

   * Any locking needed by the cache must be provided internally by the cache provider.
   *
   * @return A ReadWriteLock
   */
  default ReadWriteLock getReadWriteLock() {
    return null;
  }

}

public class PerpetualCache implements Cache {

  private final String id;

  private final Map cache = new HashMap<>();

  public PerpetualCache(String id) {
    this.id = id;
  }

  @Override
  public String getId() {
    return id;
  }

  @Override
  public int getSize() {
    return cache.size();
  }

  @Override
  public void putObject(Object key, Object value) {
    cache.put(key, value);
  }

  @Override
  public Object getObject(Object key) {
    return cache.get(key);
  }

  @Override
  public Object removeObject(Object key) {
    return cache.remove(key);
  }

  @Override
  public void clear() {
    cache.clear();
  }

  @Override
  public boolean equals(Object o) {
    if (getId() == null) {
      throw new CacheException("Cache instances require an ID.");
    }
    if (this == o) {
      return true;
    }
    if (!(o instanceof Cache)) {
      return false;
    }

    Cache otherCache = (Cache) o;
    return getId().equals(otherCache.getId());
  }

  @Override
  public int hashCode() {
    if (getId() == null) {
      throw new CacheException("Cache instances require an ID.");
    }
    return getId().hashCode();
  }

}

/**
 * Lru (least recently used) cache decorator.
 *
 * @author Clinton Begin
 */
public class LruCache implements Cache {

  private final Cache delegate;
  private Map keyMap;
  private Object eldestKey;

  public LruCache(Cache delegate) {
    this.delegate = delegate;
    setSize(1024);
  }

  @Override
  public String getId() {
    return delegate.getId();
  }

  @Override
  public int getSize() {
    return delegate.getSize();
  }

  public void setSize(final int size) {
    keyMap = new LinkedHashMap(size, .75F, true) {
      private static final long serialVersionUID = 4267176411845948333L;

      @Override
      protected boolean removeEldestEntry(Map.Entry eldest) {
        boolean tooBig = size() > size;
        if (tooBig) {
          eldestKey = eldest.getKey();
        }
        return tooBig;
      }
    };
  }

  @Override
  public void putObject(Object key, Object value) {
    delegate.putObject(key, value);
    cycleKeyList(key);
  }

  @Override
  public Object getObject(Object key) {
    keyMap.get(key); //touch
    return delegate.getObject(key);
  }

  @Override
  public Object removeObject(Object key) {
    return delegate.removeObject(key);
  }

  @Override
  public void clear() {
    delegate.clear();
    keyMap.clear();
  }

  private void cycleKeyList(Object key) {
    keyMap.put(key, key);
    if (eldestKey != null) {
      delegate.removeObject(eldestKey);
      eldestKey = null;
    }
  }

}

参考资料

mybatis（六）：设计模式 - 装饰器模式
Mybatis之设计模式之装饰者模式
mybatis 缓存的使用， 看这篇就够了