Typically object caching in Java is managed by the container or framework in use. Occasionally however there is a need to manually cache domain-specific objects, whereby a java.util.Map implementation will not suffice.

Using the popular ehcache framework as an example, the following pattern is typically observed:

public class SomeClass {

  private final Cache cache = ...

  ...

  public void doSomethingWithObject(Object key) {
    SomeObject o = getSomeObject(key);
    o.doSomething();
  }

  public SomeObject getSomeObject(Object key) {
    SomeObject o = null;

    Element element = cache.get(key);
    if (element != null) {
      o = element.getValue();
    }
    else {
      o = load(key);
      cache.put(new Element(key, o));
    }
    return o;
  }

  private Object load(Object key) {
    ...
  }
}

The common aspects of this pattern are as follows:

• Cache – the cache instance
• Key – the unique key of the cachable object
• load() – the mechanism for loading objects not in the cache

Key Uniformity

As most caching frameworks will allow any object to be used as a key, there is potential for different types of errors, such as a value specified as a key, mixing object types in a single cache, added to the wrong cache instance, etc. We can avoid some of these problems by enforcing a uniform approach to defining cache keys:

public enum CacheEntry {

  SomeObject("org.mnode.example.someObject.%s");

  private String key;

  public String getKey(Object uid) {
    return String.format(key, uid);
  }
}

public class SomeClass {
  ...

  public SomeObject getSomeObject(Object uid) {
    SomeObject o = null;
    String key = CacheEntry.SomeObject.getKey(uid);
    ...
  }
}

As this approach enforces a key ‘namespace’ for specific object types, it also makes it easier to store mixed data in a single cache, thus simplifying the management of cached objects:

public class SomeClass {
  ...

  public <T> T get(CacheEntry entry, Object uid) {
    T o = null;
    String key = entry.getKey(uid);

    Element element = cache.get(key);
    if (element != null) {
      o = (T) element.getValue();
    }
    else {
      o = (T) load(key);
      cache.put(new Element(key, o));
    }
    return o;
  }
}

Object Loading

Different types of cached data will also usually require specific code for loading the data initially. We can refactor this to be defined as part of the CacheEntry:

interface Loader<T> {
  T load(Object...args);
}

public enum CacheEntry {

  SomeObject("org.mnode.example.someObject.%s", new Loader<SomeObject> {
    SomeObject load(Object...args) {
      Object uid = args[0];
      // load data from backing store..
      ...
    }
  });

  private String key;

  private Loader<?> loader;

  public String getKey(Object...args) {
    return String.format(key, args);
  }

  public Object load(Object...args) {
    loader.load(args);
  }
}

Using this combined object loader and key namespace support we can extract the caching logic to a generic adapter:

public class CacheAdapter {

  private final Cache cache;

  public CacheAdapter(Cache cache) {
    this.cache = cache;
  }

  public <T> T get(CacheEntry entry, Object...args) {
    T o = null;
    String key = entry.getKey(args);

    Element element = cache.get(key);
    if (element != null) {
      o = (T) element.getValue();
    }
    else {
      o = (T) entry.load(args);
      if (o != null) {
        cache.put(new Element(key, o));
      }
    }
    return o;
  }
}

public class SomeClass {

  private final CacheAdapter cache = ...

  public void doSomethingWithObject(Object uid) {
    SomeObject o = cache.get(CacheEntry.SomeObject, uid);
    o.doSomething();
  }

  public SomeObject getSomeObject(Object uid) {
    return cache.get(CacheEntry.SomeObject, uid);
  }
}

A Real Example

Caching XMPP VCard objects:

import org.jivesoftware.smack.XMPPConnection;
import org.jivesoftware.smack.XMPPException;
import org.jivesoftware.smackx.packet.VCard;

public enum CacheEntry {

  VCard("vcard.%s", new Loader<VCard> {
    VCard load(Object...args) {
      String user = (String) args[0];
      XMPPConnection connection = (XMPPConnection) args[1];
      try {
        VCard card = new VCard();
        card.load(connection, user);
      } catch (XMPPException e) {
        return null;
      }
    }
  });
}

public class AvatarRepository {

  private final CacheAdapater vcardCache = ...

  private final XMPPConnection connection = ...

  public Image getAvatar(String user) {
    Image avatar = null;

    VCard vcard = vcardCache.get(CacheEntry.VCard, user, connection);
    if (vcard != null) {
      avatar = new ImageIcon(vcard.getAvatar()).getImage();
    }

    return avatar;
  }
}

Conclusion

By defining a key namespace and object loading mechanism for cachable data types we can improve the manageability of object caching in the following ways:

• Improved support for mixed data type caching
• Increased decoupling from the caching implementation
• Uniformity in object loading and caching