Mybatis Cache缓存机制

剖析两个经典的缓存策略：最近最少被使用(LRU)和软引用缓存

LRU，Mybatis巧妙的使用了LinkedHashMap.removeEldestEntry，此方法是一个受保护的方法，继承并重写这个方法，在方法中eldestKey持有被回收的对象，在做相关操作的使用轮训eldestKey，发现对象存在的时候，回收缓存容器中的对象。

 

public class LruCache implements Cache {

  private final Cache delegate;
  private Map<Object, Object> keyMap;
  private Object eldestKey;

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

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

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

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

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

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

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

  }

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

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

  public ReadWriteLock getReadWriteLock() {
    return delegate.getReadWriteLock();
  }

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

}

软引用缓存，个人认为比较高效的缓存，也是结合了Java垃圾回收的特点，也就是说如果对象不被引用，一旦开始垃圾回收，则该缓存对象会被回收。Mybatis使用了ReferenceQueue，会根据垃圾回收结果，在操作缓存是，回收缓存容器对象。

 

public class SoftCache implements Cache {
  private final LinkedList hardLinksToAvoidGarbageCollection;
  private final ReferenceQueue queueOfGarbageCollectedEntries;
  private final Cache delegate;
  private int numberOfHardLinks;

  public SoftCache(Cache delegate) {
    this.delegate = delegate;
    this.numberOfHardLinks = 256;
    this.hardLinksToAvoidGarbageCollection = new LinkedList();
    this.queueOfGarbageCollectedEntries = new ReferenceQueue();
  }

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

  public int getSize() {
    removeGarbageCollectedItems();
    return delegate.getSize();
  }

  public void setSize(int size) {
    this.numberOfHardLinks = size;
  }

  public void putObject(Object key, Object value) {
    removeGarbageCollectedItems();
    delegate.putObject(key, new SoftEntry(key, value, queueOfGarbageCollectedEntries));
  }

  public Object getObject(Object key) {
    Object result = null;
    SoftReference softReference = (SoftReference) delegate.getObject(key);
    if (softReference != null) {
      result = softReference.get();
      if (result == null) {
        delegate.removeObject(key);
      } else {
        hardLinksToAvoidGarbageCollection.addFirst(result);
        if (hardLinksToAvoidGarbageCollection.size() > numberOfHardLinks) {
          hardLinksToAvoidGarbageCollection.removeLast();
        }
      }
    }
    return result;
  }

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

  public void clear() {
    hardLinksToAvoidGarbageCollection.clear();
    removeGarbageCollectedItems();
    delegate.clear();
  }

  public ReadWriteLock getReadWriteLock() {
    return delegate.getReadWriteLock();
  }

  private void removeGarbageCollectedItems() {
    SoftEntry sv;
    while ((sv = (SoftEntry) queueOfGarbageCollectedEntries.poll()) != null) {
      delegate.removeObject(sv.key);
    }
  }

  private static class SoftEntry extends SoftReference {
    private final Object key;

    private SoftEntry(Object key, Object value, ReferenceQueue garbageCollectionQueue) {
      super(value, garbageCollectionQueue);
      this.key = key;
    }
  }

}