弱引用与ThreadLocal的结合

一直没看到基于弱引用（java.lang.ref.WeakReference）的合适的例子，直到阅读Lucene-core的代码，发现了这么一段：

 

 

public class CloseableThreadLocal<T> implements Closeable {

  private ThreadLocal<WeakReference<T>> t = new ThreadLocal<WeakReference<T>>();

  // Use a WeakHashMap so that if a Thread exits and is
  // GC'able, its entry may be removed:
  private Map<Thread,T> hardRefs = new WeakHashMap<Thread,T>();
  
  // Increase this to decrease frequency of purging in get:
  private static int PURGE_MULTIPLIER = 20;

  // On each get or set we decrement this; when it hits 0 we
  // purge.  After purge, we set this to
  // PURGE_MULTIPLIER * stillAliveCount.  This keeps
  // amortized cost of purging linear.
  private final AtomicInteger countUntilPurge = new AtomicInteger(PURGE_MULTIPLIER);

  protected T initialValue() {
    return null;
  }
  
  public T get() {
    WeakReference<T> weakRef = t.get();
    if (weakRef == null) {
      T iv = initialValue();
      if (iv != null) {
        set(iv);
        return iv;
      } else {
        return null;
      }
    } else {
      maybePurge();
      return weakRef.get();
    }
  }

... ....

 

这里面用的很好：由于ThreadLocal在使用上并么有数量的限制，需要靠使用者手工删除的方式才能释放对象的引用，因此实际上很容易造成内存泄露，而CloseableThreadLocal使用WeekReference的方式缓存ThreadLocal的数据，很好地解决了这个问题，Perfect！