guava:LoadingCache缓存机制支持弱引用(WeakReference)

前几天写过一篇博客：《java:基于弱引用(WeakReference)的FunctionCached实现》介绍在guava 缓存机制上实在基于弱引用(WeakReference)的缓存机制。
当时还挺得意，然而在我因为其他原因又进一步看了guava 缓存机制相关的代码后，才知道guava 缓存机制本身就支持弱引用(WeakReference)模式，还非常方便，根本不需要再发明一遍轮子。唉，这走的弯路又是吃了不看文档的亏。
com.google.common.cache.CacheBuilder.weakValues()方法就是在构建一个缓存实例时指定弱引用(WeakReference)模式，以下是方法简要说明：

指定存储在缓存中的每个值（而不是键）都应封装在WeakReference中（默认情况下，使用强引用）。
弱值一旦是弱可访问的，就会被垃圾收集。

  /**
   * Specifies that each value (not key) stored in the cache should be wrapped in a
   * {@link WeakReference} (by default, strong references are used).
   *
   * <p>Weak values will be garbage collected once they are weakly reachable. This makes them a poor
   * candidate for caching; consider {@link #softValues} instead.
   *
   * <p><b>Note:</b> when this method is used, the resulting cache will use identity ({@code ==})
   * comparison to determine equality of values.
   *
   * <p>Entries with values that have been garbage collected may be counted in {@link Cache#size},
   * but will never be visible to read or write operations; such entries are cleaned up as part of
   * the routine maintenance described in the class javadoc.
   *
   * @return this {@code CacheBuilder} instance (for chaining)
   * @throws IllegalStateException if the value strength was already set
   */
  @GwtIncompatible // java.lang.ref.WeakReference
  public CacheBuilder<K, V> weakValues() {
    return setValueStrength(Strength.WEAK);
  }

所以要创建弱引用(WeakReference)模式的缓存对象就很简单：

	@Test
	public void test2() {
		LoadingCache<String, Integer> cache = CacheBuilder.newBuilder()
				/** 指定值存储使用弱引用(WeakReference)模式 */
				.weakValues()
				.build(new CacheLoader<String, Integer>(){

					@Override
					public Integer load(String key) throws Exception {
						/** 只会输出一次 */
						System.out.printf("parse:%s", key);
						return Integer.parseInt(key);
					}});
		cache.getUnchecked("3333");
		cache.getUnchecked("3333");
		cache.getUnchecked("3333");
	}

CacheBuilder不仅支持弱引用模式，还支持软引用(SoftReference),即softValues()方法
进一步，guava缓存机制的键存储也支持弱引用和软引用模式(对应CacheBuilder的weakKeys()和softKeys()方法)
更多说明参见guava的WIKI(https://github.com/google/guava/wiki)或源码。

参考资料

《Reference-based Eviction》