弱引用

• 弱引用weak reference，表示可有可无, gc扫描到随时回收
  • WeakReference weakRef = new WeakReference(s)
• 只要GC就回收，用做容器如WeakHashMap(key是弱引用), ThreadLocalMap的key

• WeakReference<M> m = new WeakReference<>(new M());
  System.gc();
  m.get();