如上图所示
每个Thread 都有一个map,里面存着Entry<Key,value>,而key是实现了WeakReference的ThreadLocal,如果不是WeakReference，那么可能Entry里面的key和value在线程结束才会进行GC，但是由于是WeakReference,因此当Key被设置为null时,key就会被 gc 回收.
但是由于value还在currentThread->Map->Entry->value中，因此导致了内存泄漏
解决方法hreadLocalMap类的设计本身已经有了这一问题的解决方案，那就是在每次get()/set()/remove()ThreadLocalMap中的值的时候，会自动清理key为null的value。如此一来，value也能被回收了。
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) { Entry[] tab = table; int len = tab.length; while (e != null) { ThreadLocal<?> k = e.get(); if (k == key) return e; if (k == null) expungeStaleEntry(i); else i = nextIndex(i, len); e = tab[i]; } return null; }中的
if (k == null) expungeStaleEntry(i);expungeStaleEntry保证了key为null的情况下value也置为null
private int expungeStaleEntry(int staleSlot) { Entry[] tab = table; int len = tab.length; // expunge entry at staleSlot tab[staleSlot].value = null; tab[staleSlot] = null; size--; // Rehash until we encounter null Entry e; int i; for (i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal<?> k = e.get(); if (k == null) { e.value = null; tab[i] = null; size--; } else { int h = k.threadLocalHashCode & (len - 1); if (h != i) { tab[i] = null; // Unlike Knuth 6.4 Algorithm R, we must scan until // null because multiple entries could have been stale. while (tab[h] != null) h = nextIndex(h, len); tab[h] = e; } } } return i; }