ThreadLocal中的WeakReference

​ Thread對象有一個變量名爲threadLocals的ThreadLocalMap對象，裏面定義了一個Entry數組，Entry對象弱引用了ThreadLocal。

​

private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();

                if (k == key) {
                    e.value = value;
                    return;
                }

                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

				if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }

​ Entry弱引用了ThreadLocal，因此Entry是否存活，不會影響ThreadLocal的生命週期，ThreadLocal在沒有其它對象引用後被回收，但是它對應的Entry對象中的value實際還在被Entry引用，如果不清掉Entry，value就會一直強引用，無法釋放，那麼就可能會內存溢出。（用完要remove()或者下次set()時會檢查）

​ 弱引用在下次GC時會被回收掉，前提條件是弱引用的對象不存在其它的強引用和軟引用。