ThreadLocal

在Thread 上保存了一个ThreadLocalMap，以ThreadLocal object 的hash 为key, thread local object 保存的值为value.

Thread.threadLocals --->  ThreadLocalMap<ThreadLocalHash, value>

ThreadLocal.get()  ----> cur_thread.threadLocals.get(cur_thrd_local_hash)

ThreadLocal.set() ----> cur_thread.threadLocals.set(cur_thrd_local_hash, value)

ThreadLocal细节

• ThreadLocalMap size 为 2 ^ n. rehash 阈值为2/3, resize 为3/4
• ThreadLocalMap 使用线性探测实现
• Entry 中对于threadlocal 使用弱引用，避免threadlocal 内存泄漏

        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

• 当entry 中的threadlocal 变为null时，应当将此entry 回收。会在set/get过程中尝试清除这样的entry （并将这个entry 后面的entry rehash）