ThreadLocal

ThreadLocal原理：
每个Thread都有一个内部类ThreadLocal.ThreadLocalMap threadLocals，ThreadLocalMap 有内部类Entry

ThreadLocal通过线程来获取ThreadLocalMap(返回当前线程的threadLocals)，ThreadLocalMap又通过TheadLocal对象来获取Entry，进而得到value(好像Entry中有一个key和一个value，key是ThreadLocal对象，value是就是要存储的值，但是这样理解并不准确)

public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

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

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

public class WeakReference<T> extends Reference<T> {
      public WeakReference(T referent) {
            super(referent);
      }
}

ThreadLocal通过线程来获取ThreadLocalMap(返回当前线程的threadLocals)，ThreadLocalMap将ThreadLocal对象和value保存进Entry

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
}

void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
 }