相信很多人知道ThreadLocal是针对每个线程的，但是其中的原理相信大家不是很清楚，那咱们就一块看一下源码。
首先，我们先看看它的set方法。非常简单，从当前Thread中获取map。那么这个getMap方法是什么样的呢？咱们继续看。
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
*
this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}