关于threadlocal我有一个问题。
一般认为，threadlocal中有一个map,（上文中的private Map storage = Collections.synchronizedMap(new HashMap())），ｍａｐ中的ｋｅｙ为Thread.currentThread,我觉得这个ｍａｐ是不是只会增大而不会减小。每多建一个新的线程，就会多一个。而当线程结束时，如果不是线程池的话，线程对象就没用了，但ｍａｐ中没有除去以该线程做ｋｅｙ的元素。后果很可能就是，在ＪＶＭ中有许多线程对象，它们分配的空间无法被收回。
在java sdk api ThreadLocal中有这样一段，是什么意思
Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the ThreadLocal instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).
Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the ThreadLocal instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).
我觉得：它好象是说在thread里有一个隐含的对thread-local变量的引用。（在源码中找到以下代码）
Thread.java
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
而在ThreadLocal.java中，有以下
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Many applications will have no need for
* this functionality, relying solely on the {@link initialValue()}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current threads' copy of
* this thread-local.
*/
public void set(Object value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t); <--- 这里，等于是说把key-value也放在了当前thread中的thread-local中了
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @return the map
*/
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
综上，是不是其实threadlocal中自己并没有map,而是从调用它的thread中取map?
这样是不是ＯＫ了？
不会，按照那段英文:after a thread goes away,当一个线程结束，他的引用拷贝将由Thread Local交由垃圾回收机制回收。
after a thread goes away, all of its copies of thread-local instances are subject to garbage collection .
翻译：线程结束后，该线程中所持有的thread-local实例的拷贝交垃圾回收机制处理。