ThreadLocal的主要实现如下:
ThreadLocal中有一个静态内部类，名为ThreadLocalMap,主要定义如下：
* ThreadLocalMap is a customized hash map suitable only for
* maintaining thread local values. No operations are exported
* outside of the ThreadLocal class. The class is package private to
* allow declaration of fields in class Thread. To help deal with
* very large and long-lived usages, the hash table entries use
* WeakReferences for keys. However, since reference queues are not
* used, stale entries are guaranteed to be removed only when
* the table starts running out of space.
*/
static class ThreadLocalMap {
/**
* The entries in this hash map extend WeakReference, using
* its main ref field as the key (which is always a
* ThreadLocal object). Note that null keys (i.e. entry.get()
* == null) mean that the key is no longer referenced, so the
* entry can be expunged from table. Such entries are referred to
* as "stale entries" in the code that follows.
*/
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
注意看其中的说明：
- ThreadLocalMap是个定制的HashMap，所以很多HashMap的实现，这里其实都有类似的东西。
- ThreadLocalMap的Entrty元素，注意是WeakReference类型，想想weakReference的特点嘞，它跟SoftReference的区别就是:
- SoftReference指向的对象，只有在内存不足时才会被GC回收掉。
- 只有WeakReference指向的对象，在下次GC的时候一般就会回收掉。
所以这个Entry继承WeakReference，应该只是为了当一个线程结束时，其对应的ThreadLocal数据会尽快的被释放掉。
- Entry中的key值是ThreadLocal本身，value则是你需要存储的数据（Object类型），所以对应到HashMap，可以想到其hashcode就是key的hashcode。
- 每个线程独享一个ThreadLocalMap，比如一个Thread的实现，里面定义了4个ThreadLocal，那么在实际代码运行过程中，只会有一个ThreadLocalMap实例，里面包含了至少4个Entry，分别对应上述的几个ThreadLocal对象。而多个Thread之间，ThreadLocalMap是不共用的，每个线程对象自己维护自己的ThreadLocalMap。
- 关于ThreadLocalMap存放在哪儿，其实跟正常的java对象一样，都是存放在堆空间里，通过线程来获取，如下：
class Thread implements Runnable {
/* Make sure registerNatives is the first thing <clinit> does. */
private static native void registerNatives();
static {
registerNatives();
}
private volatile String name;
...
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
...
通过线程对象 t.threadLocals就可以获取到整个ThreadLocalMap，然后再根据key值获取存入的结果。
- 这里之前看的时候，没有仔细去看对应的实现，以为是放在线程在初始化的时候指定的-Xss分配的堆空间内，但是仔细看过代码之后，发现两者并无关联。这里还是理解不够深，-Xss指定的是Stack space，也就是栈空间，其中是不会存放对象的。
