ThreadLocal可以很方便的在一个线程运行周期内传递各种数据，而不用在调用栈的每个方法上添加新的参数
它的运行机制也比较简单，每个线程实例内部都会有一个 ThreadLocal.ThreadLocalMap threadLocals（类型具体定义在ThreadLocal.class 内部）类型的字段，在调用ThreadLocal.set的时候会将自己实例作为key和value包装到一个Entry类型实例里一起存储在threadLocals的table中，因为是数组类型，所以会根据ThreadLocal实例的hashcode和数组长度做 & 运算，计算出位置，这里解决hash冲突的方式用的是开放定址法（好像是？），在发现一个位置已经存放了数据后，会向后移动一个位置，直到那里没有被使用或者ThreadLocal的实例被回收。
private void set(ThreadLocal<?> key, Object value) {
...
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
...
}
为什么ThreadLocal在table中有引用还是会被回收呢？由于在Entry类型中k是WeakReference，当ThreadLocal实例不被强引用后就会被回收，但对应Entry实例并不会被回收，依然还在table里（似乎会产生泄漏，一部分内存没有被使用也无法被回收），下面看一下threadLocals的类型定义
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;
}
}
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
}