特点：线程隔离，相互是隔离的，不影响的,每个线程都自己玩自己的，只操作自己的单独的变量副本
内部维护的是ThreadLocalMap，所以有key和value
当key指向的值为null时，value不被清理，由于key是弱引用，此时就会造成内存泄露，所以必须要通过ThreadLocal里面的remove方法手动删除值，避免内存泄露。
ThreadLocalMap里面面由一个个由数组组成的key-value，组成了Entry
弱引用非常容易GC，很容易造成内存泄露，所以需要将key为null的entry清除
引用关系：Thread-> ThreadLocal->Entry（弱）->key（弱）->value(强)（不清除时容易发生内存泄露）
//内部类，实=实现
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
static class ThreadLocalMap {
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
private static final int INITIAL_CAPACITY = 16;
private Entry[] table;
private int size = 0;
private int threshold; // Default to 0
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
private static int nextIndex(int i, int len) {
return ((i + 1 < len) ? i + 1 : 0);
}
private static int prevIndex(int i, int len) {
return ((i - 1 >= 0) ? i - 1 : len - 1);
}
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
for (Entry e : parentTable) {
if (e != null) {
@SuppressWarnings("unchecked")
ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
if (key != null) {
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}
}