ThreadLocal可以在线程内的定义一个变量，该变量是在该线程内的，所以是线程安全的。
(This class provides thread-local variables. These variables differ from * their normal counterparts in that each thread that accesses one (via its* <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
* copy of the variable.)
ThreadLocal类怎么使用：
public class ThreadLocalTest {
public class ThreadLocalVariableHolder {
private static ThreadLocal<Integer> value = new ThreadLocal<Integer>(){
private Random rand = new Random(47);
//覆盖父类的initialValue方法，初始化线程内的变量
@Override
protected Integer initialValue() {
return rand.nextInt(10000);
}
};
public static void increment(){
value.set(value.get() + 1);
}
public static int get(){
return value.get();
}
}
ThreadLocal类实现原理：
ThreadLocal类实现线程原理是，每个线程中都有一个ThreadLocalMap存储该线程所有的ThreadLocal变量。
Thread:
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
ThreadLocalMap的key是ThreadLocal，value是线程存储对应ThreadLocal的变量。这里特别注意，Entry为一个WeakReference，这样HashMap就不回溢出了。
/**
* 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;
}
}
ThreadLocal的get方法
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);//获取线程内的ThreadLocalMap
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}