ThreadLocal类提供的几个方法：
public T get() { }
public void set(T value) { }
public void remove() { }
protected T initialValue() { } get()方法用来获取ThreadLocal当前线程中保存的变量副本；
set()用来设置当前线程中变量的副本；
remove()用来移除当前线程中变量的副本；
initialValue()是一个protected方法，一般是用来在使用时进行重写，它是一个延迟加载的方法。
首先来看下ThreadLocal类是如何为每个线程创建一个变量的副本的。
先看下get()方法的实现：
/**
* 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);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
} 第一句话是取得当前线程，然后通过getMap(t)方法获取一个ThreadLocalMap类型的map，接着获取到该map中的<key,value>键值对，注意这里获取键值对传进去的是this，而不是当前线程t。（？）
如果获取成功，则返回value；
如果map为空，则调用setInitialValue()方法返回值。
我们来根据上面的每一句来仔细分析：
首先看getMap()方法中做了什么：
/**
* 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;
} 在getMap(t)方法中，是调用当前线程t，返回当前线程t中的一个成员变量threadlocals，注意：这个threadlocals是Thread中的成员变量，而不是ThreadLocal中。
那么我们继续看下Thread类中的成员变量threadlocals是什么：
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null; 实际上就是一个ThreadLocalMap，这个类型是ThreadLocal当中的一个静态内部类，我们看下ThreadLocalMap的实现：
/**
* 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;
}
} 可以看到ThreadLocalMap中的Entry继承了WeakReference，并且使用ThreadLocal作为键值。
然后再继续看setInitialValue()方法的具体实现：
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
} 如果map不为空，就设置键值对；为空，再创建Map，跟下去看createMap()的实现：
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}