ThreadLocal是什么

1、概念
ThreadLocal是一个线程内部数据存储类，每一个使用该变量的线程都获得该变量的一个副本，每个副本之间相互独立，使得每一个线程可以任意的修改和访问自己的变量副本，而不会对其他线程产生任何影响，即不同线程使用同一个ThreadLocal变量，但是他们通过TheadLocal获取到的值不同。
2、原理
ThreadLocal是一个泛型类，下面看一下set和get方法 public class ThreadLocal<T>;

public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}

   ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

    ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {  // key指Thread对象引用
        table = new Entry[INITIAL_CAPACITY];
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        setThreshold(INITIAL_CAPACITY);
    }

Thread内部有一个ThreadLocalMap，用来存储线程的ThreadLocal数据，由ThreadLocal来维护，ThreadLocal内部有一个初始容量为16的Entry数组 /* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null; // Thread.class成员变量

Entry(ThreadLocal<?> k, Object v) {
reference = k;  // k是ThreadLocal
value = v;
}

下面看一下map如何set的 /**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal<?> key, Object value) {

// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.

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);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}可以看出存储规则：由key计算在table中的位置，更新或者重新创建Entry
下面看一下get方法：首先通过reference（ThreadLocal）计算ThreadLocal在table中的位置并返回该位置处的Entry，Entry.value即是所需要的值 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();
}总结：每一个Thread内部有一个ThreadLocalMap，其实是一个默认大小为16的Entry数组，Entry中保存了reference（该ThreadLocal对象）和对应的值value，ThreadLocal的get和set方法所操作的都是当前线程的Entry数组table