给每个线程拷贝一个线程自己本地的变量副本，每个线程就直接操作自己的本地副本就ok了，然后就跟其他的线程就没有冲突了，避免多个线程并发的访问同一个共享的数据。重点就是用一个变量，同时多个线程操作的时候不让变量是共享。

源码剖析

设置值流程

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

1. Thread t = Thread.currentThread();获取当前线程
2. ThreadLocalMap map = getMap(t);获取Thread的ThreadLocalMap，这个map只能是自己线程内部可以使用。一个Thread可以放多个ThreadLocal

1. map != null；如果map不为空就可以直接set，没有map就得创建

   1. map.set(this, value); key就是当前线程的ThreadLocal对象，value就是设置的值

      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();
        }

      获取值流程

      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();
      }

2. Thread t = Thread.currentThread();获取当前线程

3. ThreadLocalMap map = getMap(t);拿到线程的ThreadLocalMap
4. ThreadLocalMap.Entry e = map.getEntry(this); 通过当前线程的ThreadLocal对象找到那个entry数组
5. T result = (T)e.value;从entry数组中获取值，然后返回。