学习ThreadLocal

JDK早期设计：每个ThreadLocal都创建一个Map, 然后用Thread(线程) 作为Map的key, 要存储的局部变量作为Map的value, 这样就能达到各个线程的局部变量隔离的效果。

JDK8 优化设计：每个Thread维护一个ThreadLocalMap, 这个Map的key是ThreadLocal实例本身,value才是真正要存储的值Object。ThreadLocal内部维护的是一个类似Map的ThreadLocalMap数据结构，key为当前对象的Thread对象，值为泛型的Object。使用ThreadLocal比较方便的就是当thread不变的情况下，可以很方便的设置或者获取对象。

static class ThreadLocalMap {

        
        static class Entry extends WeakReference> {
            
            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