ThreadLocal

ThreadLocal这个类，定义了方法但是没有数据。

1. 它的对象作为key在使用，而map放在Thread类中；2. 这个类里面封装了操作map的方法

Thread类

public class Thread implements Runnable {
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

ThreadLocal类

public class ThreadLocal<T> {
    public T get() {
        Thread t = Thread.currentThread();
        //map = t.threadLocals;
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            //以ThreadLocal对象作key
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }
    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;
    }
    protected T initialValue() {
        return null;
    }
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
    
}

java.lang.ThreadLocal.ThreadLocalMap

public class ThreadLocal<T> {
    static class ThreadLocalMap {
        static class Entry extends WeakReference<ThreadLocal> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }
        
        private Entry getEntry(ThreadLocal key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }
        private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
            Entry[] tab = table;
            int len = tab.length;

            while (e != null) {
                ThreadLocal k = e.get();
                if (k == key)
                    return e;
                if (k == null)
                    expungeStaleEntry(i);
                else
                    i = nextIndex(i, len);
                e = tab[i];
            }
            return null;
        }
    }
}

 ThreadLocalMap 里面的 Entry 继承了 WeakReference，为什么呢？为了方便垃圾回收