读ThreadLocal源代码

//可以存取线程局部变量

//先看构造函数
public ThreadLocal() {
    }

//设置值
public void set(T value) {
        Thread t = Thread.currentThread();
	//这个Map以this为key
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

//继承了WeakReference说明这个key是自动回收的
static class ThreadLocalMap {
        static class Entry extends WeakReference<ThreadLocal> {
           
            Object value;

            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }

private void set(ThreadLocal key, Object value) {

            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();
        }

//获取保存的值
public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);

        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
	    //有值则返回
            if (e != null)
                return (T)e.value;
        }
	//返回初始化的值
        return setInitialValue();
    }

//设置初始化的值
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;
    }

//覆盖这个方法获取初始值
protected T initialValue() {
        return null;
    }

//删除值
 public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }


/**
总结：ThreadLocal主要用于传递变量。如果要设置初始化的值可以直接覆盖initialValue()方法即可。
其原理是：将值存在一个ThreadLocalMap中。以自身为key,值为value。这个map继承了WeakReference。
说明其对于key是弱引用(相当于WeakHashMap)。
*/