java并发编程之ThreadLocal详解

private static ThreadLocal<String> threadLocal = new ThreadLocal<>();

public void setAndGet(String name) {
    threadLocal.set(name);
    String s = threadLocal.get();
}

public void remove() {
    threadLocal.remove();
}

public void set(T value) {
    Thread t = Thread.currentThread();
    // 通过当前线程获取出来一个ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

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();
}

ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

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

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 static final int INITIAL_CAPACITY = 16;
    private Entry[] table;
    private int size = 0;
    private int threshold; // Default to 0
    // 省略其他代码
}

private void set(ThreadLocal<?> key, Object value) {
    Entry[] tab = table;
    int len = tab.length;
    // 计算key对应放在tab中的下标
    int i = key.threadLocalHashCode & (len-1);

    //循环遍历tab,首先获取到下标i对应的对象，如果不为空,则执行循环体
    // 如果不是相同的threadLocal或者位置失效，则要寻找下一个位置。
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        ThreadLocal<?> k = e.get();
	    // 判断与当前ThreadLocal是否是同一个对象，如果是则进行值替换，结束
        if (k == key) {
            e.value = value;
            return;
        }
		// 如果k==null代表threadLocal的key失效了，将失效的key进行替换
        if (k == null) {
            replaceStaleEntry(key, value, i);
            return;
        }
    }

    tab[i] = new Entry(key, value);
    int sz = ++size;
    if (!cleanSomeSlots(i, sz) && sz >= threshold)
        rehash();
}

private static int nextIndex(int i, int len) {
    return ((i + 1 < len) ? i + 1 : 0);
}