ThreadLocal 原理分析

用法

ThreadLocal<String> threadLocal = new ThreadLocal<>(); // 无初始值
ThreadLocal<String> threadLocal = ThreadLocal.withInitial(() -> "123"); // 有初始值

threadLocal.set("123"); // set操作
threadLocal.get(); // get操作
threadLocal.remove(); // remove操作

一个小例子：

public static void main(String[] args) throws InterruptedException {
    ThreadLocal threadLocal = new InheritableThreadLocal();
    threadLocal.set("Hello");
    System.out.println("现在线程是" + Thread.currentThread().getName() + ", 尝试获取：" + threadLocal.get());
    new Thread(() -> {
        threadLocal.set("World");
        System.out.println("现在线程是" + Thread.currentThread().getName() + ", 尝试获取：" + threadLocal.get());
        threadLocal.remove();
    }).start();
    Thread.sleep(3000);
    System.out.println("现在线程是" + Thread.currentThread().getName() + ", 尝试获取：" + threadLocal.get());
    threadLocal.remove();
}

输出：
现在线程是main, 尝试获取：Hello
现在线程是Thread-0, 尝试获取：World
现在线程是main, 尝试获取：Hello

实现

set操作

public void set(T value) {
    Thread t = Thread.currentThread(); // 获取当前线程
    ThreadLocalMap map = getMap(t); // 获取ThreadLocalMap
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value); // 创建map
}

// getMap
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}
public class Thread implements Runnable {
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

// createMap
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
    table = new Entry[INITIAL_CAPACITY]; // INITIAL_CAPACITY = 16;
    int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); // 计算下标
    table[i] = new Entry(firstKey, firstValue);
    size = 1;
    setThreshold(INITIAL_CAPACITY); // 设置阈值
}

// 哈希值
private final int threadLocalHashCode = nextHashCode();
private static int nextHashCode() {
    return nextHashCode.getAndAdd(HASH_INCREMENT); // HASH_INCREMENT = 0x61c88647;
}

// ThreadLocalMap数据结构
static class ThreadLocalMap {
    private Entry[] table;

    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); // 下标

    // 使用线性探测法来解决哈希冲突
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        ThreadLocal<?> k = e.get(); // 获取弱引用ThreadLocal

        if (k == key) { // 对于已经存在的key，直接赋值
            e.value = value;
            return;
        }

        if (k == null) { // 弱引用ThreadLocal为null，进行替换
            replaceStaleEntry(key, value, i);
            return;
        }
    }

    tab[i] = new Entry(key, value); // 上面两种情况不是，直接赋值
    int sz = ++size; // size+1
    if (!cleanSomeSlots(i, sz) && sz >= threshold) // 清除无效entry，大于阈值，扩容并重新散列化
        rehash();
}

get操作

public T get() {
    Thread t = Thread.currentThread(); // 获取当前线程
    ThreadLocalMap map = getMap(t); // 获取ThreadLocalMap
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this); // 传入自己，也就是threadlocal对象，得到entry
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

private Entry getEntry(ThreadLocal<?> key) {
    int i = key.threadLocalHashCode & (table.length - 1); // 获得下标
    Entry e = table[i]; // 取值
    if (e != null && e.get() == key) // 正好取到，直接返回
        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) { // 依次加一往后取值
        ThreadLocal<?> k = e.get();
        if (k == key) // 取到返回
            return e;
        if (k == null) // 弱引用ThreadLocal为null，清除无效的entry
            expungeStaleEntry(i); //
        else
            i = nextIndex(i, len); // 下一个
        e = tab[i];
    }
    return null;
}

remove操作

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

private void remove(ThreadLocal<?> key) {
    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)]) {
        if (e.get() == key) {
            e.clear(); // 弱引用的引用置为null
            expungeStaleEntry(i); // 清除entry，并重新散列化
            return;
        }
    }
}

public void clear() {
    this.referent = null;
}

private int expungeStaleEntry(int staleSlot) {
    Entry[] tab = table;
    int len = tab.length;

    // 槽置为null
    tab[staleSlot].value = null;
    tab[staleSlot] = null;
    size--;

    // 后面的进行重新散列化
    Entry e;
    int i;
    for (i = nextIndex(staleSlot, len);
         (e = tab[i]) != null;
         i = nextIndex(i, len)) {
        ThreadLocal<?> k = e.get();
        if (k == null) {
            e.value = null;
            tab[i] = null;
            size--;
        } else {
            int h = k.threadLocalHashCode & (len - 1);
            if (h != i) {
                tab[i] = null;

                while (tab[h] != null)
                    h = nextIndex(h, len);
                tab[h] = e;
            }
        }
    }
    return i;
}

内存泄露

当ThreadLocal没有强依赖，ThreadLocal会在下一次发生GC时被回收，key是被回收了，但是value却没有被回收，为了防止这个问题出现，最好手动调用remove方法。