ThreadLocal

可以使用 java.lang.ThreadLocal 类来实现线程本地存储功能。

ThreadLocal 从理论上讲并不是用来解决多线程并发问题的，因为根本不存在多线程竞争。

在一些场景 (尤其是使用线程池) 下，由于 ThreadLocal.ThreadLocalMap 的底层数据结构导致 ThreadLocal 有内存泄漏的情况，应该尽可能在每次使用 ThreadLocal 后手动调用 remove()，以避免出现 ThreadLocal 经典的内存泄漏甚至是造成自身业务混乱的风险。

对于以下代码，thread1 中设置 threadLocal 为 1，而 thread2 设置 threadLocal 为 2。过了一段时间之后，thread1 读取 threadLocal 依然是 1，不受 thread2 的影响。

public class ThreadLocalExample {
    public static void main(String[] args) {
        ThreadPoolExecutor executor = new ThreadPoolExecutor(
                5, 10, 1, TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(100), new ThreadPoolExecutor.CallerRunsPolicy());
        ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
        executor.execute(() ->{
            threadLocal.set(1);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(threadLocal.get());
            threadLocal.remove();
        });
        executor.execute(() ->{
            threadLocal.set(2);
            threadLocal.remove();
        });
        executor.shutdown();
    }
}

public class ThreadLocalExample {
    public static void main(String[] args) {
        ThreadPoolExecutor executor = new ThreadPoolExecutor(
                5, 10, 1, TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(100), new ThreadPoolExecutor.CallerRunsPolicy());
        ThreadLocal<Integer> threadLocal1 = new ThreadLocal<>();
        ThreadLocal<Integer> threadLocal2 = new ThreadLocal<>();
        executor.execute(() ->{
            threadLocal1.set(1);
            threadLocal2.set(1);
            threadLocal1.remove();
            threadLocal2.remove();
        });
        executor.execute(() ->{
            threadLocal1.set(2);
            threadLocal2.set(2);
            threadLocal1.remove();
            threadLocal2.remove();
        });
        executor.shutdown();
    }
}

它所对应的底层结构图为：

每个 Thread 都有一个 ThreadLocal.ThreadLocalMap 对象。

/* ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;

当调用一个 ThreadLocal 的 set(T value) 方法时，先得到当前线程的 ThreadLocalMap 对象，然后将 ThreadLocal->value 键值对插入到该 Map 中。

public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

get() 方法类似。

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();
}