Java 有线程安全的 set 吗？

在多线程环境下，要使用线程安全的集合，比如，ConcurrentHashMap是线程安全的HashMap，CopyOnWriteArrayList是线程安全的ArrayList。

那么HashSet对应的线程安全集合，是什么呢？java有没有提供默认实现呢？

在java的concurrent包中，我找到了CopyOnWriteArraySet，那么它是线程安全的吗？下面是测试代码。

public static void main(String[] args) {
    Set<String> set = new CopyOnWriteArraySet<>();
    ExecutorService service = Executors.newFixedThreadPool(12);
    int times = 10000;
    AtomicInteger flag = new AtomicInteger(0);
    for(int i = 0; i < times; i ++){
        service.execute(()->{
            set.add("a" + flag.getAndAdd(1));
        });
    }
    service.shutdown();
    try {
        service.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
    }catch (Exception e){
        e.printStackTrace();
    }
    System.out.println(set.size());
}

经过多次执行，结果都是10000。

可以说明，CopyOnWriteArraySet是线程安全的Set。

那么CopyOnWriteArraySet是如何保证写入时的线程安全呢？以下是CopyOnWriteArraySet的add源码。

public boolean add(E e) {
    return al.addIfAbsent(e);
}
public boolean addIfAbsent(E e) {
    Object[] snapshot = getArray();
    return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :
        addIfAbsent(e, snapshot);
}
private static int indexOf(Object o, Object[] elements,
                           int index, int fence) {
    if (o == null) {
        for (int i = index; i < fence; i++)
            if (elements[i] == null)
                return i;
    } else {
        for (int i = index; i < fence; i++)
            if (o.equals(elements[i]))
                return i;
    }
    return -1;
}
private boolean addIfAbsent(E e, Object[] snapshot) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] current = getArray();
        int len = current.length;
        if (snapshot != current) {
            // Optimize for lost race to another addXXX operation
            int common = Math.min(snapshot.length, len);
            for (int i = 0; i < common; i++)
                if (current[i] != snapshot[i] && eq(e, current[i]))
                    return false;
            if (indexOf(e, current, common, len) >= 0)
                    return false;
        }
        Object[] newElements = Arrays.copyOf(current, len + 1);
        newElements[len] = e;
        setArray(newElements);
        return true;
    } finally {
        lock.unlock();
    }
}