HashSet线程不安全，证明它，并且提出线程安全的解决办法

1、HashSet线程不安全

public class testHashSetUnsafe {

    public static void main(String[] args) {
        HashSet set = new HashSet<>();
        for (int i = 0; i <30; i++) {
            new Thread(()-> {
                set.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(set);
            },String.valueOf(i)).start();
        }
    }
}

2、报错

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1469)
	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
	at java.lang.String.valueOf(String.java:2994)
	at java.io.PrintStream.println(PrintStream.java:821)
	at com.miller.mybatis.collectionUnsafe.testHashSetUnsafe.lambda$main$0(testHashSetUnsafe.java:22)

3、为什么？

 

4、怎么办？

public class testHashSetUnsafe {

    public static void main(String[] args) {
        Set set = new CopyOnWriteArraySet();
        for (int i = 0; i <30; i++) {
            new Thread(()-> {
                set.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(set);
            },String.valueOf(i)).start();
        }
    }
}

5、源码分析：写时复制，读写分离

 /**
     * A version of addIfAbsent using the strong hint that given
     * recent snapshot does not contain e.
     */
    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();
        }
    }