CopyOnWriteArrayList

List 不是线程安全的

public static void main(String[] args) {

    List<String> list = new ArrayList<>();

    for (int i = 1; i <= 10; i++) {
        new Thread(() -> {
            list.add(UUID.randomUUID().toString().substring(0, 5));
            System.out.println(list);
        }, String.valueOf(i)).start();
    }

    System.out.println(">>" + list);
}

并发执行下会出现 Exception in thread "8" java.util.ConcurrentModificationException（并发修改异常）

解决方案

• 使用线程安全的 Vector 类，List<String> list = new Vector<>();

• 将 ArrayList 类转换为线程安全的，List<String> list = Collections.synchronizedList(new ArrayList<>());

• 使用 juc 包中的 CopyOnWriteArrayList，List<String> list = new CopyOnWriteArrayList<>();

CopyOnWriteArrayList 与 Vector 区别

/**
 * Vector 类中方法加的 synchronized 锁，效率相对较低
 * Appends the specified element to the end of this Vector.
 *
 * @param e element to be appended to this Vector
 * @return {@code true} (as specified by {@link Collection#add})
 * @since 1.2
 */
public synchronized boolean add(E e) {
    modCount++;
    ensureCapacityHelper(elementCount + 1);
    elementData[elementCount++] = e;
    return true;
}

/**
 * CopyOnWriteArrayList 采用 Lock 锁，写时复制
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return {@code true} (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        Object[] newElements = Arrays.copyOf(elements, len + 1);
        newElements[len] = e;
        setArray(newElements);
        return true;
    } finally {
        lock.unlock();
    }
}

CopyOnWriteArraySet

Set 不是线程安全的

public static void main(String[] args) {
    Set<String> set = new HashSet<>();

    for (int i = 1; i <= 10; i++) {
        new Thread(() -> {
            set.add(UUID.randomUUID().toString().substring(0, 5));
            System.out.println(set);
        }, String.valueOf(i)).start();
    }
}

并发执行下会出现 Exception in thread "3" java.util.ConcurrentModificationException(并发修改异常)

解决方案

• 将 HashSet 类转换为线程安全的，Set<String> set = Collections.synchronizedSet(new HashSet<>());

• 使用 juc 包中的 CopyOnWriteArraySet Set<String> set = new CopyOnWriteArraySet<>();

ConcurrentHashMap

Map 不是线程安全的

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();

    for (int i = 1; i <= 30; i++) {
        new Thread(() -> {
            map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0, 5));
            System.out.println(map);
        }, String.valueOf(i)).start();
    }
}

并发执行下会出现 Exception in thread "16" Exception in thread "21" java.util.ConcurrentModificationException（并发修改异常）

解决方案

• 将 map 转为线程安全的，Map<String, String> map = Collections.synchronizedMap(new HashMap<>());

• 使用 juc 包中的 ConcurrentHashMap，Map<String, String> map = new ConcurrentHashMap<>();