集合的线程安全

list集合线程不安全演示

List中的add方法没有添加Synchronized，因此是线程不安全的：

public boolean add(E e) {
    modCount++;
    add(e, elementData, size);
    return true;
}

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

执行会出现并发修改异常：

Exception in thread "17" java.util.ConcurrentModificationException

Vector解决

public static void main(String[] args) {
    var list = new Vector<String>();
    for (int i = 0; i < 300; i++) {
        new Thread(() -> {
            list.add(UUID.randomUUID().toString().substring(0, 8));
            System.out.println(list);
        }, String.valueOf(i)).start();
    }
}

Vector源码使用synchronized关键字，因此不存在线程安全问题，但是效率较低比较古老，不经常使用

Collections.synchronizedList

public static void main(String[] args) {
    var list = Collections.synchronizedList(new ArrayList<>());
    for (int i = 0; i < 300; i++) {
        new Thread(() -> {
            list.add(UUID.randomUUID().toString().substring(0, 8));
            System.out.println(list);
        }, String.valueOf(i)).start();
    }
}

效率较低比较古老，不经常使用

JUC 解决方案：CopyOnWriteArrayList

public static void main(String[] args) {
    var list = new CopyOnWriteArrayList<String>();
    for (int i = 0; i < 300; i++) {
        new Thread(() -> {
            list.add(UUID.randomUUID().toString().substring(0, 8));
            System.out.println(list);
        }, String.valueOf(i)).start();
    }
}

CopyOnWriteArrayList：写时复制技术，允许并发读取集合，但是只能单独写，即在写的时候首先复制一份集合，写完之后覆盖之前的内容

HashSet和HashMap线程不安全演示

public static void main(String[] args) {
    var set = new HashSet<String>();
    for (int i = 0; i < 300; i++) {
        new Thread(() -> {
            set.add(UUID.randomUUID().toString().substring(0, 8));
            System.out.println(set);
        }, String.valueOf(i)).start();
    }
}

public static void main(String[] args) {
    var map = new HashMap<String, String>();
    for (int i = 0; i < 300; i++) {
        int loop = i;
        new Thread(() -> {
            map.put(String.valueOf(loop), UUID.randomUUID().toString().substring(0, 8));
            System.out.println(map);
        }, String.valueOf(i)).start();
    }
}

JUC 解决方案：CopyOnWriteArrayList

public static void main(String[] args) {
    var set = new CopyOnWriteArraySet<String>();
    for (int i = 0; i < 300; i++) {
        new Thread(() -> {
            set.add(UUID.randomUUID().toString().substring(0, 8));
            System.out.println(set);
        }, String.valueOf(i)).start();
    }
}

JUC 解决方案：ConcurrentHashMap

public static void main(String[] args) {
    var map = new ConcurrentHashMap<String, String>();
    for (int i = 0; i < 300; i++) {
        int loop = i;
        new Thread(() -> {
            map.put(String.valueOf(loop), UUID.randomUUID().toString().substring(0, 8));
            System.out.println(map);
        }, String.valueOf(i)).start();
    }
}