CopyOnWriteArrayList & CopyOnWriteArraySet & ConcurrentHashMap & 写时复制

写时复制

CopyOnWrite容器即写时复制的容器。往一个容器添加元素的时候，不直接往当前容器Object[]添加，而是先将当前容器Object[]进行Copy，复制出一个新的容器Object[] newElements，然后新的容器Object[] newElements里添加元素，添加完元素之后，再将原容器的引用指向新的容器 setArray(newElements);。这样做的好处是可以对CopyOnWrite容器进行并发的读，而不需要加锁，因为当前容器不会添加任何元素。所以CopyOnWrite容器也是一种读写分离的思想，读和写不同的容器。

CopyOnWriteArrayList 源码

 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();
     }
 }

应用

public class NotSafeDemo
{
    public static void main(String[] args)
    {
        Map<String,String> map = new ConcurrentHashMap<>();
        for (int i = 1; i <=30; i++)
        {
            new Thread(() -> {
                map.put(Thread.currentThread().getName(),UUID.randomUUID().toString().substring(0,8));
                System.out.println(map);
            },String.valueOf(i)).start();
        }
    }

    public static void setNotSafe()
    {
        Set<String> set = new CopyOnWriteArraySet<>();
        for (int i = 1; i <=30; i++)
        {
            new Thread(() -> {
                set.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(set);
            },String.valueOf(i)).start();
        }
    }

    public static void listNotSafe()
    {
        List<String> list = new CopyOnWriteArrayList();//Collections.synchronizedList(new ArrayList<>());//new Vector<>();//new ArrayList<>();

        for (int i = 1; i <=30; i++)
        {
            new Thread(() -> {
                list.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(list);
            },String.valueOf(i)).start();
        }
    }
}