[JUC] Thread safety of collections

List thread insecurity and solution

/**
 * list 线程不安全，可能出现ConcurrentModificationException并发修改异常
 */
public class ThreadDemo4 {
    
    
    public static void main(String[] args) {
    
    
        //创建ArrayList集合
//        List<String> list = new ArrayList<>();
        //解决方案1： Vector
//        List<String> list = new Vector<>();
        //解决方案2： Collections.synchronizedList
//        List<String> list = Collections.synchronizedList(new ArrayList<>());
        //解决方案3： CopyOnWriteArrayList
        List<String> list = new CopyOnWriteArrayList<>();
        for (int i = 0; i < 300; i++) {
    
    
            new Thread(()->{
    
    
                //向集合添加内容
                list.add(UUID.randomUUID().toString().substring(0,8));
                //从集合索引内容
                System.out.println(list);
            },String.valueOf(i)).start();
        }
    }
}

Solution - Vector

The add method of Vector is added with the synchronized keyword, which is a very old solution

Solutions - Collections

List<String> list = Collections.synchronizedList(new ArrayList<>());

Not very efficient and not used much

Solution - CopyOnWriteArrayList

It comes from the principle of CopyOnWriteArrayList under the JUC package : copy-on-write technology, copy the same copy when writing, add new content, and then overwrite the previous one.

HashSet, HashMap thread insecurity and solutions

//        Set<String> set = new HashSet<>();
        //HashSet解决方案:CopyOnWriteArraySet
//        Set<String> set = new CopyOnWriteArraySet<>();
//        Map<String, String> map = new HashMap<>();
        //HashMap解决方案:ConcurrentHashMap
        Map<String, String> map = new ConcurrentHashMap<>();
        for (int i = 0; i < 300; i++) {
    
    
            String key = String.valueOf(i);
            new Thread(()->{
    
    
                //向集合添加内容
                map.put(key, UUID.randomUUID().toString().substring(0,8));
                //从集合索引内容
                System.out.println(map);
            },String.valueOf(i)).start();
        }

HashSet solution - CopyOnWriteArraySet

HashMap Solution - ConcurrentHashMap