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集合类线程不安全问题

Posted on In Valine:
Symbols count in article: 2k Reading time ≈ 2 mins.

ArrayList

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/**
 * @author xixing
 * @version 1.0
 * @date 2020/7/2 14:53
 */
public class ContainerNotSafeDemo {

    public static void main(String[] args) {
        List<String> list= new CopyOnWriteArrayList<>();
        for(int i=0;i<100;i++){
            new Thread(()->{
                list.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(list);
            },String.valueOf(i)).start();
        }
        //java.util.ConcurrentModificationException

        /**
         *1.故障现象
         *  java.util.ConcurrentModificationException
         *
         *2.导致原因
         *  并发争抢修改导致，参考花名册签名，一个正在写，另一个抢导致
         *3.解决方案
         *     1.new Vector<>() 不建议，这个类已经被不建议使用。
         *     2.Collections.synchronizedList(new ArrayList())
         *     3.new CopyOnWriteArrayList<>();
         *
         *
         */
    }
}

image-20200702152923248

HashSet

HashSet底层是HashMap，以key为set，value是一个object的常量。

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/**
 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
 * default initial capacity (16) and load factor (0.75).
 */
public HashSet() {
    map = new HashMap<>();
}
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/**
 * Adds the specified element to this set if it is not already present.
 * More formally, adds the specified element <tt>e</tt> to this set if
 * this set contains no element <tt>e2</tt> such that
 * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
 * If this set already contains the element, the call leaves the set
 * unchanged and returns <tt>false</tt>.
 *
 * @param e element to be added to this set
 * @return <tt>true</tt> if this set did not already contain the specified
 * element
 */
public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}
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Set<String> set= new CopyOnWriteArraySet<>();
for(int i=0;i<100;i++){
    new Thread(()->{
        set.add(UUID.randomUUID().toString().substring(0,8));
        System.out.println(set);
    },String.valueOf(i)).start();
}

HashMap

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Map<String,String> map= new ConcurrentHashMap<>();
for(int i=0;i<100;i++){
    new Thread(()->{
        map.put(Thread.currentThread().getName(),UUID.randomUUID().toString().substring(0,8));
        System.out.println(map);
    },String.valueOf(i)).start();
}