集合类不安全

package com.marlowe.unsafe;

import java.util.*;
import java.util.concurrent.CopyOnWriteArrayList;

/**
 * @program: juc
 * @description: java.util.ConcurrentModificationException 并发修改异常
 * @author: Marlowe
 * @create: 2020-12-03 21:00
 **/
public class ListTest {
    public static void main(String[] args) {

        /**
         * 并发下ArrayList 不安全的
         *
         * 解决方法：
         * 1、List<String> list = new Vector<>();
         * 2、List<String> list = Collections.synchronizedList(new ArrayList<>());
         * 3、List<String> list = new CopyOnWriteArrayList<>();
         */

        /**
         * CopyOnWrite 写入时复制 COW 计算机程序设计领域的一种优化策略
         * 多个线程调用的时候，list，读取的时候，固定的，写入的（覆盖）
         * 在写入的时候避免覆盖，造成数据问题！
         * CopyOnWriteArrayList 比 Vector 好在那里 前者是lock，后者是是synchronized
         */
        List<String> list = new CopyOnWriteArrayList<>();

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

package com.marlowe.unsafe;

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.UUID;
import java.util.concurrent.CopyOnWriteArraySet;

/**
 * @program: juc
 * @description: 同理可证 ：java.util.ConcurrentModificationException
 * 1、Set<String> set = Collections.synchronizedSet(new HashSet<>());
 * 2、Set<String> set = new CopyOnWriteArraySet<>();
 * @author: Marlowe
 * @create: 2020-12-03 21:53
 **/
public class SetTest {
    public static void main(String[] args) {
        // Set<String> set = new HashSet<>();
        // Set<String> set = Collections.synchronizedSet(new HashSet<>());
        Set<String> set = new CopyOnWriteArraySet<>();
        for (int i = 0; i < 30; i++) {
            new Thread(() -> {
                set.add(UUID.randomUUID().toString().substring(0, 5));
                System.out.println(set);
            }, String.valueOf(i)).start();
        }
    }
}

public HashSet() {
    map = new HashMap<>();
}

// add set本质就是map    key是无法重复的
public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

// PRESENT
private static final Object PRESENT = new Object();

import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;

/**
 * @program: juc
 * @description: java.util.ConcurrentModificationException
 * @author: Marlowe
 * @create: 2020-12-03 22:11
 **/
public class MapTest {
    public static void main(String[] args) {

        // map是这样用的吗？不是，工作中不用HashMap

        // HashMap<String, String> map = new HashMap<>();

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