JUC并发编程中的集合不安全问题源码解析

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CMMKK 2022-10-20 10:11:52 博主文章分类：Java ©著作权

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JUC并发编程四:集合不安全（Java）

1、List不安全！

代码示例：

package unsafe;

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

//java.util.ConcurrentModificationException 并发修改异常！
public class ListTest {
    public static void main(String[] args) {
//        List<String>  list = Arrays.asList("1", "2", "3");
//        list.forEach(System.out::println); // 1 2 3

        //并发的情况下 ArrayList是不安全 可以用Synchronized解决；
        /*
        解决方案：
         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是计算机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（覆盖）
        //在写入的时候避免覆盖，造成数据问题
        List<String> list=new CopyOnWriteArrayList<>();
        for (int i = 1; i <=10; i++) {
            
            new Thread(()->{
                list.add(UUID.randomUUID().toString().substring(0,5));
                System.out.println(list);
            },String.valueOf(i)).start();
            
        }
    }
}
/*
[8d63f, 915bb]
[8d63f, 915bb, 1b436, 4bcdb, 783e4, 8ec24]
[8d63f, 915bb, 1b436, 4bcdb, 783e4]
[8d63f, 915bb, 1b436, 4bcdb]
[8d63f, 915bb, 1b436]
[8d63f, 915bb, 1b436, 4bcdb, 783e4, 8ec24, f6001]
[8d63f, 915bb, 1b436, 4bcdb, 783e4, 8ec24, f6001, 220a3]
[8d63f, 915bb, 1b436, 4bcdb, 783e4, 8ec24, f6001, 220a3, e7c59]
[8d63f, 915bb, 1b436, 4bcdb, 783e4, 8ec24]
[8d63f, 915bb, 1b436, 4bcdb, 783e4, 8ec24, f6001, 220a3, e7c59, dcf72]
 */

2.、Set不安全！

代码示例：

package unsafe;

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

/*
ConcurrentModificationException 并发修改异常！
 */
public class SetTest {
    public static void main(String[] args) {
//        HashSet<String> set = new HashSet<>();
        /*
        解决方案:
        1.Set<Object> set = Collections.synchronizedSet(new HashSet<>());
        2. Set<Object> set = new CopyOnWriteArraySet<>();
         */
       Set<Object> 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();
        }
    }
}

hashSet底层是什么？

public HashSet() {    
    map = new HashMap<>();
}
//add set 本质就是map kye是无法重复的！
 public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
private static final Object PRESENT = new Object(); //不变的值

3、HashMap不安全！

代码示例：

package unsafe;

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

/*
ConcurrentModificationException  并发异常！
 */
public class MapTest {
    public static void main(String[] args) {
        //map是这样用的吗？ 不是，工作中不用HashMap
        // 默认等价于什么？new HashMap<>(16,0.75);
//        HashMap<String, String> map = new HashMap<>();

        /*
        解决方案:
        1.Map<String,String> map = new ConcurrentHashMap<>();
         */
       Map<String,String> map = new ConcurrentHashMap<>();
        //加载因子，初始化容量
        for (int i = 1; i <= 30; i++) {
            new Thread(()->{
            map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0,5));
                System.out.println(map);
            },String.valueOf(i)).start();
        }
    }
}
/*
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d}
{11=9163c, 12=c6962, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{11=9163c, 12=c6962, 13=f5049, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{11=9163c, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 10=f8834}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 10=f8834}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 10=f8834, 21=bc829}
{1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1}
{11=9163c, 22=f46c5, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 10=f8834, 21=bc829}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{22=f46c5, 23=8ff09, 24=b6066, 10=f8834, 11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 21=bc829}
{22=f46c5, 23=8ff09, 24=b6066, 25=faa0d, 10=f8834, 11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 21=bc829}
{11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 10=f8834}
{22=f46c5, 23=8ff09, 24=b6066, 25=faa0d, 26=50ce2, 10=f8834, 11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 21=bc829}
{22=f46c5, 23=8ff09, 24=b6066, 25=faa0d, 26=50ce2, 27=ac5f9, 10=f8834, 11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 21=bc829}
{11=9163c, 22=f46c5, 12=c6962, 23=8ff09, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 10=f8834, 21=bc829}
{22=f46c5, 23=8ff09, 24=b6066, 25=faa0d, 26=50ce2, 27=ac5f9, 28=27ea1, 10=f8834, 11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 21=bc829}
{22=f46c5, 23=8ff09, 24=b6066, 25=faa0d, 26=50ce2, 27=ac5f9, 28=27ea1, 29=61c81, 10=f8834, 11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 21=bc829}
{22=f46c5, 23=8ff09, 24=b6066, 25=faa0d, 26=50ce2, 27=ac5f9, 28=27ea1, 29=61c81, 30=b14ed, 10=f8834, 11=9163c, 12=c6962, 13=f5049, 14=ad8ae, 15=dff72, 16=d51e5, 17=0d764, 18=74566, 19=b47e3, 1=6e980, 2=ebcba, 3=9f691, 4=23762, 5=d3faa, 6=586bf, 7=3062d, 8=970e9, 9=7e8a1, 20=db82f, 21=bc829}
 */