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JUC：6_1集合类并发问题、集合类并发不安全解决方案2:map、set

悬灸人雪洋 2022-03-21 阅读 41
java经验分享面试多线程

JUC：6_1集合类并发问题、集合类并发不安全解决方案2:map、set

集合类并发不安全解决方案1:list

集合类并发不安全解决方案2:map、set

HashSet的本质就是HashMap，

public HashSet() {
        map = new HashMap<>();
    }
public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

在这里插入图片描述

代码

package juc.hashmap;

import org.apache.commons.lang3.time.DateFormatUtils;

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

public class Test {

    public static void main(String[] args) {

        Set<String> set = new HashSet<>();
        /*并发安全解决方案1：Collections（集合工具类）提供的一些安全集合类*/
//        Set<String> set = Collections.synchronizedSet(new HashSet<>());
        /*并发安全解决方案2：CopyOnWriteArraySet*/
//        Set<String> set = new CopyOnWriteArraySet<>();
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                set.add(UUID.randomUUID().toString().substring(0, 8));
                System.out.println(Thread.currentThread().getName() + "==>" + set);
            }, "set线程执行" + i).start();
        }

        Map<String, String> map = new HashMap<>();//等价于：Map<String, String> map1 = new HashMap<>(1<<4,0.75f);//1<<4，位运算，往左移动4位
        /*并发安全解决方案1：Collections（集合工具类）提供的一些安全集合类*/
//        Map<String,String> map = Collections.synchronizedMap(new HashMap<>());
        /*并发安全解决方案2：ConcurrentHashMap*/
//        Map<String, String> map = new ConcurrentHashMap<>();

        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                map.put(UUID.randomUUID().toString().substring(0, 8), DateFormatUtils.ISO_8601_EXTENDED_DATETIME_FORMAT.format(new Date()));
                System.out.println(Thread.currentThread().getName() + "==>" + map);
            }, "map线程执行" + i).start();
        }
    }
}

报错

Exception in thread "set线程执行2" Exception in thread "set线程执行9" Exception in thread "set线程执行0" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1469)
	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$0(Test.java:22)
	at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1469)
	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$0(Test.java:22)
	at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1469)
	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$0(Test.java:22)
	at java.lang.Thread.run(Thread.java:748)
set线程执行1==>[3eb8630f, d27fe9bb, 0f7e6b22]
set线程执行7==>[3eb8630f, d27fe9bb, b13fac67, e06a4584, 0f7e6b22, c415245c]
set线程执行8==>[3eb8630f, 8d434916, d27fe9bb, b13fac67, e06a4584, 0f7e6b22, 35519a70, c415245c]
set线程执行3==>[3eb8630f, 8d434916, d27fe9bb, b13fac67, e06a4584, 0f7e6b22, 35519a70, 02deed6c, c415245c]
set线程执行5==>[3eb8630f, 8d434916, d27fe9bb, b13fac67, e06a4584, 0f7e6b22, c98d10e9, 35519a70, 02deed6c, c415245c]
set线程执行6==>[3eb8630f, d27fe9bb, b13fac67, 0f7e6b22, c415245c]
set线程执行4==>[3eb8630f, d27fe9bb, b13fac67, 0f7e6b22]
Exception in thread "map线程执行5" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1479)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1477)
	at java.util.AbstractMap.toString(AbstractMap.java:554)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$1(Test.java:35)
	at java.lang.Thread.run(Thread.java:748)
Exception in thread "map线程执行7" Exception in thread "map线程执行6" Exception in thread "map线程执行8" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1479)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1477)
	at java.util.AbstractMap.toString(AbstractMap.java:554)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$1(Test.java:35)
	at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1479)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1477)
	at java.util.AbstractMap.toString(AbstractMap.java:554)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$1(Test.java:35)
	at java.lang.Thread.run(Thread.java:748)
Exception in thread "map线程执行1" Exception in thread "map线程执行0" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1479)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1477)
	at java.util.AbstractMap.toString(AbstractMap.java:554)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$1(Test.java:35)
	at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1479)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1477)
	at java.util.AbstractMap.toString(AbstractMap.java:554)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$1(Test.java:35)
	at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1479)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1477)
	at java.util.AbstractMap.toString(AbstractMap.java:554)
	at java.lang.String.valueOf(String.java:2994)
	at java.lang.StringBuilder.append(StringBuilder.java:131)
	at juc.hashmap.Test.lambda$main$1(Test.java:35)
	at java.lang.Thread.run(Thread.java:748)
map线程执行4==>{3d31715e=2022-03-20T19:54:54, f8d1523a=2022-03-20T19:54:54, 03895c72=2022-03-20T19:54:54, 73c3295f=2022-03-20T19:54:54, fba70c52=2022-03-20T19:54:54}
map线程执行3==>{3d31715e=2022-03-20T19:54:54, f8d1523a=2022-03-20T19:54:54, 03895c72=2022-03-20T19:54:54, 19872028=2022-03-20T19:54:54, 4b00f689=2022-03-20T19:54:54, 73c3295f=2022-03-20T19:54:54, fba70c52=2022-03-20T19:54:54}
map线程执行9==>{03895c72=2022-03-20T19:54:54, 19872028=2022-03-20T19:54:54, 4b00f689=2022-03-20T19:54:54, fba70c52=2022-03-20T19:54:54, 3d31715e=2022-03-20T19:54:54, f8d1523a=2022-03-20T19:54:54, a652aaec=2022-03-20T19:54:54, 73c3295f=2022-03-20T19:54:54}
map线程执行2==>{a22b2cd0=2022-03-20T19:54:54, 03895c72=2022-03-20T19:54:54, 19872028=2022-03-20T19:54:54, 4b00f689=2022-03-20T19:54:54, fba70c52=2022-03-20T19:54:54, e60199d5=2022-03-20T19:54:54, 3d31715e=2022-03-20T19:54:54, f8d1523a=2022-03-20T19:54:54, a652aaec=2022-03-20T19:54:54, 73c3295f=2022-03-20T19:54:54}
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