12、JUC:ConcurrentHashMap

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https://www.bilibili.com/video/BV1B7411L7tE?p=13
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常用的hashmap存在并发安全问题

package com.unsafe;

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

/**
 * Created by zjl
 * 2020/11/24
 **/
public class MapTest {
    
    

    public static void main(String[] args) {
    
    
        HashMap<String, String> map = new HashMap<>();
        
        for (int i = 0; i < 10; i++) {
    
    
            new Thread(()->{
    
    
                map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0,5));
                System.out.println(map);
            },String.valueOf(i)).start();
        }
    }
}

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/bin/java "-javaagent:/Users/zhangjianlong/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/193.5662.53/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=61632:/Users/zhangjianlong/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/193.5662.53/IntelliJ IDEA.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/tools.jar:/Users/zhangjianlong/testCode/JUC-study/target/classes:/Users/zhangjianlong/apache-maven-3.6.3/repository/org/projectlombok/lombok/1.18.8/lombok-1.18.8.jar com.unsafe.MapTest
{
    
    0=f6291, 1=415ea, 4=fb59e}
{
    
    0=f6291, 1=415ea, 3=1264a, 4=fb59e, 7=3eb5f, 8=b0543, 9=c582f}
{
    
    0=f6291, 1=415ea, 3=1264a, 4=fb59e, 9=c582f}
{
    
    0=f6291, 1=415ea, 3=1264a, 4=fb59e, 9=c582f}
{
    
    0=f6291, 1=415ea, 3=1264a, 4=fb59e, 7=3eb5f, 9=c582f}
{
    
    0=f6291, 1=415ea, 2=ff46f, 3=1264a, 4=fb59e, 5=15599, 6=55ef8, 7=3eb5f, 8=b0543, 9=c582f}
{
    
    0=f6291, 1=415ea, 3=1264a, 4=fb59e, 5=15599, 7=3eb5f, 8=b0543, 9=c582f}
Exception in thread "0" Exception in thread "1" Exception in thread "6" 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$EntryIterator.next(HashMap.java:1477)
	at java.util.AbstractMap.toString(AbstractMap.java:554)
	at java.lang.String.valueOf(String.java:2994)
	at java.io.PrintStream.println(PrintStream.java:821)
	at com.unsafe.MapTest.lambda$main$0(MapTest.java:18)
	at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationException
	at java.util.HashMap$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.io.PrintStream.println(PrintStream.java:821)
	at com.unsafe.MapTest.lambda$main$0(MapTest.java:18)
	at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationException
	at java.util.HashMap$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.io.PrintStream.println(PrintStream.java:821)
	at com.unsafe.MapTest.lambda$main$0(MapTest.java:18)
	at java.lang.Thread.run(Thread.java:748)

Process finished with exit code 0

使用ConcurrentHashMap

package com.unsafe;

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

/**
 * Created by zjl
 * 2020/11/24
 **/
public class MapTest {
    
    

    public static void main(String[] args) {
    
    

        Map<String, String> map = new ConcurrentHashMap<String,String>();

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

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/bin/java "-javaagent:/Users/zhangjianlong/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/193.5662.53/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=61731:/Users/zhangjianlong/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/193.5662.53/IntelliJ IDEA.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/tools.jar:/Users/zhangjianlong/testCode/JUC-study/target/classes:/Users/zhangjianlong/apache-maven-3.6.3/repository/org/projectlombok/lombok/1.18.8/lombok-1.18.8.jar com.unsafe.MapTest
{
    
    0=4d6ff, 2=98486, 3=2f835, 4=28880, 6=9ccdb, 7=47e6d}
{
    
    0=4d6ff, 1=33fee, 2=98486, 3=2f835, 4=28880, 5=a11c9, 6=9ccdb, 7=47e6d, 8=c393d, 9=75910}
{
    
    0=4d6ff, 1=33fee, 2=98486, 3=2f835, 4=28880, 5=a11c9, 6=9ccdb, 7=47e6d, 8=c393d}
{
    
    0=4d6ff, 2=98486, 3=2f835, 4=28880, 5=a11c9, 6=9ccdb, 7=47e6d}
{
    
    0=4d6ff, 1=33fee, 2=98486, 3=2f835, 4=28880, 5=a11c9, 6=9ccdb, 7=47e6d}
{
    
    0=4d6ff, 2=98486, 3=2f835, 4=28880, 6=9ccdb, 7=47e6d}
{
    
    0=4d6ff, 2=98486, 3=2f835, 4=28880, 6=9ccdb, 7=47e6d}
{
    
    0=4d6ff, 2=98486, 3=2f835, 4=28880, 6=9ccdb, 7=47e6d}
{
    
    0=4d6ff, 2=98486, 3=2f835, 4=28880, 6=9ccdb, 7=47e6d}
{
    
    0=4d6ff, 2=98486, 3=2f835, 4=28880, 6=9ccdb, 7=47e6d}

Process finished with exit code 0