java - AtomicInteger 线程安全

Question

我怎样才能实现这里的 while 循环总是精确执行 100 次。当我执行代码时，在极少数情况下它会在控制台上打印 99 或 98 行而不总是 100，我不明白。

import java.util.ArrayList;
import java.util.concurrent.atomic.AtomicInteger;

public class Print implements Runnable {
    static AtomicInteger atomicInteger = new AtomicInteger(0);

    @Override
    public void run() {
        while (atomicInteger.getAndIncrement() < 100)
            System.out.println(Thread.currentThread());
    }

    public static void main(String[] args) throws InterruptedException {
        ArrayList<Thread> threads = new ArrayList<>();

        for (int i = 0; i < 5; i++)
            threads.add(new Thread(new Print()));

        for (Thread thread : threads)
            thread.start();

        for (Thread thread : threads)
            thread.join();
    }
}

Answer 1

无法复制您报告的体验。
在我的调试器中，这似乎是正确的，因为我在您的代码中看不到任何线程安全错误。
为了自动化进一步的测试，我按如下方式更改了您的代码。而不是打电话System.out.println , 我将线程 ID 添加到 List的 Long .我将列表设为 CopyOnWriteArrayList 为了线程安全。我可以以编程方式检测结果列表的大小是否不完全是 100 个元素。

package work.basil.demo;

import java.time.Instant;
import java.util.ArrayList;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.atomic.AtomicInteger;

public class Print implements Runnable
{
    static AtomicInteger atomicInteger = new AtomicInteger( 0 );
    static CopyOnWriteArrayList < Long > list = new CopyOnWriteArrayList <>();

    @Override
    public void run ( )
    {
        while ( atomicInteger.getAndIncrement() < 100 )
            //System.out.println(Thread.currentThread());
            list.add( Thread.currentThread().getId() );
    }

    public static void main ( String[] args ) throws InterruptedException
    {
        System.out.println( "INFO - demo starting. " + Instant.now() );
        for ( int cycle = 0 ; cycle < 1_000_000 ; cycle++ )
        {
            ArrayList < Thread > threads = new ArrayList <>();

            for ( int i = 0 ; i < 5 ; i++ )
                threads.add( new Thread( new Print() ) );

            for ( Thread thread : threads )
                thread.start();

            for ( Thread thread : threads )
                thread.join();

//            System.out.println( "list.size() = " + list.size() );
//            if ( list.size() == 100 ) { System.out.println( "DEBUG list.size() = " + ( list.size() ) ); }
            if ( list.size() != 100 ) { System.out.println( "DEBUG list.size() = " + ( list.size() ) ); }
        }
        System.out.println( "INFO - demo done. " + Instant.now() );
    }
}

在 Mac mini (2018) 3 GHz Intel Core i5 上运行时，该 i5 具有六个真实内核且无超线程，以及 32 GB 2667 MHz DDR4，在 IntelliJ 中使用 Java 16。运行 cycle到一百万大约需要 5 分钟。

INFO - demo starting. 2021-06-08T22:11:56.010181Z
INFO - demo done. 2021-06-08T22:16:26.982616Z

ExecutorService顺便说一句，在现代 Java 中，我们很少需要解决 Thread直接上课。相反，使用 Executors框架添加到 Java 5。
这是上面代码的修订版，重新调整以使用执行程序服务。

public static void main ( String[] args ) throws InterruptedException
{
    System.out.println( "INFO - demo starting. " + Instant.now() );
    for ( int cycle = 0 ; cycle < 1_000_000 ; cycle++ )
    {
        ExecutorService executorService = Executors.newFixedThreadPool( 5 );

        int countTasks = 5;
        for ( int i = 0 ; i < countTasks ; i++ )
        {
            executorService.submit( new Print2() );
        }

        executorService.shutdown();
        executorService.awaitTermination( Duration.ofMinutes( 7 ).toSeconds() , TimeUnit.SECONDS );

//            System.out.println( "list.size() = " + list.size() );
//            if ( list.size() == 100 ) { System.out.println( "DEBUG list.size() = " + ( list.size() ) ); }
        if ( list.size() != 100 ) { System.out.println( "DEBUG list.size() = " + ( list.size() ) ); }
    }
    System.out.println( "INFO - demo done. " + Instant.now() );
}