为什么对空原子类调用方法不会产生异常？ import java.util.concurrent.*； 导入java.util.concurrent.AtomicInteger； 公共类\u 7 \u同步\u数据\u访问{ 私有原子整数计数； 私有void incrementAndReport（）{ System.out.print（count.incrementAndGet（）+“here”）；//不打印 } 公共静态void main（字符串[]args）{ ExecutorService=null； 试一试{ 服务=Executors.newFixedThreadPool（20）； _7_Synchronizing_Data_Access manager=新建_7_Synchronizing_Data_Access（）； 对于（int i=0；imanager.incrementAndReport（））； }最后{ if（服务！=null） service.shutdown（）； } } }

运行此程序不会输出任何内容。甚至不是NullPointerException。如您所见，我没有实例化

count

。我认为它会出错。这是为什么？

会抛出并捕获

NullPointerException

异常，但为了查看它们，您需要检查

服务返回的
未来的
实例。提交
调用：

按如下方式更改循环：

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

public class _7_Synchronizing_Data_Access {
    private AtomicInteger count;

    private void incrementAndReport() {
        System.out.print(count.incrementAndGet() + "here"); //does not print
    }

    public static void main(String[] args) {
        ExecutorService service = null;
        try {
            service = Executors.newFixedThreadPool(20);
            _7_Synchronizing_Data_Access manager = new _7_Synchronizing_Data_Access();
            for (int i = 0; i < 10; i++)
                service.submit(() -> manager.incrementAndReport());
        } finally {
            if (service != null)
                service.shutdown();
        }
    }
}

如果围绕
System.out.println（count.incrementAndGet（）+“here”）语句：

java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException

您的主线程很可能在提交的任务开始运行之前结束
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException

private void incrementAndReport() {
    try {
        System.out.println(count.incrementAndGet() + "here"); //does not print
    }
    catch (Exception exc) {
        System.out.println (exc);
    }
}