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java - 为什么调用 null Atomic 类的方法不会产生异常？

问题描述

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();
        }
    }
}

运行这个程序什么也没输出。甚至没有 NullPointerException。如您所见，我没有实例化count. 我希望它会引发错误。这是为什么？

标签： javaconcurrencyatomic

解决方案

异常被NullPointerException抛出和捕获，但为了查看它们，您需要检查调用Future返回的实例：service.submit

改变你的循环如下：

for (int i = 0; i < 10; i++) {
    Future f = service.submit(() -> manager.incrementAndReport());
    try {
        System.out.println (f.get ());
    }
    catch (ExecutionException exex) {
        System.out.println (exex);
    }
    catch (InterruptedException intEx) {
        System.out.println (intEx);
    }
}

将输出：

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

System.out.println(count.incrementAndGet() + "here");如果用 try-catch 包围语句，您还会看到抛出异常：

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

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