为什么对空原子类调用方法不会产生异常？

Question

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();
        }
    }
}

运行这个程序没有输出任何结果。甚至连NullPointerException都没有。如您所见，我没有实例化count。我想它会造成一个错误。为什么会这样呢？

Answer 1

抛出并捕获NullPointerException异常，但要查看它们，需要检查service.submit调用返回的Future实例：

按以下方式更改您的循环：

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);
    }
}

将产出：

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

您还将看到，如果将System.out.println(count.incrementAndGet() + "here");语句围绕在try-catch中，则会引发异常：

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