Question

几天前，我获得了此NullPointerException的支持票：

com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract com.redacted.SalesResponsePagination com.redacted.StatisticsService.findSalesData(com.redacted.ConfStats) throws com.redacted.AsyncException' threw an unexpected exception: java.lang.NullPointerException
    at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure(RPC.java:389)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:579)
    at ... (typical GWT + Tomcat stacktrace)
Caused by: java.lang.NullPointerException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at java.util.concurrent.ForkJoinTask.getThrowableException(Unknown Source)
    at java.util.concurrent.ForkJoinTask.reportException(Unknown Source)
    at java.util.concurrent.ForkJoinTask.invoke(Unknown Source)
    at java.util.stream.ForEachOps$ForEachOp.evaluateParallel(Unknown Source)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateParallel(Unknown Source)
    at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
    at java.util.stream.ReferencePipeline.forEach(Unknown Source)
    at com.redacted.StatisticsControllerImpl.replacePrices(StatisticsControllerImpl.java:310)
    at com.redacted.StatisticsControllerImpl.findSalesData(StatisticsControllerImpl.java:288)
    at com.redacted.StatisticsServiceImpl.findSalesData(StatisticsServiceImpl.java:83)
    at sun.reflect.GeneratedMethodAccessor752.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:561)
    ... 33 more
Caused by: java.lang.NullPointerException
    at com.redacted.StatisticsControllerImpl.lambda$replacePrices$27(StatisticsControllerImpl.java:317)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(Unknown Source)
    at ... (typical stream.forEach stacktrace)

现在，这很容易，因为 NPE的确切行号很明显；我要做的就是去StatisticsControllerImpl.java:317：

    salesResponsePagination.getSalesResponses().parallelStream()
            .peek(sr -> /*...*/)
            .filter(sr -> /*...*/)
/*310*/     .forEach(sr -> {
                final List<CartElement> sentCEs = DaoService.getCartElementDAO().getSentCEs(/*...*/);
                if (sentCEs != null && !sentCEs.isEmpty() && sentCEs.get(0) != null) {
                    final CartElement ce = sentCEs.get(0);
                    // some more non-NPE lines...
/*317*/             if (sr.getCurrency().equals(ce.getPurchaseCurrency()) && sr.getPrice().equals(ce.getPurchasePrice().intValue()) && !ce.getCurrency().equals(ce.getPurchaseCurrency())) {
                        // Some currency exchanging
                    }
                    // Etcetera (about 12 lines more)
            });

然后将.equals()调用替换为Object.equals()，以避免出现NPE（调查后来出现一些以NULL价格或货币注册的销售的原因）。测试，提交，推送和发送票证以进行质量检查。

但是，第二天质量检查人员返回了该票证，说NPE仍然存在，并且其中包括了一个新的，几乎相似的堆栈跟踪：

com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract com.redacted.SalesResponsePagination com.redacted.StatisticsService.findSalesData(com.redacted.ConfStats) throws com.redacted.AsyncException' threw an unexpected exception: java.lang.NullPointerException
    at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure(RPC.java:389)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:579)
    at ... (typical GWT + Tomcat stacktrace)
Caused by: java.lang.NullPointerException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at java.util.concurrent.ForkJoinTask.getThrowableException(Unknown Source)
    at java.util.concurrent.ForkJoinTask.reportException(Unknown Source)
    at java.util.concurrent.ForkJoinTask.invoke(Unknown Source)
    at java.util.stream.ForEachOps$ForEachOp.evaluateParallel(Unknown Source)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateParallel(Unknown Source)
    at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
    at java.util.stream.ReferencePipeline.forEach(Unknown Source)
    at com.redacted.StatisticsControllerImpl.replacePrices(StatisticsControllerImpl.java:310)
    at com.redacted.StatisticsControllerImpl.findSalesData(StatisticsControllerImpl.java:288)
    at com.redacted.StatisticsServiceImpl.findSalesData(StatisticsServiceImpl.java:83)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:561)
    ... 33 more
Caused by: java.lang.NullPointerException

此堆栈跟踪与上一个完全相同，除了两件事：

此呼叫使用的是NativeMethodAccessorImpl而不是GeneratedMethodAccessor752 。比较：
at com.redacted.StatisticsServiceImpl.findSalesData(StatisticsServiceImpl.java:83)
at sun.reflect.GeneratedMethodAccessor752.invoke(Unknown Source)
vs
at com.redacted.StatisticsServiceImpl.findSalesData(StatisticsServiceImpl.java:83)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
这是缺少lambda的堆栈跟踪。 发生NPE的行缺失。

这最初使我失望。为什么堆栈跟踪中的最后一部分丢失了？我要求QA重新测试并重新连接堆栈跟踪几次，直到获得完整的堆栈跟踪为止。但是，我最后得到的完整答案再次基于GeneratedMethodAccesor 。

现在，如果我理解正确，将sun.reflect.NativeMethodAccessorImpl用于方法的第一次调用，直到JIT有足够的信息以sun.reflect.GeneratedMethodAccessorNNN的形式为该方法生成优化的Accesor。 /> 我不明白的是：如果Native使用我的代码，而Generated使用JIT生成的代码，则Native不应该显示有关我的 more 信息代码，不少吗？

所以我的问题是：
为什么sun.reflect.NativeMethodAccessorImpl中抛出的lambda运行时异常似乎丢失了lambda的堆栈跟踪？

这可能是JDK源代码中的错误吗？尤其是当在sun.reflect.GeneratedMethodAccessor中引发的同一异常包含lambda堆栈跟踪而没有问题时。

以下代码设法获得与第一个类似的堆栈跟踪。我可以通过分别用足够低或高的第一个参数（即NativeMethodAccessor或GeneratedMethodAccesor）运行java test.Main 1或java test.Main 30来强制使用。
但是，无论使用Native还是Generated，它的lambda部分始终存在。

package test;

import java.lang.reflect.Method;
import java.util.stream.IntStream;

class MyOtherClass {
    public void methodWithLambda(boolean fail) {
        IntStream.range(0, 1000).parallel().forEach(k -> {
            if (fail && k % 500 == 0)
                throw new NullPointerException();
        });
    }
    public String methodProxy(boolean fail) {
        methodWithLambda(fail);
        return "OK";
    }
}

class MyClass {
    public String methodReflected(Boolean fail) {
        return new MyOtherClass().methodProxy(fail);
    }
}

class Main {
    public static void main(String[] args) throws Exception {
        Class<MyClass> clazz = MyClass.class;
        Object instance = clazz.newInstance();
        Method method = clazz.getMethod("methodReflected", Boolean.class);
        int reps = args.length >= 1 ? Integer.valueOf(args[0]) : 20; 
        for (; reps --> 0;) {
            // Several non-failing calls to force creation of GeneratedMethodAccesor
            System.out.println((String) method.invoke(instance, false));
        }
        // Failing call
        System.out.println((String) method.invoke(instance, true));
    }
}

使用NativeMethodAccesor时上述代码的堆栈跟踪：

Exception in thread "main" java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at test.Main.main(Main.java:36)
Caused by: java.lang.NullPointerException
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
        at java.lang.reflect.Constructor.newInstance(Unknown Source)
        at java.util.concurrent.ForkJoinTask.getThrowableException(Unknown Source)
        at java.util.concurrent.ForkJoinTask.reportException(Unknown Source)
        at java.util.concurrent.ForkJoinTask.invoke(Unknown Source)
        at java.util.stream.ForEachOps$ForEachOp.evaluateParallel(Unknown Source)
        at java.util.stream.ForEachOps$ForEachOp$OfInt.evaluateParallel(Unknown Source)
        at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
        at java.util.stream.IntPipeline.forEach(Unknown Source)
        at java.util.stream.IntPipeline$Head.forEach(Unknown Source)
        at test.MyOtherClass.methodWithLambda(Main.java:8)
        at test.MyOtherClass.methodProxy(Main.java:14)
        at test.MyClass.methodReflected(Main.java:21)
        ... 5 more
Caused by: java.lang.NullPointerException
        at test.MyOtherClass.lambda$methodWithLambda$0(Main.java:10)
        at java.util.stream.ForEachOps$ForEachOp$OfInt.accept(Unknown Source)
        at java.util.stream.Streams$RangeIntSpliterator.forEachRemaining(Unknown Source)
        at java.util.Spliterator$OfInt.forEachRemaining(Unknown Source)
        at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
        at java.util.stream.ForEachOps$ForEachTask.compute(Unknown Source)
        at java.util.concurrent.CountedCompleter.exec(Unknown Source)
        at java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
        at java.util.concurrent.ForkJoinPool$WorkQueue.execLocalTasks(Unknown Source)
        at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(Unknown Source)
        at java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
        at java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)

编辑：需要明确的是：我不是在寻找解决此NPE的方法，也不是在强制打印lambda的堆栈跟踪的方法。我想知道的是发生上述情况的原因：不同的实现？有毛病吗与forEach()有关系吗？

Answer 1

这可能是JDK-6678999的问题，“ 空字符串比较后缺少堆栈跟踪 ”：

将字符串比较为null并捕获异常并重复操作之后，JVM开始引发“无栈” NullPointerException（它在9000次循环之后发生，但这是可变的）

对该问题的评估是

当服务器编译器编译方法时，堆栈跟踪将引发异常   出于性能目的，可以省略该方法的任何操作。

[…]如果用户始终需要堆栈跟踪，请对VM使用-XX：-OmitStackTraceInFastThrow选项。

因此，选项-XX:-OmitStackTraceInFastThrow可以解决问题。

请注意，该错误报告是针对Java 6的，但由于已将其关闭为“无法修复”，因此它可能仍然有用，尽管您必须在Windows 7中将“服务器编译器”替换为“ c2编译器”。现在解释。

使用NativeMethodAccessorImpl或GeneratedMethodAccessor…与该问题无关，但两者都有共同的原因；更多的执行可能会触发优化。

使用NativeMethodAccessor而不是GeneratedMethodAccessor时缺少Lambda堆栈跟踪

1 个答案: