使用NativeMethodAccessor而不是GeneratedMethodAccessor时,Lambda堆栈跟踪丢失

Question

几天前,我得到了一张支持票NullPointerException:

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,这很简单,因为NPE的确切行号很明显 ; 我所要做的就是去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)
            });

并替换.equals()呼叫Object.equals()以避免NPE(调查一些销售以NULL价格或货币注册的原因后来出现).测试,提交,推送,向QA发送票证.

然而,第二天QA返回票证说NPE持续存在,并且它们包括一个新的,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

除了两件事之外,这个堆栈跟踪与前一个完全相同:

• 此调用使用的是NativeMethodAccessorImpl代替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)

• 这个缺少lambda的堆栈跟踪.NPE发生的线路不见了.

这最初让我失望了.为什么堆栈跟踪的las部分缺失？我让QA重新测试并重新连接堆栈跟踪几次,直到我得到一个完整的; 然而,我最终得到的完整的一个GeneratedMethodAccesor再次基于.

现在,如果我理解正确,sun.reflect.NativeMethodAccessorImpl将用于方法的第一次调用,直到JIT有足够的信息以形式生成该方法的优化访问者sun.reflect.GeneratedMethodAccessorNNN.
我不明白的是:如果Native使用我的代码并Generated使用JIT生成的代码,不应该Native显示有关我的代码的更多信息,而不是更少？

所以我的问题是:
为什么抛出lambda运行时异常sun.reflect.NativeMethodAccessorImpl似乎缺少lambda的堆栈跟踪？

这可能是JDK源代码中的错误吗？特别是当内部抛出的同一异常sun.reflect.GeneratedMethodAccessor包括lambda堆栈跟踪没有问题时.

---

以下代码设法获得类似于第一个的堆栈跟踪.我可以分别强制使用NativeMethodAccessor或GeneratedMethodAccesor运行足够低或高的第一参数(即java test.Main 1或java test.Main 30).
然而,它的lambda部分总是存在,无论是使用Native还是Generated.

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));
    }
}

使用时堆栈跟踪以上代码NativeMethodAccesor:

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:只是要清楚:我不是在寻找修复这个NPE的方法,也没有办法强制lambda的堆栈跟踪打印.我想知道的是上面发生的原因:不同的实现？一个bug？与此有关forEach()？

Answer 1

这可能是JDK-6678999的问题，“空字符串比较后缺少 Stacktrace ”：

在将字符串与 null 进行比较并捕获异常并重复操作后，JVM 开始抛出“无堆栈”NullPointerException（它发生在 9000 次循环之后，但这是可变的）

问题的评价是

当服务器编译器编译一个方法时，出于性能目的，该方法抛出的异常中的堆栈跟踪可能会被省略。

[...] 如果用户总是想要堆栈跟踪，请对 VM 使用 -XX:-OmitStackTraceInFastThrow 选项。

因此，该选项-XX:-OmitStackTraceInFastThrow可能会解决问题。

请注意，错误报告是针对 Java 6 的，但由于它已被关闭为“不会修复”，因此它可能仍然相关，尽管您现在必须在解释中将“服务器编译器”替换为“c2 编译器”。

使用NativeMethodAccessorImpl或GeneratedMethodAccessor…与此问题无关，除非两者有共同的原因；更多的执行次数可能会触发优化。