Not being able to stop at exception when using consecutive stream.flatMap

Question

When debugging my code in IntelliJ IDEA I can't stop at an exception in certain cases when consecutive flatMap methods are used on a stream. Instead of stopping at exception, it stops in API's ReferencePipeline#flatMap method at line 271, with frames stack not showing the exception at all. Here's the simplest example possible:

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class AssertTest {
    public static void main(String[] args) {
        Arrays.asList(
            Arrays.asList(0, 0, 0),
            Arrays.asList(1, 1, 1),
            Arrays.asList(2, 2, 2)
        )
            .stream()
            .flatMap(a -> test1(a).stream())
            .flatMap(a -> test2(a).stream())
            .collect(Collectors.toSet());
    }
    private static List<List<Integer>> test1(List<Integer> ints) {
        return Arrays.asList(ints);
    }
    private static List<List<Integer>> test2(List<Integer> ints) {
        throw new RuntimeException();
    }
}

Here's what I see in my IDE when I run the code in debugger:

If I comment out the line that says .flatMap(a -> test1(a).stream()) , then it stops at the exception as intended. The same occurs with assertions (replace throw new RuntimeException() with assert false and run with -ea ).

This issue concerns only the debugger: after the program crashes on exception, the stack trace contains the right place where the exception originated.

Exception in thread "main" java.lang.RuntimeException
    at mypkg.AssertTest.test2(AssertTest.java:23)
    at mypkg.AssertTest.lambda$main$1(AssertTest.java:16)
    at mypkg.AssertTest$$Lambda$2/14008398.apply(Unknown Source)
    at java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:267)
    at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
    at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)
    at java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:270)
    at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
    at mypkg.AssertTest.main(AssertTest.java:17)

Is this a bug? If so, is it mentioned anywhere? If not, where should I report it: Jetbrains, Oracle? Or is this the correct behavior?

Answer 1

Here's the answer I got from a Jetbrains developer after reporting a bug:

http://youtrack.jetbrains.com/issue/IDEA-126981?replyTo=27-763411

I can reproduce it only if I set any exception breakpoint to stop only on uncaught exceptions, is this the case?

Your exception is caught and re-thrown inside java, it is a downside of streams :( You'll need to stop on caught exceptions as well to be able to stop in your code.

Enabling stopping at caught exceptions fixes this for me.