Java 8: Debugging Stream Pipelines

I've found that stream pipelines can be difficult to debug because stack traces involving lambda expressions are quite cryptic. Consider the following contrived example:

import java.util.Arrays;
import java.util.List;
import java.util.function.Function;

public class Test {
  public static void main(final String[] args) {
    final List<String> list = Arrays.asList("foo", null, "bar");
    list.stream()
        .map(Function.identity())
        .filter(x -> true)
        .map(String::length)
        .forEach(System.out::println);
  }
}

You may have already guessed that the code above will throw a NullPointerException when String.length is called on the null element in the list. I've added extra map and filter operations, which do nothing, just to make the example a bit more interesting. In the real world, you will probably have a number of different operations in your stream pipeline.

Running the code, produces the following stack trace:

Exception in thread "main" java.lang.NullPointerException
  at Test$$Lambda$3/455659002.apply(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$2$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.Spliterators$ArraySpliterator.forEachRemaining(Unknown Source)
  at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
  at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(Unknown Source)
  at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
  at java.util.stream.ReferencePipeline.forEach(Unknown Source)
  at Test.main(Test.java:12)

The stack trace shows that a NullPointerException occurred but it doesn't tell you which operation in the pipeline failed. What does Test$$Lambda$3/455659002.apply(Unknown Source) mean and why is there no line number?! Since lambda expressions don't have a name, the compiler makes one up (similar to anonymous classes). In this case, it is Test$$Lambda$3 but that doesn't help us track the bug in our code.

So, what can we do? Let's go old-school and add some logging to our code! We can use peek to print out each element before it is consumed by the next operation in the pipeline.

import java.util.Arrays;
import java.util.List;
import java.util.function.Function;

public class Test {
  public static void main(final String[] args) {
    final List<String> list = Arrays.asList("foo", null, "bar");
    list.stream()
        .peek(x -> System.out.println("Running identity on: " + x))
        .map(Function.identity())
        .peek(x -> System.out.println("Running filter on: " + x))
        .filter(x -> true)
        .peek(x -> System.out.println("Running string length on: " + x))
        .map(String::length)
        .peek(x -> System.out.println("Running print on: " + x))
        .forEach(System.out::println);
  }
}

Running it produces the following output:

Running identity map on: foo
Running filter on: foo
Running string length on: foo
Running print on: 3
3
Running identity map on: null
Running filter on: null
Running string length on: null
Exception in thread "main" java.lang.NullPointerException
  at Test$$Lambda$6/295530567.apply(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$11$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$2$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$11$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$11$1.accept(Unknown Source)
  at java.util.Spliterators$ArraySpliterator.forEachRemaining(Unknown Source)
  at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
  at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(Unknown Source)
  at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
  at java.util.stream.ReferencePipeline.forEach(Unknown Source)
  at Test.main(Test.java:16)

Great! Now we know that the NullPointerException was thrown by the string length lambda!

In general, I think stack traces involving lambdas could be improved in future versions of Java.