Java/Dataflow - Unable to use ClassLoader to detect classpath elements

Question

I'm guessing this is more of a general Java/Eclipse question, but I'm not a Java guy and this isn't clicking for me. Stack trace at the end.

https://github.com/apache/beam/blob/9d9ffa5f1a3a9f280dfafae15944764a568515ef/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowRunner.java

Above is the file where the error is being thrown and it's pretty straightforward - are we using a URLClassLoader or not? Not being a Java guy, I don't understand how to switch to a URLClassLoader and googling this makes me feel like it's not something I need to be concerned with.

"Unable to use ClassLoader to detect classpath elements"

Makes me feel like I added a classpath incorrectly and Eclipse is doing something weird.

None of the Google dataflow/apache beam documentation mention anything about a URLClassLoader and now I can't even get their example project to run. I'm pretty sure I did something, but I'm not sure what.

Any help would be appreciated.

Jan 16, 2018 8:03:48 PM org.apache.beam.runners.dataflow.DataflowRunner 

detectClassPathResourcesToStage
SEVERE: Unable to use ClassLoader to detect classpath elements. Current ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@3b764bce, only URLClassLoaders are supported.
Exception in thread "main" java.lang.RuntimeException: Failed to construct instance from factory method DataflowRunner#fromOptions(interface org.apache.beam.sdk.options.PipelineOptions)
    at org.apache.beam.sdk.util.InstanceBuilder.buildFromMethod(InstanceBuilder.java:233)
    at org.apache.beam.sdk.util.InstanceBuilder.build(InstanceBuilder.java:162)
    at org.apache.beam.sdk.PipelineRunner.fromOptions(PipelineRunner.java:52)
    at org.apache.beam.sdk.Pipeline.create(Pipeline.java:142)
    at twitchInsights.MainPipeline.main(MainPipeline.java:59)
Caused by: java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at org.apache.beam.sdk.util.InstanceBuilder.buildFromMethod(InstanceBuilder.java:222)
    ... 4 more
Caused by: java.lang.IllegalArgumentException: Unable to use ClassLoader to detect classpath elements. Current ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@3b764bce, only URLClassLoaders are supported.
    at org.apache.beam.runners.dataflow.DataflowRunner.detectClassPathResourcesToStage(DataflowRunner.java:1365)
    at org.apache.beam.runners.dataflow.DataflowRunner.fromOptions(DataflowRunner.java:244)
    ... 9 more

Full Exception:

Thread [main] (Suspended (exception IllegalArgumentException))  
DataflowRunner.detectClassPathResourcesToStage(ClassLoader) line: 1365  
DataflowRunner.fromOptions(PipelineOptions) line: 244   
NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not available [native method]  
NativeMethodAccessorImpl.invoke(Object, Object[]) line: 62  
DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 43  
Method.invoke(Object, Object...) line: 564  
InstanceBuilder<T>.buildFromMethod(Class<?>[]) line: 222    
InstanceBuilder<T>.build() line: 162    
PipelineRunner<ResultT>.fromOptions(PipelineOptions) line: 52   
Pipeline.create(PipelineOptions) line: 142  
MainPipeline.main(String[]) line: 60    

Answer 1

A workaround that works for me in Java 9 is defining filesToStage programmatically based on the classpath.

Her is an example how you can generate the file set from the class path:

dataflowPipelineOptions.setFilesToStage( 
  Arrays.
    asList(System.getProperty("java.class.path").split(File.pathSeparator)).
    stream().
    map(entry -> new File(entry).toString()).
    collect(Collectors.toList())
)

Answer 2

I think this is a Java 9 specific issue. Beam does not officially support Java 9 yet - see https://issues.apache.org/jira/browse/BEAM-2530 for current progress.

You can try working around this specific issue by specifying the classpath elements via --filesToStage instead (this will bypass the autodetection based on current classloader - which is what's failing). However you may hit other issues.