How do I eliminate concerning warnings about missing classes when using JanusGraph 1.0.0 with CQL in embedded mode

[pete-gillin-privitar]

We are using JanusGraph in embedded mode with the CQL backend. On upgrading to JanusGraph 1.0.0, we are getting some concerning warnings about missing classes when we do our first traversal. I am not sure whether I should be worried about these, or what combination of maven dependencies (and possibly exclusions?) I need to get rid of them.

I can reproduce this by doing anything which causes the class JanusGraphTraverserUtil to be loaded, in a maven project with org.janusgraph:janusgraph-core and org.janusgraph:janusgraph-cql as dependencies. I am providing the full text of the warnings and a minimal standalone reproduction below, but I'll summarize quickly here.

We have a dependency on janusgraph-cql but not on janusgraph-hadoop. That means that we have org.janusgraph.hadoop.scan.CQLHadoopScanRunner on the classpath, but not org.janusgraph.hadoop.scan.AbstractHadoopScanRunner which CQLHadoopScanRunner extends. That means that we'll never be able to load the class CQLHadoopScanRunner. That doesn't seem to matter in itself, since (in our testing so far) we have not hit a code path which actually uses that class. However, the first time we do a traversal, the static initializer of org.janusgraph.graphdb.util.JanusGraphTraverserUtil uses the Reflections utility to scan every class under org.janusgraph, and that tries to expand the supertypes, and that logs a warning could not get type for name org.janusgraph.hadoop.scan.AbstractHadoopScanRunner from any class loader.

We get a similar story for a few other classes (see the below). As well as several from janusgraph-hadoop, this includes one in org.apache.hadoop:hadoop-mapreduce-client-core, and one in org.apache.tinkerpop:gremlin-server.

The upgrade instructions contain this note, which seems potentially relevant:

If you are running embedded JanusGraph with Cassandra, you have to exclude the cassandra-hadoop-util from janusgraph-cql

However, that makes no difference that I can tell. In my reproduction below, I put

      <exclusions>
        <exclusion>
          <groupId>org.janusgraph</groupId>
          <artifactId>cassandra-hadoop-util</artifactId>
        </exclusion>
      </exclusions>

in the janusgraph-cql dependency in my pom.xml and nothing changed. I assume that this is because the classes like CQLHadoopScanRunner are in janusgraph-cql, not in cassandra-hadoop-util.

In my toy reproduction, I can make these warnings go away by adding dependencies on janusgraph-hadoop and gremlin-server. In my real application, adding janusgraph-hadoop causes a different problem (to do with Spring initialization). I can try to debug that if adding this dependency is the correct thing to do, but I'd like confirmation that this really is the correct approach. We're not using Hadoop to the best of my knowledge, so being forced to add the janusgraph-hadoop dependency would feel like an odd thing to do.

Alternatively, we could ignore the warnings, or we could even repackage the janusgraph-cql JAR to exclude the classes with missing superclasses. But I'd like confirmation that this is going to be safe — I don't know whether they'd be used on some code path that we just haven't happened to hit in our testing so far. (Repackaging the JAR would also seem like an odd thing to do.)

Any advice on how to proceed would be very welcome! Thanks.

---

Full text of warnings:

[com.example.Repro.main()] WARN org.reflections8.Reflections - could not get type for name org.janusgraph.hadoop.scan.AbstractHadoopScanRunner from any class loader
org.reflections8.ReflectionsException: could not get type for name org.janusgraph.hadoop.scan.AbstractHadoopScanRunner
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:412)
	at org.reflections8.Reflections.expandSuperTypes(Reflections.java:421)
	at org.reflections8.Reflections.<init>(Reflections.java:158)
	at org.reflections8.Reflections.<init>(Reflections.java:200)
	at org.reflections8.Reflections.<init>(Reflections.java:173)
	at org.janusgraph.graphdb.util.JanusGraphTraverserUtil.<clinit>(JanusGraphTraverserUtil.java:35)
	at com.example.Repro.main(Repro.java:11)
	at org.codehaus.mojo.exec.ExecJavaMojo.doMain(ExecJavaMojo.java:385)
	at org.codehaus.mojo.exec.ExecJavaMojo.doExec(ExecJavaMojo.java:374)
	at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0(ExecJavaMojo.java:296)
	at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.ClassNotFoundException: org.janusgraph.hadoop.scan.AbstractHadoopScanRunner
	at org.codehaus.mojo.exec.URLClassLoaderBuilder$ExecJavaClassLoader.loadClass(URLClassLoaderBuilder.java:181)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:410)
	... 10 more
[com.example.Repro.main()] WARN org.reflections8.Reflections - could not get type for name org.janusgraph.hadoop.formats.util.HadoopInputFormat from any class loader
org.reflections8.ReflectionsException: could not get type for name org.janusgraph.hadoop.formats.util.HadoopInputFormat
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:412)
	at org.reflections8.Reflections.expandSuperTypes(Reflections.java:421)
	at org.reflections8.Reflections.<init>(Reflections.java:158)
	at org.reflections8.Reflections.<init>(Reflections.java:200)
	at org.reflections8.Reflections.<init>(Reflections.java:173)
	at org.janusgraph.graphdb.util.JanusGraphTraverserUtil.<clinit>(JanusGraphTraverserUtil.java:35)
	at com.example.Repro.main(Repro.java:11)
	at org.codehaus.mojo.exec.ExecJavaMojo.doMain(ExecJavaMojo.java:385)
	at org.codehaus.mojo.exec.ExecJavaMojo.doExec(ExecJavaMojo.java:374)
	at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0(ExecJavaMojo.java:296)
	at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.ClassNotFoundException: org.janusgraph.hadoop.formats.util.HadoopInputFormat
	at org.codehaus.mojo.exec.URLClassLoaderBuilder$ExecJavaClassLoader.loadClass(URLClassLoaderBuilder.java:181)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:410)
	... 10 more
[com.example.Repro.main()] WARN org.reflections8.Reflections - could not get type for name org.apache.tinkerpop.gremlin.server.GraphManager from any class loader
org.reflections8.ReflectionsException: could not get type for name org.apache.tinkerpop.gremlin.server.GraphManager
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:412)
	at org.reflections8.Reflections.expandSuperTypes(Reflections.java:421)
	at org.reflections8.Reflections.<init>(Reflections.java:158)
	at org.reflections8.Reflections.<init>(Reflections.java:200)
	at org.reflections8.Reflections.<init>(Reflections.java:173)
	at org.janusgraph.graphdb.util.JanusGraphTraverserUtil.<clinit>(JanusGraphTraverserUtil.java:35)
	at com.example.Repro.main(Repro.java:11)
	at org.codehaus.mojo.exec.ExecJavaMojo.doMain(ExecJavaMojo.java:385)
	at org.codehaus.mojo.exec.ExecJavaMojo.doExec(ExecJavaMojo.java:374)
	at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0(ExecJavaMojo.java:296)
	at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.ClassNotFoundException: org.apache.tinkerpop.gremlin.server.GraphManager
	at org.codehaus.mojo.exec.URLClassLoaderBuilder$ExecJavaClassLoader.loadClass(URLClassLoaderBuilder.java:181)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:410)
	... 10 more
[com.example.Repro.main()] WARN org.reflections8.Reflections - could not get type for name org.janusgraph.hadoop.formats.util.AbstractBinaryInputFormat from any class loader
org.reflections8.ReflectionsException: could not get type for name org.janusgraph.hadoop.formats.util.AbstractBinaryInputFormat
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:412)
	at org.reflections8.Reflections.expandSuperTypes(Reflections.java:421)
	at org.reflections8.Reflections.<init>(Reflections.java:158)
	at org.reflections8.Reflections.<init>(Reflections.java:200)
	at org.reflections8.Reflections.<init>(Reflections.java:173)
	at org.janusgraph.graphdb.util.JanusGraphTraverserUtil.<clinit>(JanusGraphTraverserUtil.java:35)
	at com.example.Repro.main(Repro.java:11)
	at org.codehaus.mojo.exec.ExecJavaMojo.doMain(ExecJavaMojo.java:385)
	at org.codehaus.mojo.exec.ExecJavaMojo.doExec(ExecJavaMojo.java:374)
	at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0(ExecJavaMojo.java:296)
	at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.ClassNotFoundException: org.janusgraph.hadoop.formats.util.AbstractBinaryInputFormat
	at org.codehaus.mojo.exec.URLClassLoaderBuilder$ExecJavaClassLoader.loadClass(URLClassLoaderBuilder.java:181)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:410)
	... 10 more
[com.example.Repro.main()] WARN org.reflections8.Reflections - could not get type for name org.janusgraph.hadoop.HadoopStoreManager from any class loader
org.reflections8.ReflectionsException: could not get type for name org.janusgraph.hadoop.HadoopStoreManager
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:412)
	at org.reflections8.Reflections.expandSuperTypes(Reflections.java:421)
	at org.reflections8.Reflections.<init>(Reflections.java:158)
	at org.reflections8.Reflections.<init>(Reflections.java:200)
	at org.reflections8.Reflections.<init>(Reflections.java:173)
	at org.janusgraph.graphdb.util.JanusGraphTraverserUtil.<clinit>(JanusGraphTraverserUtil.java:35)
	at com.example.Repro.main(Repro.java:11)
	at org.codehaus.mojo.exec.ExecJavaMojo.doMain(ExecJavaMojo.java:385)
	at org.codehaus.mojo.exec.ExecJavaMojo.doExec(ExecJavaMojo.java:374)
	at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0(ExecJavaMojo.java:296)
	at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.ClassNotFoundException: org.janusgraph.hadoop.HadoopStoreManager
	at org.codehaus.mojo.exec.URLClassLoaderBuilder$ExecJavaClassLoader.loadClass(URLClassLoaderBuilder.java:181)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:410)
	... 10 more
[com.example.Repro.main()] WARN org.reflections8.Reflections - could not get type for name org.apache.hadoop.mapreduce.RecordReader from any class loader
org.reflections8.ReflectionsException: could not get type for name org.apache.hadoop.mapreduce.RecordReader
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:412)
	at org.reflections8.Reflections.expandSuperTypes(Reflections.java:421)
	at org.reflections8.Reflections.<init>(Reflections.java:158)
	at org.reflections8.Reflections.<init>(Reflections.java:200)
	at org.reflections8.Reflections.<init>(Reflections.java:173)
	at org.janusgraph.graphdb.util.JanusGraphTraverserUtil.<clinit>(JanusGraphTraverserUtil.java:35)
	at com.example.Repro.main(Repro.java:11)
	at org.codehaus.mojo.exec.ExecJavaMojo.doMain(ExecJavaMojo.java:385)
	at org.codehaus.mojo.exec.ExecJavaMojo.doExec(ExecJavaMojo.java:374)
	at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0(ExecJavaMojo.java:296)
	at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.mapreduce.RecordReader
	at org.codehaus.mojo.exec.URLClassLoaderBuilder$ExecJavaClassLoader.loadClass(URLClassLoaderBuilder.java:181)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
	at org.reflections8.ReflectionUtils.forName(ReflectionUtils.java:410)
	... 10 more

---

Standalone reproduction:

In src/main/java/com/example/Repro.java I have:

package com.example;

import org.janusgraph.graphdb.util.JanusGraphTraverserUtil;

public class Repro {

    public static void main(String[] args) {
        // Construct a new JanusGraphTraverserUtil to trigger its static initializer block.
        // Obviously we would never do this in real life — but the static initializer will be triggered in normal usage.
        // This is the simplest way to reproduce.
        JanusGraphTraverserUtil unused = new JanusGraphTraverserUtil();
    }
}

In pom.xml I have:

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.example</groupId>
  <artifactId>repro</artifactId>
  <version>1.0-SNAPSHOT</version>
  <name>repro</name>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.janusgraph</groupId>
      <artifactId>janusgraph-core</artifactId>
      <version>1.0.0</version>
    </dependency>
    <dependency>
      <groupId>org.janusgraph</groupId>
      <artifactId>janusgraph-cql</artifactId>
      <version>1.0.0</version>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-simple</artifactId>
      <version>1.7.36</version>
    </dependency>
  </dependencies>

  <build>
    <pluginManagement>
      <plugins>
        <plugin>
          <artifactId>maven-clean-plugin</artifactId>
          <version>3.1.0</version>
        </plugin>
        <plugin>
          <artifactId>maven-resources-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-compiler-plugin</artifactId>
          <version>3.8.0</version>
        </plugin>
        <plugin>
          <artifactId>maven-jar-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-install-plugin</artifactId>
          <version>2.5.2</version>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
</project>

And then I do mvn clean compile exec:java -Dexec.mainClass=com.example.Repro to get the output shown above.

[FlorinAsavoaie]

Found any solution to this?

[porunov]

I will try to explain the decisions we made and why you see those warnings when updating to 1.0.0.
For reference, the changes come from this PR: #3245
You can check original comment here: #3245 (comment)

Before the changes were merged from #3245 and ongoing related PRs, janusgraph-cql had the following dependencies:

• janusgraph-hadoop - Yes, you would pull all hadoop dependencies regardless if you use OLAP or not.
• cassandra-all - needed for OLAP to get record format
• cassandra-driver-core version 3 - needed for OLAP (CqlRecordReader).
• cassandra-driver-core version 4 - needed for OLTP (the most common usage)

Obviously, the reason you didn't get any warnings before upgrading is because you would get all these libraries into your classpath regardless of your JanusGraph usage.

However, the problem with such structure was:

• You bundle more dependencies that you actually don't use in runtime, and when we talk about Hadoop, the difference was noticeable.
• It's actually a bit complicated to have both cassandra driver dependencies in the classpath (v3 and v4). Even so, datastax cassandra driver 4 uses different namespace, they do relay on other transitive dependencies. In some cases, we couldn't simply upgrade cassandra driver 3 to the next minor or patch version without an additional headache resolving transitive dependencies conflicts between cassandra driver 3 and 4.

To resolve these challenges, the decision was made: making Hadoop and relative OLAP dependencies optional.

As such, we created a new module called cassandra-hadoop-util and moved most of the Hadoop related classes as well as all cassandra driver 3 usage into that module. However, some of the classes are a bit harder to be moved and requires a bit deeper refactoring. As such, you can still find this in janusgraph-cql.
We made cassandra-hadoop-util optional, so that people who don't use Hadoop functionality in runtime - don't pull unnecessary dependencies. However, this resulted in this warning messages you can find. The root cause is the reflection scanner as you pointed out. It will scan all classes, but then it will print warnings if there are any imports to classes that are not in the classpath.

Are these warnings safe? - Yes. it doesn't influence JanusGraph OLTP functionality at all. If you really need OLAP functionality then it woult make sense to include hadoop dependencies and this will automatically solve the warnings issue.

I don't recommend including these unnecessary dependencies if you don't use Hadoop in your use-case. Either ignoring these warnings or somehow silencing them would be more preferable.

As for the next steps it would be nice to move all classes in this package out from janusgraph-cql to cassandra-hadoop-util or janusgraph-hadoop, so that the scan of these classes don't cause these warnings. PRs are always welcome and if you are willing to dig into this refactoring I would be more than happy to review it.