NoClassDefFoundError:scala/Serializable when using multiple versions of Scala

This is esaulgd, application engineer at Geisha Tokyo.

A while ago, I was working on a web app developed with Scalatra and Maven. Suddenly, the following error started to pop up anytime I tried to build: (edited for brevity)

java.lang.NoClassDefFoundError: scala/Serializable
  at java.lang.ClassLoader.defineClass1(Native Method)
  at java.lang.ClassLoader.defineClassCond(ClassLoader.java)
  at java.lang.ClassLoader.defineClass(ClassLoader.java)
  at java.security.SecureClassLoader.defineClass(SecureClassLoader.java)
  at java.net.URLClassLoader.defineClass(URLClassLoader.java)
  at java.net.URLClassLoader.access$000(URLClassLoader.java)
  at java.net.URLClassLoader$1.run(URLClassLoader.java)

A Google search turned up no easily identifiable cause. Most people with similar problems found them to be related to building Lift, so I wondered whether there was an analogous issue with Scalatra. Just when it looked like I would have to roll up my sleeves and start messing with the build process, I found this article written in Japanese.

The article places the blame on simultaneous usage of multiple versions of Scala. When I checked, indeed there it was on the log output:

[WARNING] Multiple versions of scala libraries detected!

The reason I hadn’t made the connection is that this particular warning had been sitting there for months without any ill consequences. Well, until now.

As the problem had roots in the dependency chain deeper than the component I was in charge of, I brought the project lead into the fold. After some deliberation, we decided to make everything rely on the then-current Scala 2.9.0.

The “easy” part was to make sure that the web app, and all internally-developed upstream components, compiled using 2.9.0. In most cases this was as simple as editing the pom.xml as shown below and recompiling.

  <project ...>
    <properties>
-      <scala.version>2.8.1</scala.version>
+      <scala.version>2.9.0</scala.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-library</artifactId>
            <version>${scala.version}</version>
        </dependency>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-compiler</artifactId>
            <version>${scala.version}</version>
        </dependency>
    </dependencies>
  </project>

For externally-made components where a version for Scala 2.9 was available, we updated the dependency info. When this was not the case, for example with Scalatra (scalatra_2.8.1 ver 2.0.0.M3 at the time), we deployed a custom build to our internal repository.

<project ...>
  <properties>
-   <scalatra.version>2.0.0.M3</scalatra.version>
+   <scalatra.version>2.0.0-SNAPSHOT</scalatra.version>
    <scala.version>2.9.0</scala.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.scalatra</groupId>
-     <artifactId>scalatra_2.8.1</artifactId>
+     <artifactId>scalatra_${scala.version}</artifactId>
      <version>${scalatra.version}</version>
    </dependency>
    <dependency>
      <groupId>org.scalatra</groupId>
-     <artifactId>scalatra-scalate_2.8.1</artifactId>
+     <artifactId>scalatra-scalate_${scala.version}</artifactId>
      <version>${scalatra.version}</version>
    </dependency>
    <dependency>
      <groupId>org.scalatra</groupId>
-     <artifactId>scalatra-scalatest_2.8.1</artifactId>
+     <artifactId>scalatra-scalatest_${scala.version}</artifactId>
      <version>${scalatra.version}</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

Once the entire chain was updated, both the warning and the error disappeared.

A possible explanation for this phenomenon lies with the compiler daemon. However, the Maven build wasn’t supposed to be using the daemon in the first place, so either there’s a fault there or another factor is at play.

In any case, having this information spelled out in English will hopefully help someone else.

Good luck and happy scala-ing!

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