Maven compilation issue with Java 9

Question

Trying to compile a Maven project using JDK 9.0.1 I'm facing this stacktrace without much of an explanation:

Exception in thread "main" java.lang.AssertionError at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155) at jdk.compiler/com.sun.tools.javac.util.Assert.check(Assert.java:46) at jdk.compiler/com.sun.tools.javac.comp.Modules.enter(Modules.java:250) at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.readSourceFile(JavaCompiler.java:821) at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$ImplicitCompleter.complete(JavacProcessingEnvironment.java:1510) at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:633) at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1314) at jdk.compiler/com.sun.tools.javac.code.Type$ClassType.complete(Type.java:1139) at jdk.compiler/com.sun.tools.javac.code.Type$ClassType.getTypeArguments(Type.java:1065) at jdk.compiler/com.sun.tools.javac.code.Printer.visitClassType(Printer.java:237) at jdk.compiler/com.sun.tools.javac.code.Printer.visitClassType(Printer.java:52) at jdk.compiler/com.sun.tools.javac.code.Type$ClassType.accept(Type.java:992) at jdk.compiler/com.sun.tools.javac.code.Printer.visit(Printer.java:136) at jdk.compiler/com.sun.tools.javac.util.AbstractDiagnosticFormatter.formatArgument(AbstractDiagnosticFormatter.java:197) at jdk.compiler/com.sun.tools.javac.util.AbstractDiagnosticFormatter.formatArguments(AbstractDiagnosticFormatter.java:165) at jdk.compiler/com.sun.tools.javac.util.BasicDiagnosticFormatter.formatMessage(BasicDiagnosticFormatter.java:111) at jdk.compiler/com.sun.tools.javac.util.BasicDiagnosticFormatter.formatMessage(BasicDiagnosticFormatter.java:67) at jdk.compiler/com.sun.tools.javac.util.AbstractDiagnosticFormatter.formatArgument(AbstractDiagnosticFormatter.java:183) at jdk.compiler/com.sun.tools.javac.util.AbstractDiagnosticFormatter.formatArguments(AbstractDiagnosticFormatter.java:165) at jdk.compiler/com.sun.tools.javac.util.BasicDiagnosticFormatter.formatMessage(BasicDiagnosticFormatter.java:111) at jdk.compiler/com.sun.tools.javac.util.BasicDiagnosticFormatter.formatMessage(BasicDiagnosticFormatter.java:67) at jdk.compiler/com.sun.tools.javac.util.JCDiagnostic.getMessage(JCDiagnostic.java:771) at jdk.compiler/com.sun.tools.javac.api.ClientCodeWrapper$DiagnosticSourceUnwrapper.getMessage(ClientCodeWrapper.java:799) at org.codehaus.plexus.compiler.javac.JavaxToolsCompiler.compileInProcess(JavaxToolsCompiler.java:131) at org.codehaus.plexus.compiler.javac.JavacCompiler.performCompile(JavacCompiler.java:174) at org.apache.maven.plugin.compiler.AbstractCompilerMojo.execute(AbstractCompilerMojo.java:1075) at org.apache.maven.plugin.compiler.CompilerMojo.execute(CompilerMojo.java:168) at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134) at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208) at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:154) at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:146) at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117) at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81) at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51) at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128) at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:309) at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:194) at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:107) at org.apache.maven.cli.MavenCli.execute(MavenCli.java:993) at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:345) at org.apache.maven.cli.MavenCli.main(MavenCli.java:191) 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.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289) at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229) at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415) at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356) 

Not really sure what's causing this, is this a bug in the JDK?

Additional details:

• Maven 3.5.0 with maven-compiler-plugin 3.7.0
• I'm just executing mvn clean install
• The source code is not open source unfortunately, so I'm not at liberty to share it
• There are no module-info.java files yet, I'm just trying to compile a project using Java 9
• Strangely enough if I leave the source level on 1.8, the code compiles, but it fails with the above exception if I specify it as 9

Answer 1

Just add this

<forceJavacCompilerUse>true</forceJavacCompilerUse> 

to your maven compiler build plugin in your POM and you'll see all the javac errors! Source with more details

Answer 2

Debugging

Step one should be to add the maven-compiler-plugin and enable <forceJavacCompilerUse>true</forceJavacCompilerUse> as the top answer suggests.

<project>   <build>     <plugins>       <plugin>         <groupId>org.apache.maven.plugins</groupId>         <artifactId>maven-compiler-plugin</artifactId>         <version>3.8.1</version>         <configuration>           <forceJavacCompilerUse>true</forceJavacCompilerUse>         </configuration>       </plugin>       ... 

This will point out the actual compilation error.

[main] INFO org.apache.maven.plugin.compiler.CompilerMojo - ------------------------------------------------------------- [main] ERROR org.apache.maven.plugin.compiler.CompilerMojo - COMPILATION ERROR :  [main] INFO org.apache.maven.plugin.compiler.CompilerMojo - ------------------------------------------------------------- [main] ERROR org.apache.maven.plugin.compiler.CompilerMojo -    last round: true /home/vsts/work/1/s/src/main/java/com/company/services/TemplateService.java:[3,61] error: cannot find symbol   symbol:   class VariableNotFoundException 

Root Cause

For me the root cause was that I made a commit and pushed it to the server which triggered CI, but did not include one class in the commit that was being used somewhere. Hence the compiler was not able to find it in the CI environment.

throws VariableNotFoundException { 

The solution is to make sure you don't have any Git staged files that you forgot to include as part of your commit.