Details
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Bug
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Status: Closed
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Major
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Resolution: Fixed
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None
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None
Description
Before the compilation starts, the compiler plugin prepares the module path and the class path. Unfortunately, this process is very lenient. If the plugin cannot find a module that is specified in the module-info file, it doesn't report this problem, and it just carries on. In the end, the compiler will give a "module not found", giving no clue about the root cause.
As an example, I attached a simple maven project with a module info file that refers to an automatic module named jython-standalone. The dependencies contain a jar file jython-standalone-2.7.0.jar. That jar file contains no module information, and it would normally be resolvable as an automatic module. However, when building the project, the Java compiler fails with this error:
module not found: jython.standalone
The root cause of the error is that the jar file contains a class at top level, which is illegal for a module. So, the error message "module not found" is misleading.
I think this is a bug, because the maven-compiler-plugin fails to perform its task of building a correct module path, but doesn't tell, and just carries on. Instead, it should stop, and give an informative error message.
Relevant source code:
- https://github.com/apache/maven-compiler-plugin, class CompilerMojo
- https://github.com/codehaus-plexus/plexus-languages, class LocationManager and MainClassModuleNameExtractor
- https://docs.oracle.com/javase/9/docs/api/java/lang/module/ModuleFinder.html
The MainClassModuleNameExtractor uses the ModuleFinder class of the JDK to find modules. I think that these two error situations should lead to an error and a clear description of possible causes:
- ModuleFinder#findAll returns an empty set.
- ModuleFinder#findAll results in a FindException. When this happens is well documented in the ModuleFinder#of method. This exception is currently swallowed.