[MCOMPILER-510] NullPointerException when using --patch-module compiler argument

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Duplicate
• Affects Version/s: 3.10.1
• Assignee: Unassigned
• Reporter: Artem K.
• Created: 01/Nov/22 13:33
• Updated: 30/Oct/23 02:40
• Resolved: 23/Oct/23 10:06

Description

Summary

When using --patch-module compiler argument, there is a NullPointerException due to field pathElements not being initialized in CompilerMojo.

Description

The method preparePaths in CompilerMojo has two code paths:

@Override
protected void preparePaths( Set<File> sourceFiles )
{
    boolean hasModuleDescriptor = false;
    // ...
    if ( hasModuleDescriptor )
    {
        modulepathElements = new ArrayList<>( compilePath.size() );
        classpathElements = new ArrayList<>( compilePath.size() );
        pathElements = new LinkedHashMap<>( compilePath.size() );
    }
    else
    {
        classpathElements = new ArrayList<>();
        modulepathElements = Collections.emptyList();
        // ...
    }
}

Notice that in the else part, pathElements is not initialized. This field is used when the compiler arguments contain --patch-module switch. So whenever the compiler is used with this switch and there is no module descriptor in the project, NPE occurs.

Proof-of-concept

Attached is a simple proof-of-concept to illustrate the issue. The project consists of a single class with raises NullPointerException using a new constructor NullPointerException(Throwable), which does not exist in base JDK.

• When executing javac{} src\main\java\poc\Main.java, the compilation fails as expected.
• Using a replacement NullPointerException class in java_base.jar with the added constructor, the compilation works fine:{} javac --patch-module java.base=java_base.jar src\main\java\poc\Main.java.
• Attempting to do the same using Maven compiler plugin results in NullPointerException: mvn -X compile

Walkaround

I was able to fix the problem by adding the line

pathElements = Collections.emptyMap(); 

to the else part of the CompilerMojo.preparePaths.

The compilation started working, it does however produce a warning

[WARNING] Can't locate java_base.jar

every single time. I did not investigate much the cause of this message.

Also note that while similar, this issues is not the same as MCOMPILER-311.

Issue Links

• duplicates: MCOMPILER-547 NPE when --patch-module is used in non-module project