Summary

A new warning is produced when static-ness of a class recorded in an InnerClasses attribute is not matching the static-ness of the same-named class read from source.

Problem

Consider these three files:

OriginalVersion.java:
class Test {
    public class Nested {}
}

Use.java:
public class Use {
    Test.Nested n;
}

Test.java:
class Test {
    Use use;
    public static class Nested {}
}

First compiling OriginalVersion.java and Use.java:

$ ~/tools/jdk/jdk-25/bin/javac -d out OriginalVersion.java Use.java 

this will record the Test.Nested as a non-static innerclass inside the InnerClasses attribute of Use. Then compiling Test.java with the classfile on classpath leads to:

$ ~/tools/jdk/jdk-25/bin/javac -classpath out Test.java 
An exception has occurred in the compiler (25-internal). Please file a bug against the Java compiler via the Java bug reporting page (https://bugreport.java.com) after checking the Bug Database (https://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.NullPointerException: Cannot read field "sym" because "mdef.params.head" is null
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitClassDef(Lower.java:2240)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:859)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2062)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitClassDef(Lower.java:2204)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:859)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2062)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2081)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translateTopLevelClass(Lower.java:4366)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1688)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1505)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:968)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:319)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:178)
        at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:66)
        at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:52)
printing javac parameters to: /tmp/javac.20251031_131057.args

The reason is that when Use.class is read, it changes the flags of Test.Nested to a state that is incompatible with the actual source file being compiled.

Solution

The solution is twofold:

• while reading the InnerClasses attribute, javac will ignore the flags for classes that are being read from the source file

• javac will produce a new warning under the -Xlint:classfile category:

  $ javac -classpath out -Xlint:classfile Test.java warning: [classfile] InnerClasses attribute for Nested in out/Use.class inconsistent with source code 1 warning

Specification

There will be a new warning under the -Xlint:classfile category when a mismatch is detected between the static-ness recorded in an InnerClasses attribute and the static-ness in the source file.

Please note the -Xlint:classfile is not enabled by default.