ClassLoader.defineClass throws incorrect exception (ClassFormatError instead of LinkageError) on duplicate class definition

[Shawn-Patrick-Star]

Java -version output

openjdk version "1.8.0_492"
IBM Semeru Runtime Open Edition (build 1.8.0_492-b09)
Eclipse OpenJ9 VM (build 1.8.0_492-b09-openj9-0.59.0, JRE 1.8.0 Linux amd64-64-Bit Compressed References 20260428_1524 (JIT enabled, AOT enabled)
OpenJ9   - c53b6b93f4
OMR      - 6426f03fa
JCL      - 0a800bff02 based on jdk8u492-b09)

Summary of problem

According to the Java Virtual Machine Specification (JVMS §5.3.5), attempting to define a class with the same name twice using the same ClassLoader instance must result in a LinkageError.

When dynamically loading a valid class bytecode twice via the same ClassLoader instance, HotSpot strictly follows the specification and throws a LinkageError, whereas OpenJ9 incorrectly throws a ClassFormatError.

Source Code

public class Test extends ClassLoader {
    private static int[] DATA = new int[] {
        -54, -2, -70, -66, 0, 0, 0, 52, 0, 13,
        10, 0, 3, 0, 10, 7, 0, 11, 7, 0,
        12, 1, 0, 6, 60, 105, 110, 105, 116, 62,
        1, 0, 3, 40, 41, 86, 1, 0, 4, 67,
        111, 100, 101, 1, 0, 15, 76, 105, 110, 101,
        78, 117, 109, 98, 101, 114, 84, 97, 98, 108,
        101, 1, 0, 10, 83, 111, 117, 114, 99, 101,
        70, 105, 108, 101, 1, 0, 18, 84, 101, 115,
        116, 67, 97, 115, 101, 48, 48, 48, 48, 48,
        46, 106, 97, 118, 97, 12, 0, 4, 0, 5,
        1, 0, 13, 84, 101, 115, 116, 67, 97, 115,
        101, 48, 48, 48, 48, 48, 1, 0, 16, 106,
        97, 118, 97, 47, 108, 97, 110, 103, 47, 79,
        98, 106, 101, 99, 116, 0, 32, 0, 2, 0,
        3, 0, 0, 0, 0, 0, 1, 0, 0, 0,
        4, 0, 5, 0, 1, 0, 6, 0, 0, 0,
        29, 0, 1, 0, 1, 0, 0, 0, 5, 42,
        -73, 0, 1, -79, 0, 0, 0, 1, 0, 7,
        0, 0, 0, 6, 0, 1, 0, 0, 0, 1,
        0, 1, 0, 8, 0, 0, 0, 2, 0, 9
    };

    public Test() {}

    public void load() {
        byte[] bytes = new byte[DATA.length];
        for (int i = 0; i < bytes.length; i++) {
            bytes[i] = (byte) DATA[i];
        }
        defineClass(bytes, 0, bytes.length);
    }

    public static void main(String[] args) {
        Test test = new Test();
        test.load(); 
        test.load();
    }
}

Steps to reproduce

1. Compile the Test.java.
2. Run with OpenJ9.

[hangshao0]

Could you please look at this @lzhou2025 ?

[lzhou2025]

Could you please look at this @lzhou2025 ?

I will look into it.