OpenJ9: Compatibility Bug – Missing Deprecation Warning for HotSpot Flag -XX:+TLABStats

[xwqmary]

The OpenJ9 JVM is failing a common compatibility test because it does not output the required deprecation warning when launched with the obsolete HotSpot flag -XX:+TLABStats.

This behavior prevents test suites from validating command-line compatibility and offers poor guidance to users migrating from OpenJDK.

``

public class test {
     public static void main(String[] args) {
          String javaCommand = System.getProperty("java.home") + "/bin/java";
                try {

                 ProcessBuilder pb = new ProcessBuilder(javaCommand, "-XX:+TLABStats", "-version");
                 pb.redirectErrorStream(true);
                 Process process = pb.start();

                 String output = new BufferedReader(new InputStreamReader(process.getInputStream()))
                            .lines()
                            .reduce("", (acc, line) -> acc + line + "\n");
        
                   int exitCode = process.waitFor();

                  boolean foundDeprecationWarning = output.toLowerCase().contains("tlabstats") && 
                                          output.toLowerCase().contains("deprecat");

                  if (!foundDeprecationWarning) {
                       throw new AssertionError("Expected deprecation warning for TLABStats flag was not found. Exit Code: " + exitCode + "\nOutput:\n" + output);
               }
        

             } catch (Exception e) {
               e.printStackTrace();
               System.exit(1);
    }
}

}

``

The output shows that the JVM runs normally, but no deprecation warning is printed:
`
Exception in thread "main" java.lang.AssertionError: Expected deprecation warning for TLABStats flag was not found. Exit Code: 0
Output:
openjdk version "17.0.13" 2024-10-15
IBM Semeru Runtime Open Edition 17.0.13.11 (build 17.0.13+11)
Eclipse OpenJ9 VM 17.0.13.11 (build openj9-0.48.0, JRE 17 Linux amd64-64-Bit Compressed References 20241015_886 (JIT enabled, AOT enabled)
OpenJ9 - 1d58314
OMR - d10a4d553
JCL - d17dd58f8d7 based on jdk-17.0.13+11)

On HotSpot, deprecated or obsolete options like -XX:+TLABStats typically produce explicit warnings such as:

“Warning: Option -XX:+TLABStats was deprecated…”

However, OpenJ9 currently handles this option silently, which breaks command-line compatibility expectations.

[pshipton]

The default behavior for OpenJ9 is to ignore unrecognized options that start with -XX:. You can change this behavior via https://eclipse.dev/openj9/docs/xxignoreunrecognizedxxcolonoptions/, i.e. -XX:-IgnoreUnrecognizedXXColonOptions, in which case specifying -XX:+TLABStats will cause an error.