oracle/graal · critical · GraalError

Multiple garbage collectors selected: %s %s

Error message

Multiple garbage collectors selected: %s %s

What it means

getSelectedGC requires that exactly one HotSpotGC reports isSelected(this). If a second collector also reports selected, it throws GraalError('Multiple garbage collectors selected: A B'). Normally HotSpot guarantees a single -XX:+Use*GC, so this indicates contradictory VM flags or a broken/ported VM where several Use flags are simultaneously true.

Source

Thrown at compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/GraalHotSpotVMConfig.java:109

    public boolean useSerialGC() {
        return gc == HotSpotGraalRuntime.HotSpotGC.Serial;
    }

    public boolean useG1GC() {
        return gc == HotSpotGraalRuntime.HotSpotGC.G1;
    }

    public final HotSpotGraalRuntime.HotSpotGC gc = getSelectedGC();

    private HotSpotGraalRuntime.HotSpotGC getSelectedGC() throws GraalError {
        HotSpotGraalRuntime.HotSpotGC selected = null;
        for (HotSpotGraalRuntime.HotSpotGC value : HotSpotGraalRuntime.HotSpotGC.values()) {
            if (value.isSelected(this)) {
                if (!value.supported) {
                    throw new GraalError(value.name() + " garbage collector is not supported by Graal");
                }
                if (selected != null) {
                    throw new GraalError("Multiple garbage collectors selected: " + selected + " " + value);
                }
                selected = value;
            }
        }
        if (selected == null) {
            // Exactly one GC flag is guaranteed to be selected.
            selected = HotSpotGraalRuntime.HotSpotGC.Serial;
        }
        return selected;
    }

    /**
     * Determines if {@code -Xcomp} (or the equivalent thereof) was specified as a JVM argument.
     */
    public final boolean xcompMode = !access.getFlag("UseInterpreter", Boolean.class);

    public final boolean ropProtection = access.getFieldValue("VM_Version::_rop_protection", Boolean.class, "bool", false);

View on GitHub (pinned to a66e9ccd1d)

Solutions

1. Leave exactly one GC-selection flag (let the VM default pick one) and remove duplicates from scripts, JAVA_OPTS, and JVM_OPTIONS env vars
2. Query the effective GC with -XX:+PrintFlagsFinal -version | grep Use.*GC to see which flags ended up true
3. If the VM really defaults multiple collectors to true, that is a VM build bug — fix the port/build

Example fix

# before
java -XX:+UseG1GC -XX:+UseParallelGC -XX:+UseJVMCICompiler MyApp

# after
java -XX:+UseG1GC -XX:+UseJVMCICompiler MyApp

// ensure exactly one Use*GC flag in the effective command line before enabling JVMCI
long gcFlags = Stream.of("UseG1GC", "UseParallelGC", "UseSerialGC", "UseZGC", "UseShenandoahGC").filter(f -> args.contains("-XX:+" + f)).count();
if (gcFlags > 1) throw new IllegalArgumentException("multiple GC flags");

When it happens

Trigger: Passing multiple -XX:+Use...GC flags (e.g. -XX:+UseG1GC -XX:+UseSerialGC) on a VM build that does not override later flags to false, or running a custom/modified HotSpot where more than one collector's selection check (flag or platform default) returns true; failure happens while GraalHotSpotVMConfig fields initialize.

Common situations: Benchmark scripts concatenating GC flags from different templates; custom JVM builds or ports with buggy flag defaults; flags injected both by JAVA_OPTS and the command line in container entrypoints.

Related errors

• Compiler configuration '%s' not found. Available configurati
• %s garbage collector is not supported by Graal
• too many decompiles: {} {}
• No options specified for MethodFilter:
• Option name must follow '<prefix>' prefix

AI-assisted analysis of oracle/graal@a66e9ccd1d (2026-08-14). Data as JSON: /api/errors/f1bc69ec914d7b3e. Report an issue: GitHub.