Template extension method declared on must not be private:

Error message

Template extension method declared on  must not be private:

What it means

Template extension methods are invoked statically from generated code without an instance of the declaring class. Qute rejects any @TemplateExtension method that is private because it cannot be invoked from the generated resolver class.

Source

Thrown at independent-projects/qute/generator/src/main/java/io/quarkus/qute/generator/ExtensionMethodGenerator.java:89

    public static final String PRIORITY = "priority";
    public static final String NAMESPACE = "namespace";
    public static final String PATTERN = "pattern";

    public ExtensionMethodGenerator(IndexView index, ClassOutput classOutput) {
        super(index, classOutput);
    }

    public static void validate(MethodInfo method, String namespace) {
        if (!Modifier.isStatic(method.flags())) {
            throw new IllegalStateException(
                    "Template extension method declared on " + method.declaringClass().name() + "  must be static: " + method);
        }
        if (method.returnType().kind() == Kind.VOID) {
            throw new IllegalStateException("Template extension method declared on " + method.declaringClass().name()
                    + " must not return void: " + method);
        }
        if (Modifier.isPrivate(method.flags())) {
            throw new IllegalStateException("Template extension method declared on " + method.declaringClass().name()
                    + " must not be private: " + method);
        }
    }

    /**
     *
     * @param method
     * @param matchName
     * @param matchNames
     * @param matchRegex
     * @param priority
     * @return the fully qualified name of the generated class
     */
    public String generate(MethodInfo method, String matchName, List<String> matchNames, String matchRegex, Integer priority) {

        LOG.debugf("Generating resolver for extension method declared on %s: %s", method.declaringClass(), method);

        AnnotationInstance extensionAnnotation = method.annotation(TEMPLATE_EXTENSION);

View on GitHub (pinned to e1c734241f)

Solutions

Make the method at least package-private or public (keep it static)
Remove the @TemplateExtension annotation if the method should not be exposed to templates
Move the method to a dedicated extension class with appropriate visibility

Example fix

// before
@TemplateExtension
private static String slugify(String val) { ... }
// after
@TemplateExtension
static String slugify(String val) { ... }

Defensive patterns

Strategy: validation

Validate before calling

if (java.lang.reflect.Modifier.isPrivate(m.getModifiers()))
    throw new IllegalStateException("@TemplateExtension method must not be private: " + m);

Prevention

Keep extension methods package-private or public
Avoid narrowing visibility of annotated methods during refactoring
Grep for @TemplateExtension on private methods in CI

When it happens

Trigger: Annotating a private static method with @TemplateExtension, or making a previously package-private/public extension method private; detected by ExtensionMethodGenerator.validate during build.

Common situations: Narrowing visibility during refactoring; helper methods annotated by mistake; extension methods defined inside another class as private helpers.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/45fa43a4936e0877. Report an issue: GitHub.