quarkusio/quarkus · error · IllegalArgumentException

Method + method.name() + of interface + ifaceDotName + i

Error message

Method  + method.name() +  of interface  + ifaceDotName +  is not a getter method since it returns void

What it means

Same interface-generation path: a non-default/static/private method on a @ConfigurationProperties interface that returns void cannot map to a config property, so IllegalArgumentException 'Method <name> of interface <iface> is not a getter method since it returns void' is thrown.

Source

Thrown at extensions/spring-boot-properties/deployment/src/main/java/io/quarkus/spring/boot/properties/deployment/InterfaceConfigurationPropertiesUtil.java:165

                });
            });

            for (DotName ifaceDotName : allInterfaces) {
                ClassInfo classInfo = index.getClassByName(ifaceDotName);
                List<MethodInfo> methods = classInfo.methods();
                for (MethodInfo method : methods) {
                    Type returnType = method.returnType();
                    short methodModifiers = method.flags();
                    if (isDefault(methodModifiers) || Modifier.isStatic(methodModifiers)
                            || Modifier.isPrivate(methodModifiers)) {
                        continue;
                    }
                    if (!method.parameterTypes().isEmpty()) {
                        throw new IllegalArgumentException("Method " + method.name() + " of interface " + ifaceDotName
                                + " is not a getter method since it defined parameters");
                    }
                    if (returnType.kind() == Type.Kind.VOID) {
                        throw new IllegalArgumentException("Method " + method.name() + " of interface " + ifaceDotName
                                + " is not a getter method since it returns void");
                    }

                    NameAndDefaultValue nameAndDefaultValue = determinePropertyNameAndDefaultValue(method, namingStrategy);
                    String fullConfigName = prefixStr + "." + nameAndDefaultValue.getName();

                    cc.method(method.name(), mc -> {
                        mc.returning(ClassDesc.of(method.returnType().name().toString()));
                        mc.public_();

                        mc.body(bc -> {
                            if ((returnType.kind() == Type.Kind.CLASS)) {
                                ClassInfo returnTypeClassInfo = index.getClassByName(returnType.name());
                                if ((returnTypeClassInfo != null) && Modifier.isInterface(returnTypeClassInfo.flags())) {
                                    String generatedSubInterfaceImp = generateImplementationForInterfaceConfigPropertiesRec(
                                            originalInterface, returnTypeClassInfo,
                                            fullConfigName, namingStrategy, interfaceToGeneratedClass);

View on GitHub (pinned to e1c734241f)

Solutions

1. Remove the void method — config property interfaces are read-only.
2. Give the method a body and mark it default if it is a helper.
3. Convert mutators into setters on a separate POJO-based config class if mutation is genuinely needed.

Example fix

// before
public interface AppProps {
    void setHost(String host);
}

// after
public interface AppProps {
    String getHost();
}

for (Method m : AppProps.class.getMethods()) {
  if (!m.isDefault() && !Modifier.isStatic(m.getModifiers())
      && m.getReturnType() == void.class) {
    throw new IllegalStateException("Void method in config interface: " + m.getName());
  }
}

When it happens

Trigger: Declaring a void-returning abstract method in an interface annotated with @ConfigurationProperties — e.g. a mutator-style method like setHost(String) or a command-style method; raised while generating the implementing class.

Common situations: Adding setters by habit from JavaBean style (config interfaces are read-only); porting interfaces that had write methods; leftover abstract methods after refactoring.

Related errors

• Method + method.name() + of interface + ifaceDotName + i
• Failed to open path tree with root %s
• Dev services for ${request.getName()} requires a startable s
• Name cannot start with '/':${name}
• The class (${name}) cannot be created during deployment.

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/dfd4ef79bcc7d7a2. Report an issue: GitHub.