spring-projects/spring-framework · error · IllegalStateException

Failed to find Java constructor for Kotlin primary…

Error message

Failed to find Java constructor for Kotlin primary constructor: {clazz.getName()}

What it means

Thrown by the internal KotlinDelegate.findPrimaryConstructor when a Kotlin primary constructor was found via the Kotlin reflection API but ReflectJvmMapping.getJavaConstructor(primaryCtor) returned null, so Spring cannot map the Kotlin constructor to a Java Constructor for instantiation. This is an IllegalStateException, indicating an unusual mismatch between the Kotlin reflection model and the JVM constructor model.

Solutions

1. Align kotlin-reflect and the Kotlin compiler version used to compile the class on the same Kotlin release.
2. Avoid passing Kotlin value/inline (@JvmInline) classes through BeanUtils.instantiateClass; instantiate them directly.
3. If you must instantiate, resolve a constructor explicitly via clazz.getDeclaredConstructors() and call instantiateClass(Constructor, args...).

Example fix

// before
MyBean bean = BeanUtils.instantiateClass(MyKotlinValueClass.class);

// after - resolve a concrete constructor
Constructor<MyBean> ctor = MyKotlinValueClass.class.getDeclaredConstructors()[0];
ReflectionUtils.makeAccessible(ctor);
MyBean bean = ctor.newInstance();

// before BeanUtils.instantiateClass on a possibly-Kotlin class
Constructor<?> ctor = BeanUtils.findPrimaryConstructor(type);
if (ctor == null) {
    ctor = type.getDeclaredConstructors()[0]; // explicit fallback
}
ReflectionUtils.makeAccessible(ctor);
Object instance = ctor.newInstance();

static boolean hasResolvableJavaConstructor(Class<?> clazz) {
    try {
        return BeanUtils.findPrimaryConstructor(clazz) != null
            || clazz.getDeclaredConstructors().length > 0;
    } catch (IllegalStateException ex) {
        return false;
    }
}

try {
    return BeanUtils.instantiateClass(kotlinType);
} catch (IllegalStateException ex) {
    // Kotlin primary constructor unmappable - resolve manually
    Constructor<?> c = kotlinType.getDeclaredConstructors()[0];
    ReflectionUtils.makeAccessible(c);
    return c.newInstance();
}

When it happens

Trigger: Instantiating a Kotlin class through BeanUtils.instantiateClass(clazz) / BeanUtils.findPrimaryConstructor(clazz) on a class compiled by Kotlin where the primary constructor exists in Kotlin metadata but has no mappable Java Constructor (e.g. certain inline/value classes with unusual shapes, compiler-version mismatches, or classes generated by older/newer kotlin-reflect).

Common situations: Mixing kotlin-reflect versions (kotlin-reflect version != compiler version that produced the classes), instantiating a Kotlin value/inline class with an atypical constructor layout, or running on a Kotlin class whose primary constructor is synthetic and not directly mappable.

Related errors

• Constructor threw exception
• Illegal arguments for constructor
• Cannot create default implementation for
• Cannot resolve method
• Could not copy property

AI-assisted analysis of spring-projects/spring-framework@69bf83ad71 (2026-08-09). Data as JSON: /api/errors/275f2e1bb2f51d16. Report an issue: GitHub.

Appendix: source

Thrown at spring-beans/src/main/java/org/springframework/beans/BeanUtils.java:889

		 * https://kotlinlang.org/docs/reference/classes.html#constructors</a>
		 */
		@SuppressWarnings("unchecked")
		public static <T> @Nullable Constructor<T> findPrimaryConstructor(Class<T> clazz) {
			try {
				KClass<T> kClass = JvmClassMappingKt.getKotlinClass(clazz);
				KFunction<T> primaryCtor = KClasses.getPrimaryConstructor(kClass);
				if (primaryCtor == null) {
					return null;
				}
				if (KotlinDetector.isInlineClass(clazz)) {
					Constructor<?>[] constructors = clazz.getDeclaredConstructors();
					Assert.state(constructors.length == 1,
							"Kotlin value classes annotated with @JvmInline are expected to have a single JVM constructor");
					return (Constructor<T>) constructors[0];
				}
				Constructor<T> constructor = ReflectJvmMapping.getJavaConstructor(primaryCtor);
				if (constructor == null) {
					throw new IllegalStateException(
							"Failed to find Java constructor for Kotlin primary constructor: " + clazz.getName());
				}
				return constructor;
			}
			catch (UnsupportedOperationException ex) {
				return null;
			}
		}

		/**
		 * Instantiate a Kotlin class using the provided constructor.
		 * @param ctor the constructor of the Kotlin class to instantiate
		 * @param args the constructor arguments to apply
		 * (use {@code null} for unspecified parameter if needed)
		 */
		public static <T> T instantiateClass(Constructor<T> ctor, @Nullable Object... args)
				throws IllegalAccessException, InvocationTargetException, InstantiationException {

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