spring-projects/spring-framework · error · IllegalArgumentException

Cannot create default implementation for '{interfaceType.get

Error message

Cannot create default implementation for '{interfaceType.getName()}' mixin ({defaultImplType.getName()}): {ex}

What it means

DelegatePerTargetObjectIntroductionInterceptor.createNewDelegate throws IllegalArgumentException when the defaultImplType cannot be instantiated via ReflectionUtils.accessibleConstructor(defaultImplType).newInstance(). This per-target mixin creates a fresh delegate object per advised target; if defaultImplType has no accessible no-arg constructor (or is abstract/interface) the eager delegate creation in the constructor fails.

Source

Thrown at spring-aop/src/main/java/org/springframework/aop/support/DelegatePerTargetObjectIntroductionInterceptor.java:139

	private Object getIntroductionDelegateFor(@Nullable Object targetObject) {
		synchronized (this.delegateMap) {
			if (this.delegateMap.containsKey(targetObject)) {
				return this.delegateMap.get(targetObject);
			}
			else {
				Object delegate = createNewDelegate();
				this.delegateMap.put(targetObject, delegate);
				return delegate;
			}
		}
	}

	private Object createNewDelegate() {
		try {
			return ReflectionUtils.accessibleConstructor(this.defaultImplType).newInstance();
		}
		catch (Throwable ex) {
			throw new IllegalArgumentException("Cannot create default implementation for '" +
					this.interfaceType.getName() + "' mixin (" + this.defaultImplType.getName() + "): " + ex);
		}
	}

}

View on GitHub (pinned to e8729d0438)

Solutions

1. Ensure defaultImplType is a concrete class with an accessible (public or at least package-private) no-arg constructor.
2. Add an explicit no-arg constructor or mark one as accessible.
3. For Kotlin, annotate a no-arg constructor requirement or provide a JVM-friendly default constructor.

Example fix

// before
public class LockableImpl {
  public LockableImpl(boolean locked) { ... } // no no-arg ctor -> error 119
}
new DelegatePerTargetObjectIntroductionInterceptor(LockableImpl.class, Lockable.class);

// after
public class LockableImpl {
  public LockableImpl() { this(false); }
  public LockableImpl(boolean locked) { ... }
}

try {
    ReflectionUtils.accessibleConstructor(defaultImplType).newInstance();
} catch (Exception ex) {
    throw new IllegalArgumentException("defaultImplType needs an accessible no-arg ctor", ex);
}

boolean hasAccessibleNoArgCtor(Class<?> type) {
    if (type == null || type.isInterface() || Modifier.isAbstract(type.getModifiers())) return false;
    try { ReflectionUtils.accessibleConstructor(type); return true; }
    catch (NoSuchMethodException e) { return false; }
}

When it happens

Trigger: new DelegatePerTargetObjectIntroductionInterceptor(NoDefaultCtorImpl.class, Lockable.class) where NoDefaultCtorImpl has no public/accessible no-arg constructor, or passing an abstract class / interface as defaultImplType.

Common situations: Delegate class with only parameterised constructors; private no-arg constructor; accidentally passing the interface as defaultImplType; Kotlin class without a no-arg constructor.

Related errors

• Spring AOP cannot handle constructor advice
• 'defaultImpl' attribute must be set on DeclareParents
• DynamicIntroductionAdvice may only be added as part of Intro
• Both 'constructorArgs' and 'constructorArgTypes' need to be
• Number of 'constructorArgs' ({constructorArgs.length}) must

AI-assisted analysis of spring-projects/spring-framework@e8729d0438 (2026-08-04). Data as JSON: /data/errors/31f883517c9f8b98.json. Report an issue: GitHub.