Spring AI The isFunctionalType method of MethodToolVNet Provider is incorrect, resulting in some cases where the tool cannot be registered

 private boolean isFunctionalType(Method toolMethod) {
        var isFunction = ClassUtils.isAssignable(toolMethod.getReturnType(), Function.class)
                || ClassUtils.isAssignable(toolMethod.getReturnType(), Supplier.class)
                || ClassUtils.isAssignable(toolMethod.getReturnType(), Consumer.class);
        if (isFunction) {
            logger.warn("Method {} is annotated with @Tool but returns a functional type. "
                    + "This is not supported and the method will be ignored.", toolMethod.getName());
        }
        return isFunction;
    }

The isFunctionalType method of class MethodToolCallbackProvider has a bug ( Unable to correctly determine function type )，fixed：

 private boolean isFunctionalType(Method toolMethod) {
        var isFunction = ClassUtils.isAssignable(Function.class, toolMethod.getReturnType())
                || ClassUtils.isAssignable(Supplier.class, toolMethod.getReturnType())
                || ClassUtils.isAssignable(Consumer.class, toolMethod.getReturnType());
        if (isFunction) {
            log.warn("Method {} is annotated with @Tool but returns a functional type. "
                    + "This is not supported and the method will be ignored.", toolMethod.getName());
        }

        return isFunction;
    }

Comment From: jcgouveia

This is a very serious bug. I have tools that return just the "Object" type and I cannot use them, because isFunctionalType returns true, which is obviously wrong.

Comment From: sunyuhan1998

Hi @TheEterna @jcgouveia , I pushed a PR to fix the issue, can you guys help review the PR https://github.com/spring-projects/spring-ai/pull/3479 ? Thank you.