nested | A memory efficient container for rust nested collections

This problem's caused by a bug in Springfox. It's making an assumption about how Spring MVC is set up that doesn't always hold true. Specifically, it's assuming that MVC's path matching will use the Ant-based path matcher and not the PathPattern-based matcher. PathPattern-based matching has been an option for some time now and is the default as of Spring Boot 2.6.

As described in Spring Boot 2.6's release notes, you can restore the configuration that Springfox assumes will be used by setting spring.mvc.pathmatch.matching-strategy to ant-path-matcher in your application.properties file. Note that this will only work if you are not using Spring Boot's Actuator. The Actuator always uses PathPattern-based parsing, irrespective of the configured matching-strategy. A change to Springfox will be required if you want to use it with the Actuator in Spring Boot 2.6 and later.

Please note that Spring Cloud Gateway is not compatible with Spring MVC (spring-boot-starter-web). This is outlined in section "How to include Spring Cloud Gateway in the official reference documentation":

Spring Cloud Gateway is built on Spring Boot 2.x, Spring WebFlux, and Project Reactor. As a consequence, many of the familiar synchronous libraries (Spring Data and Spring Security, for example) and patterns you know may not apply when you use Spring Cloud Gateway.

Additionally, it is stated that:

Spring Cloud Gateway requires the Netty runtime provided by Spring Boot and Spring Webflux. It does not work in a traditional Servlet Container or when built as a WAR.

As already suggested by the error message, you would need to remove the dependency on spring-boot-starter-web. You can list all your direct and transitive dependencies with the following command:

Am I right?

Yes!

The double loop:

You can use rrapply::rrapply:

You can not turn arbitrary classes into hidden classes.

The documentation of defineHiddenClass contains the sentence

• On any attempt to resolve the entry in the run-time constant pool indicated by this_class, the symbolic reference is considered to be resolved to C and resolution always succeeds immediately.

What it doesn’t spell out explicitly is that this is the only place where a type resolution ever ends up at the hidden class.

But it has been said unambiguously in bug report JDK-8222730:

For a hidden class, its specified hidden name should only be accessible through the hidden class's 'this_class' constant pool entry.

The class should not be accessible by specifying its original name in, for example, a method or field signature even within the hidden class.

Which we can check. Even a simple case like

I think this gets you pretty close; might just need to adjust the various name columns and drop the extra data (I kept the grouping column).

The main idea is to recursively use pd.json_normalize with pd.concat for all availalable children levels.

EDIT: Put everything into a single function and added section to collapse the name columns like the expected output.

I know this does not solve your problem directly, but consider moving to springdoc which most recent release supports Spring Boot 2.6.0. Springfox is so buggy at this point that is a pain to use. I've moved to springdoc 2 years ago because of its Spring WebFlux support and I am very happy about it. Additionally, it also supports Kotlin Coroutines, which I am not sure Springfox does.

If you decide to migrate, springdoc even has a migration guide.

A third attempt:

I've taken your code, and at last got it to run properly (in python):

Your title says you are using Java 9. With Activiti 6 you will have to use JDK 1.8 (Java 8).