"springdoc-openapi-starter-webflux-ui" for sprint boot 3.x can't parse nested POJO well, but "springdoc-openapi-webflux-ui" for spring boot 2.x works well.

[softenwareness]

Describe the bug
It was working for nested POJO as openAPI request or response definition before(Spring boot 2.7.18), but upgrade to spring boot 3.x, not working any more, throw exception.

To Reproduce
Steps to reproduce the behavior:

1. To define a POJO

@Data
public class FieldDescriptor {
    private String name;
    private String type;
    @ArraySchema(schema = @Schema(name = "fields", implementation = FieldDescriptor.class, ref = "FieldDescriptor"))
    private List<FieldDescriptor> fields = new ArrayList<>();

}

//To use it to Controller
`@Tag(name = "Demo")
@RestController("demo/api")
@RequiredArgsConstructor
public class DemoController {

    @GetMapping("/get")
    public Mono<ResponseEntity<FieldDescriptor>> get() {
        FieldDescriptor fieldDescriptor = new FieldDescriptor();
        fieldDescriptor.setName("fields");
        fieldDescriptor.setType("String");
        FieldDescriptor subFieldDescriptor = new FieldDescriptor();
        subFieldDescriptor.setName("sub");
        subFieldDescriptor.setType("Integer");

        fieldDescriptor.getFields().add(subFieldDescriptor);

       return Mono.just(ResponseEntity.ok(fieldDescriptor));
    }
}

pom:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>3.4.4</version>
<!--		<version>2.7.18</version>-->
	</parent>
	<groupId>com.example</groupId>
	<artifactId>demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>demo</name>
	<description>Demo project for Spring Boot</description>

	<properties>
		<java.version>17</java.version>

	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-webflux</artifactId>
		</dependency>
		<!-- For Spring Boot 3.4.x, throw exception -->
		<dependency>
			<groupId>org.springdoc</groupId>
			<artifactId>springdoc-openapi-starter-webflux-ui</artifactId>
			<version>2.8.6</version>
		</dependency>
		<!-- For Spring Boot 2.7.18, it works well -->
<!--		<dependency>-->
<!--			<groupId>org.springdoc</groupId>-->
<!--			<artifactId>springdoc-openapi-webflux-ui</artifactId>-->
<!--			<version>1.8.0</version>-->
<!--		</dependency>-->

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
		<dependency>
			<groupId>org.projectlombok</groupId>
			<artifactId>lombok</artifactId>
			<scope>provided</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>
</project>

• What version of spring-boot you are using? 3.4.4
• What modules and versions of springdoc-openapi are you using? 2.8.6
• What is the actual and the expected result using OpenAPI Description (yml or json)?

Throw Exception

Warning

java.lang.StackOverflowError: null
at org.springdoc.core.converters.WebFluxSupportConverter.resolve(WebFluxSupportConverter.java:89) ~[springdoc-openapi-starter-common-2.8.6.jar:2.8.6]
at org.springdoc.core.converters.AdditionalModelsConverter.resolve(AdditionalModelsConverter.java:163) ~[springdoc-openapi-starter-common-2.8.6.jar:2.8.6]
at org.springdoc.core.converters.FileSupportConverter.resolve(FileSupportConverter.java:72) ~[springdoc-openapi-starter-common-2.8.6.jar:2.8.6]
at org.springdoc.core.converters.ResponseSupportConverter.resolve(ResponseSupportConverter.java:84) ~[springdoc-openapi-starter-common-2.8.6.jar:2.8.6]
at org.springdoc.core.converters.SchemaPropertyDeprecatingConverter.resolve(SchemaPropertyDeprecatingConverter.java:95) ~[springdoc-openapi-starter-common-2.8.6.jar:2.8.6]
....

Expected behavior
For below nested POJO, can work well .
It was working well for

@Data
public class FieldDescriptor {
    private String name;

    private String type;

    @ArraySchema(schema = @Schema(name = "fields", implementation = FieldDescriptor.class, ref = "FieldDescriptor"))
    private List<FieldDescriptor> fields = new ArrayList<>();

}

• Provide with a sample code (HelloController) or Test that reproduces the problem
  Additional context
  Please download below zip file and have a try.
  demo.zip

[bnasslahsen]

@softenwareness,

v1 of springdoc was only covering OAS 3.0
If you use same version of OpenAPI by setting springdoc.api-docs.version=openapi_3_0, generation will work.

The issue you are getting is bug in swagger-core, that you should report.

Use this sample and will get StackOverflowError.

ModelConverters.getInstance(true)
				.resolveAsResolvedSchema(
						new AnnotatedType(FieldDescriptor.class));

Also the following syntax, avoids the issue:

  @ArraySchema(schema = @Schema(name = "fields", ref = "FieldDescriptor"))
   private List<FieldDescriptor> fields = new ArrayList<>();

[softenwareness]

Thanks.