[BUG] [JAVA] microprofile client generator makes readOnly properties de-facto required

Description

Java code generated from a readOnly-marked property causes a bug when the property is not present in the response json payload.

openapi-generator version

6.0.0

OpenAPI declaration file content or url

  "components": {
    "schemas": {
      "Foo": {
        "type": "object",
        "properties": {
          "a": {
            "type": "string"
          },
          "b": {
            "type": "string",
            "readOnly": true
          }
        }
      }
    }
  }

Full sample openapi spec: https://gist.github.com/mak-100/cb924fca623211814585f21c8e3038a1

Generation Details

        <plugin>
          <groupId>org.openapitools</groupId>
          <artifactId>openapi-generator-maven-plugin</artifactId>
          <version>6.0.0</version>
        </plugin>

...

          <execution>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              <inputSpec>.../openapi.json</inputSpec>
              <generatorName>java</generatorName>
              <apiPackage>de.integration</apiPackage>
              <modelPackage>de.integration.model</modelPackage>
              <generateApis>false</generateApis>
              <generateModels>true</generateModels>
              <generateModelDocumentation>false</generateModelDocumentation>
              <generateModelTests>false</generateModelTests>
              <generateSupportingFiles>false</generateSupportingFiles>
              <library>microprofile</library>
              <configOptions>
                <sourceFolder>src/gen/java/main</sourceFolder>
                <dateLibrary>java8</dateLibrary>
              </configOptions>
            </configuration>
          </execution>

Steps to reproduce

Run mvn compile to generate the source code from the openapi spec.

The generated class Foo.java has a constructor with a non-nullable parameter "b":

@JsonbCreator
  public Foo(
    @JsonbProperty("b") String b
  ) {
    this.b = b;
  }

The problem is that JsonbProperty.nillable() is false by default:

package javax.json.bind.annotation;

...

@JsonbAnnotation
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE, ElementType.METHOD, ElementType.FIELD, ElementType.PARAMETER})
public @interface JsonbProperty {

    ...

    /**
     * Switches o/off serialization of null values.
     *
     * @return True if field with null value should be serialized as key/value pair into JSON with null value.
     */
    boolean nillable() default false;
}

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/10086

Suggest a fix

Generate constructur where properties which are readOnly but not required have the attribute nillable = true, in the example above:

  @JsonbCreator
  public Foo(
    @JsonbProperty(value = "b", nillable = true) String b
  ) {
    this.b = b;
  }

Start by running mvn compile with the linked OpenAPI specification and inspect the generated Foo.java. Trace the Java generator's microprofile library handling of the @JsonbCreator constructor, then verify that an absent, non-required readOnly property can be deserialized without failure and that the generated annotation reflects this behavior.