[BUG] [JAVA] [CLIENT] [JERSEY2] Using default constructor for generated API uses a not Threadsafe ApiClient

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used? 4.3.1
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When generating a Jersey2 Client, using the default constructor for the api uses a single static isntance of the ApiClient Class.

import generated.api.testlogging.api.LogsApi;
import generated.api.testlogging.api.invoker.ApiClient;

public Class ServiceApi {
   private final LogsApi logsApiNotThreadSafe;
   private final LogsApi logsApiThreadSafe;

   public ServiceApi{
      this.logsApiNotThreadSafe = new LogsApi(); // <-- Default Constructor, not Threadsafe!

      this.logsApiThreadSafe = new LogsApi(new ApiClient()); // <-- Threadsafe
   }

Snipped from generated LogsApi.class

public class LogsApi {
  private ApiClient apiClient;

  public LogsApi() {
    this(Configuration.getDefaultApiClient());
  }

  public LogsApi(ApiClient apiClient) {
    this.apiClient = apiClient;
  }
}

Generated Configuration.class

public class Configuration {
    private static ApiClient defaultApiClient = new ApiClient();

    /**
     * Get the default API client, which would be used when creating API
     * instances without providing an API client.
     *
     * @return Default API client
     */
    public static ApiClient getDefaultApiClient() {
        return defaultApiClient;
    }

    /**
     * Set the default API client, which would be used when creating API
     * instances without providing an API client.
     *
     * @param apiClient API client
     */
    public static void setDefaultApiClient(ApiClient apiClient) {
        defaultApiClient = apiClient;
    }
}

openapi-generator version

4.3.1 (I assume all versions are affected)

OpenAPI declaration file content or url

Input
https://gist.github.com/FloKaemmerer/eba930a252ddf7f4c6ce6638994a6175

Configuration

 <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.3.1</version>
                <executions>
                    <execution>
                        <id>testlogging</id>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${testlogging-api-input-spec-file}</inputSpec>
                            <generatorName>java</generatorName>
                            <invokerPackage>generated.api.testlogging.invoker</invokerPackage>
                            <modelPackage>generated.api.testlogging.model</modelPackage>
                            <apiPackage>generated.api.testlogging.api</apiPackage>
                            <templateDirectory>swagger/java</templateDirectory>
                            <generateApiTests>false</generateApiTests>
                            <generateModelTests>false</generateModelTests>
                            <generateModelDocumentation>false</generateModelDocumentation>
                            <configOptions>
                                <library>jersey2</library>
                                <hideGenerationTimestamp>true</hideGenerationTimestamp>
                                <delegatePattern>true</delegatePattern>
                                <java8>true</java8>
                                <dateLibrary>java8</dateLibrary>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Command line used for generation

mvn clean compile

Steps to reproduce

Generate the Api and check LogsApi.java

Related issues/PRs

Suggest a fix

Don't make the Default constructor use a static instance of the ApiClient

Start by reproducing the generation with the provided Maven configuration and inspect the generated LogsApi.java and Configuration.class. Compare the default and ApiClient-taking constructors and trace how the default client is obtained. Done means the generated Jersey2 client no longer shares an unsafe static ApiClient through its default constructor, with the generated output verified after mvn clean compile.