Retrofit2, maven project, Illegal reflective access warning

Question

I just started a new maven project and did a simple implementation of the Retrofit client. I'm getting the following warnings:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by retrofit2.Platform (file:/C:/Users/Admin/.m2/repository/com/squareup/retrofit2/retrofit/2.8.1/retrofit-2.8.1.jar) to constructor java.lang.invoke.MethodHandles$Lookup(java.lang.Class,int)
WARNING: Please consider reporting this to the maintainers of retrofit2.Platform
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Process finished with exit code 0

here is the code

import retrofit2.Retrofit;
import retrofit2.SimpleXmlConverterFactory;
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory;

public class RetrofitClient {

    private static Retrofit retrofit = null;
    private RetrofitClient() { }

    public static EndPoints getAPI(String baseUrl) {
        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(baseUrl)
                    .addConverterFactory(SimpleXmlConverterFactory.create())
                    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                    .build();
        }
        return retrofit.create(EndPoints.class);
    }
}

Interface is simply

import retrofit2.Call;
import retrofit2.http.GET;

public interface EndPoints {
    @GET("teststatuses")
    Call<String> testStatus();
}

Call looks like this:

EndPoints endPoints = RetrofitClient.getAPI("http://localhost:8080/");
Call<String> repos = endPoints.testStatus();
System.out.println(repos.execute());

The project runs java language level 11 with SDK 11

Answer 1

There is an issue filed about it to which one of the Retrofit maintainers responded:

The reflection works around a bug in the JDK which was fixed in 14 but it's only used for default methods. As it's only a warning, it's not preventing your call from working.

So your options are either to

1. stick with Retrofit 2.8.x, and
   • ignore the warning, or
   • upgrade to Java 14
2. downgrade to Retrofit 2.7.*

Answer 2

If you are between Java versions 9 and 13 and want to stick with Retrofit versions at or higher than 2.8, you can run your jar file like this:

java --add-opens=java.base/java.lang.invoke=ALL_UNNAMED my_jar.jar

Note that this won't work on Java 8, so if you really need to, you can hack up a solution like this (works on *nix):

if java --add-opens 2>&1 | grep 'requires modules' >/dev/null; then
  java --add-opens=java.base/java.lang.invoke=ALL-UNNAMED -jar my_jar.jar
else
  java -jar my_jar.jar
fi

If you're interested in looking into this further, you can look at the code that causes this.

I also found that this has some good information: https://blog.codefx.org/java/five-command-line-options-hack-java-module-system/

If you're using Gradle, you might also try adding default jvm args: How do I add default JVM arguments with Gradle. However, that still doesn't get you a plain jar file that just works.