Retrofit2, maven project, Illegal reflective access warning

today I have got the same error. I imported Retrofit 2.8.1 to my project and it appeared. I tried everything, but one thing helped - I changed my Retrofit version to 2.7.2 and now everything works. Good luck!) If you use maven:

<dependency>
    <groupId>com.squareup.retrofit2</groupId>
    <artifactId>retrofit</artifactId>
    <version>2.7.2</version>
</dependency>

Gradle:

compile group: 'com.squareup.retrofit2', name: 'retrofit', version: '2.7.2'

I've added this block to my module build.gradle file.

Gradle

tasks.withType(Test) {
  /**
   * fix for retrofit https://github.com/square/retrofit/issues/3341
   */
  jvmArgs = ["--add-opens", "java.base/java.lang.invoke=ALL-UNNAMED"]
}

Kotlin DSL

tasks.withType<Test> {
  /**
   * fix for retrofit https://github.com/square/retrofit/issues/3341
   */
  jvmArgs = listOf("--add-opens", "java.base/java.lang.invoke=ALL-UNNAMED")
}

and the warning is not showing anymore. I think it's safe to remove it in this way because it is affecting test tasks only

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.

There was an issue filed about this, 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.*