ADDITIONAL SYSTEM INFORMATION :
$ java -version
openjdk version "17.0.1" 2021-10-19
OpenJDK Runtime Environment Homebrew (build 17.0.1+1)
OpenJDK 64-Bit Server VM Homebrew (build 17.0.1+1, mixed mode, sharing)


A DESCRIPTION OF THE PROBLEM :
Attempting to use reflection to invoke a default method inherited in a public class from a non-public interface that extends a public interface fails. Yet this method can be invoked just fine using plain java.

As a result, without using setAccessible(true), it's impossible to perform the same operation using reflection that is permitted when using plain Java.

Is this expected? It seems like in the past Java reflection has always provided a way to do whatever is possible using plain Java, albeit sometimes you have to find the right Method to use.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Files:

============= mypackage/PublicInterface.java
package mypackage;
public interface PublicInterface {
    void setFoo(String value);
}
============= mypackage/PackagePrivateInterface.java
package mypackage;
interface PackagePrivateInterface extends PublicInterface {
    default void setFooBar() {
        this.setFoo("bar");
    }
}
============= mypackage/PublicClass.java
package mypackage;
public class PublicClass implements PackagePrivateInterface {
    @Override
    public void setFoo(String value) {
    }
}
============= DefaultMethodReflection.java
import java.lang.reflect.*;
import mypackage.*;
public class DefaultMethodReflection {

    public static void main(String[] args) throws Exception {

        // We can access PublicClass.setFooBar() using normal java no problem
        PublicClass obj = new PublicClass();
        obj.setFooBar();

        // But doing the same thing through reflection doesn't work
        Method method = PublicClass.class.getMethod("setFooBar");
        method.invoke(obj); // boom
    }
}
============= Shell commands
$ javac -d classes *.java mypackage/*.java
$ java -classpath classes DefaultMethodReflection


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No output
ACTUAL -
Exception in thread "main" java.lang.IllegalAccessException: class DefaultMethodReflection cannot access a member of interface mypackage.PackagePrivateInterface with modifiers "public"
at java.base/jdk.internal.reflect.Reflection.newIllegalAccessException(Reflection.java:392)
at java.base/java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:674)
at java.base/java.lang.reflect.Method.invoke(Method.java:560)
at DefaultMethodReflection.main(DefaultMethodReflection.java:13)


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See "Steps to Reproduce"
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CUSTOMER SUBMITTED WORKAROUND :
Method.setAccessible(true)

FREQUENCY : always