Elements.getConstantExpression() doesn't throw any exception when argument is not a primitive value or string while the spec states exception throwing.

You may reproduce it using following code:
===
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.element.TypeElement;
import javax.lang.model.SourceVersion;
import java.util.Set;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;

@SupportedSourceVersion(SourceVersion.RELEASE_6)
@SupportedAnnotationTypes("*")
public class Proc extends AbstractProcessor {
   public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
       if(roundEnv.processingOver()) {
           System.out.println("object: " + processingEnv.getElementUtils().getConstantExpression(new Object()));
       }
       return true;
   }
}
===

The result is like this:
# javac -processor tmp.Proc java.lang.Object
object: java.lang.Object@ab853b
Failed test:
api/javax_lang/model/util/Elements/index.html#GetConstantExpression0002