[FIXED] Proguard - Can't read StackUtilsImpl.class

Issue

I'm having an error while compiling proguard. I'm using flatlaf-1.6.4.jar for GUI Java Aplication

Can't process class [META-INF/versions/9/com/formdev/flatlaf/ui/StackUtilsImpl.class] (Unsupported class version number [53.0] (maximum 52.0, Java 1.8))

Solution

You haven't provided any details about your environment, but based on the error message ("Unsupported class version number [53.0] (maximum 52.0, Java 1.8)") it looks like your code was compiled using JRE 9 (i.e. version 53.0).

There is an old Proguard feature request on this. See #181 Java 9 major number 53 support:

Proguard does not support Java 9 class file format. The major version number for the class file in Java 9 is 53 and in Java 8 it was 52. Please enhance Proguard to support Java 9 class files.

The request was submitted on 12/21/16, and closed on 2018-03-29. Presumably you are using ProGuard 5.x (or earlier), right? If so, your solution is to use Proguard 6.0 or later. From the Proguard 6.0 release documentation:

Added backporting and support for Java 9 code

However, if you are already using a version of Proguard > 5.x then update your question to provide full details of the compiled version of your code, and your versions of Proguard, Java and NetBeans.

Also see the old SO question Error Proguard Unsupported class version number. Though not a duplicate of your question, it details a similar issue on earlier versions of Java and Proguard.

Answered By - skomisa
Answer Checked By - Clifford M. (JavaFixing Volunteer)