ProGuard says Unsupported class version number [52.0] (maximum 51.0, Java 1.7) with sbt-proguard

Question 1

It appears that ProGuard and hence sbt-proguard don't support Java 8 yet and changing the version of Java used in the script to launch sbt helped.

[sbt-updates]> show proguard:proguard
[info] Compiling 8 Scala sources to /Users/jacek/oss/sbt-updates/target/scala-2.10/sbt-0.13/classes...
[warn] there were 6 feature warning(s); re-run with -feature for details
[warn] one warning found
[info] ProGuard, version 4.9
[info] Reading program directory [/Users/jacek/oss/sbt-updates/target/scala-2.10/sbt-0.13/classes] (filtered)
[info] Reading program jar [/Users/jacek/.ivy2/cache/org.scalaz/scalaz-concurrent_2.10/bundles/scalaz-concurrent_2.10-7.1.0-M6.jar] (filtered)
...

This is with the following version of Java 7:

$ /Library/Java/JavaVirtualMachines/jdk1.7.0_51.jdk/Contents/Home/bin/java -version
java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)

Question 2

The question is really interesting and i was facing the same problem with the only difference that i added Proguard programmatically by using maven. Consequently, i considered that it will be helpful to post my solution although it is a bit different from the main question. For all those that using maven and faces the same problem, my workaround fix was to update the version of Proguard by using it as runtime inside the plugin, so the working pom.xml looks like this

  <plugin>
                    <groupId>com.github.wvengen</groupId>
                    <artifactId>proguard-maven-plugin</artifactId>
                    <version>2.0.14</version>
                    <executions>
                        <execution>
                            <phase>package</phase>
                            <goals>
                                <goal>proguard</goal>
                            </goals>
                        </execution>
                    </executions>
                    <configuration>
                        <obfuscate>true</obfuscate>
                        <attach>true</attach>
                        <appendClassifier>false</appendClassifier>
                        <addMavenDescriptor>true</addMavenDescriptor>
                        <injar>${project.build.finalName}-jar-with-dependencies.jar</injar>
                        <injarNotExistsSkip>true</injarNotExistsSkip>
                        <libs>
                            <lib>${java.home}/lib/rt.jar</lib>
                            <lib>${java.home}/lib/jce.jar</lib>
                            <lib>${java.home}/lib/ext/sunjce_provider.jar</lib>
                        </libs>
                        <options>
                        <option>-allowaccessmodification</option>
                        <option>-optimizationpasses 3</option>
                        <option>-overloadaggressively</option>
                        <option>-repackageclasses ''</option>
                        <option>-dontusemixedcaseclassnames</option>
                        <option>-dontskipnonpubliclibraryclasses</option>
                        <option>-flattenpackagehierarch</option>
                        <option>-dontwarn</option> <!-- added option to ignore com.sun missing classes -->
                            <option>-keep public class com.StocksNews.App {
                                public static void main(java.lang.String[]);
                                }
                            </option>
                        </options>
                    </configuration>
                    <dependencies>
                        <dependency>
                            <groupId>net.sf.proguard</groupId>
                            <artifactId>proguard-base</artifactId>
                            <version>6.1.1</version>
                            <scope>runtime</scope>
                        </dependency>
                    </dependencies>
                </plugin>

Question 3

It's possible to update the version of Proguard that sbt-proguard uses by changing the key proguardVersion in build.sbt to a setting newer than 5.0, e.g.

ProguardKeys.proguardVersion in Proguard := "5.2.1"

See: https://github.com/sbt/sbt-proguard/issues/5

Question 4

I fixed that by update to latest version of ProGuad from link below

Sourceforge ProGuard page

Question 5

Add classpath "com.guardsquare:proguard-gradle:7.0.1" in the project level build.gradle to resolve the error. I have solved this issue by adding the same.

I will recommend you to upgrade the Gradle Version to 7.x.x, which will remove ProGuard and you have to migrate to R8, as R8 is backward compatible with your exisiting code ProGuard rules. For Migration please read docs other you will waste alot time in figuring out some feature is not working after migration same happened to me.