java.io.IOException: The output jar is empty. Did you specify the proper '-keep' options?

执行Maven Install打包的时候，出现以下错误信息：

 [proguard] java.io.IOException: The output jar is empty. Did you specify the proper ‘-keep‘ options?
 [proguard]     at proguard.shrink.Shrinker.execute(Shrinker.java:171)
 [proguard]     at proguard.ProGuard.shrink(ProGuard.java:297)
 [proguard]     at proguard.ProGuard.execute(ProGuard.java:113)
 [proguard]     at proguard.ProGuard.main(ProGuard.java:538)

这是因为’-keep’选项使用的不正确，把所有的类都过虑掉了。