This is because SLF4J jar file needs to be added into your application's classpath. In order to fix this issue you have to provide one of the various SLF4J implementation .jar files in the classpath, as well as the interface .jar file.
A few things cause this exception:
1) Check if you have all jars and if they're in the correct path.
2) Your classpath might be broken, you can define it in the command line with
java -cp yourClassPath or at your IDE if you're using one.
java.lang.NoClassDefFoundError: org/slf4j/LoggerFactoryat java.net.URLClassLoader.findClass(Unknown Source)at java.lang.ClassLoader.loadClass(Unknown Source)at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)at java.lang.ClassLoader.loadClass(Unknown Source)at de.javawi.jstun.header.MessageHeader.