Error java.lang.ClassNotFoundException: org.slf4j.LoggerFactory when trying to execute the jar

When I try to do java -jar app.jar I get this:

Exception in thread "JavaFX Application Thread" Exception in thread "main" 
java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
    at tao.qaflash.GUI.<clinit>(GUI.java:14)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplicationWithArgs$156(LauncherImpl.java:352)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 12 more
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.NullPointerException
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:383)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    ... 5 more

My environment is: Java 1.8.0_92, Gradle 2.16, Eclipse Mars 4.5.0, Java FX. Also I have googled this error and I have tried everything from there , but with no success. I tried to add to the build.gradle as dependencies slfj-simple, log4j, logback-classic, slf4j-jdk14,junit,vaadin-slf4j-jdk14 but the error kept appearing.

When I run the application from Eclipse, everything works fine, the logs are written, only when I try to run the jar from command line, I get this ClassNotFoundException.

build.gradle looks like :

dependencies {
  compile 'net.sf.staf:jstaf:3.4.4'

  // SLF4J API module which will be used for logging
  compile 'org.slf4j:slf4j-api:1.7.21'

  // Log4J2 logging framework including the binding module to slf4j
  compile 'org.apache.logging.log4j:log4j-slf4j-impl:2.5'
  compile 'org.apache.logging.log4j:log4j-api:2.5'
  compile 'org.apache.logging.log4j:log4j-core:2.5'

  testCompile 'junit:junit:4.12'
  testCompile "org.testfx:testfx-core:4.0.+"
  testCompile "org.testfx:testfx-junit:4.0.+"
}

Any help is greately appreciated.

Simple: the classpath that you are using when trying to execute your JAR file does not contain the required sl4j logger library.

That is all there is to this. This is not about your build setup - it is about the setup that gets used when you run java on the command line!

See here for further reading.