无法使用java -jar命令执行jar文件

Question

I have created a jar file usign maven2 build. I am trying to run that jar file using the command:

java -jar sample.jar com.app.Test

Test being the class which is having the main method. But i am getting this exception:

Exception in thread "main" java.lang.NullPointerException
        at sun.launcher.LauncherHelper.getMainClassFromJar(Unknown Source)
        at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

Can any one help me to solve this exception and run the jar file?

Thanks in advance.

Answer 1

If you want to run the Test class, you should use

 java -cp sample.jar com.app.Test

This way, you add the jar to the classpath and then run the specified main class.

What java -jar does is that it executes a runnable jar file (which defines its own main class in the manifest file). Any parameters after that would not be used to specify the class, but end up in the String array passed to the main method.

So if you have a properly constructed runnable jar file, it should just be

java -jar sample.jar 

Answer 2

If you are using Maven, you may need to use maven's install command. You can find the format here http://maven.apache.org/guides/mini/guide-3rd-party-jars-local.html

Answer 3

Looks like you do need a manifest and a Main-Class attribute, if you don't have one. If you are using java 7 a bug is filed here http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7067922