Running a Java program from the command line that depends on a jar file?

Question

I have a simple Java file Q.java that depends on an external library file X.jar. Both Q.java and X.jar are in the same directory. I can compile Q.java from the command line by doing: "javac -cp X.jar Q.java". This generates a Q.class file. How do I run this now? I tried all these:

1) java Q 2) java -cp X.jar Q

I keep getting a Exception in thread "main" java.lang.NoClassDefFoundError: Q Caused by: java.lang.ClassNotFoundException: Q

So how do I run this from the command line now that I have the class file?

Answer 1

java -cp X.jar:. Q

You have to specify in the classpath that you want to use the JAR dependency AND the current local directory to resolve classes.

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Edit suggested in the comments:

On Windows, replace : by ;:

java -cp X.jar;. Q

Answer 2

Set the current directory in your classpath, it should solve the problem. Most of the time, we need current directory in the classpath, so generally I advice to set "." (without quotes) in your system CLASSPATH environment variable instead of setting for each run.