Java Command Line Trouble with Reading a Class from a Jar Archive

Question

I am trying to run a java based tool using a command line syntax as the following: java -cp archive.jar archiveFolder.theMainClassName.Although the class I am searching for, a main class, "theMainClassName" is in the archive.jar and in the archiveFolder given at input, I keep getting the error that my class is not seen. Does anybody have any ideas concerning this problem? Thank you in advance

Answer 1

Perhaps with java -jar archive.jar?

Of course, it supposes the manifest points to the right class...

You should give the exact message you got, it might shed more light.

EDIT: See Working with Manifest Files: The Basics for information on setting the application entry point (Main class) in your jar manifest file.

Answer 2

Does theMainClassName class have the following package line at the top:

package archiveFolder

You need the class file to be in the same directory structure as the declared package. So if you had something like:

org/jc/tests/TestClass.class

its source file would have to look like this:

package org.jc.tests;

public class TestClass {
  public static void main(String[] args) {
    System.out.printf("This is a test class!\n");
  }
}

Then you could use the following to create the jar file and run it from the command line (assuming the current directory is at the top level, just above org):

$ jar -cf testJar.jar org/jc/tests/*.class
$ java -cp testJar.jar org.jc.tests.TestClass

Answer 3

Here's a concrete example of what does work, so you can compare your own situation.

Take this code and put it anywhere, in a file called MainClass.java. (I've assumed a directory called src later. Normally you'd arrange the source to match the package, of course.)

package archiveFolder;

public class MainClass
{
    public static void main(String[] args)
    {
        System.out.println("I'm MainClass");
    }
}

Then run each of these commands:

# Compile the source
javac -d . src/MainClass.java

# Build the jar file
jar cf archive.jar archiveFolder

# Remove the unpackaged binary, to prove it's not being used
rm -rf archiveFolder # Or rmdir /s /q archiveFolder on Windows

# Execute the class
java -cp archive.jar achiveFolder.MainClass

The result:

I'm MainClass

How are you building your jar file? Is the code in the appropriate package?

Answer 4

Usually this happens when a dependent class (static member) is not found - like this, using log4j:

public class MyClass {
  private static Logger log = Logger.getLogger("com.example");
}

The reason is that the initialization of such a static member can be understood as part of the class loading - errors causing the class not to be available (loadable), resulting in the error you described.

Static constructors are another possible reason:

public class MyClass {
  static {
     // <b>any</b> error caused here will cause the class to 
     // not be loaded. Demonstrating with stupid typecast.
     Object o = new String();
     Integer i = (Integer) o;
  }
}

Answer 5

I think others have covered some common stuff here. I'd jar tf the jar and make sure the class is listed. I'd also double-check that the class is public and the method is "public static void main(String[] arg)".