JAR - Package directory - can't find or load main method

[Quazi Irfan]

I am trying to understand how package(directory structure) and JAR files work interchangeably. From my understanding JAR is a package file format typically used to aggregate many Java classes. Since many classes are usually contained inside a package, I should be able to create a JAR file from a package, and use them interchangeably. A single JAR file is easier to handle then multiple classes and folders.

To test my understanding of the concept, I am working with the following project structure,

/development_directory
____/BookPackage
________Book.java
____BookDemo.java

Book.java contains the following code,

package BookPackage;

public class Book{
	private String bookname;
	public Book(String str){
		bookname = str;
	}

	public String getBookName(){
		return bookname;
	}
}

and BookDemo.java contains the following code,

import BookPackage.Book;

class BookDemo{
	public static void main(String ... args){
		Book b = new Book("Lord of the Rings");
		System.out.println(b.getBookName());
	}
}

From the development_directory, I used the following commands to compile and run the project,
development_directory/ javac BookDemo.java
development_directory/ java BookDemo
...and it compiles and works as expected.

-------------------------------------------------------------------------------------------------

Now, I will try to convert the BookPackage folder(package) into single jar file for portability, and I am using the following command to do that,
development_directory/ jar -cf bookpkg.jar BookPackage
...this command generates the bookpkg.jar file.

I am going to remove the BookPackage folder to see if bookpkg.jar file really works as a replacement for the BookPackage directory structure.

After removal of BookPackage folder, the development_directory structure looks like this,
/development_directory
____bookpkg.jar
____BookDemo.java

Now, I tried to use the following commands to see if the program works as before,

// to compile
development_directory/ javac -cp ./bookpkg.jar BookDemo.java
...it compiles without any error - this means the replacement of bookpkg.jar for BookPackage(package) folder structure is working - Am I correct?

// to run
development_directory/ java -cp ./bookpkg.jar BookDemo
...And I hit an error,

Error: Could not find or load main class BookDemo

But the BookDemo class is at default namespace - why it JRE won't be able to find the main method?

And, the way I am trying to replace package folder structure with a single JAR file - am I getting the concept correctly?

-

[Dave Tolls]

Your class path in the java call only contains the jar file.
It does not contain '.' (ie the current directory) so the JVM can't find the BookDemo class.

[Quazi Irfan]

Thanks Dave. I used the following command and it works,

development_directory/ java -cp .:./bookpkg.jar BookDemo

I found the explanation in another thread : Classpath and 'Current Directory'

So, I am getting the concept or Package and JAR correctly, right?