Question

Problem: I wrote a hello world java program, compiled and created a jar file of it – PERFECT, no problems there.

I now added the following to the top of the source code

package za.ac.uj.csc2a;

saved the code, place all the files in the right directories and every time i compile with this batch file

del *.class
del *.jar
del *.txt

echo compiling HelloWorld.java.....

javac HelloWorld.java

javap -c HelloWorld.class > ByteCode.txt

jar cfe HelloWorld.jar HelloWorld HelloWorld.class

java -jar za.ac.uj.csc2a.HelloWorld.jar

i get,

Error: Unable to acces jarfile za.ac.uj.csc2a.HelloWorld.jar

Any Advice….

---

Here is the final bat file that works

del *.class
del *.jar
del *.txt
RD /Q /S bin

echo compiling HelloWorld.java.....

MD bin

javac -d bin za/ac/uj/csc2a/HelloWorld.java
javap -c bin/za/ac/uj/csc2a/HelloWorld.class > ByteCode.txt
jar cfe HelloWorld.jar za.ac.uj.csc2a.HelloWorld -C bin .
java -jar HelloWorld.jar
pause

thanks alot

Aiden

Answer 1

Your jar is named Helloworld.jar:

 jar cfe HelloWorld.jar HelloWorld HelloWorld.class

But you call it with the package name as prefix to the jar:

 java -jar za/ac/uj/csc2a/HelloWorld.jar

 java -jar HelloWorld.jar

should be right.

Normally, you put your classes and sources in different directories, if you use packages. A typical tree looks like

proj
|-- src
|   |-- za
|       |-- ac
|           `-- HelloWorld.java
|-- bin
    |-- za
        |-- ac
            `-- HelloWorld.class

You can move to the src-directory, and compile with:

 javac -d ../bin za/ac/HelloWorld.java

(reduced the package/path length for convenience) This will produce the directory structure in bin if needed (or often classes)

Then, to jar the whole thing with the correct directory structure, you change to proj, and issue:

jar cfe HelloWorld.jar za.ac.HelloWorld -C bin . 

The dot stands for the whole directory.

jar -help 

will tell you more options. To create a runnable jar you need a manifest too. Else you can start your app:

java -cp HelloWorld.jar za.ac.HelloWorld 

(shortened the path a bit again).

Note, that the structure of the directory has to reflect the package declaration. You can’t omit a part (za), and step into the next directory (ac). The class to run is za.ac.HelloWorld and therefor it is searched for in za/ac/ – no matter whether in the file system or in the jar.

If you learn to issue the commands from the right base, it is easy. If not, you’ll get years of trial and error.

For a jar which can be startet with java -jar, you need a manifest. A template is generated by the jar command. There you add

Main-Class: za.ac.HelloWorld

with a following blank line. Again: the whole name with package is needed. Main-Class can’t be written as Mainclass. HelloWorld must not be followed by .class or .java – common mistakes! But as you know, you may use the -e switch to specify the main class, but again: Include the package name.