Java: Setting Classpath

Question

I have a project due soon and everything is coming together really nicely, but java classpaths are getting seriously in the way. I'll try to explain the situation as clearly and thoroughly as I can.

So we are using Javacc to write a programming language. The javacc file compiles into several java files. The Parser.java file includes references and calls to the other generated java files. So, after generation, we compile the Parser.java file. The problem is we get many errors, including not being able to recognize the calls to the other java files as well as our own files. We asked on a classroom discussion board about our problem and the professor responded with "you need to have the class files in your classpath". Ok, great, so the question is, how do we do that? Basically we have a single directory with the generated java files and our other helper files.

So, what have we tried?

I have tried changing my .bashrc (Ubuntu) file to include the correct classpath but that doesn't work. ie

 CLASSPATH=Home/project

(something like that I had the syntax right in the file)

I've tried on compilation executing

javac -cp . Parser.java

and

javac -cp "." Parser.java

neither works.

I have tried to edit the xml (I think xml) .classpath file in the directory of our files. Still doesn't work.

Somehow, I was able to compile Parser.java in one of the directories I have (we ended up making multiple directories with the same files in it in a futile effort to make something work) but when I try to run

java -cp . Parser.java

or

java Parser.java

It says it can not find the main and throws (I believe, its on my other computer) ClassNotFound or ClassNotDefined exception (something like that, it cannot find the main in the Parser file eventhough it IS there).

We have tried adding packages deceleration and import statements to our file, nothing seems to work.

BASICALLY: How can I successfully change the Classpath so that my java files (all in one directory and not jarred) can be compiled and run on my machine?

Thank you for your help. I greatly appreciate it.

Answer 1

I highly suggest you look into Ant . A simple build file can solve all these problems for you.

You don't need to edit your .bashrc or CLASSPATH .

From the command line you need to build ALL the java files together. I am not sure if JavaCC needs javacc.jar after it's generated your Lexer and Parser. But let's assume it does for some generic AST support.

javacc.jar is located in ~/javacc-5.0/lib/javacc.jar

Scenario 1: Simple directory structure, all Java files are in the root folder with no package.

root
  | Parser.java
  | Lexer.java
  | Program.java

To compile these I need to run:

javac -cp ~/javacc-5.0/lib/javacc.jar Parser.java Lexer.java Program.java

then I can execute Program like so if Program has main

java -cp ~/javacc-5.0/lib/javacc.jar:. Program

Scenario 2: Medium directory structure, code in root but with packages.

root
  | org
     | myproject
          | Parser.java
          | Lexer.java
          | Program.java

then you need to execute javac like so:

javac -cp ~/javacc-5.0/lib/javacc.jar org/myproject/Parser.java org/myproject/Lexer.java org/myproject/Program.java

and to execute

java -cp ~/javacc-5.0/lib/javacc.jar:. org.myproject.Program

Scenario 3: Complex directory structure, specific source directory with packages.

root
   | src
      | org
          | myproject
              | Parser.java
              | Lexer.java
              | Program.java

then you need to execute javac like so:

javac -cp ~/javacc-5.0/lib/javacc.jar -sourcepath src src/org/myproject/Parser.java src/org/myproject/Lexer.java src/org/myproject/Program.java

and to execute

java -cp ~/javacc-5.0/lib/javacc.jar:src org.myproject.Program

Answer 2

There is a difference between javac and java . You try to use them in wrong way. javac is a compiler which produces *.class files (in your case Parser.class). This *.class file must be then run by java .

So if I have file Parser.java

class Parser {

  public static void main(String[] args) {
    System.out.println("Hi");
  }

}

I would go the directory where this files resides and run in shell:

javac Parser.java

Than run the compiled file

java Parser

And that's it. Note that the name of the class in java file must be same as the name of the file.

Answer 3

1. Create a Jar file of all your compiled classes. Refer to this tutorial on how to create a jar file.

2. Start you program with this command

     java -classpath pathToJarFile com.abc.MainClass