Add a .jar file in classpath with Java

Question

I am trying to develop a script in Java which finds all .jar files in a specified directory, then them to classpath and under certain conditions, invokes their main() method. Here is my Java info:

java version "1.6.0_24"
OpenJDK Runtime Environment (IcedTea6 1.11.5) (6b24-1.11.5-0ubuntu1~12.04.1)
OpenJDK Server VM (build 20.0-b12, mixed mode)

Here is the ls of the current working dir:

clojure.jar 
loader.class
loader.java 

I am doing the following in order to add clojure.jar to the classpath and invoke its main method:

import java.io.File;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.MalformedURLException;

import java.lang.reflect.Method;

public final class loader {

  public static void main (String[] args) {
    try {
      printClasspathString();
      System.out.println ("**********");
      URL[] classesRepo = { new File("clojure.jar").toURI ().toURL (),
                            new File(System.getProperty("user.dir")).toURI ().toURL ()};
      ClassLoader currentThreadClassLoader = Thread.currentThread().getContextClassLoader();
      URLClassLoader urlClassLoader = new URLClassLoader( classesRepo, currentThreadClassLoader);
      Thread.currentThread().setContextClassLoader(urlClassLoader);
      printClasspathString();
    } catch (Exception ex) {
      System.out.println(ex.getMessage ());
    }

    //Do I miss something here?

    String mainClassName="clojure.main";

    Class<?> mainClass = null;
    try {
      mainClass = Class.forName(mainClassName);
    }
    catch (Exception ex) {
      throw new IllegalArgumentException("class not found in your jar file " + mainClassName);
    }

    Method mainMethod = null;
    try {
      mainMethod = mainClass.getMethod("main", String[].class);
    }
    catch (Exception ex) {
      throw new IllegalArgumentException("class to launch (" + mainClassName + ") does not have a public static void main(String[]) method");
    }

    try {
      mainMethod.invoke(null, (Object) args);
    } catch (Exception ex) {
      System.out.println(ex.getMessage ());
    }

  }

  public static void printClasspathString() {
    ClassLoader applicationClassLoader = Thread.currentThread().getContextClassLoader();
    if (applicationClassLoader == null) {
      applicationClassLoader = ClassLoader.getSystemClassLoader();
    }
    URL[] urls = ((URLClassLoader)applicationClassLoader).getURLs();
    for(int i=0; i < urls.length; i++) {
      System.out.println (urls[i].getFile());
    }
  }

}

Unfortunately, the loader doesn't work as expected:

$ java -cp . loader
/home/proofit404/data/downloads/clojure-loader/
**********
/home/proofit404/data/downloads/clojure-loader/clojure.jar
/home/proofit404/data/downloads/clojure-loader/
Exception in thread "main" java.lang.IllegalArgumentException: class not found in your jar file clojure.main
    at loader.main(loader.java:37)

If I use the -cp option, though, everything works fine:

$ java -cp .:clojure.jar loader
/home/proofit404/data/downloads/clojure-loader/
/home/proofit404/data/downloads/clojure-loader/clojure.jar
**********
/home/proofit404/data/downloads/clojure-loader/clojure.jar
/home/proofit404/data/downloads/clojure-loader/
Clojure 1.4.0
user=> (System/exit 0)

So - what is it that I need to change in my code to make it work as expected?

Answer 1

I think the problem is that the Class.forName(String) method does not use the threads contextclassloader, but the classloader of the current class:

public static Class<?> forName(String className)
                    throws ClassNotFoundException

Returns the Class object associated with the class or interface with the given string name. Invoking this method is equivalent to:

    Class.forName(className, true, currentLoader)

where currentLoader denotes the defining class loader of the current class.

This means your URLClassLoader wont be used. Try instead to explicitly pass the classloader by using Class.forName(String,boolean, ClassLoader):

mainClass = Class.forName(mainClassName, true, urlClassLoader);