How to Add Jars at Runtime?¶

| java |

Suppose you have the next project structure:

blog_post_image_inline

Here is the source code:

package org.halyph.one;

import java.io.File;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.security.CodeSource;

import org.halyph.three.MyClassThree;
import org.halyph.two.MyClassTwo;

public class DynamicClassLoader {
 public static void main(String[] args) throws Exception {
  System.out.println("DynamicClassLoader.main()");

  MyClassTwo myClassTwo = new MyClassTwo();
  MyClassThree myClassThree = new MyClassThree();
  myClassTwo.printMessage();
  myClassThree.printMessage();

 }
}

package org.halyph.two;

public class MyClassTwo {
 public void printMessage() {
  System.out.println("MyClassTwo.printMessage()");
 }
}

package org.halyph.three;

public class MyClassThree {
 public void printMessage() {
  System.out.println("MyClassThree.printMessage()");
 }
}

And each of these classes are packed in separate jar:

org.halyph.one.DynamicClassLoader - dynone.jar
org.halyph.two.MyClassTwo - dyntwo.jar
org.halyph.three.MyClassThree - dynthree.jar

To run this application you should put all these jars on classpath, E.g. the application output

# java -cp dynone.jar;dyntwo.jar;dynthree.jar org.halyph.one.DynamicClassLoader
DynamicClassLoader.main()
MyClassTwo.printMessage()
MyClassThree.printMessage()

But there are some cases when dyntwo.jar and dynthree.jar jars location can be known during runtime only. And we have to run our application in the next way:

# java -cp dynone.jar org.halyph.one.DynamicClassLoader

Of cause, you’ll get the next (or similar) error:

# java -cp dynone.jar org.halyph.one.DynamicClassLoader
DynamicClassLoader.main()
Exception in thread "main" java.lang.NoClassDefFoundError: org/halyph/two/MyClassTwo
        at org.halyph.one.DynamicClassLoader.main(DynamicClassLoader.java:28)
Caused by: java.lang.ClassNotFoundException: org.halyph.two.MyClassTwo
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        ... 1 more

So, how can we solve this problem? Here is the patched org.halyph.one.DynamicClassLoader:

package org.halyph.one;

import java.io.File;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.security.CodeSource;

import org.halyph.three.MyClassThree;
import org.halyph.two.MyClassTwo;

public class DynamicClassLoader {

 public static void main(String[] args) throws Exception {
  System.out.println("DynamicClassLoader.main()");

  String[] jars = new String[] { "dyntwo.jar",  "dynthree.jar"};
  addJarsToClasspath(getRootFolder(), jars);
  MyClassTwo myClassTwo = new MyClassTwo();
  MyClassThree myClassThree = new MyClassThree();
  myClassTwo.printMessage();
  myClassThree.printMessage();

 }

 public static void addJarsToClasspath(String rootFolder, String[] jarNames)
   throws SecurityException, NoSuchMethodException,
   MalformedURLException, IllegalArgumentException,
   IllegalAccessException, InvocationTargetException {

  Method addURL = URLClassLoader.class.getDeclaredMethod("addURL",
    new Class[] { URL.class });
  addURL.setAccessible(true); // you're telling the JVM to override the
         // default visibility

  File[] files = new File[jarNames.length];
  int i = 0;
  for (String jarName : jarNames) {
   files[i++] = new File(rootFolder + jarName);
  }

  // returning the jars to add
  ClassLoader cl = ClassLoader.getSystemClassLoader();
  for (i = 0; i < files.length; i++) {
   URL url = files[i].toURL();
   addURL.invoke(cl, new Object[] { url });
  }
 }

 public static String getRootFolder() {
  String result = null;
  try {
   String qualifiedClassName = DynamicClassLoader.class.getName();

   Class qc = Class.forName(qualifiedClassName);
   CodeSource source = qc.getProtectionDomain().getCodeSource();
   if (source != null) {
    URL location = source.getLocation();
    File file = new File(location.toURI());

    result = file.getParent() + "/";
   } else {
    System.out.println(qualifiedClassName + " : "
      + "unknown source");
   }
  } catch (Exception e) {
   System.err.println("Unable to locate class on command line.");
   e.printStackTrace();
  }
  return result;
 }

}

Here is the output of patched application:

# java -cp dynone.jar org.halyph.one.DynamicClassLoader
DynamicClassLoader.main()
MyClassTwo.printMessage()
MyClassThree.printMessage()

So, two additional methods were added to support this “magic”:

public static String getRootFolder() - this method simply identifies the folder where dynone.jar (on behalf of org.halyph.one.DynamicClassLoader class) is located. Because for this demo we put all application-related jars in one folder
public static void addJarsToClasspath(String rootFolder, String[] jarNames) - and here the “magic” happen. Based on this post on www.velocityreviews.com forums:

There is a well-known hack for dynamically extending the class path. It generally works, and we use it in production releases of our software. It uses reflection, plus the knowledge that the default ClassLoader is a URLClassLoader.
…
As I said, this is a hack, undocumented, and subject to change at any time. Indeed, there’s no guarantee that extant JREs use a URLClassLoader as a default class loader. Moreover, it only works if your program doesn’t have a security manager (it probably doesn’t), or your code is trusted.
…
We’ve never seen problems with it, although our software is used on a plethora of different boxes and OSs.

It works for me and I hope it will be useful for you as well.
Happy hacking!