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Posted 20 Jan 2011

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Adding class paths to SystemClassLoader

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23 Jan 2011CPOL
How to add class paths to SystemClassLoader.


This article is a non-recommended hack for dynamically adding a class path to SystemClassLoader. You should re-think your design before using this hack; usually, you should be able to use URLClassLoader to load whatever you want.

About SystemClassLoader

SystemClassLoaders are usually URLClassLoaders, and in URLClassLoader, there is a method called addURL(), so we can make use of this to add paths/jars to SystemClassLoader dynamically at runtime. However, since "addURL()" is a protected method, we have to use Reflection to call it.

The sample code is as follows:

public static void addURLs(URL[] urls) {
  ClassLoader cl = ClassLoader.getSystemClassLoader();
  if ( cl instanceof URLClassLoader ) {
    URLClassLoader ul = (URLClassLoader)cl;
    // addURL is a protected method, but we can use reflection to call it
    Class<?>[] paraTypes = new Class[1];
    paraTypes[0] = URL.class;
    Method method = URLClassLoader.class.getDeclaredMethod("addURL", paraTypes);
    // change access to true, otherwise, it will throw exception
    Object[] args = new Object[1];
    for(int i=0; i<urls.length; i++) {
      args[0] = urls[i];
      method.invoke(ul, args);
  } else {
    // SystemClassLoader is not URLClassLoader....


I have written a test case, and it will automatically load all "lib/*.jar" and instantiate an object as specified in args[0]. The test output is as follows. Note: I only included "bin" as my class path, and I have commons-collections.jar under my "lib" directory, but the program is still able to dynamically load the required class.

Image 1


  • 2010.01.20: First draft.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Sam NG
Hong Kong Hong Kong
No Biography provided

Comments and Discussions

GeneralInteresting, but well known and not recommended hack Pin
Alexandre Nikolov20-Jan-11 22:19
MemberAlexandre Nikolov20-Jan-11 22:19 
GeneralRe: Interesting, but well known and not recommended hack Pin
Sam NG23-Jan-11 16:50
MemberSam NG23-Jan-11 16:50 
Hi Nikolov,

I completely agree with your comment and I updated the introduction to remind people they should try to rethink their design instead of just hacking the SystemClassLoader.

For me, I am extending JUnit to do some special testing, and the system requires some jars from a commercial software, and of course I can't redistribute those jars. Because I want to make the unit test easier to use (so it is easier to force developers to use this), the program will do some auto-discovery by searching the PATH to detect the install path of the commercial software if the classpath is not setup properly. I have tried using URLClassLoader to load the jars I need, but since "this" is still loaded by the SystemClassLoader, lots of the classes still can't be resolved. Unless I change my code to remove all the "compile-time dependency", i.e. loading everything by Class.forName, and use reflection to call all methods, and I have used this method on another (simpler) project, I can't think of another solution that can solve this problem.

If you have some other ideas/recommendations on how to handle this properly, please share with me because I come across of this problem very often. Thanks.


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