Adding class paths to SystemClassLoader

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Introduction

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
    method.setAccessible(true);
    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....
  }
}

Test

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.

History

• 2010.01.20: First draft.