Reload used classes at runtime Java

Question

I\'m working on a program that watches a directory and runs all tests in the directory when it sees changes in the directory.

This requires the program to dynamicall

Answer 1

If you used the custom ClassLoader exactly like in the linked answer it is not overriding the method protected Class<?> loadClass(String name, boolean resolve). This implies that when the JVM is resolving dependencies it will still delegate to the parent class loader. And, of course, when it was not delegating to the parent ClassLoader it had the risk of missing some required classes.

The easiest solution is to set up the right parent class loader. You are currently passing Thread.currentThread().getContextClassLoader() which is a bit strange as your main intention is that the delegation should not delegate to that loader but load the changed classes. You have to think about which class loaders exist and which to use and which not. E.g. if the class Foo is within the scope of your current code but you want to (re)load it with the new ClassLoader, Foo.class.getClassLoader().getParent() would be the right delegate parent for the new ClassLoader. Note that it might be null but this doesn’t matter as in this case it would use the bootstrap loader which is the correct parent then.

Note that when you set up the right parent ClassLoader matching your intentions you don’t need that custom ClassLoader anymore. The default implementation (see URLClassLoader) already does the right thing. And with current Java versions it is Closeable making it even more suitable for dynamic loading scenarios.

Here is a simple example of a class reloading:

import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;

public class ReloadMyClass
{
  public static void main(String[] args)
  throws ClassNotFoundException, IOException {
    Class<?> myClass=ReloadMyClass.class;
    System.out.printf("my class is Class@%x%n", myClass.hashCode());
    System.out.println("reloading");
    URL[] urls={ myClass.getProtectionDomain().getCodeSource().getLocation() };
    ClassLoader delegateParent = myClass.getClassLoader().getParent();
    try(URLClassLoader cl=new URLClassLoader(urls, delegateParent)) {
      Class<?> reloaded=cl.loadClass(myClass.getName());
      System.out.printf("reloaded my class: Class@%x%n", reloaded.hashCode());
      System.out.println("Different classes: "+(myClass!=reloaded));
    }
  }
}