Custom Classpath JUnit Test Runner

ClasspathTestRunner.java

import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;

import org.junit.runner.notification.RunNotifier;
import org.junit.runners.BlockJUnit4ClassRunner;
import org.junit.runners.model.InitializationError;

public class ClasspathTestRunner extends BlockJUnit4ClassRunner {

  static ClassLoader customClassLoader;

  public ClasspathTestRunner(Class<?> clazz) throws InitializationError {
    super(loadFromCustomClassloader(clazz));
  }

  // Loads a class in the custom classloader
  private static Class<?> loadFromCustomClassloader(Class<?> clazz) throws InitializationError {
    try {
      // Only load once to support parallel tests
      if (customClassLoader == null) {
        customClassLoader = new CustomClassLoader();
      }
      return Class.forName(clazz.getName(), true, customClassLoader);
    } catch (ClassNotFoundException e) {
      throw new InitializationError(e);
    }
  }


  // Runs junit tests in a separate thread using the custom class loader
  @Override
  public void run(final RunNotifier notifier) {
    Runnable runnable = () -> {
      super.run(notifier);
    };
    Thread thread = new Thread(runnable);
    thread.setContextClassLoader(customClassLoader);
    thread.start();
    try {
      thread.join();
    } catch (InterruptedException e) {
      throw new RuntimeException(e);
    }
  }


  // Custom class loader.  
  // Loads classes that match pattern, otherwise load from context loader
  public static class CustomClassLoader extends URLClassLoader {

    ClassLoader parent = null;

    public CustomClassLoader() {

      super(getClasspathUrls(), null);

      parent = Thread.currentThread()
                     .getContextClassLoader();

    }

    @Override
    public synchronized Class<?> loadClass(String name) throws ClassNotFoundException {

      // Only use custom classloader for classes from these packages
      if (name.startsWith("org.elasticsearch") || name.startsWith("org.apache.lucene")) {
        Class<?> c = findLoadedClass(name);
        if (c == null) {
          c = findClass(name);
        }
        return c;
      }

      // Otherwise load from the parent classloader
      return parent.loadClass(name);
    }


    private static URL[] getClasspathUrls() {

      // Start with the original classpath URLs
      ArrayList<URL> classpathUrls = new ArrayList<>();

      // TODO: Add the custom urls your unit test needs
      // classpathUrls.add()

      return classpathUrls.toArray(new URL[classpathUrls.size()]);
    }
  }
}

UnitTest.java

import org.junit.Test;
import org.junit.runner.RunWith;

@RunWith(ClasspathTestRunner.class)
public class MyTests {

  @Test
  public void someTest() {
  }
}