Play Framework/Scala example source code file (DelegatingClassLoader.java)

This example Play Framework source code file (DelegatingClassLoader.java) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Play Framework (and Scala) source code examples by using tags.

All credit for the original source code belongs to Play Framework; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Play Framework tags/keywords

classloader, core, delegatingclassloader, enumeration, illegalaccessexception, illegalstateexception, invocationtargetexception, list, method, override, play, play framework, url

The DelegatingClassLoader.java Play Framework example source code

package play.core.classloader;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.util.Collections;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.Vector;

public class DelegatingClassLoader extends ClassLoader {

  private static final List<String> buildSharedClasses;
  static {
    List<String> list = new ArrayList<String>();
    list.add(play.core.BuildLink.class.getName());
    list.add(play.core.BuildDocHandler.class.getName());
    list.add(play.core.server.ServerWithStop.class.getName());
    list.add(play.api.UsefulException.class.getName());
    list.add(play.api.PlayException.class.getName());
    list.add(play.api.PlayException.InterestingLines.class.getName());
    list.add(play.api.PlayException.RichDescription.class.getName());
    list.add(play.api.PlayException.ExceptionSource.class.getName());
    list.add(play.api.PlayException.ExceptionAttachment.class.getName());
    buildSharedClasses = Collections.unmodifiableList(list);
  }

  private ClassLoader buildLoader;
  private ApplicationClassLoaderProvider applicationClassLoaderProvider;

  public DelegatingClassLoader(ClassLoader commonLoader, ClassLoader buildLoader, ApplicationClassLoaderProvider applicationClassLoaderProvider) {
    super(commonLoader);
    this.buildLoader = buildLoader;
    this.applicationClassLoaderProvider = applicationClassLoaderProvider;
  }

  public static boolean isSharedClass(String name) {
    return buildSharedClasses.contains(name);
  }

  @Override
  public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
    if (isSharedClass(name)) {
      return buildLoader.loadClass(name);
    } else {
      return super.loadClass(name, resolve);
    }
  }

  @Override
  public URL getResource(String name) {
    // -- Delegate resource loading. We have to hack here because the default implementation is already recursive.
    Method findResource;
    try {
      findResource = ClassLoader.class.getDeclaredMethod("findResource", String.class);
    } catch (NoSuchMethodException e) {
      throw new IllegalStateException(e);
    }
    findResource.setAccessible(true);
    ClassLoader appClassLoader = applicationClassLoaderProvider.get();
    URL resource = null;
    if (appClassLoader != null) {    
      try {
        resource = (URL) findResource.invoke(appClassLoader, name);
      } catch (IllegalAccessException e) {
        throw new IllegalStateException(e);
      } catch (InvocationTargetException e) {
        throw new IllegalStateException(e);
      }
    }
    return resource != null ? resource : super.getResource(name);
  }

  @Override
  public Enumeration<URL> getResources(String name) throws IOException {
    Method findResources;
    try {
      findResources = ClassLoader.class.getDeclaredMethod("findResources", String.class);
    } catch (NoSuchMethodException e) {
      throw new IllegalStateException(e);
    }
    findResources.setAccessible(true);
    ClassLoader appClassLoader = applicationClassLoaderProvider.get();
    Enumeration<URL> resources1;
    if (appClassLoader != null) {
      try {
        resources1 = (Enumeration<URL>) findResources.invoke(appClassLoader, name);
      } catch (IllegalAccessException e) {
        throw new IllegalStateException(e);
      } catch (InvocationTargetException e) {
        throw new IllegalStateException(e);
      }
    } else {
      resources1 = new Vector<URL>().elements();
    }
    Enumeration<URL> resources2 = super.getResources(name);
    return combineResources(resources1, resources2);
  }

  private Enumeration<URL> combineResources(Enumeration<URL> resources1, Enumeration<URL> resources2) {
    Set<URL> set = new HashSet<URL>();
    while (resources1.hasMoreElements()) {
      set.add(resources1.nextElement());
    }
    while (resources2.hasMoreElements()) {
      set.add(resources2.nextElement());
    }
    return new Vector<URL>(set).elements();
  }

  @Override
  public String toString() {
    return "DelegatingClassLoader, using parent: " + getParent();
  }

}