content: /*
   * The Alluxio Open Foundation licenses this work under the Apache License, version 2.0
   * (the "License"). You may not use this work except in compliance with the License, which is
   * available at www.apache.org/licenses/LICENSE-2.0
   *
   * This software is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
   * either express or implied, as more fully set forth in the License.
   *
   * See the NOTICE file distributed with this work for information regarding copyright ownership.
   */
  
  package alluxio.extensions;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.io.IOException;
  import java.net.URL;
  import java.net.URLClassLoader;
  import java.util.Enumeration;
  
  /**
   * An isolated {@link ClassLoader} for loading extensions to core Alluxio. This class loader first
   * scans the provided URLs to define a class and in case the class is not found it will fallback to
   * the provided default class loader.
   */
  public class ExtensionsClassLoader extends URLClassLoader {
    private static final Logger LOG = LoggerFactory.getLogger(ExtensionsClassLoader.class);
  
    /**
     * A {@link ClassLoader} to make the protected methods accessible.
     */
    private static class DefaultClassLoader extends ClassLoader {
      /**
       * @param classLoader the parent class loader
       */
      public DefaultClassLoader(ClassLoader classLoader) {
        super(classLoader);
      }
  
      @Override
      public Class<?> findClass(String name) throws ClassNotFoundException {
        return super.findClass(name);
      }
  
      @Override
      public Class<?> loadClass(String name) throws ClassNotFoundException {
        return super.loadClass(name);
      }
  
      @Override
      public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        return super.loadClass(name, resolve);
      }
    }
  
    private final DefaultClassLoader mDefaultClassloader;
  
    /**
     * @param urls array of URLs of jars
     * @param defaultClassLoader the default class loader to fall back
     */
    public ExtensionsClassLoader(URL[] urls, ClassLoader defaultClassLoader) {
      // Pass null to override parent first delegation
      super(urls, null);
      mDefaultClassloader = new DefaultClassLoader(defaultClassLoader);
      LOG.debug("Created ExtensionsClassLoader with jars={}", urls);
    }
  
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
      try {
        return super.findClass(name);
      } catch (ClassNotFoundException e) {
        return mDefaultClassloader.findClass(name);
      }
    }
  
    @Override
    public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
      try {
        return super.loadClass(name, resolve);
      } catch (ClassNotFoundException e) {
        return mDefaultClassloader.loadClass(name, resolve);
      }
    }
  
    @Override
    public Enumeration<URL> getResources(String name) throws IOException {
      Enumeration<URL> resources = super.getResources(name);
      // Falls back to default class loader if not found in the URL class loader
      if (!resources.hasMoreElements()) {
        LOG.debug("Falling back to default class loader for loading resource {}", name);
        return mDefaultClassloader.getResources(name);
      }
      return resources;
    }
  }
  