/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.util;

import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;

/**
 * A variant of the URLClassLoader that first loads from the URLs and only after that from the parent.
 *
 * <p>{@link #getResourceAsStream(String)} uses {@link #getResource(String)} internally so we
 * don't override that.
 */
public final class ChildFirstClassLoader extends URLClassLoader {

	/**
	 * The classes that should always go through the parent ClassLoader. This is relevant
	 * for Flink classes, for example, to avoid loading Flink classes that cross the
	 * user-code/system-code barrier in the user-code ClassLoader.
	 */
	private final String[] alwaysParentFirstPatterns;

	public ChildFirstClassLoader(URL[] urls, ClassLoader parent, String[] alwaysParentFirstPatterns) {
		super(urls, parent);
		this.alwaysParentFirstPatterns = alwaysParentFirstPatterns;
	}

	@Override
	protected synchronized Class<?> loadClass(
		String name, boolean resolve) throws ClassNotFoundException {

		// First, check if the class has already been loaded
		Class<?> c = findLoadedClass(name);

		if (c == null) {
			// check whether the class should go parent-first
			for (String alwaysParentFirstPattern : alwaysParentFirstPatterns) {
				if (name.startsWith(alwaysParentFirstPattern)) {
					return super.loadClass(name, resolve);
				}
			}

			try {
				// check the URLs
				c = findClass(name);
			} catch (ClassNotFoundException e) {
				// let URLClassLoader do it, which will eventually call the parent
				c = super.loadClass(name, resolve);
			}
		}

		if (resolve) {
			resolveClass(c);
		}

		return c;
	}

	@Override
	public URL getResource(String name) {
		// first, try and find it via the URLClassloader
		URL urlClassLoaderResource = findResource(name);

		if (urlClassLoaderResource != null) {
			return urlClassLoaderResource;
		}

		// delegate to super
		return super.getResource(name);
	}

	@Override
	public Enumeration<URL> getResources(String name) throws IOException {
		// first get resources from URLClassloader
		Enumeration<URL> urlClassLoaderResources = findResources(name);

		final List<URL> result = new ArrayList<>();

		while (urlClassLoaderResources.hasMoreElements()) {
			result.add(urlClassLoaderResources.nextElement());
		}

		// get parent urls
		Enumeration<URL> parentResources = getParent().getResources(name);

		while (parentResources.hasMoreElements()) {
			result.add(parentResources.nextElement());
		}

		return new Enumeration<URL>() {
			Iterator<URL> iter = result.iterator();

			public boolean hasMoreElements() {
				return iter.hasNext();
			}

			public URL nextElement() {
				return iter.next();
			}
		};
	}
}
