flink-core/src/main/java/org/apache/flink/util/ChildFirstClassLoader.java

/*
 * 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.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.function.Consumer;

/**
 * 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 FlinkUserCodeClassLoader {

    /**
     * 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,
            Consumer<Throwable> classLoadingExceptionHandler) {
        super(urls, parent, classLoadingExceptionHandler);
        this.alwaysParentFirstPatterns = alwaysParentFirstPatterns;
    }

    @Override
    protected Class<?> loadClassWithoutExceptionHandling(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.loadClassWithoutExceptionHandling(name, resolve);
                }
            }

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

    static {
        ClassLoader.registerAsParallelCapable();
    }
}