co.cask.cdap.common.lang.CombineClassLoader.java Source code

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Here is the source code for co.cask.cdap.common.lang.CombineClassLoader.java

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/*
 * Copyright  2015 Cask Data, Inc.
 *
 * Licensed 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 co.cask.cdap.common.lang;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterators;
import com.google.common.collect.Sets;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.Enumeration;
import java.util.List;
import java.util.Set;
import javax.annotation.Nullable;

/**
 * A {@link ClassLoader} that load classes from list of other {@link ClassLoader}s. Note that
 * this ClassLoader just delegates to other ClassLoaders, but never define class, hence no Class
 * loaded by this class would have {@link Class#getClassLoader()}} returning this ClassLoader.
 */
public class CombineClassLoader extends ClassLoader {

    private static final Logger LOG = LoggerFactory.getLogger(CombineClassLoader.class);
    private final List<ClassLoader> delegates;

    /**
     * Creates a CombineClassLoader with the given parent and a list of ClassLoaders for delegation.
     *
     * @param parent parent ClassLoader. If null, bootstrap ClassLoader will be the parent.
     * @param delegates list of ClassLoaders for delegation
     */
    public CombineClassLoader(@Nullable ClassLoader parent, Iterable<? extends ClassLoader> delegates) {
        super(parent);
        this.delegates = ImmutableList.copyOf(delegates);
    }

    /**
     * Returns the list of {@link ClassLoader}s that this class delegates to.
     */
    public List<ClassLoader> getDelegates() {
        return delegates;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        for (ClassLoader classLoader : delegates) {
            try {
                return classLoader.loadClass(name);
            } catch (ClassNotFoundException e) {
                LOG.trace("Class {} not found in ClassLoader {}", name, classLoader);
            }
        }

        throw new ClassNotFoundException("Class not found in all delegated ClassLoaders: " + name);
    }

    @Override
    protected URL findResource(String name) {
        for (ClassLoader classLoader : delegates) {
            URL url = classLoader.getResource(name);
            if (url != null) {
                return url;
            }
        }
        return null;
    }

    @Override
    protected Enumeration<URL> findResources(String name) throws IOException {
        // Using LinkedHashSet to preserve the ordering
        Set<URL> urls = Sets.newLinkedHashSet();
        for (ClassLoader classLoader : delegates) {
            Iterators.addAll(urls, Iterators.forEnumeration(classLoader.getResources(name)));
        }
        return Iterators.asEnumeration(urls.iterator());
    }

    @Override
    public InputStream getResourceAsStream(String name) {
        for (ClassLoader classLoader : delegates) {
            InputStream is = classLoader.getResourceAsStream(name);
            if (is != null) {
                return is;
            }
        }
        return null;
    }
}