org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader Maven / Gradle / Ivy

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/*
 * 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.kafka.connect.runtime.isolation;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.net.URL;
import java.net.URLClassLoader;
import java.util.HashMap;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

/**
 * A custom classloader dedicated to loading Connect plugin classes in classloading isolation.
 *
 * 
 * Under the current scheme for classloading isolation in Connect, the delegating classloader loads
 * plugin classes that it finds in its child plugin classloaders. For classes that are not plugins,
 * this delegating classloader delegates its loading to its parent. This makes this classloader a
 * child-first classloader.
 * 
 * This class is thread-safe and parallel capable.
 */
public class DelegatingClassLoader extends URLClassLoader {
    private static final Logger log = LoggerFactory.getLogger(DelegatingClassLoader.class);

    private final ConcurrentMap, ClassLoader>> pluginLoaders;
    private final ConcurrentMap aliases;

    // Although this classloader does not load classes directly but rather delegates loading to a
    // PluginClassLoader or its parent through its base class, because of the use of inheritance in
    // in the latter case, this classloader needs to also be declared as parallel capable to use
    // fine-grain locking when loading classes.
    static {
        ClassLoader.registerAsParallelCapable();
    }

    public DelegatingClassLoader(ClassLoader parent) {
        super(new URL[0], parent);
        this.pluginLoaders = new ConcurrentHashMap<>();
        this.aliases = new ConcurrentHashMap<>();
    }

    public DelegatingClassLoader() {
        // Use as parent the classloader that loaded this class. In most cases this will be the
        // System classloader. But this choice here provides additional flexibility in managed
        // environments that control classloading differently (OSGi, Spring and others) and don't
        // depend on the System classloader to load Connect's classes.
        this(DelegatingClassLoader.class.getClassLoader());
    }

    /**
     * Retrieve the PluginClassLoader associated with a plugin class
     * @param name The fully qualified class name of the plugin
     * @return the PluginClassLoader that should be used to load this, or null if the plugin is not isolated.
     */
    // VisibleForTesting
    PluginClassLoader pluginClassLoader(String name) {
        if (!PluginUtils.shouldLoadInIsolation(name)) {
            return null;
        }
        SortedMap, ClassLoader> inner = pluginLoaders.get(name);
        if (inner == null) {
            return null;
        }
        ClassLoader pluginLoader = inner.get(inner.lastKey());
        return pluginLoader instanceof PluginClassLoader
               ? (PluginClassLoader) pluginLoader
               : null;
    }

    ClassLoader connectorLoader(String connectorClassOrAlias) {
        String fullName = aliases.getOrDefault(connectorClassOrAlias, connectorClassOrAlias);
        ClassLoader classLoader = pluginClassLoader(fullName);
        if (classLoader == null) classLoader = this;
        log.debug(
            "Getting plugin class loader: '{}' for connector: {}",
            classLoader,
            connectorClassOrAlias
        );
        return classLoader;
    }

    public void installDiscoveredPlugins(PluginScanResult scanResult) {
        pluginLoaders.putAll(computePluginLoaders(scanResult));
        for (String pluginClassName : pluginLoaders.keySet()) {
            log.info("Added plugin '{}'", pluginClassName);
        }
        aliases.putAll(PluginUtils.computeAliases(scanResult));
        for (Map.Entry alias : aliases.entrySet()) {
            log.info("Added alias '{}' to plugin '{}'", alias.getKey(), alias.getValue());
        }
    }

    @Override
    protected Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
        String fullName = aliases.getOrDefault(name, name);
        PluginClassLoader pluginLoader = pluginClassLoader(fullName);
        if (pluginLoader != null) {
            log.trace("Retrieving loaded class '{}' from '{}'", fullName, pluginLoader);
            return pluginLoader.loadClass(fullName, resolve);
        }

        return super.loadClass(fullName, resolve);
    }

    private static Map, ClassLoader>> computePluginLoaders(PluginScanResult plugins) {
        Map, ClassLoader>> pluginLoaders = new HashMap<>();
        plugins.forEach(pluginDesc ->
                pluginLoaders.computeIfAbsent(pluginDesc.className(), k -> new TreeMap<>())
                        .put(pluginDesc, pluginDesc.loader()));
        return pluginLoaders;
    }
}