自定义classLoader实现加载外部jar, 以skywalking agent 类加载器为例子

整体思路

• 扩展findClass ，解决loadClass可以查找到
• 扩展findResource，解决getResources可以获取到资源

基本原理

ClassLoader loadClass的加载顺序
findLoadedClass 加载本地已经加载过的class
parent.loadClass 通过父类去加载class
findClass 前面都没有加载到，就通过findClass去搜索jar并加载class

扩展findClass

扩展findResource

扩展后AgentClassLoader.getDefault().getResources(“skywalking-plugin.def”); 就可以获取插件定义

完整源代码

AgentClassLoader

/*
 * 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.skywalking.apm.agent.core.plugin.loader;

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.locks.ReentrantLock;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
import lombok.RequiredArgsConstructor;
import org.apache.skywalking.apm.agent.core.boot.AgentPackageNotFoundException;
import org.apache.skywalking.apm.agent.core.boot.AgentPackagePath;
import org.apache.skywalking.apm.agent.core.boot.PluginConfig;
import org.apache.skywalking.apm.agent.core.conf.Config;
import org.apache.skywalking.apm.agent.core.conf.SnifferConfigInitializer;
import org.apache.skywalking.apm.agent.core.logging.api.ILog;
import org.apache.skywalking.apm.agent.core.logging.api.LogManager;
import org.apache.skywalking.apm.agent.core.plugin.PluginBootstrap;

/**
 * The <code>AgentClassLoader</code> represents a classloader, which is in charge of finding plugins and interceptors.
 */
public class AgentClassLoader extends ClassLoader {

    static {
        /*
         * Try to solve the classloader dead lock. See https://github.com/apache/skywalking/pull/2016
         */
        registerAsParallelCapable();
    }

    private static final ILog LOGGER = LogManager.getLogger(AgentClassLoader.class);
    /**
     * The default class loader for the agent.
     */
    private static AgentClassLoader DEFAULT_LOADER;

    private List<File> classpath;
    private List<Jar> allJars;
    private ReentrantLock jarScanLock = new ReentrantLock();

    public static AgentClassLoader getDefault() {
        return DEFAULT_LOADER;
    }

    /**
     * Init the default class loader.
     *
     * @throws AgentPackageNotFoundException if agent package is not found.
     */
    public static void initDefaultLoader() throws AgentPackageNotFoundException {
        if (DEFAULT_LOADER == null) {
            synchronized (AgentClassLoader.class) {
                if (DEFAULT_LOADER == null) {
                    DEFAULT_LOADER = new AgentClassLoader(PluginBootstrap.class.getClassLoader());
                }
            }
        }
    }

    public AgentClassLoader(ClassLoader parent) throws AgentPackageNotFoundException {
        super(parent);
        File agentDictionary = AgentPackagePath.getPath();
        classpath = new LinkedList<>();
        Config.Plugin.MOUNT.forEach(mountFolder -> classpath.add(new File(agentDictionary, mountFolder)));
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        List<Jar> allJars = getAllJars();
        String path = name.replace('.', '/').concat(".class");
        for (Jar jar : allJars) {
            JarEntry entry = jar.jarFile.getJarEntry(path);
            if (entry == null) {
                continue;
            }
            try {
                URL classFileUrl = new URL("jar:file:" + jar.sourceFile.getAbsolutePath() + "!/" + path);
                byte[] data;
                try (final BufferedInputStream is = new BufferedInputStream(
                    classFileUrl.openStream()); final ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
                    int ch;
                    while ((ch = is.read()) != -1) {
                        baos.write(ch);
                    }
                    data = baos.toByteArray();
                }
                return processLoadedClass(defineClass(name, data, 0, data.length));
            } catch (IOException e) {
                LOGGER.error(e, "find class fail.");
            }
        }
        throw new ClassNotFoundException("Can't find " + name);
    }

    @Override
    protected URL findResource(String name) {
        List<Jar> allJars = getAllJars();
        for (Jar jar : allJars) {
            JarEntry entry = jar.jarFile.getJarEntry(name);
            if (entry != null) {
                try {
                    return new URL("jar:file:" + jar.sourceFile.getAbsolutePath() + "!/" + name);
                } catch (MalformedURLException ignored) {
                }
            }
        }
        return null;
    }

    @Override
    protected Enumeration<URL> findResources(String name) throws IOException {
        List<URL> allResources = new LinkedList<>();
        List<Jar> allJars = getAllJars();
        for (Jar jar : allJars) {
            JarEntry entry = jar.jarFile.getJarEntry(name);
            if (entry != null) {
                allResources.add(new URL("jar:file:" + jar.sourceFile.getAbsolutePath() + "!/" + name));
            }
        }

        final Iterator<URL> iterator = allResources.iterator();
        return new Enumeration<URL>() {
            @Override
            public boolean hasMoreElements() {
                return iterator.hasNext();
            }

            @Override
            public URL nextElement() {
                return iterator.next();
            }
        };
    }

    private Class<?> processLoadedClass(Class<?> loadedClass) {
        final PluginConfig pluginConfig = loadedClass.getAnnotation(PluginConfig.class);
        if (pluginConfig != null) {
            // Set up the plugin config when loaded by class loader at the first time.
            // Agent class loader just loaded limited classes in the plugin jar(s), so the cost of this
            // isAssignableFrom would be also very limited.
            SnifferConfigInitializer.initializeConfig(pluginConfig.root());
        }

        return loadedClass;
    }

    private List<Jar> getAllJars() {
        if (allJars == null) {
            jarScanLock.lock();
            try {
                if (allJars == null) {
                    allJars = doGetJars();
                }
            } finally {
                jarScanLock.unlock();
            }
        }

        return allJars;
    }

    private LinkedList<Jar> doGetJars() {
        LinkedList<Jar> jars = new LinkedList<>();
        for (File path : classpath) {
            if (path.exists() && path.isDirectory()) {
                String[] jarFileNames = path.list((dir, name) -> name.endsWith(".jar"));
                for (String fileName : jarFileNames) {
                    try {
                        File file = new File(path, fileName);
                        Jar jar = new Jar(new JarFile(file), file);
                        jars.add(jar);
                        LOGGER.info("{} loaded.", file.toString());
                    } catch (IOException e) {
                        LOGGER.error(e, "{} jar file can't be resolved", fileName);
                    }
                }
            }
        }
        return jars;
    }

    @RequiredArgsConstructor
    private static class Jar {
        private final JarFile jarFile;
        private final File sourceFile;
    }
}

PluginResourcesResolver 获取skywalking-plugin.def插件定义

/*
 * 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.skywalking.apm.agent.core.plugin;

import java.io.IOException;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import org.apache.skywalking.apm.agent.core.logging.api.ILog;
import org.apache.skywalking.apm.agent.core.logging.api.LogManager;
import org.apache.skywalking.apm.agent.core.plugin.loader.AgentClassLoader;

/**
 * Use the current classloader to read all plugin define file. The file must be named 'skywalking-plugin.def'
 */
public class PluginResourcesResolver {
    private static final ILog LOGGER = LogManager.getLogger(PluginResourcesResolver.class);

    public List<URL> getResources() {
        List<URL> cfgUrlPaths = new ArrayList<URL>();
        Enumeration<URL> urls;
        try {
            urls = AgentClassLoader.getDefault().getResources("skywalking-plugin.def");

            while (urls.hasMoreElements()) {
                URL pluginUrl = urls.nextElement();
                cfgUrlPaths.add(pluginUrl);
                LOGGER.info("find skywalking plugin define in {}", pluginUrl);
            }

            return cfgUrlPaths;
        } catch (IOException e) {
            LOGGER.error("read resources failure.", e);
        }
        return null;
    }
}