FatJar技术

概念

　　将一个jar及其依赖的三方jar全部打到一个包中，这个包即为FatJar。

作用

　　作用：　Jar包隔离，避免Jar冲突。

打包方式

1. maven-shade-plugin插件；
2. spring-boot-maven-plugin插件（Spring Boot打包插件）；

嵌套Jar资源加载方案

　　思路：扩展Jar URL协议+定制ClassLoader;

扩展Jar URL协议

　　问题： JDK内置的Jar URL协议只支持一个’!/’，需要扩展此协议使其支持多个’!/’，以便能够加载jar in jar的资源，如下所示：

 jar:file:/data/spring-boot-theory/BOOT-INF/lib/spring-aop-5.0.4.RELEASE.jar!/org/springframework/aop/SpringProxy.class

jar:file:/data/spring-boot-theory.jar!/BOOT-INF/lib/spring-aop-5.0.4.RELEASE.jar!/org/springframework/aop/SpringProxy.class

　　解决方案：定制协议处理器Handler，定制规则查看另一篇，SpringBoot实现如下图所示：

定制ClassLoader

　　加载class：重载loadclass，添加对应的包路径；

　　加载其它资源：使用URLClassLoader原有逻辑即可；

SpringBoot提供了LaunchedURLClassLoader ，实现如下所示：

/*
 * Copyright 2012-2017 the original author or authors.
 *
 * 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 org.springframework.boot.loader;

import java.io.IOException;
import java.net.JarURLConnection;
import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLConnection;
import java.security.AccessController;
import java.security.PrivilegedExceptionAction;
import java.util.Enumeration;
import java.util.jar.JarFile;

import org.springframework.boot.loader.jar.Handler;

/**
 * {@link ClassLoader} used by the {@link Launcher}.
 *
 * @author Phillip Webb
 * @author Dave Syer
 * @author Andy Wilkinson
 */
public class LaunchedURLClassLoader extends URLClassLoader {

    static {
        ClassLoader.registerAsParallelCapable();
    }

    /**
     * Create a new {@link LaunchedURLClassLoader} instance.
     * @param urls the URLs from which to load classes and resources
     * @param parent the parent class loader for delegation
     */
    public LaunchedURLClassLoader(URL[] urls, ClassLoader parent) {
        super(urls, parent);
    }

    @Override
    public URL findResource(String name) {
        Handler.setUseFastConnectionExceptions(true);
        try {
            return super.findResource(name);
        }
        finally {
            Handler.setUseFastConnectionExceptions(false);
        }
    }

    @Override
    public Enumeration<URL> findResources(String name) throws IOException {
        Handler.setUseFastConnectionExceptions(true);
        try {
            return new UseFastConnectionExceptionsEnumeration(super.findResources(name));
        }
        finally {
            Handler.setUseFastConnectionExceptions(false);
        }
    }

    @Override
    protected Class<?> loadClass(String name, boolean resolve)
            throws ClassNotFoundException {
        Handler.setUseFastConnectionExceptions(true);
        try {
            try {
                definePackageIfNecessary(name);
            }
            catch (IllegalArgumentException ex) {
                // Tolerate race condition due to being parallel capable
                if (getPackage(name) == null) {
                    // This should never happen as the IllegalArgumentException indicates
                    // that the package has already been defined and, therefore,
                    // getPackage(name) should not return null.
                    throw new AssertionError("Package " + name + " has already been "
                            + "defined but it could not be found");
                }
            }
            return super.loadClass(name, resolve);
        }
        finally {
            Handler.setUseFastConnectionExceptions(false);
        }
    }

    /**
     * Define a package before a {@code findClass} call is made. This is necessary to
     * ensure that the appropriate manifest for nested JARs is associated with the
     * package.
     * @param className the class name being found
     */
    private void definePackageIfNecessary(String className) {
        int lastDot = className.lastIndexOf('.');
        if (lastDot >= 0) {
            String packageName = className.substring(0, lastDot);
            if (getPackage(packageName) == null) {
                try {
                    definePackage(className, packageName);
                }
                catch (IllegalArgumentException ex) {
                    // Tolerate race condition due to being parallel capable
                    if (getPackage(packageName) == null) {
                        // This should never happen as the IllegalArgumentException
                        // indicates that the package has already been defined and,
                        // therefore, getPackage(name) should not have returned null.
                        throw new AssertionError(
                                "Package " + packageName + " has already been defined "
                                        + "but it could not be found");
                    }
                }
            }
        }
    }

    private void definePackage(String className, String packageName) {
        try {
            AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                String packageEntryName = packageName.replace('.', '/') + "/";
                String classEntryName = className.replace('.', '/') + ".class";
                for (URL url : getURLs()) {
                    try {
                        URLConnection connection = url.openConnection();
                        if (connection instanceof JarURLConnection) {
                            JarFile jarFile = ((JarURLConnection) connection)
                                    .getJarFile();
                            if (jarFile.getEntry(classEntryName) != null
                                    && jarFile.getEntry(packageEntryName) != null
                                    && jarFile.getManifest() != null) {
                                definePackage(packageName, jarFile.getManifest(), url);
                                return null;
                            }
                        }
                    }
                    catch (IOException ex) {
                        // Ignore
                    }
                }
                return null;
            }, AccessController.getContext());
        }
        catch (java.security.PrivilegedActionException ex) {
            // Ignore
        }
    }

    /**
     * Clear URL caches.
     */
    public void clearCache() {
        for (URL url : getURLs()) {
            try {
                URLConnection connection = url.openConnection();
                if (connection instanceof JarURLConnection) {
                    clearCache(connection);
                }
            }
            catch (IOException ex) {
                // Ignore
            }
        }

    }

    private void clearCache(URLConnection connection) throws IOException {
        Object jarFile = ((JarURLConnection) connection).getJarFile();
        if (jarFile instanceof org.springframework.boot.loader.jar.JarFile) {
            ((org.springframework.boot.loader.jar.JarFile) jarFile).clearCache();
        }
    }

    private static class UseFastConnectionExceptionsEnumeration
            implements Enumeration<URL> {

        private final Enumeration<URL> delegate;

        UseFastConnectionExceptionsEnumeration(Enumeration<URL> delegate) {
            this.delegate = delegate;
        }

        @Override
        public boolean hasMoreElements() {
            Handler.setUseFastConnectionExceptions(true);
            try {
                return this.delegate.hasMoreElements();
            }
            finally {
                Handler.setUseFastConnectionExceptions(false);
            }

        }

        @Override
        public URL nextElement() {
            Handler.setUseFastConnectionExceptions(true);
            try {
                return this.delegate.nextElement();
            }
            finally {
                Handler.setUseFastConnectionExceptions(false);
            }
        }

    }

}

加载步骤

1. 注册定制Handler;
2. 获取当前Jar包及其嵌套Jar包URL;
3. 创建ClassLoader，进行资源加载；

使用示例代码，如下所示：

import java.net.URL;
import java.util.List;
import com.cainiao.iots.client.utils.loader.ExecutableArchiveLauncher;
import com.cainiao.iots.client.utils.loader.archive.Archive;
import com.cainiao.iots.client.utils.loader.jar.JarFile;

public class IotClientLauncher extends ExecutableArchiveLauncher {
    static final String BOOT_INF_LIB = "sar/jars/";

    private ClassLoader classLoader;

    @Override
    protected boolean isNestedArchive(Archive.Entry entry) {
        return entry.getName().startsWith(BOOT_INF_LIB);
    }

    @Override
    protected void launch(String[] args) throws Exception {
        //step1:注册handler
        JarFile.registerUrlProtocolHandler();

        //step2:获取当前Jar包及其嵌套Jar包URL
        List<Archive> archives = getClassPathArchives();
        for(int i = 0; i < archives.size(); i++){
            System.out.println("Archive url: " + archives.get(i).getUrl());
        }

        //step3:创建ClassLoader
        this.classLoader = createClassLoader(archives);
    }

    public ClassLoader getClassLoader() {
        return classLoader;
    }

    public static void main(String[] args) throws Exception {
        //1. 创建ClassLoader
        IotClientLauncher launcher = new IotClientLauncher();
        launcher.launch(args);
        ClassLoader loader = launcher.getClassLoader();

        //2. 加载jar in jar的资源
        URL url = loader.getResource("1.jpg");
        Class<?> clazz = loader.loadClass("*.*.*");
    }
}

参考：

1. https://segmentfault.com/a/1190000013532009；

原文地址：https://blog.csdn.net/yangguosb/article/details/80764971