我需要在运行时为java 8和java 9+加载一个jar文件(上面的注释对这两个版本都不起作用)。下面是实现它的方法(如果相关的话，使用Spring Boot 1.5.2)。

public static synchronized void loadLibrary(java.io.File jar) {
    try {            
        java.net.URL url = jar.toURI().toURL();
        java.lang.reflect.Method method = java.net.URLClassLoader.class.getDeclaredMethod("addURL", new Class[]{java.net.URL.class});
        method.setAccessible(true); /*promote the method to public access*/
        method.invoke(Thread.currentThread().getContextClassLoader(), new Object[]{url});
    } catch (Exception ex) {
        throw new RuntimeException("Cannot load library from jar file '" + jar.getAbsolutePath() + "'. Reason: " + ex.getMessage());
    }
}

下面是一个未弃用的版本。我修改了原始版本，删除了不推荐使用的功能。

/**************************************************************************************************
 * Copyright (c) 2004, Federal University of So Carlos                                           *
 *                                                                                                *
 * All rights reserved.                                                                           *
 *                                                                                                *
 * Redistribution and use in source and binary forms, with or without modification, are permitted *
 * provided that the following conditions are met:                                                *
 *                                                                                                *
 *     * Redistributions of source code must retain the above copyright notice, this list of      *
 *       conditions and the following disclaimer.                                                 *
 *     * Redistributions in binary form must reproduce the above copyright notice, this list of   *
 *     * conditions and the following disclaimer in the documentation and/or other materials      *
 *     * provided with the distribution.                                                          *
 *     * Neither the name of the Federal University of So Carlos nor the names of its            *
 *     * contributors may be used to endorse or promote products derived from this software       *
 *     * without specific prior written permission.                                               *
 *                                                                                                *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS                            *
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT                              *
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR                          *
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR                  *
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,                          *
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,                            *
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR                             *
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF                         *
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING                           *
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS                             *
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.                                   *
 **************************************************************************************************/
/*
 * Created on Oct 6, 2004
 */
package tools;

import java.io.File;
import java.io.IOException;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

/**
 * Useful class for dynamically changing the classpath, adding classes during runtime. 
 */
public class ClasspathHacker {
    /**
     * Parameters of the method to add an URL to the System classes. 
     */
    private static final Class<?>[] parameters = new Class[]{URL.class};

    /**
     * Adds a file to the classpath.
     * @param s a String pointing to the file
     * @throws IOException
     */
    public static void addFile(String s) throws IOException {
        File f = new File(s);
        addFile(f);
    }

    /**
     * Adds a file to the classpath
     * @param f the file to be added
     * @throws IOException
     */
    public static void addFile(File f) throws IOException {
        addURL(f.toURI().toURL());
    }

    /**
     * Adds the content pointed by the URL to the classpath.
     * @param u the URL pointing to the content to be added
     * @throws IOException
     */
    public static void addURL(URL u) throws IOException {
        URLClassLoader sysloader = (URLClassLoader)ClassLoader.getSystemClassLoader();
        Class<?> sysclass = URLClassLoader.class;
        try {
            Method method = sysclass.getDeclaredMethod("addURL",parameters);
            method.setAccessible(true);
            method.invoke(sysloader,new Object[]{ u }); 
        } catch (Throwable t) {
            t.printStackTrace();
            throw new IOException("Error, could not add URL to system classloader");
        }        
    }

    public static void main(String args[]) throws IOException, SecurityException, ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException, NoSuchMethodException{
        addFile("C:\\dynamicloading.jar");
        Constructor<?> cs = ClassLoader.getSystemClassLoader().loadClass("test.DymamicLoadingTest").getConstructor(String.class);
        DymamicLoadingTest instance = (DymamicLoadingTest)cs.newInstance();
        instance.test();
    }
}

困难的原因在于安全。类加载器是不可变的;您不应该在运行时随意地向它添加类。我很惊讶它能与系统类加载器一起工作。下面是如何创建你自己的子类加载器:

URLClassLoader child = new URLClassLoader(
        new URL[] {myJar.toURI().toURL()},
        this.getClass().getClassLoader()
);
Class classToLoad = Class.forName("com.MyClass", true, child);
Method method = classToLoad.getDeclaredMethod("myMethod");
Object instance = classToLoad.newInstance();
Object result = method.invoke(instance);

很痛苦，但事实就是这样。

虽然这里列出的大多数解决方案要么是hack(在JDK 9之前)难以配置(代理)，要么就是不再工作(在JDK 9之后)，但我发现没有人提到一个清晰的文档化方法，这真的很令人惊讶。

您可以创建一个自定义系统类装入器，然后您可以自由地做任何您想做的事情。不需要反射，所有类共享相同的类加载器。

当启动JVM时添加这个标志:

java -Djava.system.class.loader=com.example.MyCustomClassLoader

类加载器必须有一个接受类加载器的构造函数，必须将类加载器设置为父类加载器。构造函数将在JVM启动时调用，真正的系统类加载器将被传递，主类将由自定义加载器加载。

要添加jar，只需调用ClassLoader.getSystemClassLoader()并将其转换为您的类。

请查看此实现，以获得精心制作的类装入器。请注意，您可以将add()方法更改为public。

如果你在Android上工作，下面的代码可以工作:

String jarFile = "path/to/jarfile.jar";
DexClassLoader classLoader = new DexClassLoader(jarFile, "/data/data/" + context.getPackageName() + "/", null, getClass().getClassLoader());
Class<?> myClass = classLoader.loadClass("MyClass");

在Java 9中，URLClassLoader的答案现在会给出如下错误:

java.lang.ClassCastException: java.base/jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to java.base/java.net.URLClassLoader

这是因为所使用的类装入器发生了变化。相反，要添加到系统类装入器，您可以通过代理使用Instrumentation API。

创建代理类:

package ClassPathAgent;

import java.io.IOException;
import java.lang.instrument.Instrumentation;
import java.util.jar.JarFile;

public class ClassPathAgent {
    public static void agentmain(String args, Instrumentation instrumentation) throws IOException {
        instrumentation.appendToSystemClassLoaderSearch(new JarFile(args));
    }
}

添加meta - inf / MANIFEST。MF并将其与代理类一起放在JAR文件中:

Manifest-Version: 1.0
Agent-Class: ClassPathAgent.ClassPathAgent

运行代理:

使用bytes -buddy-agent库将代理添加到运行的JVM中:

import java.io.File;

import net.bytebuddy.agent.ByteBuddyAgent;

public class ClassPathUtil {
    private static File AGENT_JAR = new File("/path/to/agent.jar");

    public static void addJarToClassPath(File jarFile) {
        ByteBuddyAgent.attach(AGENT_JAR, String.valueOf(ProcessHandle.current().pid()), jarFile.getPath());
    }
}