下面的解决方案有点笨拙，因为它使用反射来绕过封装，但它工作得完美无缺:

File file = ...
URL url = file.toURI().toURL();

URLClassLoader classLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);
method.invoke(classLoader, url);

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。

请看看我开始的这个项目:proxy-object lib

这个库将从文件系统或任何其他位置加载jar。它将为jar专用一个类装入器，以确保没有库冲突。用户将能够从加载的jar中创建任何对象，并在其上调用任何方法。 这个库被设计用来从支持Java 7的代码库中加载用Java 8编译的jar。

创建一个对象:

    File libDir = new File("path/to/jar");

    ProxyCallerInterface caller = ObjectBuilder.builder()
            .setClassName("net.proxy.lib.test.LibClass")
            .setArtifact(DirArtifact.builder()
                    .withClazz(ObjectBuilderTest.class)
                    .withVersionInfo(newVersionInfo(libDir))
                    .build())
            .build();
    String version = caller.call("getLibVersion").asString();

ObjectBuilder支持工厂方法、调用静态函数和回调接口实现。 我将在自述页面上发布更多的例子。

jodonnell提出的解决方案很好，但应该稍加改进。我用这篇文章成功地开发了我的应用程序。

分配当前线程

首先我们要加上

Thread.currentThread().setContextClassLoader(classLoader);

否则你将无法加载资源(如spring/context.xml)存储到jar。

不包括

你的罐子放入父类装入器中，否则你将无法理解谁装入了什么。

请参见使用URLClassLoader重新加载jar的问题

然而，OSGi框架仍然是最好的方法。

下面的解决方案有点笨拙，因为它使用反射来绕过封装，但它工作得完美无缺:

File file = ...
URL url = file.toURI().toURL();

URLClassLoader classLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);
method.invoke(classLoader, url);

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();
    }
}