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");

我个人认为java.util.ServiceLoader做得很好。你可以在这里得到一个例子。

以防将来有人搜索这个，这种方法对我来说适用于OpenJDK 13.0.2。

我有许多需要在运行时动态实例化的类，每个类都可能具有不同的类路径。

在这段代码中，我已经有了一个名为pack的对象，它保存了一些关于我试图加载的类的元数据。getObjectFile()方法返回类的类文件的位置。getObjectRootPath()方法返回bin/目录的路径，其中包含我试图实例化的类的类文件。getLibPath()方法返回一个目录的路径，该目录包含构成该类所在模块类路径的jar文件。

File object = new File(pack.getObjectFile()).getAbsoluteFile();
Object packObject;
try {
    URLClassLoader classloader;

    List<URL> classpath = new ArrayList<>();
    classpath.add(new File(pack.getObjectRootPath()).toURI().toURL());
    for (File jar : FileUtils.listFiles(new File(pack.getLibPath()), new String[] {"jar"}, true)) {
        classpath.add(jar.toURI().toURL());
    }
    classloader = new URLClassLoader(classpath.toArray(new URL[] {}));

    Class<?> clazz = classloader.loadClass(object.getName());
    packObject = clazz.getDeclaredConstructor().newInstance();

} catch (Exception e) {
    e.printStackTrace();
    throw e;
}
return packObject;

我以前使用Maven依赖项:org.xeustechnologies:jcl-core:2.8来做这件事，但在JDK 1.8之后，它有时会冻结，并且永远不会返回在Reference::waitForReferencePendingList()中卡住“等待引用”。

我还保留了类加载器的映射，以便在我试图实例化的类与我已经实例化的类在同一个模块中时可以重用它们，我建议这样做。

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();
    }
}

JCL类装入器框架怎么样?我必须承认，我还没有用过，但它看起来很有前途。

使用的例子:

JarClassLoader jcl = new JarClassLoader();
jcl.add("myjar.jar"); // Load jar file  
jcl.add(new URL("http://myserver.com/myjar.jar")); // Load jar from a URL
jcl.add(new FileInputStream("myotherjar.jar")); // Load jar file from stream
jcl.add("myclassfolder/"); // Load class folder  
jcl.add("myjarlib/"); // Recursively load all jar files in the folder/sub-folder(s)

JclObjectFactory factory = JclObjectFactory.getInstance();
// Create object of loaded class  
Object obj = factory.create(jcl, "mypackage.MyClass");

我找到的最好的是org.apache.xbean.classloader.JarFileClassLoader，它是XBean项目的一部分。

下面是我过去使用的一个简短方法，用于从特定目录中的所有lib文件创建一个类装入器

public void initialize(String libDir) throws Exception {
    File dependencyDirectory = new File(libDir);
    File[] files = dependencyDirectory.listFiles();
    ArrayList<URL> urls = new ArrayList<URL>();
    for (int i = 0; i < files.length; i++) {
        if (files[i].getName().endsWith(".jar")) {
        urls.add(files[i].toURL());
        //urls.add(files[i].toURI().toURL());
        }
    }
    classLoader = new JarFileClassLoader("Scheduler CL" + System.currentTimeMillis(), 
        urls.toArray(new URL[urls.size()]), 
        GFClassLoader.class.getClassLoader());
}

然后使用类加载器，只需执行:

classLoader.loadClass(name);