如何在运行时从下载的jar文件中加载未知类？

要从实现某个接口的jar文件动态加载类,但事先并不知道哪个类和jar文件本身没有指定任何默认的"插件"类,您可以遍历下载的jar和获取jar中包含的类列表,如下所示:

    /**
     * Scans a JAR file for .class-files and returns a {@link List} containing
     * the full name of found classes (in the following form:
     * packageName.className)
     *
     * @param file
     * JAR-file which should be searched for .class-files
     * @return Returns all found class-files with their full-name as a List of
     *         Strings
     * @throws IOException If during processing of the Jar-file an error occurred
     * @throws IllegalArgumentException If either the provided file is null, does 
     *                                  not exist or is no Jar file 
     */
    public List<String> scanJarFileForClasses(File file) throws IOException, IllegalArgumentException
    {
            if (file == null || !file.exists())
                    throw new IllegalArgumentException("Invalid jar-file to scan provided");
            if (file.getName().endsWith(".jar"))
            {
                    List<String> foundClasses = new ArrayList<String>();
                    try (JarFile jarFile = new JarFile(file))
                    {
                            Enumeration<JarEntry> entries = jarFile.entries();
                            while (entries.hasMoreElements())
                            {
                                    JarEntry entry = entries.nextElement();
                                    if (entry.getName().endsWith(".class"))
                                    {
                                            String name = entry.getName();
                                            name = name.substring(0,name.lastIndexOf(".class"));
                                            if (name.indexOf("/")!= -1)
                                                    name = name.replaceAll("/", ".");
                                            if (name.indexOf("\\")!= -1)
                                                    name = name.replaceAll("\\", ".");
                                            foundClasses.add(name);
                                    }
                            }
                    }
                    return foundClasses;
            }
            throw new IllegalArgumentException("No jar-file provided");
    }

一旦知道了包含在jar文件中的类,就需要加载每个类并检查它们是否实现了所需的接口,如下所示:

    /**
     * <p>
     * Looks inside a jar file and looks for implementing classes of the provided interface.
     * </p>
     *
     * @param file
     * The Jar-File containing the classes to scan for implementation of the given interface
     * @param iface
     * The interface classes have to implement
     * @param loader
     * The class loader the implementing classes got loaded with
     * @return A {@link List} of implementing classes for the provided interface
     * inside jar files of the <em>ClassFinder</em>s class path
     *
     * @throws Exception If during processing of the Jar-file an error occurred
     */
    public List<Class<?>> findImplementingClassesInJarFile(File file, Class<?> iface, ClassLoader loader) throws Exception
    {
        List<Class<?>> implementingClasses = new ArrayList<Class<?>>();
        // scan the jar file for all included classes
        for (String classFile : scanJarFileForClasses(file))
        {
            Class<?> clazz;
            try
            {
                // now try to load the class
                if (loader == null)
                    clazz = Class.forName(classFile);
                else
                    clazz = Class.forName(classFile, true, loader);

                // and check if the class implements the provided interface
                if (iface.isAssignableFrom(clazz) && !clazz.equals(iface))
                    implementingClasses.add(clazz);
            }
            catch (ClassNotFoundException e)
            {
                e.printStackTrace();
            }
        }
        return implementingClasses;
    }

因为您现在可以收集某个接口的所有实现,您可以通过简单的初始化新实例

public void executeImplementationsOfAInJarFile(File downloadedJarFile)
{
    If (downloadedJarFile == null || !downloadedJarFile.exists())
        throw new IllegalArgumentException("Invalid jar file provided");

    URL downloadURL = downloadedJarFile.toURI().toURL();
    URL[] downloadURLs = new URL[] { downloadURL };
    URLClassLoader loader = URLClassLoader.newInstance(downloadURLs, getClass().getClassLoader());
    try
    {
        List<Class<?>> implementingClasses = findImplementingClassesInJarFile(downloadedJarFile, A.class, loader);
        for (Class<?> clazz : implementingClasses)
        {
            // assume there is a public default constructor available
            A instance = clazz.newInstance();
            // ... do whatever you like here
        }
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }
}

请注意,此示例假定A是接口.如果在Jar-File中找不到实现类,则jar文件将由类加载器加载,但不会发生对象的实例化.

另请注意,提供父类加载器始终是一种好习惯 - 尤其是使用URLClassLoader.否则,可能会发生某些未包含在Jar-File中的类可能会丢失,因此您将ClassNotFoundException尝试访问它们.这是由于类加载器使用的委托机制,它首先询问其父级是否知道所需类的类定义.如果是,则该类将由父级加载; 如果没有,该类将由创建的URLClassLoader加载.

请记住,使用不同的ClassLoader可以多次加载同一个类(peer-classloaders).但是虽然类的名称和字节可能相同,但由于使用了不同的类加载器实例,因此类不兼容 - 因此尝试将类加载器A加载的实例强制转换为类加载器B加载的类型将失败.

@Edit:修改代码以避免返回空值,而是抛出或多或少的适当异常.@ Edit2:因为我无法接受代码审查建议,所以我将评论直接编辑到帖子中