Class<?> cls = null;
try {
     // 尝试第一种方法
     cls = loadClass1(mainContext, pkg, entryCls);
} catch (Exception e) {
     // 尝试第二种方法
     cls = loadClass2(mainContext, pkg, entryCls);
}
// 示例代码
Object obj = cls.newInstance();
Method method = cls.getDeclaredMethod("someMethod");
method.invoke(obj);

// 第一种加载方法
private Class<?> loadClass1(Context mainContext, String pkg, String entryCls) throws Exception {
    Context pluginContext = mainContext.createPackageContext(pkg, Context.CONTEXT_INCLUDE_CODE | Context.CONTEXT_IGNORE_SECURITY);
    ClassLoader loader = pluginContext.getClassLoader();
    return loader.loadClass(entryCls); 
}

// 第二种加载方法
private Class<?> loadClass2(Context mainContext, String pkg, String entryCls) throws Exception {
    Context pluginContext = mainContext.createPackageContext(pkg, Context.CONTEXT_INCLUDE_CODE | Context.CONTEXT_IGNORE_SECURITY);
    String path = generatePluginDexPath(mainContext, pkg);
    ensureFileExist(pluginContext, pkg, path);
    // cacheDir必须是主程序的私有目录，否则DexClassLoader可能会拒绝加载
    String cacheDir = mainContext.getApplicationInfo().dataDir;
    ClassLoader parentLoader = pluginContext.getClassLoader();
    DexClassLoader loader = new DexClassLoader(path, cacheDir, null, parentLoader);
    return loader.loadClass(entryCls);
}

// 获取程序版本号
private int getVersionCode(Context context, String pkg) {
    PackageInfo info = null;
    int versionCode = 0;
    try {
         info = context.getPackageManager().getPackageInfo(pkg, PackageManager.GET_ACTIVITIES);
         versionCode = info.versionCode;
    } catch (Exception e) {} 
    return versionCode;
}

// 获取插件二进制代码的存储位置，注意做好版本控制；路径必须是以.dex结束，否则加载会出问题
private String generatePluginDexPath(Context context, String pkg) {
    int version = getVersionCode(context, pkg);
    String path = getMyAppPath() + ".classes/" + pkg + version + ".dex";
    return path;
}

// 主程序在SD卡上的数据目录
private String getMyAppPath() {
    return Environment.getExternalStorageDirectory().getAbsolutePath() + "/MyApp/";
}