Update

This works both on Dalvik and ART: new DexClassLoader(jarredDex.getAbsolutePath(), context.getDir("outdex", Context.MODE_PRIVATE).getAbsolutePath(), null, context.getClassLoader()); where jarredDex is a jar-file with classes.dex. Jar can be obtained by running dx --dex --output=filename.jar your/classes/dir.

Original answer

I've took a code sample from this article. But ART uses PathClassLoader instead of Dalvik's DexClassLoader. This code is tested on emulator with Android 6 and on Xiaomi with Android 5.1 and works fine:

// Before the secondary dex file can be processed by the DexClassLoader,// it has to be first copied from asset resource to a storage location.File dexInternalStoragePath = new File(getDir("dex", Context.MODE_PRIVATE), SECONDARY_DEX_NAME);try (BufferedInputStream bis = new BufferedInputStream(getAssets().open(SECONDARY_DEX_NAME));     OutputStream dexWriter = new BufferedOutputStream(new FileOutputStream(dexInternalStoragePath))) {    byte[] buf = new byte[BUF_SIZE];    int len;    while((len = bis.read(buf, 0, BUF_SIZE)) > 0) {        dexWriter.write(buf, 0, len);    }} catch (IOException e) {    throw new RuntimeException(e);}try {    PathClassLoader loader = new PathClassLoader(dexInternalStoragePath.getAbsolutePath(), getClassLoader());    Class<?> toasterClass = loader.loadClass("my.package.ToasterImpl");    Toaster toaster = (Toaster) toasterClass.newInstance();    toaster.show(this, "Success!");} catch (ReflectiveOperationException e) {    throw new RuntimeException(e);}

It is the problem with making dex file. Change the command as below then will work in all versions of android.

dx.bat --dex --output path/output.jar  path/input.jar

The above method in windows will generate a .jar file with classes.dex file inside that jar file. The DexClassLoader is searching for classes.dex file inside, that is why it is throwing ClassNotFoundException