ClassInjector.from(new ClassSource() {
    @Nullable
    @Override
    public ClassFile getClassFile(@Nonnull String className) throws IOException {
        // Find bytecode for the given class name from arbitrary sources.
        byte[] bytecode = ...
        // Return the ClassFile object with the given class name and its bytecode.
        // If no bytecode was found, return null.
        return bytecode == null ? null : new JvmClassFile(className, bytecode);
    }
}).into(getClass().getClassLoader());

// Get a Context object.
final Context context = ...
ClassInjector.from(new ClassSource() {
    @Nullable
    @Override
    public ClassFile getClassFile(@NonNull String className) throws IOException {
        // Find a zip file containing "classes.dex" with the given class name.
        String sourcePath = ...
        // If not found, return null.
        if (sourcePath == null) {
            return null;
        }
        // Construct the path for the optimized dex data.
        File cacheDir = context.getDir("dex_cache", Context.MODE_PRIVATE);
        String optimizedPath = new File(cacheDir, "classes.dex").getCanonicalPath();
        // Open the dex file.
        DexFile dexFile = DexFile.loadDex(sourcePath, optimizedPath, 0);
        // Return the ClassFile object with the given class name and its dex file.
        return new DexClassFile(className, dexFile);
    }
}).into(context.getClassLoader());