Is there any way to make an Android application to download and use a Java library at runtime?

Here is an example:

Imagine that the application needs to make some calculations depending on the input values. The application asks for these input values and then checks if the required Classes or Methods are available.

If not, it connects to a server, downloads the needed library, and loads it at runtime to calls the required methods using reflection techniques. The implementation could change depending on various criteria such as the user who is downloading the library.

Solution 1

Sorry, I'm late and the question has already an accepted answer, but yes, you can download and execute external libraries. Here is the way I did:

I was wondering whether this was feasible so I wrote the following class:


import android.util.Log;

public class MyClass {
    public MyClass() {
        Log.d(MyClass.class.getName(), "MyClass: constructor called.");

    public void doSomething() {
        Log.d(MyClass.class.getName(), "MyClass: doSomething() called.");

And I packaged it in a DEX file that I saved on my device's SD card as /sdcard/shlublu.jar.

Then I wrote the "stupid program" below, after having removed MyClass from my Eclipse project and cleaned it:

public class Main extends Activity {

    protected void onCreate(Bundle savedInstanceState) {

        try {
            final String libPath = Environment.getExternalStorageDirectory() + "/shlublu.jar";
            final File tmpDir = getDir("dex", 0);

            final DexClassLoader classloader = new DexClassLoader(libPath, tmpDir.getAbsolutePath(), null, this.getClass().getClassLoader());
            final Class<Object> classToLoad = (Class<Object>) classloader.loadClass("");

            final Object myInstance  = classToLoad.newInstance();
            final Method doSomething = classToLoad.getMethod("doSomething");


        } catch (Exception e) {

It basically loads the class MyClass that way:

  • create a DexClassLoader

  • use it to extract the class MyClass from "/sdcard/shlublu.jar"

  • and store this class to the application's "dex" private directory (internal storage of the phone).

Then, it creates an instance of MyClass and invokes doSomething() on the created instance.

And it works... I see the traces defined in MyClass in my LogCat:

I've tried on both an emulator 2.1 and on my physical HTC cellphone (which is running Android 2.2 and which is NOT rooted).

This means you can create external DEX files for the application to download and execute them. Here it was made the hard way (ugly Object casts, Method.invoke() ugly calls...), but it must be possible to play with Interfaces to make something cleaner.

Wow. I'm the first surprised. I was expecting a SecurityException.

Some facts to help investigating more:

  • My DEX shlublu.jar was signed, but not my app
  • My app was executed from Eclipse / USB connection. So this is an unsigned APK compiled in DEBUG mode

Solution 2

Shlublu's anwser is really nice. Some small things though that would help a beginner:

  • for library file "MyClass" make a separate Android Application project which has the MyClass file as only file in the src folder (other stuff, like, manifest, res, etc. should also be there)
  • in library project manifest make sure you have: <application android:icon="@drawable/icon" android:label="@string/app_name"> <activity android:name=".NotExecutable" android:label="@string/app_name"> </activity> </application> (".NotExecutable" is not a reserved word. It is just that I had to put something here)

  • For making the .dex file, just run the library project as android application (for the compiling) and locate .apk file from the bin folder of the project.

  • Copy the .apk file to your phone and rename it as shlublu.jar file (an APK is actually a specialization of a jar, though)

Other steps are the same as described by Shlublu.

  • Big thanks to Shlublu for cooperation.

Solution 3

Technically should work but what about Google rules? From:

An app distributed via Google Play may not modify, replace or update itself using any method other than Google Plays update mechanism. Likewise, an app may not download executable code (e.g. dex, JAR, .so files) from a source other than Google Play. This restriction does not apply to code that runs in a virtual machine and has limited access to Android APIs (such as JavaScript in a WebView or browser).

Solution 4

I am not sure if you can achieve this by dynamically loading java code. May be you can try embedding a script engine your code like rhino which can execute java scripts which can be dynamically downloaded and updated.

Solution 5

sure, it is possible. apk which is not installed can be invoked by host android application.generally,resolve resource and activity's lifecircle,then,can load jar or apk dynamically. detail,please refer to my open source research on github:

also,DexClassLoader and reflection is needed, now look at some key code:

 * Load a apk. Before start a plugin Activity, we should do this first.<br/>
 * NOTE : will only be called by host apk.
 * @param dexPath
public DLPluginPackage loadApk(String dexPath) {
    // when loadApk is called by host apk, we assume that plugin is invoked by host.
    mFrom = DLConstants.FROM_EXTERNAL;

    PackageInfo packageInfo = mContext.getPackageManager().
            getPackageArchiveInfo(dexPath, PackageManager.GET_ACTIVITIES);
    if (packageInfo == null)
        return null;

    final String packageName = packageInfo.packageName;
    DLPluginPackage pluginPackage = mPackagesHolder.get(packageName);
    if (pluginPackage == null) {
        DexClassLoader dexClassLoader = createDexClassLoader(dexPath);
        AssetManager assetManager = createAssetManager(dexPath);
        Resources resources = createResources(assetManager);
        pluginPackage = new DLPluginPackage(packageName, dexPath, dexClassLoader, assetManager,
                resources, packageInfo);
        mPackagesHolder.put(packageName, pluginPackage);
    return pluginPackage;

your demands is only partly of function in the open source project mentioned at the begining.

Solution 6

If you're keeping your .DEX files in external memory on the phone, such as the SD card (not recommended! Any app with the same permissions can easily overwrite your class and perform a code injection attack) make sure you've given the app permission to read external memory. The exception that gets thrown if this is the case is 'ClassNotFound' which is quite misleading, put something like the following in your manifest (consult Google for most up to date version).

<manifest ...>

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"
                 android:maxSdkVersion="18" />

Solution 7

I think @Shlublu answer is correct but i just want to highlight some key points.

  1. We can load any classes from external jar and apk file.
  2. In Any way, we can load Activity from external jar but we can not start it because of the context concept.
  3. To load the UI from external jar we can use fragment. Create the instance of the fragment and embedded it in the Activity. But make sure fragment creates the UI dynamically as given below.

    public class MyFragment extends Fragment {
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup 
      container, @Nullable Bundle savedInstanceState)
      super.onCreateView(inflater, container, savedInstanceState);
      LinearLayout layout = new LinearLayout(getActivity());
    Button button = new Button(getActivity());
    button.setText("Invoke host method");
    layout.addView(button, LinearLayout.LayoutParams.MATCH_PARENT,
    return layout;