Interacting with dynamically loaded library using DexClassLoader

Question

I am loading a jar file at runtime from SD card using DexClassLoader class

  final String libPath = Environment.getExternalStorageDirectory() +     "/test.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("org.shlublu.android.sandbox.MyClass");

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



        doSomething.invoke(myInstance);

In my jar file i am printing few logs which is working fine.Now what i am trying to do is print a Toast from jar file. While doing this the exception i am getting is java.lang.reflect.InvocationTargetException. I know why we get this exception and reason behind it is nullpointer exception at context using it while printing toast. So its not a duplicate of

What could cause java.lang.reflect.InvocationTargetException?

cause behind it is

  java.lang.NullPointerException: Attempt to invoke virtual method      'android.content.Context android.content.Context.getApplicationContext()' on a null object reference 

code in jar file is

public class MyClass extends Activity {


@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
}


public void doSomething() {

    Toast.makeText(getApplicationContext(), "MyClass: doSomething() called.", Toast.LENGTH_LONG).show();

    Log.e(MyClass.class.getName(), "MyClass: doSomething() called.");
}}

Can anyone help me to achieve this.Any help will be appreciated.

Edit: what i am trying to do is I have a library of my own which many of my clients are using...what i want is that to load my library from user's SD card..and i want to update the library when ever i want without user's knowledge and without any version updates.Library includes few interfaces,Fragments and Activities. so now i am able to load my library from sd card and can invoke basic functions.Now the main challenge is to implement interfaces from library and invoking functions having usage of context in them.

Any samples or hint involving such operations will be much helpful.

Answer 1

I see two ways of solving this, depending on what you are willing to do:

• If MyClass has to be an Activity

An Activity has to be started properly, using startActivity() or any variant. It also has to be declared in your manifest. So the following works only if all your MyClass variants have the same signature.

I'm assuming your startup code sits in an Activity. Right after having loaded classToLoad, you could do the following:

final File tmpDir = getDir("dex", 0);
final String libPath = Environment.getExternalStorageDirectory() + "/test.jar";
final DexClassLoader classloader = new DexClassLoader(libPath, tmpDir.getAbsolutePath(), null, this.getClass().getClassLoader());

try {
    final Class<Object> classToLoad = (Class<Object>) classloader.loadClass("org.shlublu.android.sandbox.MyClass");

    // CHANGED: THIS STARTS PROPERLY YOUR ACTIVITY ONCE THE CLASS LOADED
    final Intent intent = new Intent(this, classToLoad);
    startActivity(intent); 

} catch (ClassNotFoundException e) {
    // handle that Exception properly here
}

Now change doSomething() in a way it uses the underlying Context of your new Activity instead of getApplicationContext(). Then call it from MyClass.onCreate() for example, and see that it works:

public class MyClass extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        doSomething(); // CHANGED: just as an example
    }

    private void doSomething() {
        // CHANGED: now the following line uses 'this' instead of `getApplicationContext()`
        Toast.makeText(this, "MyClass: doSomething() called.", Toast.LENGTH_LONG).show(); 

        Log.d(MyClass.class.getName(), "MyClass: doSomething() called.");
    }
}

The rest - when to call doSomething(), why, etc... - depends all on what you are willing to do.

• If MyClass just has to display a Toast

No need to create another Activity in that case. doSomething() just needs to receive the proper Context to display the Toast.

Change MyClass as follows:

public class MyClass {
    private void doSomething(Context ctx) {
        Toast.makeText(ctx, "MyClass: doSomething() called.", Toast.LENGTH_LONG).show();

        Log.d(MyClass.class.getName(), "MyClass: doSomething() called.");
    }
}

And change your startup code to pass this to doSomething(), assuming it is run from an Activity:

final File tmpDir = getDir("dex", 0);
final String libPath = Environment.getExternalStorageDirectory() + "/test.jar";
final DexClassLoader classloader = new DexClassLoader(libPath, tmpDir.getAbsolutePath(), null, this.getClass().getClassLoader());

try {
    final Class<Object> classToLoad = (Class<Object>) classloader.loadClass("org.shlublu.android.sandbox.MyClass");

    // CHANGED: LOADS THE METHOD doSomething(Context). EXECUTES IT WITH this AS AN ARGUMENT
    final Class[] args = new Class[1];
    args[0] = Context.class;
    final Method doSomething = classToLoad.getMethod("doSomething", args);

    final Object myInstance  = classToLoad.newInstance();

    doSomething.invoke(myInstance, this);    
} catch (ClassNotFoundException e) {
    // handle that Exception properly here
}