accessing to classes of app from dex file by classloader

Question

I have a app that recieve a dex file from server then save it on sdcard and load it for

doing some functions.I am loading a my app's class from my dex file as following

dex file

public class Main  {


    public void onCreate() 
    {
        System.out.print("------------onCreate------------");
        try {
             final ClassLoader classloader = ClassLoader.getSystemClassLoader();
            final Class<Object> classToLoad = (Class<Object>) classloader.loadClass("com.example.myapp.M");
            final Object myInstance  = classToLoad.newInstance();
            final Method doSomething = classToLoad.getMethod("doSomething");
            doSomething.invoke(myInstance);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

app code

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        try {
            final String libPath = Environment.getExternalStorageDirectory() + "/Lib/lib.apk";
            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("com.example.lib.Main");
            final Object myInstance  = classToLoad.newInstance();
            final Method doSomething = classToLoad.getMethod("onCreate");
            doSomething.invoke(myInstance);
            } catch (Exception e) {
            e.printStackTrace();
        }
    }

M class in my app

public class M {
public void doSomething()
{
    Log.e("doSomething", "--------------------doSomething----");
}
}

but it have me following error.what is wrong in my code

11-19 08:24:12.718: W/System.err(10867): java.lang.ClassNotFoundException: Didn't find class "com.example.myapp2.M" on path: .
11-19 08:24:12.738: W/System.err(10867):    at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:65)
11-19 08:24:12.747: W/System.err(10867):    at java.lang.ClassLoader.loadClass(ClassLoader.java:501)
11-19 08:24:12.758: W/System.err(10867):    at java.lang.ClassLoader.loadClass(ClassLoader.java:461)
11-19 08:24:12.758: W/System.err(10867):    at com.example.lib.Main.onCreate(Main.java:15)
11-19 08:24:12.768: W/System.err(10867):    at java.lang.reflect.Method.invokeNative(Native Method)
11-19 08:24:12.788: W/System.err(10867):    at java.lang.reflect.Method.invoke(Method.java:511)
11-19 08:24:12.788: W/System.err(10867):    at com.example.myapp2.MainActivity.onCreate(MainActivity.java:28)
11-19 08:24:12.798: W/System.err(10867):    at android.app.Activity.performCreate(Activity.java:5104)
11-19 08:24:12.818: W/System.err(10867):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
11-19 08:24:12.828: W/System.err(10867):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
11-19 08:24:12.828: W/System.err(10867):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
11-19 08:24:12.828: W/System.err(10867):    at android.app.ActivityThread.access$600(ActivityThread.java:141)
11-19 08:24:12.828: W/System.err(10867):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
11-19 08:24:12.828: W/System.err(10867):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-19 08:24:12.838: W/System.err(10867):    at android.os.Looper.loop(Looper.java:137)
11-19 08:24:12.847: W/System.err(10867):    at android.app.ActivityThread.main(ActivityThread.java:5041)
11-19 08:24:12.847: W/System.err(10867):    at java.lang.reflect.Method.invokeNative(Native Method)
11-19 08:24:12.847: W/System.err(10867):    at java.lang.reflect.Method.invoke(Method.java:511)
11-19 08:24:12.857: W/System.err(10867):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
11-19 08:24:12.898: W/System.err(10867):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
11-19 08:24:12.898: W/System.err(10867):    at dalvik.system.NativeStart.main(Native Method)

What is ambiguous to you in message `ClassNotFoundException: Didn't find class on path .` — Marek Sebera, Nov 19 '13 at 09:46
"path".can I access to classes of my app from dex file by classloader.if yes how specify path for accessing? — zohreh, Nov 19 '13 at 09:49

score 1 · Answer 1 · edited May 23 '17 at 10:28

1

See if you're following these steps.

Do you have the library file as an APK or a JAR containing (Classes.dex) file on the SD Card?
Copy the APK file into a temporary folder into your application director /data/data/your.app.name/tmp. This is important as for security reasons Android doesn't let you load something off the SD card.
Use DexClassLoader as described here -> How to load a Java class dynamically on android/dalvik?
Use this classloader instance for loading any dynamic class files.

If you have more than 1 JAR files then merge them into a single DEX file/APK.

To convert your Java JAR file or classes into DEX you need to use the Android SDK's dx.bat. Here's a sample ANT script to do. This can also be done via Command line too with similar arguments.

<exec executable="dx.bat">
    <arg value="--dex" />
    <arg value="--output=${target.directory.fullpath}/${apk.name}" />
    <arg value="--positions=lines" />
    <arg path="${classes.directory.full.path}" />
</exec>

edited May 23 '17 at 10:28

Community

1
1

answered Nov 19 '13 at 10:02

Swaroop

908
1
12
25

can i access to my app's classes from my jar file?can you explain how I can do it? – zohreh Nov 19 '13 at 10:06
I want from a my jar file's class call a method of my app – zohreh Nov 19 '13 at 10:08
Yes your plugged-in JAR (DEX format) files can call the methods of your application without explicitly doing anything as it must act as a Parent Class Loader. – Swaroop Nov 19 '13 at 10:28
See the constructor for `DexClassLoader` where you set the parent class loader as one of the input. [DexClassLoader](http://developer.android.com/reference/dalvik/system/DexClassLoader.html). You can get the Parent Class Loader using activity.getClassLoader() method to pass into the `DexClassLoader()` constructor – Swaroop Nov 19 '13 at 10:31
i used above code in jar file and i get that error again.what is path in error? – zohreh Nov 19 '13 at 10:45

zohreh · Accepted Answer · 2013-11-21T09:07:06.637

1

I found my answer I need change

final Class<Object> classToLoad = (Class<Object>)classloader.loadClass("com.example.myapp.M");

to

final Class<Object> classToLoad = (Class<Object>)Class.forName("com.example.myapp.M");

in my dex file

edited Nov 21 '13 at 09:07

answered Nov 19 '13 at 19:54

zohreh

1,055
1
9
26

its strange ho wthis worked for you.You are not mentioning your .jar file path anywhere... – Harry Sharma Oct 16 '15 at 12:33
So if you do this, does it then load all the other classes that com.example.myapp.M needs as well (that aren't already loaded)? – LarsH Aug 01 '16 at 01:33
Zohreh did you load class files from the SD card? Or some other External Storage Directory? I'm curious because @Swaroop said you couldn't load them from the SD card. – LarsH Aug 26 '16 at 21:41

score 0 · Answer 3 · answered May 24 '17 at 05:32

@zohreh Replace the below line

final DexClassLoader classloader = new DexClassLoader(libPath, tmpDir.getAbsolutePath(), null, this.getClass().getClassLoader());

to

final DexClassLoader classloader = new DexClassLoader(libPath, tmpDir.getAbsolutePath(), null, ClassLoader.getSystemClassLoader());

score 0 · Answer 4 · answered Nov 14 '21 at 21:29

The listed answers didn't work for me. However, getting the ClassLoader via Thread.currentThread().getContextClassLoader() turned out to work:

final Class<Object> classToLoad = (Class<Object>) Class.forName("com.example.myapp.M", true, Thread.currentThread().getContextClassLoader());

accessing to classes of app from dex file by classloader

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