Android: Dynamically loading classes

This article will explain how to dynamically load a class on Android, how it differs from desktop Java, and how to prepare files for dynamic loading.



Here is a complete program to demonstrate class loading in standard desktop Java:
import java.net.URL; import java.net.URLClassLoader; import java.lang.reflect.Method; public class Main { private void load(String filename, String classname) { try { URL[] urls = { new URL(filename) }; ClassLoader parent = getClass().getClassLoader(); ClassLoader loader = new URLClassLoader(urls, parent); Class<?> c = loader.loadClass(classname); Object o = c.newInstance(); Method m = c.getMethod("func"); m.invoke(o); } catch (Exception e) { System.err.print(String.format("DLL failed: %s: %s", e.getClass().getName(), e.getMessage())); } } public static void main(String [ ] args) { Main main = new Main(); main.load("file:/tmp/Foo.jar", "Foo"); } } This will load class Foo from /tmp/Foo.jar, create an instance of that class, and finally call the member function func().
Note:

• Loading of a regular file is done via URLClassLoader.
• The file name must be prefixed by "file:".
• The statements in the try clause throws a number of different exceptions, that are all caught by the catch-all Exception.

Test this code:$ javac *.java $ cp Foo.jar /tmp $ java Main Dynamic class loading in Android is slightly different:

• The standard class loaders don't seem to work.
• The class loader to use is dalvik.system.DexClassLoader.
• The classes must be put into a .dex file that must be embedded in a .jar file. Done using the little known utility dx.

The application does not change much: package se.sdu; import java.lang.reflect.Method; import android.app.Activity; import android.os.Bundle; import android.util.Log; import dalvik.system.DexClassLoader; public class Main extends Activity { public void dload(String filename, String classname) { try { Context ctx = getApplicationContext(); String dex_dir = ctx.getDir("dex", 0).getAbsolutePath(); ClassLoader parent = getClass().getClassLoader(); DexClassLoader loader = new DexClassLoader(filename, dex_dir, null, parent); Class c = loader.loadClass(classname); Object o = c.newInstance(); Method m = c.getMethod("func"); m.invoke(o); } catch (Exception e) { Log.e("se.sdu", String.format("DLL failed: %s: %s", e.getClass().getName(), e.getMessage())); } } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); dload("/mnt/sdcard/tmp/Foo.jar", "Foo"); } }
Note

• dex_dir is as a cache for optimized classes.
• The file name must not be prefixed by "file:".

This single-file application is built just like any other Android application. No special configuration is necessary.
Now to the dynamically loaded class:
import android.util.Log; public class Foo { public static void func() { Log.d("se.sdu", "Hello from Foo.func()"); } } To compile the dynamically loadable class, and build and push the jar file containing it:
javac -classpath /path/to/android.jar Foo.java dx --dex --output Foo.jar Foo.class adb push Foo.jar /mnt/sdcard/tmp/. The option --dex tell dx to generate a .dex file, and the file name extension .jar to put the .dex file into a .jar file.

That's all.