Accessing Java Classes from Mono for Android via JNI

Mono for Android let’s you write Android applications in a .NET language (for example, C#). It comes with wrappers for almost the entire Android API so building “standard” apps is easy enough. However, sometimes you may need to work with third party Java classes that aren’t part of Android itself. These classes usually come as a .jar file. Fortunately, Mono for Android provides a way to access those Java classes. Unfortunately, this way is a “little” bit tricky and documentation on this is (currently) quite limited.

This article will show some basic example of how to call a method of a Java class from Mono for Android. We will use Visual Studio 2010 in this article, as it is (in my opinion) currently superior to Mono Develop.

Update: Mono for Android v4.2 and higher provides an automatic way for doing the things described in this article. This is a lot easier than the manual way and thus the preferred way.

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Android Source Code in Eclipse

Google made developing an Android app fairly simple. Everything you need can be downloaded for free from Android’s development site. This includes the Android API, an Android emulator (for running Android apps directly on your computer), and an Eclipse plugin called ADT (Android Developer Tools). However, there is was one thing missing: the Java source code.

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Unit Testing Framework for MonoDroid

Currently MonoDroid (or “Mono for Android”) is lacking a unit testing framework. Since Xamarin (the guys behind MonoDroid) enable us to use C# on almost every platform it would be nice to be able to reuse C# unit tests written for Visual Studio on other platforms, such as Android.

This is the goal behind the MonoDroid Unit Testing Framework which can be obtained here:

Happy testing!

Overview over all testsDetails about one test method

Android and MTP (programmer’s view)

In Android 3.x, Google switched from USB Mass Storage to MTP for files stored in the “external storage”. For any device created before Android 3.x, creating a file and copying it off the device was easy:

File dir = Environment.getExternalStoragePublicDirectory("My App");
File file = new File(dir, "test.txt");
FileUtils.writeStringToFile(file, "Hello File");

This would place a file called test.txt in the directory My App on the SD card/external storage. You then would connect your Android device via USB with your computer, enable USB mass storage, and simply copy the file off the device.

USB mass storage on Android

With newer devices (like the Galaxy Nexus I’m using) that’s no longer enough because they use MTP instead of USB Mass Storage. If you’d execute the code above, the file would be created but it wouldn’t show up in the Windows Explorer (or whatever tool you’re using the view the device’s contents).

MTP on Android

To make the file show up, after closing it use the following code (API documentation):

MediaScannerConnection.scanFile(ctx, new String[] { file.getAbsolutePath() }, null, null);

After doing this the file should appear immediately in the Windows Explorer.


  • The external storage is scanned automatically when the Android device is booting (e.g. after a restart).
  • If you pass a directory (instead of a file) to scanFile(), the directory will show up as file in Window Explorer, so don’t do this.

Logging from C/C++ on Android (instead of debugging)

With the Android NDK Google lets us write C/C++ code for Android. I don’t like writing C/C++ code because it’s error-prone but sometimes there’s no other way.

Unfortunately (this is the “I don’t like this” part), debugging Android C/C++ code is terribly difficult. If you don’t have a week to get the debugging toolchain working, and if you only need some quick and temporary solution, logging may be an alternative.

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