Organizing your Java files in a C++ Qt project


If you develop an application for the Android platform with Qt, you may want to code some stuff in Java. In this case, you may use Android Studio in addition to Qt Creator. If you do nothing and use the default configuration generated by Qt Creator, your .java files will be put in the Qt directory and not in the directory of your project. Cross your fingers if you upgrade your version of Qt… or take your destiny in your own hands and configure your future now.

Show me the damage done

If you do like screenshots, enjoy.

I assume that you have created a new project in Qt Creator, and that you have copied the template files to your directory structure. You can now launch Android Studio to work on the Java side, it’s quite more convenient than using Qt Creator for this part, not mentionning that you may want to use the debugger1 with Android Studio GUI.

Launch Android Studio and select Import project.
Select your new Qt project.
Ask Android Studio for help in creating a new java class.
Handsomedroid Studio helps you. Create a new package.

And you’re done. It’s so good to feel like a pro :

Feeling like a pro.

Let’s have a look at your Qt directory…

Oooops (Object Oriented oops) where is my file ?

Hem, it looks like your java file is not in the best place one can consider. If you use a version control system, you probably have not had the file added in.


Let’s fix it

It took me time to figure out how to fix it.2 It’s so simple I have to fill this part with unecessary words to make it bigger. You only need to create a src directory under the android directory lying in your project directory. It looks like this :

Everything is in the right place.

If you try creating a class now, Android Studio gently asks you where should it put it :

You can now use your project directory.

And you can see the slight difference in Android Studio :

The org class hierarchy is no more merged under a single org.

Wasting some more time

If you have more knowledge on this subject, it would be very kind of you to leave a comment in order to kill the voodoo in this matter. Here is what I believe.

Android Studio uses and is tied to Gradle. Gradle uses the notion of source set. In a source set, you can put several source directories. Qt defines template files 3 in order to build an Android apk. These template files include a build.gradle script. This template Gradle script actually adds, in the main source set, the Qt directory.

    sourceSets {
        main {
            manifest.srcFile 'AndroidManifest.xml'
            java.srcDirs = [qt5AndroidDir + '/src', 'src', 'java']
            aidl.srcDirs = [qt5AndroidDir + '/src', 'srcqt', 'aidl']
            res.srcDirs = [qt5AndroidDir + '/res', 'res']
            resources.srcDirs = ['src']
            renderscript.srcDirs = ['src']
            assets.srcDirs = ['assets']
            jniLibs.srcDirs = ['libs']

I’ve not found where the default main source set is specified. It appears that Gradles finds sources that are inside the src directory if it exist in the same directory than, that is QtProjectPath/android/src by default when creating templates in Qt Creator.

Consequently, you have nothing to do if you put your Java files in this directory. If you want to add your Java files somewhere else, you just have to add source directories to your source set using one of the documented syntax.

You can even define several source sets, but it is beyond the scope of this article.

  1. But if you are this kind of programmer, you know only the concept of debugger, and you have no experience in debugging because you never need to, because you never introduce a bug in a program : it’s ineffecient, why would you do that ? Even opening someone else’s project make bugs fade away in a whisper of fear.
  2. I’ve hiked in Qt Creator documentation, in Android Studio documentation, in IntelliJ Idea documentation, in Gradle documentation, on Bogdan Vatra’s blog and found the naked truth : we’re alone in the universe. So I decided to act as a responsible individual, and consequently did a lot of stupid things. So poped up the solution. It’s just a note, for you to know. May prooves itself useful. In some way.
  3. This configuration is copied from your kit when you click on Create Templates (well, my understanding is that the template files copied in your project directory are used as templates by Qt Creator when building the apk. At least Manifest.xml is such a template.). I’m not sure about the correct way to use several kits in a project if you must have different values in the files copied from the templates…

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