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Use one "updatable" library in different Android Studio projects [duplicate]

I have started working on a project where I will need to share a bunch of Java classes across a bunch of apps. In Eclipse it was possible to create one project with all such classes and use it as a library in a workspace with all your dependent projects, but in Android Studio it doesn't seem possible to do so (At least not easily).
I have been reading a bunch of posts and a lot of them suggest setting up a library project, generating an aar file and then using that in my projects. But, as I understand it, this will make my library open-source (Am I right?), which I don't want. I am doing this for a client and I want the code base to be private.
Also, I know that a module can be imported into a new project. But this creates a COPY of the original module. This is not what I want at all. I don't wanna maintain multiple copies of the same classes, which completely defeats the purpose of 'code sharing'.
Is there any good way of achieving what I am looking for? Any help is appreciated.

You have a couple different options.
One option is to maintain your libraries as separate projects and compile them to an archive format, such as JAR or AAR; JAR files are for pure Java libraries, and AAR is for Android libraries (which contain code that accesses Android APIs and/or has Android resources). As was pointed out in the comments, AAR doesn't force you to publish your code to the world any more than JAR files would; it's just an archive file format whose files can be local to your machine or your organization.
With that archive file in hand, you can include it in other projects. If you're part of a multi-developer organization, you may find it convenient to use a repository manager to publish and maintain those libraries within your organization, and you can use Maven coordinate-style specs to include libraries in your projects, which you don't have to manually copy over to your development machine.
The disadvantage of this approach is that it makes it a little harder to make changes to those libraries: you need to load up the project, make changes, build an archive, and distribute the archive.
The other approach is to keep the library as a source module like you did in Eclipse. You observed that Android Studio will make a copy of the module if you import it via UI, but if you bypass the UI and modify the build scripts directly, you can do what you want, which is to use the module in-place and share a single copy among multiple projects. To do this, work in your settings.gradle file and add this:
include ':module_name'
project(':module_name').projectDir = new File(settingsDir, '../relative/path/to/module')
I would strongly encourage you to not use a pure relative path here; in this example, the path is anchored to the settingsDir variable supplied by Gradle, which is defined to be the directory where settings.gradle is found. If you use a pure relative path (i.e isn't anchored to anything), you're dependent on the working directory being the same in all environments where the build file is run (command line vs. Android Studio vs. CI server), which isn't a good thing to assume.

You need to think in the eclipse projects as Android Studio/IntelliJ Idea modules. Then, you can generate android (or java) libraries and then include them in your project.
To mark an Android Studio module as a library you can go to File -> Project Structure -> Facets and there click on Library Module

I was in same situation as you, and i founded an approach using git.
Steps to do, to have library:
Create project in Android Studio.
Create android library module in that project.
In that library module create git repository.
Add modulename.iml in .gitignore file
Use GitHub or Bitbucket for private cloud repository. and push your library to it.
Create new android library model in any project that you want.
Close Android Studio (not sure is that mandatory).
Using explorer go to your created module folder.
Remove all data in it, except modulename.iml.
Clone your library from "GitHub" into it.
That's all.
Now you are able to use library in multiple project whether you are at home or at work. Just after finishing you work not forget to push library changes. And after opening new one pull them.
I think you can automate this thing somehow.
The benefit is that you don't need to build any jar, or aar.

You can certainly create and use a library without making it open source or available to others.
First, you don't need to make it an aar unless it contains Resources.
If it's just plain classes, you can just make it a .jar file.
Either way, the easiest way to share these libraries (either aar or jar) is to set up your own repository. Nexus and Artifactory are the two most common repository managers.
You keep the library in its own project, and then publish it to your own, in-house repository.
Then, projects that need to use the library are configured (in gradle) to use the in-house repository, and get the library from it.

What are Pros and Cons of using Library project(In Eclipse)in my existing Android project?

I am working on a Android app in which I would like to add plug and play module functionality ,Say I have Two android project
[A] An app for capturing a image using camera and storing it in memory
[B]Enabling map and locate current location .
now I want to add add this functionality in my Another Android app.While going through android developer link and Android Library Projects - Tutorial I figure out that to use plug and play module I have to make my above project as library project so that I can use it in my new app ,now my questions are
1 Is this only way to use library project to add plug and play module
functionally in my app or there are another way also?
2 what are the pros and cons of using library project in order to add
plug and play module functionally?
basically I am researching on how to add plug and play service in my android app and trying to find best solution ,so any clarity on this topic will be extremaly helpful !!!
thanks in advance !!!

An alternative would be to use a linked source folder. In Eclipse, you can set this up in the project settings in the Java Build Path section. You can choose some source code directory outside your project that will virtually be in your project's space (and other projects that use it will do the same). I find this to be handy just from an IDE UI standpoint when I'm co-developing a module along with an app or two. What I don't like about library projects is that they sometimes don't update correctly within your project so you have to rebuild it and your project or clean your project to continue. Also, the module is simpler because it's just a directory of source code files, not an entire Eclipse project.
One other downside of libraries is that they can introduce conflicts that can be a headache to fight. For example, your library might have a different version of the Android compatibility library in it than your main project, and therefore give you compile time errors. Or there are sometimes errors with duplicate libraries, and you have to go fool with the Order and Export settings of your project.
If your module is going to rely on String, layout, and image resources, etc. that are common to all apps that implement the module, then a library project will be easier to work with so you don't have to pass all your resources in through your module's class constructors and duplicate them in each project.
A third option is Gradle, although as far as I know, that would essentially just help automate one of the above two options. I'm not very familiar with Gradle.

Managing lots of Apps

So I am working on a project where we have lots of Apps which are basically the same (Same code) but also different in terms of icons etc.
I am working with Eclipse with the Mercurial Plugin (Which I really like) which creates a local source control folder.
Now I wanted to ask which is the best way to manage all of those projects. For example I create a new feature in the Main Project and I want all of the other projects to be updated and compiled (and uploaded to the play store -> I don't think that one is possible though) with the new code automatically.
I have heard of Maven and ANT but I don't know exactly what those tools can do for me or if I'm even on the right track?
Or would I have to develop something like this myself?

This is what Library Projects are for. You create one base project and convert it to a library project (Eclipse: Project Properties / Android / is Library [x])
The other projects, which are depending on the base project have to include it as a library project (Eclipse: Project Properties / Android)
If you want to change res files like drawables you have to name the files the same and the android builder will overwrite the library project icon with the one from your project.
Thats it.

Android library project as jar file for distribution, like google analytics

I have seen this question, and have some more doubts regarding creating a jar file which I can distribute and can be used in any android applications.
What my requirement is
As I said, I want to build and distribute a closed source library. I
don't want the source code to be visible.
In that library I don't want to use any assets, layouts, resources
etc. But I want to use some android specific codes, like getting
android device id.
The most popular answer in the above linked SO question tells to create a regular java project and import android.jar in it. I tried to do that, but I don't know how to add android.jar to any java project. I would like to get some clarification on that too.
Moreover I would like to know if there are any other methods using android sdk itself (without using java project) create a closed source library jar file.
I think what I want is possible, since google analytics for android native apps seems to have done it. I am sure in the .jar file they distribute they are using android specific codes, since there seems no other way for them to get the device information to display in the analytics viewer.
EDIT : CAN SOMEONE CLARIFY THIS??
I think I have made some progress. This is what I have done
Created a regular android project (not library project, the "is
Library" checkmark is unchecked)
In the project I have coded out my logic. It uses some android
specific classes like SharedPreference, UUID, PackageManager. But
nothing related with assets, layouts also no class extending
Activity. Just a java class extending java.lang.object
Exported the project using Project->rightclick->export->Java->JAR
file. In the next screen I unchecked the checkbox near
AndroidManifest.xml. Set a destination directory to export and
clicked next thrice with keeping the default settings. Then I clicked
Finish, and got a lovely libMyLibraryName.jar at my desktop.
Then I created another android project, added this libMyLibraryName.jar to new project using project->rightclick->properties->java build path -> libraries->add external jar.
And I tried to use my class in the library, in my new project
MyLibraryClass objClass = new MyLibraryClass(this);
And I was able to compile and run successfully. I even sent the library to one of my co worker who was able to use the library in his on machine (Just making sure library project in my workspace wont influence the project using it).
Now I have 2 questions.
1) My first question is , what they meant by the term "true library" in the below given documentation ? Is it any non android java project which can be exported to a JAR file?
However, a library project differs from an standard Android
application project in that you cannot compile it directly to its own
.apk and run it on an Android device. Similarly, you cannot export
the library project to a self-contained JAR file, as you would do
for a true library. Instead, you must compile the library indirectly,
by referencing the library in the dependent application and building
that application.
Well this portion is taken from documentation under title "Library Projects".
2) My second question is, anything wrong with the way I have created the JAR file? Any possible pitfalls which might bite me back later? I would like to make sure I am not doing something terribly wrong, before using it in my important projects.
I might add that I didn't try the method of creating a JAVA project and importing android.jar. I am ready to try that one, if what I have done currently is wrong.

The android.jar will be located where you installed your Android SDK. Under the platforms directory there should be a number of other directories named android-<version>. The android.jar will be there. Choose the one specific to the minimum android version you are targeting.
Once you have that, copy it into your project. If you're using eclipse I think you can just cut and paste jars straight into your project, right click and add it to build path. If you're not using eclipse or any other IDE, you just need to ensure that the android.jar is on the classpath when building your jar file.
After that your newly built android library can be dropped into any Android project.
In answer to your additional questions:
What they mean by a true library is a jar file as opposed to an Android library project.
I don't think there's anything wrong with the way you created the jar file. I would have made it using the android.jar as I mentioned above but your way should also work. To verify this I would examine the jar contents and make sure all you have in there is .class files.

Does an Android library project also export its external libraries?

I'm developing a software layer that I would like to reuse several time for building my Android applications. Basically I want that, once installed, the software layer any other apps can use it (like a system library).
I was wondering what is the best solution for doing this, when I found that recently Android supports library projects (http://developer.android.com/guide/developing/projects/projects-eclipse.html#SettingUpLibraryProject).
So I decided to create my software layer as a library project, making the code it contains re-usable by the other applications I want to realize.
My software layer depends on a set of external jar, which are correctly located in the lib folder of the library project.
The problem is that when I create a new project referencing the library project I'm not able to see the classes defined into the external jars of the library project: i.e. it seems that they are not part of the classpath.
So when referring to a library project is possible to re-use only the source code defined there? If my library project have some other libraries I have to import these libraries also in the other ones (I want to avoid this!)?
I'm also interested to know if there are other ways for doing this, but searching around I haven't found other ways for realizing Android libraries/shared code.
Thanks.

Android library projects definitely incorporate any JARs you have in the library project's libs/ directory. However, if you are using Eclipse, you probably have to somehow manually add those to your build path of the host project (the one reusing the library).

Ok I finally figured out that for solving this is sufficient to add the jars to the host project build-path (no need to re-import them, you can just choose the path from the library project). However it is weird that they are not automatically exported in the host project classpath.