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I am trying to understand how to communicate between applications in Android - not just between Activity instances.

I set up a 'client' that sends a Messenger obj to a Service (in the Intent sent to the service); the service creates a Message obj and sends it back to the 'client' using messenger.send(message). This works fine until I try to use the Message.obj to hold an object.

I created my own Parcelable class MyParcelable in the service and put it into the message. All works until the message is unmarshalled in the 'client'. The unmarshall fails because the 'client' has no access to the MyParcelable class. That's obvious - they are in different packages (say com.whatever.myclient and com.whatever.myserver). Is this completely the wrong way to go about this?

I have also tried creating a Parcel and sending that (both applications would thereby have access to the class) - but Parcel is not Parcelable. I have read about class loaders being used but do not understand how separate class loaders in separate applications (processes, if I understand Android architecture in that regard). That is, how can one class loader be 'taught' about a class that exists in the other class loader? It certainly seems like there should be an obvious 'this is how you do it' but I have not seen it yet.

Michael
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Art Swri
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3 Answers3

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You cannot unmarshall a class at a process that has no knowledge of this class (as you already correctly pointed out). If you want MyParcelable to be sent to your Service, then you need to include MyParcelable in the Service at build, and include the right ClassLoader to be used when unmarshalling, because unmarshalling is done by a system thread, which doesn't know what MyParcelable is even if you included it at compile time.

EDIT:

(a) include MyParcelable into 2 different pkgs and have them be seen as the same class

Make a library of your common classes and include it in both applications (the application and the Service). You can either mark it as an Android library (in Eclipse this is right under the project's Android properties) or export it as a JAR and import it in both apps. It's always a good idea to have independent classes in a library instead of putting them in the apps directly. This way you can reuse them on other projects ;)

(2) include the right ClassLoader (where do I get the right class loader from, for example)

You can get your ClassLoader from the current thread when you're executing your code. Obviously this ClassLoader knows about your classes because it's executing your code ;) For example in Activity#onCreate(), you can do Thread.currentThread.getContextClassLoader(). Since your MyParcelable class is also included on the Service, you can do the same to get a valid ClassLoader to unmarshall.

Another solution for IPC is to define an AIDL interface so you don't have to implement Parcelable.

m0skit0
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    Hey m0skit0, you should expand this a bit. I was going to answer, but you have it exactly right. A little more explanation would just make it clearer. Parcelables only work if the ClassLoaders marshalling and unmarshalling the data have identical copies of the Parcelable class... – G. Blake Meike Mar 25 '13 at 02:54
  • Sounds good but I have no idea how to do either (a) include MyParcelable into 2 different pkgs and have them be seen as the same class or (2) include the right ClassLoader (where do I get the right class loader from, for example). Thanks for your answer but you're about 2 levels of expertise above my understanding. – Art Swri Mar 25 '13 at 03:04
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If you dont want to use a persistence layer somewhere ( i use cloud apis ) you could run a service in a separate process and bind to the service the respective apps that need the shared value-holder type object data. the service would be where you construct and get the obj values on behalf of connecting ( binding ) apps.

Robert Rowntree
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  • These are both very good suggestions and I will definitely keep them close by as I proceed. I am at this point mainly trying to get a grasp on the Parcelable and class loader concepts in hopes that I will understand them enough to be able to use them appropriately. Thanks much for your answer. – Art Swri Mar 25 '13 at 03:09
  • know the android fundamentals and activity lifecycle . that may be more important than CL concepts. http://developer.android.com/reference/android/app/Activity.html http://developer.android.com/guide/components/fundamentals.html – Robert Rowntree Mar 25 '13 at 03:28
  • imo - inter app, the easiest would be using 'intent-extra' fields and a i guess a chooser which would allow selection of the target app . – Robert Rowntree Mar 25 '13 at 03:56
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OK, I found the 'easy way' I was looking for. You can put a Bundle into the Message.obj field. Bundle is Parcelable so can be transported between client and server. There's no issue of the server not knowing what class it's receiving as Bundle is 'built into' Android Java. And Bundle can apparently include 'sub-Bundles' with in. This is simpler than having to deal with ensuring both parties know the definition of a user-defined class. Thanks for the answers above - I certainly appreciated the info!

Art Swri
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