Pass active objects between services through AIDL

Question

I'm trying to have a common object shared for several services in different packages. Each service must call the same object.

For example, Service A (from APK A) instantiates a custom object and I want that Service B and C (from APK B and C) retrieves the reference of this object and call some method of it.

I found in the Android reference that it should be possible by using Parcel :

Active Objects

An unusual feature of Parcel is the ability to read and write active objects. For these objects the actual contents of the object is not written, rather a special token referencing the object is written. When reading the object back from the Parcel, you do not get a new instance of the object, but rather a handle that operates on the exact same object that was originally written. There are two forms of active objects available.

Binder objects are a core facility of Android's general cross-process communication system. The IBinder interface describes an abstract protocol with a Binder object. Any such interface can be written in to a Parcel, and upon reading you will receive either the original object implementing that interface or a special proxy implementation that communicates calls back to the original object. The methods to use are writeStrongBinder(IBinder), writeStrongInterface(IInterface), readStrongBinder(), writeBinderArray(IBinder[]), readBinderArray(IBinder[]), createBinderArray(), writeBinderList(List), readBinderList(List), createBinderArrayList().

I tried to do this by passing my object (who extends binder) through AIDL but nothing works, I always gets a ClassCastException when I'm trying to retrieve the reference from the method createFromParcel(Parcel in).

An example of my code :

public class CustomObject extends Binder implements Parcelable {

  public CustomObject() {
    super();
  }

  public static final Parcelable.Creator<CustomObject> CREATOR = new Parcelable.Creator<CustomObject>() {
  public CustomObject createFromParcel(Parcel in) {
    IBinder i = in.readStrongBinder();
      // HOW TO RETRIEVE THE REFERENCE ??
      return null;
    }

    @Override
    public CustomObject[] newArray(int size) {
      return null;
    }
  };

  @Override
  public int describeContents() {
    return 0;
  }

  @Override
  public void writeToParcel(Parcel dest, int flags) {
    dest.writeStrongBinder(this);
  }
}

Is anyone has already done that ?

Thanks in advance !

Did you ever figure out how to do this? I tried years ago and gave up. Now I'm trying again with the same question/results as you. — edharned, May 05 '15 at 22:14
Unfortunately no.. For this project I changed my process in order to retrieve a copy of the object each time I wanted to call a method on the object. Not perfect but it was working. But I'm still interested to know if there is a solution to this problem — nbe_42, May 07 '15 at 07:57

score 2 · Accepted Answer · answered May 27 '13 at 17:28

Here are two approaches.

Simple: use aidl for the object itself

It seems you have an existing AIDL interface through which you pass this 'custom object' as a parcel. Don't do that. Instead:
The object which you pass through should be itself described by AIDL. Say, for example, you call it ICustomObject.aidl.
In this case you do not need to make the object Parcelable. You probably don't even need to write the above code; just use one AIDL-described type in another. For example add a line like this to the main AIDL for service A:
```
ICustomObject getCustomObject();
```
In service A, within the Stub class you've already got, you'll need to simply return something inheriting from ICustomObject.
In services B and C, you can simply call that method to get hold of an ICustomObject. Simple! No parcels, no readStrongBinder(), nothing.

Harder

If you do the above, the Android toolchain generates Java code which marshalls and unmarshalls the object. You could instead write the code yourself.

ICustomObject myObjectWhichActuallyLivesInAnotherProcess = ICustomObject.Stub.asInterface(parcel.readStrongBinder())

or even

ICustomObject myObjectWhichActuallyLivesInAnotherProcess = (ICustomObject)parcel.readStrongBinder().queryLocalInterface("com.your.custom.object");

However I think your life will be more sane if you make everything aidl.

A note on class sharing

You'll probably want to create an Android "library project" which has ICustomObject.aidl within it, such that you can share the resulting classes between the projects which build A, B and C.

score 0 · Answer 2 · answered May 27 '13 at 07:04

0

have you looked into content providers?

answered May 27 '13 at 07:04

bolvo

361
3
13

I don't want to share only data. For that, a simple AIDL mechanism is sufficient. What I need is sharing a complex object that must be unique with some methods that all services can call. – nbe_42 May 27 '13 at 11:36

score 0 · Answer 3 · answered Sep 20 '15 at 21:42

After looking into this rather extensively myself, I don't think it can actually be done. The ClassCastException you're getting is a result of casting a BinderProxy (which is a private class that extends IBinder) to your actual class (CustomObject). A BinderProxy always seems to be passed when referring to Binders across other processes and this is the object they refer to when they state "either the original object implementing that interface or a special proxy implementation". The BinderProxy does allow you to call the onTransact() method of IBinder but nothing else.

I honestly think the documentation is incorrect when they state that the "original object" is passed across processes as there are several other instances of the documentation for that class being both blatantly copy-pasted and wrong.

Pass active objects between services through AIDL

3 Answers3

Simple: use aidl for the object itself

Harder

A note on class sharing