Android Interface Definition Language (AIDL)

Android Interface Definition Language (AIDL)

IN THIS DOCUMENT

Defining an AIDL Interface
Create the .aidl file
Implement the interface
Expose the interface to clients

Passing Objects over IPC
Calling an IPC Method

SEE ALSO

Bound Services

AIDL (Android Interface Definition Language) is similar to other IDLs you might have worked with. It allows you to define the programming interface that both the client and service agree upon in order to communicate
with each other using interprocess communication (IPC). On Android, one process cannot normally access the memory of another process. So to talk, they need to decompose their objects into primitives that the operating system can understand, and marshall the
objects across that boundary for you. The code to do that marshalling is tedious to write, so Android handles it for you with AIDL.

Note: Using AIDL is necessary only if you allow clients from different applications to access your service for IPC and want to handle multithreading in your service. If you do not need to perform concurrent IPC across different applications,
you should create your interface by implementing a Binder or, if you want to perform IPC, but
do not need to handle multithreading, implement your interface using a Messenger.
Regardless, be sure that you understand Bound Services before implementing an AIDL.
Before you begin designing your AIDL interface, be aware that calls to an AIDL interface are direct function calls. You should not make assumptions about the thread in which the call occurs. What happens is different
depending on whether the call is from a thread in the local process or a remote process. Specifically:
Calls made from the local process are executed in the same thread that is making the call. If this is your main UI thread, that thread continues to execute in the AIDL interface. If it is another thread, that is the one that executes
your code in the service. Thus, if only local threads are accessing the service, you can completely control which threads are executing in it (but if that is the case, then you shouldn't be using AIDL at all, but should instead create the interface by implementing
a Binder).
Calls from a remote process are dispatched from a thread pool the platform maintains inside of your own process. You must be prepared for incoming calls from unknown threads, with multiple calls happening at the same time. In
other words, an implementation of an AIDL interface must be completely thread-safe.
The 

oneway

 keyword modifies the behavior of remote calls. When used, a remote call does not block; it simply sends the transaction data and immediately
returns. The implementation of the interface eventually receives this as a regular call from the 

Binder

 thread
pool as a normal remote call. If 

oneway

 is used with a local call, there is no impact and the call is still synchronous.

Defining an AIDL Interface

You must define your AIDL interface in an 

.aidl

 file using the Java programming language syntax, then save it in the source code (in the 

src/

 directory)
of both the application hosting the service and any other application that binds to the service.
When you build each application that contains the 

.aidl

 file, the Android SDK tools generate an 

IBinder

interface
based on the 

.aidl

 file and save it in the project's 

gen/

 directory. The service must implement the

IBinder

 interface
as appropriate. The client applications can then bind to the service and call methods from the

IBinder

 to
perform IPC.
To create a bounded service using AIDL, follow these steps:
Create the .aidl file
This file defines the programming interface with method signatures.

Implement the interface
The Android SDK tools generate an interface in the Java programming language, based on your 

.aidl

 file. This interface has an inner abstract
class named 

Stub

 that extends 

Binder

 and
implements methods from your AIDL interface. You must extend the 

Stub

 class and implement the methods.

Expose the interface to clients
Implement a 

Service

 and
override 

onBind()

 to
return your implementation of the 

Stub

 class.

Caution: Any changes that you make to your AIDL interface after your first release must remain backward compatible in order to avoid breaking other applications that use your service. That is, because your 

.aidl

 file
must be copied to other applications in order for them to access your service's interface, you must maintain support for the original interface.

1. Create the .aidl file

AIDL uses a simple syntax that lets you declare an interface with one or more methods that can take parameters and return values. The parameters and return values can be of any type, even other AIDL-generated interfaces.
You must construct the 

.aidl

 file using the Java programming language. Each 

.aidl

 file
must define a single interface and requires only the interface declaration and method signatures.
By default, AIDL supports the following data types:
All primitive types in the Java programming language (such as 

int

, 

long

, 

char

, 

boolean

,
and so on)

String

CharSequence

List

All elements in the 

List

 must
be one of the supported data types in this list or one of the other AIDL-generated interfaces or parcelables you've declared. A 

List

 may
optionally be used as a "generic" class (for example, 

List<String>

). The actual concrete class that the other side receives is always an 

ArrayList

,
although the method is generated to use the 

List

 interface.

Map

All elements in the 

Map

 must
be one of the supported data types in this list or one of the other AIDL-generated interfaces or parcelables you've declared. Generic maps, (such as those of the form 

Map<String,Integer>

 are
not supported. The actual concrete class that the other side receives is always a 

HashMap

,
although the method is generated to use the 

Map

 interface.

You must include an 

import

 statement for each additional type not listed above, even if they are defined in the same package as your interface.
When defining your service interface, be aware that:
Methods can take zero or more parameters, and return a value or void.
All non-primitive parameters require a directional tag indicating which way the data goes. Either 

in

, 

out

,
or

inout

 (see the example below).
Primitives are 

in

 by default, and cannot be otherwise.

Caution: You should limit the direction to what is truly needed, because marshalling parameters is expensive.

All code comments included in the 

.aidl

 file are included in the generated 

IBinder

 interface
(except for comments before the import and package statements).
Only methods are supported; you cannot expose static fields in AIDL.
Here is an example 

.aidl

 file:

// IRemoteService.aidl
package com.example.android;

// Declare any non-default types here with import statements

/** Example service interface */
interface IRemoteService {
/** Request the process ID of this service, to do evil things with it. */
int getPid();

/** Demonstrates some basic types that you can use as parameters
* and return values in AIDL.
*/
void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat,
double aDouble, String aString);
}

Simply save your 

.aidl

 file in your project's 

src/

 directory and
when you build your application, the SDK tools generate the 

IBinder

 interface
file in your project's 

gen/

 directory. The generated file name matches the 

.aidl

file name, but with a 

.java

 extension
(for example, 

IRemoteService.aidl

 results in 

IRemoteService.java

).
If you use Eclipse, the incremental build generates the binder class almost immediately. If you do not use Eclipse, then the Ant tool generates the binder class next time you build your application—you should build
your project with 

ant debug

 (or 

ant release

) as soon as you're finished writing the 

.aidl

 file,
so that your code can link against the generated class.

2. Implement the interface

When you build your application, the Android SDK tools generate a 

.java

 interface file named after your 

.aidl

file.
The generated interface includes a subclass named 

Stub

 that is an abstract implementation of its parent interface (for example, 

YourInterface.Stub

)
and declares all the methods from the 

.aidl

 file.

Note: 

Stub

 also defines a few helper methods, most notably 

asInterface()

, which takes an 

IBinder

 (usually
the one passed to a client's 

onServiceConnected()

 callback
method) and returns an instance of the stub interface. See the section Calling an IPC Method for more details
on how to make this cast.
To implement the interface generated from the 

.aidl

, extend the generated 

Binder

 interface
(for example,

YourInterface.Stub

) and implement the methods inherited from the 

.aidl

 file.
Here is an example implementation of an interface called 

IRemoteService

 (defined by the 

IRemoteService.aidl

example,
above) using an anonymous instance:

private final IRemoteService.Stub mBinder = new IRemoteService.Stub() {
public int getPid(){
return Process.myPid();
}
public void basicTypes(int anInt, long aLong, boolean aBoolean,
float aFloat, double aDouble, String aString) {
// Does nothing
}
};

Now the 

mBinder

 is an instance of the 

Stub

 class (a 

Binder

),
which defines the RPC interface for the service. In the next step, this instance is exposed to clients so they can interact with the service.
There are a few rules you should be aware of when implementing your AIDL interface:
Incoming calls are not guaranteed to be executed on the main thread, so you need to think about multithreading from the start and properly build your service to be thread-safe.
By default, RPC calls are synchronous. If you know that the service takes more than a few milliseconds to complete a request, you should not call it from the activity's main thread, because it might hang the application (Android
might display an "Application is Not Responding" dialog)—you should usually call them from a separate thread in the client.
No exceptions that you throw are sent back to the caller.

3. Expose the interface to clients

Once you've implemented the interface for your service, you need to expose it to clients so they can bind to it. To expose the interface for your service, extend 

Service

 and
implement 

onBind()

 to
return an instance of your class that implements the generated 

Stub

 (as discussed in the previous section). Here's an example service that exposes the 

IRemoteService

 example
interface to clients.

public class RemoteService extends Service {
@Override
public void onCreate() {
super.onCreate();
}

@Override
public IBinder onBind(Intent intent) {
// Return the interface
return mBinder;
}

private final IRemoteService.Stub mBinder = new IRemoteService.Stub() {
public int getPid(){
return Process.myPid();
}
public void basicTypes(int anInt, long aLong, boolean aBoolean,
float aFloat, double aDouble, String aString) {
// Does nothing
}
};}

Now, when a client (such as an activity) calls 

bindService()

 to
connect to this service, the client's

onServiceConnected()

 callback
receives the 

mBinder

 instance returned by the service's 

onBind()

 method.
The client must also have access to the interface class, so if the client and service are in separate applications, then the client's application must have a copy of the 

.aidl

 file
in its 

src/

 directory (which generates the

android.os.Binder

 interface—providing the client access to the AIDL
methods).
When the client receives the 

IBinder

 in
the 

onServiceConnected()

 callback,
it must call

YourServiceInterface.Stub.asInterface(service)

 to cast the returned parameter to 

YourServiceInterface

type.
For example:

IRemoteService mIRemoteService;
private ServiceConnection mConnection = new ServiceConnection() {
// Called when the connection with the service is established
public void onServiceConnected(ComponentName className, IBinder service) {
// Following the example above for an AIDL interface,
// this gets an instance of the IRemoteInterface, which we can use to call on the service
mIRemoteService = IRemoteService.Stub.asInterface(service);
}

// Called when the connection with the service disconnects unexpectedly
public void onServiceDisconnected(ComponentName className) {
Log.e(TAG, "Service has unexpectedly disconnected");
mIRemoteService = null;
}
};

For more sample code, see the 

RemoteService.java

 class
in ApiDemos.

Passing Objects over IPC

If you have a class that you would like to send from one process to another through an IPC interface, you can do that. However, you must ensure that the code for your class is available to the other side of the
IPC channel and your class must support the 

Parcelable

 interface.
Supporting the 

Parcelable

 interface
is important because it allows the Android system to decompose objects into primitives that can be marshalled across processes.
To create a class that supports the 

Parcelable

 protocol,
you must do the following:
Make your class implement the 

Parcelable

 interface.
Implement 

writeToParcel

,
which takes the current state of the object and writes it to a 

Parcel

.
Add a static field called 

CREATOR

 to your class which is an object implementing the 

Parcelable.Creator

interface.
Finally, create an 

.aidl

 file that declares your parcelable class (as shown for the 

Rect.aidl

 file,
below).
If you are using a custom build process, do not add the 

.aidl

 file to your build. Similar to a header file in the C language, this 

.aidl

 file
isn't compiled.

AIDL uses these methods and fields in the code it generates to marshall and unmarshall your objects.
For example, here is a 

Rect.aidl

 file to create a 

Rect

 class that's
parcelable:

package android.graphics;

// Declare Rect so AIDL can find it and knows that it implements
// the parcelable protocol.
parcelable Rect;

And here is an example of how the 

Rect

 class
implements the 

Parcelable

 protocol.

import android.os.Parcel;
import android.os.Parcelable;

public final class Rect implements Parcelable {
public int left;
public int top;
public int right;
public int bottom;

public static final Parcelable.Creator<Rect> CREATOR = new
Parcelable.Creator<Rect>() {
public Rect createFromParcel(Parcel in) {
return new Rect(in);
}

public Rect[] newArray(int size) {
return new Rect[size];
}
};

public Rect() {
}

private Rect(Parcel in) {
readFromParcel(in);
}

public void writeToParcel(Parcel out) {
out.writeInt(left);
out.writeInt(top);
out.writeInt(right);
out.writeInt(bottom);
}

public void readFromParcel(Parcel in) {
left = in.readInt();
top = in.readInt();
right = in.readInt();
bottom = in.readInt();
}
}

The marshalling in the 

Rect

 class is pretty simple. Take a look at the other methods on 

Parcel

 to
see the other kinds of values you can write to a Parcel.

Warning: Don't forget the security implications of receiving data from other processes. In this case, the 

Rect

reads four numbers from the 

Parcel

,
but it is up to you to ensure that these are within the acceptable range of values for whatever the caller is trying to do. See Security
and Permissions for more information about how to keep your application secure from malware.

Calling an IPC Method

Here are the steps a calling class must take to call a remote interface defined with AIDL:
Include the 

.aidl

 file in the project 

src/

 directory.
Declare an instance of the 

IBinder

 interface
(generated based on the AIDL).
Implement 

ServiceConnection

.
Call 

Context.bindService()

,
passing in your 

ServiceConnection

 implementation.
In your implementation of 

onServiceConnected()

,
you will receive an 

IBinder

 instance
(called 

service

). Call

YourInterfaceName.Stub.asInterface((IBinder)service)

 to cast the returned
parameter to YourInterfacetype.
Call the methods that you defined on your interface. You should always trap 

DeadObjectException

exceptions,
which are thrown when the connection has broken; this will be the only exception thrown by remote methods.
To disconnect, call 

Context.unbindService()

 with
the instance of your interface.
A few comments on calling an IPC service:
Objects are reference counted across processes.
You can send anonymous objects as method arguments.
For more information about binding to a service, read the Bound
Services document.
Here is some sample code demonstrating calling an AIDL-created service, taken from the Remote Service sample in the ApiDemos project.

public static class Binding extends Activity {
/** The primary interface we will be calling on the service. */
IRemoteService mService = null;
/** Another interface we use on the service. */
ISecondary mSecondaryService = null;

Button mKillButton;
TextView mCallbackText;

private boolean mIsBound;

/**
* Standard initialization of this activity.  Set up the UI, then wait
* for the user to poke it before doing anything.
*/
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.remote_service_binding);

// Watch for button clicks.
Button button = (Button)findViewById(R.id.bind);
button.setOnClickListener(mBindListener);
button = (Button)findViewById(R.id.unbind);
button.setOnClickListener(mUnbindListener);
mKillButton = (Button)findViewById(R.id.kill);
mKillButton.setOnClickListener(mKillListener);
mKillButton.setEnabled(false);

mCallbackText = (TextView)findViewById(R.id.callback);
mCallbackText.setText("Not attached.");
}

/**
* Class for interacting with the main interface of the service.
*/
private ServiceConnection mConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className,
IBinder service) {
// This is called when the connection with the service has been
// established, giving us the service object we can use to
// interact with the service.  We are communicating with our
// service through an IDL interface, so get a client-side
// representation of that from the raw service object.
mService = IRemoteService.Stub.asInterface(service);
mKillButton.setEnabled(true);
mCallbackText.setText("Attached.");

// We want to monitor the service for as long as we are
// connected to it.
try {
mService.registerCallback(mCallback);
} catch (RemoteException e) {
// In this case the service has crashed before we could even
// do anything with it; we can count on soon being
// disconnected (and then reconnected if it can be restarted)
// so there is no need to do anything here.
}

// As part of the sample, tell the user what happened.
Toast.makeText(Binding.this, R.string.remote_service_connected,
Toast.LENGTH_SHORT).show();
}

public void onServiceDisconnected(ComponentName className) {
// This is called when the connection with the service has been
// unexpectedly disconnected -- that is, its process crashed.
mService = null;
mKillButton.setEnabled(false);
mCallbackText.setText("Disconnected.");

// As part of the sample, tell the user what happened.
Toast.makeText(Binding.this, R.string.remote_service_disconnected,
Toast.LENGTH_SHORT).show();
}
};

/**
* Class for interacting with the secondary interface of the service.
*/
private ServiceConnection mSecondaryConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className,
IBinder service) {
// Connecting to a secondary interface is the same as any
// other interface.
mSecondaryService = ISecondary.Stub.asInterface(service);
mKillButton.setEnabled(true);
}

public void onServiceDisconnected(ComponentName className) {
mSecondaryService = null;
mKillButton.setEnabled(false);
}
};

private OnClickListener mBindListener = new OnClickListener() {
public void onClick(View v) {
// Establish a couple connections with the service, binding
// by interface names.  This allows other applications to be
// installed that replace the remote service by implementing
// the same interface.
bindService(new Intent(IRemoteService.class.getName()),
mConnection, Context.BIND_AUTO_CREATE);
bindService(new Intent(ISecondary.class.getName()),
mSecondaryConnection, Context.BIND_AUTO_CREATE);
mIsBound = true;
mCallbackText.setText("Binding.");
}
};

private OnClickListener mUnbindListener = new OnClickListener() {
public void onClick(View v) {
if (mIsBound) {
// If we have received the service, and hence registered with
// it, then now is the time to unregister.
if (mService != null) {
try {
mService.unregisterCallback(mCallback);
} catch (RemoteException e) {
// There is nothing special we need to do if the service
// has crashed.
}
}

// Detach our existing connection.
unbindService(mConnection);
unbindService(mSecondaryConnection);
mKillButton.setEnabled(false);
mIsBound = false;
mCallbackText.setText("Unbinding.");
}
}
};

private OnClickListener mKillListener = new OnClickListener() {
public void onClick(View v) {
// To kill the process hosting our service, we need to know its
// PID.  Conveniently our service has a call that will return
// to us that information.
if (mSecondaryService != null) {
try {
int pid = mSecondaryService.getPid();
// Note that, though this API allows us to request to
// kill any process based on its PID, the kernel will
// still impose standard restrictions on which PIDs you
// are actually able to kill.  Typically this means only
// the process running your application and any additional
// processes created by that app as shown here; packages
// sharing a common UID will also be able to kill each
// other's processes.
Process.killProcess(pid);
mCallbackText.setText("Killed service process.");
} catch (RemoteException ex) {
// Recover gracefully from the process hosting the
// server dying.
// Just for purposes of the sample, put up a notification.
Toast.makeText(Binding.this,
R.string.remote_call_failed,
Toast.LENGTH_SHORT).show();
}
}
}
};

// ----------------------------------------------------------------------
// Code showing how to deal with callbacks.
// ----------------------------------------------------------------------

/**
* This implementation is used to receive callbacks from the remote
* service.
*/
private IRemoteServiceCallback mCallback = new IRemoteServiceCallback.Stub() {
/**
* This is called by the remote service regularly to tell us about
* new values.  Note that IPC calls are dispatched through a thread
* pool running in each process, so the code executing here will
* NOT be running in our main thread like most other things -- so,
* to update the UI, we need to use a Handler to hop over there.
*/
public void valueChanged(int value) {
mHandler.sendMessage(mHandler.obtainMessage(BUMP_MSG, value, 0));
}
};

private static final int BUMP_MSG = 1;

private Handler mHandler = new Handler() {
@Override public void handleMessage(Message msg) {
switch (msg.what) {
case BUMP_MSG:
mCallbackText.setText("Received from service: " + msg.arg1);
break;
default:
super.handleMessage(msg);
}
}

};
}