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:
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.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:
This file defines the programming interface with method signatures.
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.
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.
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:
int, long,
char, boolean, and so on)StringCharSequenceList
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:
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.
.aidl file are included in the generated IBinder interface (except for comments before the import and package
statements).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.
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:
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.
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:
Parcelable interface.writeToParcel, which takes the
current state of the object and writes it to a Parcel.CREATOR to your class which is an object implementing
the Parcelable.Creator interface..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.
Here are the steps a calling class must take to call a remote interface defined with AIDL:
.aidl file in the project src/ directory.IBinder interface (generated based on the
AIDL). ServiceConnection. Context.bindService(), passing in your ServiceConnection implementation. onServiceConnected(),
you will receive an IBinder instance (called service). Call
YourInterfaceName.Stub.asInterface((IBinder)service) to
cast the returned parameter to YourInterface type.DeadObjectException exceptions, which are thrown when
the connection has broken; this will be the only exception thrown by remote
methods.Context.unbindService() with the instance of your interface. A few comments on calling an IPC service:
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);
}
}
};
}