Android Interface Definition Language (AIDL)

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

Note: Using AIDL is necessary only if you allow clients fromdifferent applications to access your service for IPC and want to handle multithreading in yourservice. If you do not need to perform concurrent IPC acrossdifferent applications, you should create your interface by implementing aBinder or, if you want to perform IPC, but do not need to handle multithreading,implement your interfaceusing a Messenger.Regardless, be sure that you understand Bound Services beforeimplementing an AIDL.

Before you begin designing your AIDL interface, be aware that calls to an AIDL interface aredirect function calls. You should not make assumptions about the thread in which the calloccurs. What happens is different depending on whether the call is from a thread in thelocal process or a remote process. Specifically:

• Calls made from the local process are executed in the same thread that is making the call. Ifthis is your main UI thread, that thread continues to execute in the AIDL interface. If it isanother thread, that is the one that executes your code in the service. Thus, if only localthreads are accessing the service, you can completely control which threads are executing in it (butif that is the case, then you shouldn't be using AIDL at all, but should instead create theinterface byimplementing aBinder).
• Calls from a remote process are dispatched from a thread pool the platform maintains inside ofyour own process. You must be prepared for incoming calls from unknown threads, with multiple callshappening at the same time. In other words, an implementation of an AIDL interface must becompletely thread-safe.
• The oneway keyword modifies the behavior of remote calls. When used, a remote call doesnot 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. Ifoneway is used with a local call,there is no impact and the call is still synchronous.

Defining an AIDL Interface

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You must define your AIDL interface in an .aidl file using the Javaprogramming language syntax, then save it in the source code (in thesrc/ directory) of boththe 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 toolsgenerate anIBinder interface based on the.aidl file and save it inthe project's gen/ directory. The service must implement theIBinderinterface as appropriate. The client applications can then bind to the service and call methods fromtheIBinder to perform IPC.

To create a bounded service using AIDL, follow these steps:

1. Create the .aidl file

   This file defines the programming interface with method signatures.

2. 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 namedStub that extendsBinder and implements methods from your AIDL interface. You must extend theStub class and implement the methods.

3. Expose the interface to clients

   Implement a Service and overrideonBind() to return your implementation of theStubclass.

Caution: Any changes that you make to your AIDL interface afteryour first release must remain backward compatible in order to avoid breaking other applicationsthat use your service. That is, because your.aidl file must be copied to other applicationsin order for them to access your service's interface, you must maintain support for the originalinterface.

1. Create the .aidl file

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

You must construct the .aidl file using the Java programming language. Each.aidlfile must define a single interface and requires only the interface declaration and methodsignatures.

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 thislist or one of the other AIDL-generated interfaces or parcelables you've declared. AList may optionally be used as a "generic" class (for example,List<String>).The actual concrete class that the other side receives is always anArrayList, although the method is generated to use theList interface.

• Map

  All elements in the Map must be one of the supported data types in thislist or one of the other AIDL-generated interfaces or parcelables you've declared. Generic maps,(such as those of the formMap<String,Integer> are not supported. The actual concrete class that the other sidereceives is always aHashMap, although the method is generated touse theMap interface.

You must include an import statement for each additional type not listed above, even ifthey 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.Eitherin, 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 trulyneeded, because marshalling parameters is expensive.

• All code comments included in the .aidl file are included in the generatedIBinder interface (except for comments before the import and packagestatements).
• Only methods are supported; you cannot expose static fields in AIDL.

Here is an example .aidl file:

// IRemoteService.aidlpackage 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 youbuild your application, the SDK tools generate theIBinder interface file in yourproject'sgen/ directory. The generated file name matches the .aidl file name, butwith a.java extension (for example, IRemoteService.aidl results inIRemoteService.java).

If you use Eclipse, the incremental build generates the binder class almost immediately. If youdo not use Eclipse, then the Ant tool generates the binder class next time you build yourapplication—you should build your project withant debug (or antrelease) as soon as you're finished writing the.aidl file, so that your code canlink against the generated class.

2. Implement the interface

When you build your application, the Android SDK tools generate a .java interface filenamed after your.aidl file. The generated interface includes a subclass named Stubthat is an abstract implementation of its parent interface (for example,YourInterface.Stub) and declares all the methods from the .aidl file.

Note: Stub alsodefines a few helper methods, most notablyasInterface(), which takes an IBinder (usually the one passed to a client'sonServiceConnected() callback method) andreturns an instance of the stub interface. See the section Calling an IPCMethod for more details on how to make this cast.

To implement the interface generated from the .aidl, extend the generatedBinder interface (for example,YourInterface.Stub) and implement the methodsinherited from the .aidl file.

Here is an example implementation of an interface called IRemoteService (defined by theIRemoteService.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 toclients 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 thinkabout 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 fewmilliseconds to complete a request, you should not call it from the activity's main thread, becauseit 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 toclients so they can bind to it. To expose the interfacefor your service, extendService and implementonBind() to return an instance of your class that implementsthe generatedStub (as discussed in the previous section). Here's an exampleservice that exposes theIRemoteService 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 themBinder 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 inseparate applications, then the client's application must have a copy of the.aidl filein its src/ directory (which generates the android.os.Binderinterface—providing the client access to the AIDL methods).

When the client receives the IBinder in theonServiceConnected() callback, it must callYourServiceInterface.Stub.asInterface(service) to cast the returnedparameter 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

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If you have a class that you would like to send from one process to another throughan IPC interface, you can do that. However, you must ensure that the code for your class isavailable to the other side of the IPC channel and your class must support theParcelable interface. Supporting theParcelable interface isimportant because it allows the Android system to decompose objects into primitives that can bemarshalled across processes.

To create a class that supports the Parcelable protocol, you must do thefollowing:

1. Make your class implement the Parcelable interface.
2. Implement writeToParcel, which takes thecurrent state of the object and writes it to aParcel.
3. Add a static field called CREATOR to your class which is an object implementingtheParcelable.Creator interface.
4. Finally, create an .aidl file that declares your parcelable class (as shown for theRect.aidl file, below).

   If you are using a custom build process, do not add the .aidl file to yourbuild. 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 unmarshallyour objects.

For example, here is a Rect.aidl file to create a Rect class that'sparcelable:

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 theParcelable 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 = newParcelable.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 othermethods onParcel to see the other kinds of values you can writeto a Parcel.

Warning: Don't forget the security implications of receivingdata from other processes. In this case, theRect reads four numbers from the Parcel, but it is up to you to ensure that these are within the acceptable range ofvalues for whatever the caller is trying to do. See Security and Permissions for moreinformation about how to keep your application secure from malware.

Calling an IPC Method

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

1. Include the .aidl file in the project src/ directory.
2. Declare an instance of the IBinder interface (generated based on theAIDL).
3. Implement ServiceConnection.
4. Call Context.bindService(), passing in yourServiceConnection implementation.
5. In your implementation of onServiceConnected(),you will receive anIBinder instance (calledservice). CallYourInterfaceName.Stub.asInterface((IBinder)service) to cast the returned parameter toYourInterface type.
6. 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.
7. 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 Servicesdocument.

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);            }        }    };}

转载于：http://developer.android.com/guide/components/aidl.html