Tuesday, 4 June 2013

Android Service

Developing own services and using system services in Android
This tutorial describes how to create and consume Android services. It is based on Eclipse 4.2, Java 1.6 and Android 4.2.

1. Android Services

1.1. Service

A service is a component which runs in the background, without direct interaction with the user. The Android platform provides and runs predefined system services and every Android application can use them, given the right permissions.
An Android application can, in addition to consuming the existing Android platform services, define and use new services.

1.2. Android Platform Service

The Android platform provides pre-defined services, usually exposed via a specific Manager class. Access to them can be gained via the getSystemService() method.

1.3. Defining new services

Every Android application can define and start new services
If you use asynchronous processing in activities or fragments the corresponding threads are still connected to the life-cycle of the corresponding activity. The Android system may decide to terminate them at any point in time.
Services run with a higher priority than inactive or invisible activities and therefore it is less likely that the Android system terminates them.
Defining your own services allows you to design very responsive applications. You can fetch the application via a service and once the application is started by the user, it can present fresh data to the user.

2. Defining services

2.1. Declaring own services

A service needs to be declared in the AndroidManifest.xml and the implementing class must extend the Service class or one of its subclasses. The following code shows an example for a service declaration and its implementation. 

<service
  android:name="MyService"
  android:icon="@drawable/icon"
  android:label="@string/service_name"
  >
</service> 


public class MyService extends Service {

  @Override
  public int onStartCommand(Intent intent, int flags, int startId) {
    //TODO do something useful
    return Service.START_NOT_STICKY;
  }

  @Override
  public IBinder onBind(Intent intent) {
  //TODO for communication return IBinder implementation
    return null;
  }
}

A service runs by default in the same process as the application. in its own thread.
Therefore you need to use asynchronous processing in the service to to perform resource intensive tasks in the background.
Services which run in the process of the application are sometimes called local services.

2.2. Running a Services in its own process

You can also specify that your Service runs in a separate process via the android:process=":process_description" attribute. 

<service
  android:name="WordService"
  android:process=":my_process" 
  android:icon="@drawable/icon"
  android:label="@string/service_name"
  >
</service>

The colon prefix before the name tells Android that the Service is private to its declaring application. If the colon is not used the Service would be a global process and can be used by other Android applications.
Running a service in its own process will not block the application in case the service performs long running operations in its main thread. But as the services runs in its own process you need to use some interprocess communication (IPC) to communicate to your service from other parts.
Even if the service runs in its own process you need to use asynchronous processing to perform network access because Android does not allow network access in the main thread of a process.

2.3. When to run a service in a separate process?

Running a service in its own process gives it its own memory address space and a garbage collector of the virtual machine in this process does not affect the application process.
Application rarely need to run a service in its own process. Running a services in its own process make the communication of the other Android components and the service harder to implement.
If you want to make a service to other Android application available, they must run in their own process.

2.4. Intent Services

You can also extend the IntentService class for your service implementation.
The IntentService is used to perform a certain task in the background. Once done, the instance of IntentService terminate itself automatically. Examples for its usage would be to download a certain resources from the Internet.
The IntentService class offers the onHandleIntent() method which will be asynchronously called by the Android system.

3. Broadcast receiver

For an introduction into BroadcastReceiver please see Android Broadcast Receiver tutorial .

4. Starting services

4.1. Start a service

An Android component (service, receiver, activity) can start and trigger a service via the startService(intent) method. This method call starts the service if it is not running. 

Intent service = new Intent(context, MyService.class);
context.startService(service);

If the service started the onCreate() method is called.
Once the service is started the method call to start the service triggers startService(intent) method in the service. It passes in the Intent for the startService(intent) call.
If startService(intent) is called while the service is running, its onStartCommand() is also called. Therefore your service needs to be prepared that onStartCommand() can be called several times. This method is called in the main user interface thread therefore it cannot be called simultaneously from two different threads.
Alternatively to startService(intent) you can also start a service via the bindService() method call. This allows you to communicate directly with the service.

4.2. Stopping a service

You stop a service via the stopService() method. No matter how frequently you started the service with startService(intent) a call to stopService() stops it.
A service can stop itself by calling the stopSelf() method.

4.3. Binding between activities and local services

If the activity wants to interact with the service it can use the bindService() method to start the service.
This method requires as parameter a ServiceConnection object which allows to connect to the service. In the service the onBind() method is called. This method returns a IBinder object to the ServiceConnection.
This IBinder object can be used by the activity to communicate with the service.
Afterwards the binding was done the onStartCommand() method is called with the Intent data provided by the activity.
startService() also allows you to provide a flag which determines the restart behavior of the services. Service.START_STICKY is used for services which are explicit started or stopped. If these services are terminated by the Android system, they are restarted if sufficient resource are available again.
Services started with Service.START_NOT_STICKY are not automatically restarted if terminated by the Android system.

4.4. Starting services regularly via AlarmManager

As with activities the Android system may terminate the process of a service at any time to save resources. For this reason you cannot simple use a TimerTask in the service to ensure that it is executed on a regular basis.
For correct scheduling of the Service use the AlarmManager class. The following code demonstrates how to do this. 

Calendar cal = Calendar.getInstance();

Intent intent = new Intent(this, MyService.class);
PendingIntent pintent = PendingIntent.getService(this, 0, intent, 0);

AlarmManager alarm = (AlarmManager)getSystemService(Context.ALARM_SERVICE);
// Start every 30 seconds
alarm.setRepeating(AlarmManager.RTC_WAKEUP, cal.getTimeInMillis(), 30*1000, pintent);

5. Communicating with Services

There are several way for an activity to communicate with an service and vice versa. This section dicusses the different ways and gives recommendation which to use.

5.1. Activity binding to local service

If the Service is started in the same process as the activity, the activity can directly bind to the service. This is a relatively simple and efficient way to communication.

5.2. Using receiver

You can also use dynamically registered receivers for the communication. For example your activity can dynamically register a receiver and the service sends outs corresponding events.

5.3. AIDL for services in a different process

To bind to a service which runs in a different process you need to use Inter Process Communication (IPC) as the data needs to be send between different processes. For this you need to create a AIDL file which looks similar to an Java interface but ends with the .aidl file extension and is only allowed to extend other AIDL files.
This approach is required if your service should be provided to other applications, otherwise you should prefer a local service.

5.4. Sending Intent data and bundle to the services

The service receives data from the starting Android component and can use this data.

5.5. Handler and Messenger

If the service should be communicating back to the activity it can receive an object of type Messenger via the Intent data it receives from the activity. If the Messenger is bound to a Handler in the activity the service can send objects of type Message to the activity.
A Messenger is parcelable, which means it can be passed to another process and you can use this object to send Messages to the Handler in the activity.
Messenger provides also the method getBinder() which allows to pass a Messenger to the activity. The activity can therefore send Messages to the service.

6. Tutorial: Using IntentService

The following example demonstrates how to use the IntentService class to download a file from the Internet. Once done the IntentService will use an instance of the Messenger class to inform the activity which started the service about the location of the downloaded file.
Create a new project called de.vogella.android.intentservice.download with a activity called MainActivity.
Create a service "DownloadService" by creating the following class and the entry in AndroidManifest.xml. Also add the permission to write to external storage and to access the Internet to the file. 

package de.vogella.android.intentservice.download;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;

import android.app.Activity;
import android.app.IntentService;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.os.Message;
import android.os.Messenger;
import android.util.Log;

public class DownloadService extends IntentService {

  private int result = Activity.RESULT_CANCELED;

  public DownloadService() {
    super("DownloadService");
  }

  // Will be called asynchronously be Android
  @Override
  protected void onHandleIntent(Intent intent) {
    Uri data = intent.getData();
    String urlPath = intent.getStringExtra("urlpath");
    String fileName = data.getLastPathSegment();
    File output = new File(Environment.getExternalStorageDirectory(),
        fileName);
    if (output.exists()) {
      output.delete();
    }

    InputStream stream = null;
    FileOutputStream fos = null;
    try {

      URL url = new URL(urlPath);
      stream = url.openConnection().getInputStream();
      InputStreamReader reader = new InputStreamReader(stream);
      fos = new FileOutputStream(output.getPath());
      int next = -1;
      while ((next = reader.read()) != -1) {
        fos.write(next);
      }
      // Sucessful finished
      result = Activity.RESULT_OK;

    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      if (stream != null) {
        try {
          stream.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
      if (fos != null) {
        try {
          fos.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }

    Bundle extras = intent.getExtras();
    if (extras != null) {
      Messenger messenger = (Messenger) extras.get("MESSENGER");
      Message msg = Message.obtain();
      msg.arg1 = result;
      msg.obj = output.getAbsolutePath();
      try {
        messenger.send(msg);
      } catch (android.os.RemoteException e1) {
        Log.w(getClass().getName(), "Exception sending message", e1);
      }

    }
  }
} 


<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.vogella.android.intentservice.download"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <service android:name="DownloadService" >
        </service>
    </application>

</manifest>

Change the main.xml layout to the following. 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClick"
        android:text="Button" />

</LinearLayout>

Change MainActivity to the following. 

package de.vogella.android.intentservice.download;

import android.app.Activity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.os.Messenger;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends Activity {
  private Handler handler = new Handler() {
    public void handleMessage(Message message) {
      Object path = message.obj;
      if (message.arg1 == RESULT_OK && path != null) {
        Toast.makeText(MainActivity.this,
            "Downloaded" + path.toString(), Toast.LENGTH_LONG)
            .show();
      } else {
        Toast.makeText(MainActivity.this, "Download failed.",
            Toast.LENGTH_LONG).show();
      }

    };
  };

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

  }

  public void onClick(View view) {
    Intent intent = new Intent(this, DownloadService.class);
    // Create a new Messenger for the communication back
    Messenger messenger = new Messenger(handler);
    intent.putExtra("MESSENGER", messenger);
    intent.setData(Uri.parse("http://www.vogella.com/index.html"));
    intent.putExtra("urlpath", "http://www.vogella.com/index.html");
    startService(intent);
  }
}

If you run your example and press the button, the download should be performed by the Service and once done the activity should show a Toast with the file name.

6. Tutorial: Using IntentService

The following example demonstrates how to use the IntentService class to download a file from the Internet. Once done the IntentService will use an instance of the Messenger class to inform the activity which started the service about the location of the downloaded file.
Create a new project called de.vogella.android.intentservice.download with a activity called MainActivity.
Create a service "DownloadService" by creating the following class and the entry in AndroidManifest.xml. Also add the permission to write to external storage and to access the Internet to the file. 

package de.vogella.android.intentservice.download;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;

import android.app.Activity;
import android.app.IntentService;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.os.Message;
import android.os.Messenger;
import android.util.Log;

public class DownloadService extends IntentService {

  private int result = Activity.RESULT_CANCELED;

  public DownloadService() {
    super("DownloadService");
  }

  // Will be called asynchronously be Android
  @Override
  protected void onHandleIntent(Intent intent) {
    Uri data = intent.getData();
    String urlPath = intent.getStringExtra("urlpath");
    String fileName = data.getLastPathSegment();
    File output = new File(Environment.getExternalStorageDirectory(),
        fileName);
    if (output.exists()) {
      output.delete();
    }

    InputStream stream = null;
    FileOutputStream fos = null;
    try {

      URL url = new URL(urlPath);
      stream = url.openConnection().getInputStream();
      InputStreamReader reader = new InputStreamReader(stream);
      fos = new FileOutputStream(output.getPath());
      int next = -1;
      while ((next = reader.read()) != -1) {
        fos.write(next);
      }
      // Sucessful finished
      result = Activity.RESULT_OK;

    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      if (stream != null) {
        try {
          stream.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
      if (fos != null) {
        try {
          fos.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }

    Bundle extras = intent.getExtras();
    if (extras != null) {
      Messenger messenger = (Messenger) extras.get("MESSENGER");
      Message msg = Message.obtain();
      msg.arg1 = result;
      msg.obj = output.getAbsolutePath();
      try {
        messenger.send(msg);
      } catch (android.os.RemoteException e1) {
        Log.w(getClass().getName(), "Exception sending message", e1);
      }

    }
  }
} 


<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.vogella.android.intentservice.download"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <service android:name="DownloadService" >
        </service>
    </application>

</manifest>

Change the main.xml layout to the following. 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClick"
        android:text="Button" />

</LinearLayout>

Change MainActivity to the following. 

package de.vogella.android.intentservice.download;

import android.app.Activity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.os.Messenger;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends Activity {
  private Handler handler = new Handler() {
    public void handleMessage(Message message) {
      Object path = message.obj;
      if (message.arg1 == RESULT_OK && path != null) {
        Toast.makeText(MainActivity.this,
            "Downloaded" + path.toString(), Toast.LENGTH_LONG)
            .show();
      } else {
        Toast.makeText(MainActivity.this, "Download failed.",
            Toast.LENGTH_LONG).show();
      }

    };
  };

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

  }

  public void onClick(View view) {
    Intent intent = new Intent(this, DownloadService.class);
    // Create a new Messenger for the communication back
    Messenger messenger = new Messenger(handler);
    intent.putExtra("MESSENGER", messenger);
    intent.setData(Uri.parse("http://www.vogella.com/index.html"));
    intent.putExtra("urlpath", "http://www.vogella.com/index.html");
    startService(intent);
  }
}

If you run your example and press the button, the download should be performed by the Service and once done the activity should show a Toast with the file name.

http://www.vogella.com/code/index.html
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