CDACians India: Android Threads, Handlers AsyncTask

This tutorial describes the usage of Threads, Handlers and AsyncTask in your application. It also covers how to handle the application lifecycle together with threads. It also describes Traceview to trace an application for performance problems. It is based on Eclipse 3.7, Java 1.6 and Android 4.0 (Ice Cream Sandwitch)

1. Android Threads and Background Processing

Android modifies the user interface and handles input events from one single user interface thread which is also called the main thread.

If the programmer does not use any concurrency constructs, all code of an Android application runs in this thread.

If you perform a long lasting operation, e.g. loading a file or accessing data from the Internet, the user interface of your Android Application will block until the corresonding code has finished.

To provide a good user experience all potentially slow running operations in an Android application should run asynchronously, e.g. via some way of concurrency constructs of the Java language or the Android framework. This includes all potential slow operations, like network, file and database access and complex calculations.

Android enforced that with an Application not responding (ANR) dialog if an Activity does not react within 5 seconds to user input. From this dialog the user can choose to stop the application.

2. Android Basics

The following assumes that you have already basic knowledge in Android development. Please check the Android development tutorial to learn the basics.

3. Background Processing

3.1. Threads

Android supports the usage of the Threads class to perform asynchronous processing.

Android also supplies the java.util.concurrent package to perform something in the background, e.g. using the ThreadPools and Executor classes.

If you need to update the user interface from a new Thread, you need to synchronize with the user interface thread.

You can use the android.os.Handler class or the AsyncTasks class for this.

3.2. Handler

The Handler class can update the user interface. A Handler provides methods for receiving instances of the Message or Runnable class.

To use a handler you have to subclass it and override the handleMessage() to process messages. To process a Runnable you can use the post() method. You only need one instance of a Handler in your Activity.

You thread can post messages via the sendMessage(Message msg) method or via thesendEmptyMessage() method.

3.3. AsyncTask

The AsyncTask class encapsulates the creation of Threads and Handlers. An AsyncTask is started via the execute() method.

The execute() method calls the doInBackground() and the onPostExecute() method.

The doInBackground() method contains the coding instruction which should be performed in a background thread. This method runs automatically in a separate Thread.

The onPostExecute() method synchronize itself again with the user interface thread and allows to update it. This method is called by the framework once the doInBackground() method finishes.

To use AsyncTask you must subclass it. AsyncTask uses generics and varargs. The parameters are the following AsyncTask <TypeOfVarArgParams , ProgressValue , ResultValue> .

TypeOfVarArgParams is passed into the doInBackground() method as input, ProgressValue is used for progress information and ResultValue must be returned from doInBackground() method and is passed to onPostExecute() as parameter.

3.4. Feedback via ProgressBar

For providing feedback to the user you can use the ProgressBar dialog, which allow to display progress to the user. The Javadoc of ProgressBar gives a nice example of its usage.

Alternatively you can provide progress feedback in the activities title bar.

3.5. Concurrency and lifecyle

One challenge in using threads is to consider the lifecycle of the application. The Android system may kill your activity or trigger a configuration change which also will restart your activity.

You also need to handle open dialogs, as dialogs are always connected to the activity which created them. In case the activity gets restarted and you access an existing dialog you receive an "View not attached to window manager" exception.

To save an object your can use the method onRetainNonConfigurationInstance() method. This method allows to save one object if the activity will be soon restarted.

To retrieve this object you can use the getLastNonConfigurationInstance() method. This way can you can save an object, e.g. a running thread, even if the activity is restarted.

getLastNonConfigurationInstance() returns null if the Activity is started the first time or if it has been finished via the finish() method.

If more then one object should be saved then you can implement the class "Application". This class can be used to access object which should be cross activities or available for the whole application lifecycle. In the onCreate() and onTerminate() you can create / destroy the object and make them available via public fields or getters. To use your application class assign the classname to the android:name attribute of your application.

<application android:icon="@drawable/icon" android:label="@string/app_name"
  android:name="MyApplicationClass">
   <activity android:name=".ThreadsLifecycleActivity"
    android:label="@string/app_name">
     <intent-filter>
      <action android:name="android.intent.action.MAIN" />
      <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
  </activity>
</application>

You can acess the Application via the getApplication() method in your activity.

4. Tutorial: Handler

In this example we use the Handler class to update a ProgressBar view in a background Thread.

Create a new Android project called "de.vogella.android.handler" with the Activity"ProgressTestActivity". Create the following layout main.xml. This layout contains the ProgressBar and sets its appearance via a style.

<?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" >

    <ProgressBar
        android:id="@+id/progressBar1"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:indeterminate="false"
        android:max="10"
        android:padding="4dip" >
    </ProgressBar>

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

</LinearLayout>

Change your Activity to the following:

package de.vogella.android.handler;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.ProgressBar;

public class ProgressTestActivity extends Activity {
  private Handler handler;
  private ProgressBar progress;

  
/** Called when the activity is first created. */
@Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    progress = (ProgressBar) findViewById(R.id.progressBar1);
    handler = new Handler();
  }

  public void startProgress(View view) {
    // Do something long
    Runnable runnable = new Runnable() {
      @Override
      public void run() {
        for (int i = 0; i <= 10; i++) {
          final int value = i;
          try {
            Thread.sleep(2000);
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
          handler.post(new Runnable() {
            @Override
            public void run() {
              progress.setProgress(value);
            }
          });
        }
      }
    };
    new Thread(runnable).start();
  }
}

Run your application. Once you press your button the ProgressBar will get updated from the background thread.

5. Tutorial: AsyncTask

In this example we will use an instance of the AsyncTask class to download the content of a webpage. We use Android HttpClient for this. Create a new Android project called de.vogella.android.asynctaskwith an Activity called ReadWebpageAsyncTask. Add the android.permission.INTERNETpermission to your <filenname>AndroidManifest.xml</filenname> file. .

Create the following layout.

<?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/readWebpage"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="readWebpage"
        android:text="Load Webpage" >
    </Button>

    <TextView
        android:id="@+id/TextView01"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="Example Text" >
    </TextView>

</LinearLayout>

Change your activity to the following:

package de.vogella.android.asynctask;

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;

import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class ReadWebpageAsyncTask extends Activity {
  private TextView textView;

  
/** Called when the activity is first created. */
@Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    textView = (TextView) findViewById(R.id.TextView01);
  }

  private class DownloadWebPageTask extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... urls) {
      String response = "";
      for (String url : urls) {
        DefaultHttpClient client = new DefaultHttpClient();
        HttpGet httpGet = new HttpGet(url);
        try {
          HttpResponse execute = client.execute(httpGet);
          InputStream content = execute.getEntity().getContent();

          BufferedReader buffer = new BufferedReader(new InputStreamReader(content));
          String s = "";
          while ((s = buffer.readLine()) != null) {
            response += s;
          }

        } catch (Exception e) {
          e.printStackTrace();
        }
      }
      return response;
    }

    @Override
    protected void onPostExecute(String result) {
      textView.setText(result);
    }
  }

  public void readWebpage(View view) {
    DownloadWebPageTask task = new DownloadWebPageTask();
    task.execute(new String[] { "http://www.vogella.com" });

  }
}

If you run your application and press your button then the content of the defined webpage should be read in the background. Once this process is done your TextView will be updated.

6. Tutorial: Activity lifecycle and Thread

The following example will download an image from the Internet in a thread and displays a dialog until the download is done. We will make sure that the thread is preserved even if the activity is restarted and that the dialog is correctly displayed and closed.

For this example create the Android project "de.vogella.android.threadslifecycle" and the Activity "ThreadsLifecycleActivity". Also add the permission to use the Internet to your app. Details for this can found here: Networking with Android.

You should have the following AndroidManifest.xml file.

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

    <uses-sdk android:minSdkVersion="10" />

    <uses-permission android:name="android.permission.INTERNET" >
    </uses-permission>

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

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

</manifest>

Change the layout main.xml 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" >

    <LinearLayout
        android:id="@+id/linearLayout1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="downloadPicture"
            android:text="Click to start download" >
        </Button>

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="resetPicture"
            android:text="Reset Picture" >
        </Button>
    </LinearLayout>

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:src="@drawable/icon" >
    </ImageView>

</LinearLayout>

Now adjust your activity. In this activity the thread is saved and the dialog is closed if the activity is destroyed.

package de.vogella.android.threadslifecycle;

import java.io.IOException;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.StatusLine;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpUriRequest;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;

import android.app.Activity;
import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.ImageView;

public class ThreadsLifecycleActivity extends Activity {
  // Static so that the thread access the latest attribute
  private static ProgressDialog dialog;
  private static ImageView imageView;
  private static Bitmap downloadBitmap;
  private static Handler handler;
  private Thread downloadThread;

  
/** Called when the activity is first created. */
@Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    // Create a handler to update the UI
    handler = new Handler();
    // get the latest imageView after restart of the application
    imageView = (ImageView) findViewById(R.id.imageView1);
    // Did we already download the image?
    if (downloadBitmap != null) {
      imageView.setImageBitmap(downloadBitmap);
    }
    // Check if the thread is already running
    downloadThread = (Thread) getLastNonConfigurationInstance();
    if (downloadThread != null && downloadThread.isAlive()) {
      dialog = ProgressDialog.show(this, "Download", "downloading");
    }
  }

  public void resetPicture(View view) {
    if (downloadBitmap != null) {
      downloadBitmap = null;
    }
    imageView.setImageResource(R.drawable.icon);
  }

  public void downloadPicture(View view) {
    dialog = ProgressDialog.show(this, "Download", "downloading");
    downloadThread = new MyThread();
    downloadThread.start();
  }

  // Save the thread
  @Override
  public Object onRetainNonConfigurationInstance() {
    return downloadThread;
  }

  // dismiss dialog if activity is destroyed
  @Override
  protected void onDestroy() {
    if (dialog != null && dialog.isShowing()) {
      dialog.dismiss();
      dialog = null;
    }
    super.onDestroy();
  }

  // Utiliy method to download image from the internet
  static private Bitmap downloadBitmap(String url) throws IOException {
    HttpUriRequest request = new HttpGet(url.toString());
    HttpClient httpClient = new DefaultHttpClient();
    HttpResponse response = httpClient.execute(request);

    StatusLine statusLine = response.getStatusLine();
    int statusCode = statusLine.getStatusCode();
    if (statusCode == 200) {
      HttpEntity entity = response.getEntity();
      byte[] bytes = EntityUtils.toByteArray(entity);

      Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0,
          bytes.length);
      return bitmap;
    } else {
      throw new IOException("Download failed, HTTP response code "
          + statusCode + " - " + statusLine.getReasonPhrase());
    }
  }

  static public class MyThread extends Thread {
    @Override
    public void run() {
      try {
        // Simulate a slow network
        try {
          new Thread().sleep(5000);
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
        downloadBitmap = downloadBitmap("http://www.vogella.com/img/lars/LarsVogelArticle7.png");
        handler.post(new MyRunnable());
      } catch (IOException e) {
        e.printStackTrace();
      } finally {

      }
    }
  }

  static public class MyRunnable implements Runnable {
    public void run() {
      imageView.setImageBitmap(downloadBitmap);
      dialog.dismiss();
    }
  }

}

Run your application and press the button to start a download. You can test the correct lifecycle behavior in the emulator via pressing "Ctrl+F11" as this changes the orientation.

It is important to note that the Thread is a static inner class. It is important to use a static inner class for your background process because otherwise the inner class will contain a reference to the class in which is was created. As the thread is passed to the new instance of your activity this would create a memory leak as the old activity would still be referred to by the Thread.

7. StrictMode

StrictMode is available as of API 9, therefore make sure to use Android 2.3.3. As discussed you should avoid performing long running operations on the UI thread. This includes file and network access. It is sometimes difficult to remember to make all the right things in your application during development. That is were StrictMode comes in. It allows to setup policies in your application to avoid doing incorrect things. For example the following setup will crash your application if it violates some of the Android policies. StrictMode should only be used during development and not in your live application.

Create the project called "de.vogella.android.strictmode" with the Activity called TestStrictMode. The following will set strict rules for your application. As the activity violates these settings you application will crash.

package de.vogella.android.strictmode;

import java.io.BufferedWriter;
import java.io.OutputStreamWriter;

import android.app.Activity;
import android.os.Bundle;
import android.os.StrictMode;

public class TestStrictMode extends Activity {
  @Override
  public void onCreate(Bundle savedInstanceState) {
    StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
        .detectAll().penaltyLog().penaltyDeath().build());
    StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder().detectAll()
        .penaltyLog().penaltyDeath().build());

    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    String eol = System.getProperty("line.separator");
    try {
      BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(openFileOutput("myfile", MODE_WORLD_WRITEABLE)));
      writer.write("This is a test1." + eol);
      writer.write("This is a test2." + eol);
      writer.write("This is a test3." + eol);
      writer.write("This is a test4." + eol);
      writer.write("This is a test5." + eol);
      writer.close();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

8. TraceView and Memory Dump

Traceview is a graphical viewer to see logs created by an Android application. Via Traceview you can find errors in your application and measure its performance.

8.1. Using TraceView in Eclipse

Eclipse supports tracing directly in the DDMS perspective. In the devices view select your application process and select "Start Method Profiling".

Use your application and re-press the same button to stop profiling. This will open a new editor which shows you the result of traceview.

You can zoom into the graphic to get more details.

8.2. Using TraceView from the command line

To start tracing some code put the following code snippet around it.

android.os.Debug.startMethodTracing("yourstring");

// ... your code is here

android.os.Debug.stopMethodTracing();

The parameter "yourstring" tells the system that it should store the data under "/sdcard/yourstring.trace". To save data on the sdcard your application needs the WRITE_EXTERNAL_STORAGE permission. After running your application you can use Traceview via adb .

adb pull /sdcard/yourstring.trace
traceview yourstring

This will start Traceview which allow you to analyse your performance data via a graphical way. The DDMS view has also a trace button available. This will trace the running application and does not require an additional authorization.

8.3. Memory Dumps

You can create a memory snapshot and analyse it with the Eclipse Memory Analyzer.

9. Thank you

Please help me to support this article:

Thanks

akm

www.cdacians.com

Cdacians

Thursday, 23 August 2012

Android Threads, Handlers AsyncTask