Understanding and Utilizing Threads in Android

Threads have been an essential part of computer science and a versatile tool for programmers for decades. Most modern operating systems support threads, and Android is no exception. Android’s support for threads enables programmers to split their programs into multiple units that execute commands and instructions in parallel. A thread is the smallest unit of processing that can be scheduled by an operating system. Your app’s main process (or program) can be broken into threads, which can then be scheduled simultaneously by the operating system. This method helps the programmer take care of longer processes in a separate thread, so that the main thread (in which the UI is running) remains quick and responsive to the user.

Main Thread and Background Threads

Whenever we create an activity, it runs by default in the main thread of your application. All the commands issued by the Android operation system (like onClick, onCreate, etc.) are sent to and processed by this main thread. But, if you include all of your processing logic in the main thread, your application might not be too busy to respond to the messages sent by the Android operating system. This might even make your application completely unresponsive, which would prompt Android to display the warning of “application not responding” and shut down your app by force. To prevent this problem, it’s good practice to perform all the tasks that might take a long time in a separate background thread rather than in the main thread. This would leave the main thread free to remain responsive to the user input sent by the Android operating system. One notable downside to this method is that the UI cannot be updated from the background thread, it has to be updated from the main thread. But, with a little planning ahead, this issue can be easily remedied.

Creating a Program to Show a Progress Bar Using Threads

As a demonstration of the versatility of threads, we are going to create a simple program that displays a background thread and passes information to a handler, which will update the progress bar.

Adding the Progress Bar

Android has a progress bar widget that can be used in the UI to show the progress in the UI. We are going to add the progress bar by adding the <ProgressBar> tag in the standard layout.xml file. The following is the layout.xml file:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="vertical"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

>

<TextView

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:text="@string/hello"

/>

<ProgressBar android:id="@+id/progress"

style="?android:attr/progressBarStyleHorizontal"

android:layout_width="fill_parent"

android:layout_height="wrap_content" />

</LinearLayout>

In the example above, we have added a “ProgressBar” with the ID as “progress” and the style as horizontal to display a horizontal progress bar. Below is the onCreate method of the activity:

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

PrgBar=(ProgressBar)findViewById(R.id.progress);

}

In the above example, we create the instance of the progress bar and store it in a variable “PrgBar”. If we run the program now, we should be able to see the following:

Adding a Handler

As mentioned before, we cannot update the UI from the background thread, so we will have to create a handler that will run in the main thread but receive messages from a background thread. To create a handler, we need to create an instance of the “Handler” class and override the handleMessage function as follows:

Handler handler=new Handler() {

@Override

public void handleMessage(Message msg) {

PrgBar.incrementProgressBy(5);

}

};

As you can see, we’ve chosen to increment the value of our progress bar by 5.

Creating the Thread

Next, we declare “AtomicBoolean” to start and end the thread in the activity.

AtomicBoolean ContinueThread = new AtomicBoolean(false);

Then, we create the thread using the onStart method. The method will look like this:

void onStart() {

super.onStart();

PrgBar.setProgress(0);

Thread background=new Thread(new Runnable() {

public void run() {

try {

while(ContinueThread.get()) {

Thread.sleep(1000);

handler.sendMessage(handler.obtainMessage());

}

}

catch (Throwable t) {

}

}

});

ContinueThread.set(true);

background.start();

}

Using this method, we set the Progress bar value to 0. Then, we create a new thread by passing it an instance of class, which implements the run() method. Within the run method, we have a “while” loop which will continue until the AtomicBoolean ContinueThread is true. Inside the while loop, we’ve made a thread to sleep for 1 second and then send a message to the handler which will increment the value of ProgressBar . At the end of onStart, we set the value of ContinueThread to true, and then we start the background thread.

Stopping the Thread When the Activity Stops

To stop the thread, we will just need to set the ContinueThread to “false” within the onStop method of the activity.

public void onStop() {

super.onStop();

ContinueThread.set(false);

}

If we run the program, you will see the progress bar updating as shown in the image below.

Creating a Program for a Digital Watch

Now, to further develop our use of threads, we are going to create a small digital watch program.

Creating the Activity and the TextView

To create a digital watch, first we will need to create an activity. In the activities main layout, declare a TextView. We are going to increase the font size of the TextView so that the digits in the watch are clear. Below is the main layout.xml file:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="vertical"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

>

<TextView

android:id="@+id/txtWatch"

android:textSize="50dp"

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:text="@string/hello"

/>

</LinearLayout>

Creating a Function to Display the Current Time

Now, we are going to write a function to display the current time on the TextView. The function is as follows:

public void displayCurrentTime()

{

Date dt = new Date();

int hours = dt.getHours();

int minutes = dt.getMinutes();

int seconds = dt.getSeconds();

String curTime = hours + ":" + minutes + ":" + seconds;

Watch.setText(curTime);

}

In the example above, we create a new “Date” object that initializes with the current time. Then, we read the hours, minutes, and seconds from that object, and we pass it on the Textiew. We’ll use the function below to call in the onCreate function and display the current time:

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

Watch=(TextView)findViewById(R.id.txtWatch);

displayCurrentTime();

}

And in the handleMessage function of our previously-created handler, we’ll add the following:

Handler handler=new Handler() {

@Override

public void handleMessage(Message msg) {

displayCurrentTime();

}

};

Creating the Thread

Now, within the onStart function, we’ll create a Thread that sends a message to the main thread every second. The main thread will then update the TextView so that it displays the current time. The onStart function is as follows:

public void onStart() {

super.onStart();

Thread background=new Thread(new Runnable() {

public void run() {

try {

while(ContinueThread.get()) {

Thread.sleep(1000);

handler.sendMessage(handler.obtainMessage());

}

}

catch (Throwable t) {

}

}

});

ContinueThread.set(true);

background.start();

}

Async Tasks

Android provides a convenient way to perform complex and time consuming tasks in the background by hiding the process of creating the background thread from the developer. This is done by the AsyncTask class. To create an AsyncTask, you’ll simply create a child class inheriting from AsyncTask and then just override some of its default methods. The AsyncTask class will take care of creating the background thread, and it will also release and destroy the thread once it’s no longer needed. We will just have to call the execute method on the subclass of AsyncTask .

Creating an Async Task to Read a File in the Background

Now, we are going to create a simple program to read a file in a background thread using AsyncTask. The code is as follows:

package asyncTaskDemo.com;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStream;

import java.io.InputStreamReader;

import android.app.Activity;

import android.os.AsyncTask;

import android.os.Bundle;

import android.widget.TextView;

public class AsyncTaskDemo extends Activity {

private TextView txtContent;

/** Called when the activity is first created. */

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

txtContent = (TextView) findViewById(R.id.txtContent);

ReadFileTask task = new ReadFileTask();

task.execute();

}

public InputStream openResourceFile(int id)

{

return getResources().openRawResource(id);

}

private class ReadFileTask extends AsyncTask<Void, Void, String> {

@Override

protected String doInBackground(Void... UnUsed) {

String response = "";

String str="";

StringBuffer buf = new StringBuffer();

InputStream is = openResourceFile(R.drawable.textfile);

BufferedReader reader = new BufferedReader(new InputStreamReader(is));

if (is!=null) {

try {

while ((str = reader.readLine()) != null) {

buf.append(str + "n" );

}

is.close();

} catch (IOException e) {

e.printStackTrace();

}

}

response = buf.toString();

return response;

}

@Override

protected void onPostExecute(String result) {

txtContent.setText(result);

}

}

}

Here we have a TextView within the UI on which we are going to display the contents of a file. This file will be read in the background using a separate thread. In the onCreate method, we simply get the TextView and store it in a member variable of the class. Finally, we create an instance of the AsyncTask class and execute it.

Async Task Arguments

While declaring the subclass to AsyncTask, we can also declare the types of arguments that we will pass to the different methods within our sub class. These arguments will override the defaults. In our ReadFileTask class, we have used void, void, and string, so void will be the argument passed to the doInBackground function, void will be the argument passed to the onProgressUpdate function, and string will be the argument passed to the onPostExecute function.

private class ReadFileTask extends AsyncTask<Void, Void, String>

The Methods of AsyncTask

Now, we will override the doInBackground method of AsyncTask — the code that will run in background thread. Here, we are just opening a file in the drawable folder, reading it line by line, and storing it within a string buffer. Then, we override the onPostExecute method, which will be called when the background thread has finished performing the task of displaying the file on the device’s screen. If we run the program now, the output will be as follows:

Conclusion

Developers always have to perform complex and time-consuming tasks within our Android apps. If these tasks aren’t segmented in separate threads, they might cause our application to hang, freeze, crash, or simply be a slow and inefficient. The benefits of using separate threads outweigh the downsides, so you should use them often to make snappy, responsive, crashproof Android apps.

Frequently Asked Questions (FAQs) about Android Threading