thread handling in android

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Jul

Android Async Task http://www.xoriant.com/blog/mobile-applicatio

1 of 14 Tuesday 30 August 2011 03



Android Async Task

ReplyAndroid, Mobile Application Development July 15th, 2010Prashant Thakkar

This blog post introduces one of the important mechanism of Async Task for

Android with sample code.

Android implements single thread model and whenever an Android application is

launched, a thread is created. Now assume you have long running operations

like a network call on a button click in your application. On button click a

request would be made to the server and response will be awaited. Now due to

the single thread model of Android, till the time response is awaited your UI

screen hangs or in other words, it is non-responsive.

We can overcome this by creating a new Thread and implement the run method

to perform the time consuming operation, so that the UI remains responsive.

As shown below a new Thread is created in onClick method

But since Android follows single thread model and Android UI toolkit is not

thread safe, so if there is a need to make some change to the UI based on theresult of the operation performed, then this approach may lead some issues.

There are various approaches via which control can be given back to UI thread

(Main thread running the application). Handler approach is one among the

various approaches.

Handler

Let us look at the code snippet below to understand Handler approach.

12345678

public void onClick(View v) {Thread t = new Thread(){public void run(){

// Long running operation}};t.start();

}

?





As seen we have modified the original run method and added code to create

Message object, which is then passed as parameter to sendMessage method of

Handler. Now let us look at the Handler. Note that the code below is present

in the main activity code.

After the execution of long running operation, the result is set in Message and

passed to sendMessage of handler. Handle will extract the response from

Message and will process and accordingly update the UI. Since Handler is part

of main activity, UI thread will be responsible for updating the UI.

Handler approach works fine, but with increasing number of long operations,

Thread needs to be created, run method needs to be implemented and Handler

needs to be created. This can be a bit cumbersome. The Android framework has

identified this pattern and has nicely enveloped it into what is called an Android

Async Task . Let us look at how it can help simplify things.

Async Task

Android Async Task takes cares of thread management and is the recommended

mechanism for performing long running operations.

Let us look at a sample class LongOperation, which extends the AsyncTask

below:

01020304050607

080910111213

public void onClick(View v) {Thread t = new Thread(){public void run(){

// Long time comsuming operationMessage myMessage=new Message();Bundle resBundle = new Bundle();resBundle.putString("status", "SUCCESS");

myMessage.obj=resBundle;handler.sendMessage(myMessage);

}};t.start();

}

123456

private Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {// Code to process the response and update UI.

}};

?

?





Modify the onClick method as shown below:

As seen class LongOperation extends AsyncTask and implements 4 methods:

doInBackground: Code performing long running operation goes in this

method. When onClick method is executed on click of button, it callsexecute method which accepts parameters and automatically calls

doInBackground method with the parameters passed.

1.

onPostExecute: This method is called after doInBackground method

completes processing. Result from doInBackground is passed to this

method.

2.

onPreExecute: This method is called before doInBackground method is

called.

3.

01020304050607

080910111213141516171819

202122232425262728293031

32

private class LongOperation extends AsyncTask<String, Void, Strin

@Overrideprotected String doInBackground(String... params) {// perform long running operation operationreturn null;

}

/* (non-Javadoc)* @see android.os.AsyncTask#onPostExecute(java.lang.Object)*/@Overrideprotected void onPostExecute(String result) {// execution of result of Long time consuming operation

} /* (non-Javadoc)* @see android.os.AsyncTask#onPreExecute()*/

@Overrideprotected void onPreExecute() {// Things to be done before execution of long running operation.}

/* (non-Javadoc)* @see android.os.AsyncTask#onProgressUpdate(Progress[])*/@Overrideprotected void onProgressUpdate(Void... values) {

// Things to be done while execution of long running operat}

}

123

public void onClick(View v) {new LongOperation().execute("");}

?

?





onProgressUpdate: This method is invoked by calling publishProgress

anytime from doInBackground call this method.

4.

Overriding onPostExecute, onPreExecute and onProgressUpdate is optional.

Points to remember:

Instance of Async Task needs to be created in UI thread. As shown in

onClick method a new instance of LongOperation is created there. Also

execute method with parameters should be called from UI thread.

1.

Methods onPostExecute, onPreExecute and onProgressUpdate should not

be explicitly called.

2.

Task can be executed only once.3.

Hope this blog helped you understand Android async task and why it is

important in Android application development.

Prashant Thakkar – Team member – Xoriant Mobile Center of Excellence.

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This entry was posted on Jul 15th, 2010 at 3:49 am and is filed under Android, Mobile Application

Development.You can follow any responses to this entry through the RSS 2.0. You can Leave a

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27 Responses to “Android Async Task”

praveen

August 7, 2010 at 5:51 pm

Thank you a lot Prashant. It is very useful.





Fredrik

August 17, 2010 at 10:09 am

Great, worked like a charm!

Lonnie VanZandt

August 29, 2010 at 5:30 pm

Prashant, this pattern appears adequate for run-to-completion asynchronous

operations. What is the recommended Android pattern for asynchronous

operations which periodically or sporadically repeat?

For example, what is the recommended approach for a background job (not

wanting to lead an answer by using “task” or “service”) which wakes up,checks query parameters, prepares a web service query, executes the query,

marshals query results, posts a notification to observers, and goes to sleep

for some short while. When it awakes, it repeats the “job”. It does this over

and over until instructed to cancel itself.

Tilsan

August 31, 2010 at 7:04 am

This is a Nice Article. Keep up ur good Work !!!!!!!!!!

Prashant Thakkar

September 1, 2010 at 1:32 am

Hi Lonnie,

As per my understanding for the kind of tasks (Job) that you want toperform, writing a service would be an better approach. Service runs in the

background for indefinite period of time and it is possible to control and

access the service via the exposed interface.

Yadavendra






Thanks a lot .This is very helpful

Matze


Hey Prashant,

does “Task can be executed only once” mean it can only be executed once

and than again if the task has finished or it can not be executed ever again?

=)

Might sound a little weird but I don’t need to call it serveral times over and

over again, so it should be executed before I call it again.

MatzeSeptember 23, 2010 at 8:09 am

Oh, thx for your tutorial by the way, it’s really good =)

Ron

October 3, 2010 at 3:28 am

Hi very useful BUT..

where you have called new LongOperation().execute(“”) I am calling new

CameraTask().execute() and getting an error ‘cannot be resolved to a type’.

I have the Async import at the top of the file. I also received an error re

‘private class CameraTask extends AsyncTask’ in that only abstract or final

are permitted. I am hoping someone can shed some light on this.

Thanks

Ron

October 3, 2010 at 3:32 am

hi found misplaced } which fixed it!

Vijay

November 29, 2010 at 4:19 am





Hi, thanks for the good tutorial. How do we get back the result to the calling

UI thread.

Prashant Thakkar

November 29, 2010 at 5:08 am

AsyncTask is either written as anonymous inner classes i.e. within the

method or as a private inner class in the same class file. So UI component to

be modified can be accessed by declaring the it as global variable.

Example:private void doSomething() {

new AsyncTask<String, Void, String>(){

@Override

protected void onPostExecute(String result) {

if (result != null && result.trim().length() == 0){

textView.setText(result);

}else{

textView.setText("Empty Input Parameter");

}

}

@Overrideprotected String doInBackground(String… param){

return param;

}

}.execute("value1");

}

Thanks

Prashant Thakkar

Dattatraya

February 1, 2011 at 12:52 am

Hi Prashant,

The code the return type of the doInBackground() is String. But i want to

load the images from url in background. it is possible…..





Thanks in advanced.

Prashant Thakkar

February 1, 2011 at 2:44 am

Yeah it possible to fetch image from server.

You can perform any operation in doInBackground() and accordingly return

string based on whether operation completed successfully or not. For

example

@Override

protected String doInBackground(String… params) {

try{// fetching image

return success;

}catch(Exception e){

return failed;

}

}

The String returned via doInBackground() is available as input parameter

for onPostExecute(String result) method.

ThanksPrashant Thakkar

P

February 5, 2011 at 5:29 pm

The problem with this basic approach is that if something causes the

activity instance to be destroyed (like a screen orientation change) while

your async task is performing its background processing, then by the time

the onPostExecute() method is invoked, back on the UI thread, you will bereferencing the old (dead) Activity instance. This leads to the ‘Leaked

Window’ error message amongst other things, same goes for

onProgressUpdate().

Mukund

March 4, 2011 at 5:24 am





All,

Is it necessary to create a service only to perform a recurring task…?

How about having a Handler…?I shall invoke sendMessageAtTime(Message, long) instead of

dispatchMessage(Message).

In handleMessage(Message), I will call the sendMessageAtTime(Message,

long) method, which is nothing but the recurring.

Mukund

March 4, 2011 at 5:27 am

All,

Is it necessary to create a service everytime for a recurring task?

How about using a Handler instead?

I shall call invoke method sendMessageAtTime(Message, long) for the

purpose.

Jason Mayor

March 21, 2011 at 4:03 am

Hi,

I am continuosly getting “This task has already been executed. A task can

be executed only once.” I want to cancel the current running task and start

a new one upon request.

Please suggest,

Json

Dattatraya

April 18, 2011 at 2:18 am

Hi Prashant,





I am new in android, i read your blog post regularly. Now I am facing one

problem, it may not related to AsyncTask, but I hope you help me. I am

doing on web base Android Application. I that one view is contain category

in table layout. when we click on any category I get the id of that category,

pass it in URL, send the request to server, get XML, Parse this XML(XML

contain thumb nail image and description), and display that information in

table layout exactly belove the selected category. In my case there are manyrecord(more than 50) so it take 1-2 minute to display thumbnail images on

table layout. I want to show the dialog box while loading and drawing the

thumbnail.

I am trying as u mentioned above. but I am not able to show dialog box at

write time and write place. It display the dialog box after displaying all

thumbnail(may be the boolean return type of On Click).

Please Give me some solution on it. If u want i will give u my source code

also.

Thanks in advance.

khangaiMay 5, 2011 at 4:10 am

thanks. it help me lot

Jay

May 11, 2011 at 7:18 am

Thabk so much!! Very helpful!!!

Vika

June 10, 2011 at 1:56 am

I’d like to show ProgressDialog when the data is passing from database to

2nd spinner.

But something wrong in my code. Could you help me, please?

Spinner marke = (Spinner) findViewById(R.id.spinner1);

marke.setOnItemSelectedListener(new OnItemSelectedListener() {

@Override

public void onItemSelected(AdapterView adapter, View view, int pos, long

id) {

new Background().execute(null);//some code





public class Background extends AsyncTask {

@Override

protected void onPreExecute() {

dialog.show();

dialog.setMessage(“Please, wait!”);

}

protected Void doInBackground(Void… params) {

return null;

}

protected void onPostExecute(Void result) {

super.onPostExecute(result);

if (dialog != null) {

dialog.dismiss();

}

}

Chris

June 14, 2011 at 2:03 pm

thanks a lot for this pretty nice article!

pratheeja

June 21, 2011 at 9:47 pm

hi Prashant,

Thank you for ths nice article. It helped me a lot.

Seth Kigen

June 29, 2011 at 10:43 pm

Thanks for this post, it was really helpful.

Saved me a lot.

Nital

July 4, 2011 at 3:06 am





Thanks a lot for this post.. very well explained!!!

Jon

July 25, 2011 at 6:49 pm

Thank you for a clear explanation of how to use a nested onpostexecute to

pass values back to the original activity.

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