Processes and Threads

Processes ?

Android operating system is built on Linux kernel,So Linux kernel process concepts are valid for android too.

When an app component start,and the application does not have any other components running,the android system initiate a new linux process for the application with a single thread of execution.This thread is called “Main Thread” .

To identify which processes should be removed or retained,The system places each processes into an “Importance Hierarchy”.The processed with the lowest importance are removed first.

Types ?

Active Processes,
Visible Processes,
Service Processes,
Background Processes,
Empty Processes,


An android application automatically creates  a “main” thread when it starts.This is also called the UI thread.

UI thread is used for user interaction.All android applications runs on the UI main application thread.

Running all the components on a single thread will result in poor response time.

If UI thread is blocked for morethan 5 secs ,“Application Not Responding” dialog box will reveal.

Reason for ANR?

1.Doing Network operation.
2.Performing Long Calculation.
3.Accessing Database Operation on the UI thread.

Using Background Thread?

Android UI is not “Thread-Safe”.
Should not access UI operations on outside main thread or any other Thread.

Single Thread Model

1.Don’t block the UI thread – Long running operations should not perform on UI thread,It should be done in Worker thread.
2.Don’t access UI from outside main thread.Use “Message Passing” methods.

Message Passing?

To establish communication between main and worker thread,Android using message passing approach.

1.Handler : Handler object registers itself with the thread in which it is created.It provides a channel to send data to the thread.

2.Looper: It is a class which turns thread to pipeline thread.It implements the “loop” – It takes assigned task,execute it,then takes the next one,So on.
Use Looper class if you want to pass multiple messages.3.PNG

Worker Thread?

1.If you want to do long running back ground operation,you should make sure to do them in separate threads (“background” or “worker” threads).
2.You cannot update the UI from any thread other than the UI thread or the “main” thread.
3.To fix this problem, Android offers several ways to access the UI thread from other threads.

View.postDelayed(Runnable, long)

public void onClick(View v) {
    new Thread(new Runnable() {
        public void run() {
            // a potentially  time consuming task
            final Bitmap bitmap =
   Runnable() {
                public void run() {

  1. The Above implementation is thread-safe.
  2. The background operation is done from a separate thread while the ImageView is always manipulated from the UI thread.
  3. However, as the complexity of the operation grows, this kind of code can get complicated and difficult to maintain.
  4. To handle more complex interactions with a worker thread, you might consider using a Handler in your worker thread, to process messages delivered from the UI thread.
  5. The best solution, is to extend the AsyncTask class, which simplifies the execution of worker thread tasks that need to interact with the UI.

Android Service & Background Thread ?

For Long running operation you can use either background thread or Async Task

Background Thread:

1.Worker Thread,
2.Both UI and background thread runs Simultaneously.
3.Used for Performing Long running operations.

Android Service:

1.An application component.
2.Performs Long running operations in the background.
3.No user interface