The Android operating system tries to maintain the application running for as long as possible, but when the available memory is low, it will try to free resources in the system by killing the processes with lower importance first.

This is when process ranking comes into the picture; the Android processes are ranked in the next five categories from the higher priority to the lower priorities:

When the system reaches a point that it needs to release resources, the processes available to be killed will be sorted, taking into account the process rank, last used processes, and components running.

The Android application always runs under a unique Linux user ID (UID) assigned to the application during the application installation so that the process runs on a sandboxed environment, which by default, isolates your data and code execution from other apps.

In some cases, a user could explicitly be required to share the UID with another application to have access to their data:

USER     PID   PPID  VSIZE  RSS  PC  NAME
root            319   1     1537236 31324 S zygote
….
u0_a221   5993  319   1731636 41504 S com.whatsapp
u0_a96    3018  319   1640252 29540 S com.dropbox.android
u0_a255   4892  319   1583828 34552 S com.accuweather.android…

The preceding table that results from running the adb shell ps command in the computer with Android SDK Table is a list of Android running processes.

The first column shows the user identifier (UID) assigned at the time of installation, the second column is the process ID (PID), the third column shows the parent process ID (PPID) that for Android applications is the Zygote process, and the last column shows the application package.

From this list, we can assure that the WhatsApp application is running under the user ID u0_a221 with the process ID 5993 and the parent process is the Zygote process with the PID 319.

When the application process starts, apart from DVM housekeeping threads, the system creates a thread of execution called main. This thread, as the name explains, plays a crucial role in the application lifetime as it is the thread that interacts with the Android UI components, updating the state and their look on the device screen.

Moreover, by default, all the Android application components (Activity, Service, ContentProvider, and BroadcastsReceiver) are also executed over the main thread line of execution. The following image shows the lists of threads running inside an application process with the main thread at the top of the list with a unique thread ID (TID) assigned by the system:

The main thread, also known as UI Thread, is also the thread where your UI event handling occurs, so to keep your application as responsible as possible, you should:

The following diagram displays the main interactions and components involved in the Looper line of execution thread:

The UI/Main thread, which has a Looper facility attached to it, holds a queue of messages (MessageQueue) with some unit of work to be executed sequentially.

When a message is ready to be processed on the queue, the Looper Thread pops the message from the queue and forwards it synchronously to the target handler specified on the message.

When the target Handler finishes its work with the current message, the Looper thread will be ready to process the next message available on the queue. Hence, if the Handler spent a noticeable amount of time processing the message, it will prevent Looper from processing other pending messages.

For example, when we write the code in an onCreate() method in the Activity class, it will be executed on the main thread. Likewise, when we attach listeners to user-interface components to handle taps and other user-input gestures, the listener callback executes on the main thread.

For apps that do little I/O or processing, such as applications that don't do complex math calculations, don't use the network to implement features, or don't use filesystem resources, this single thread model is fine. However, if we need to perform CPU-intensive calculations, read or write files from permanent storage, or talk to a web service, any further events that arrive while we're doing this work will be blocked until we're finished.

As you can imagine, if the main thread is busy with a heavy calculation or reading data from a network socket, it cannot immediately respond to user input such as a tap or swipe.

An application that doesn't respond quickly to user interaction will feel unresponsive—anything more than a couple of hundred milliseconds delay is noticeable. This is such a pernicious problem that the Android platform protects users from applications that do too much on the main thread.

The following screenshot shows a typical Android ANR dialog:

Android works hard to synchronize the user interface redraws with the hardware-refresh rate. This means that it aims to redraw at the rate of 60 frames per second—that's just 16.67 ms per frame. If we do work on the main thread that takes anywhere near 16 ms, we risk affecting the frame rate, resulting in jank—stuttering animations, jerky scrolling, and so on.

Ideally, of course, we don't want to drop a single frame. Jank, unresponsiveness, and especially the ANR, offer a very poor user experience, which translates into bad reviews and unpopular applications. A rule to live by when building Android applications is: do not block the main thread!

Further protection was added to the platform in Honeycomb (API level 11) with the introduction of a new Exception class, NetworkOnMainThreadException, a subclass of RuntimeException that is thrown if the system detects network activity initiated on the main thread.

Ideally then, we may want to offload any long-running operations from the main thread so that they can be handled in the background by another thread, and the main thread can continue to process user-interface updates smoothly and respond in a timely fashion to user interactions.

The typical time-consuming tasks that should be handled on a background thread include the following:

For this to be useful, we must be able to coordinate the work and safely pass data between cooperating threads—especially between background threads and the main thread, and it is exactly to solve this problem that asynchronous programming is used.

Let's get started with the synchronous versus asynchronous diagram:

The preceding example graphically shows the main differences between the two models of processing. On the left-hand side, the data download task occurs on the main thread, keeping the thread busy until the download data is finished. So if the user interacts with the UI and generates an event such as a touch event, the application will suffer a lag or will become unresponsive if the download task takes a substantial amount of time to finish.

On the right-hand side, the asynchronous model will hand over the download data task to another background thread, keeping the main thread available to process any event coming from the UI interaction. When the downloaded data is available, the background task could post the result to the main thread if the data handling needs to update any UI state.

When we use an asynchronous model to program our application, the Android OS will also take advantage of additional CPU cores available in the most recent devices to execute multiple background threads at the same time and increase the application's power efficiency.

A common example of a correctness problem occurs when two threads need to modify the value of the same variable based on its current value. Let's consider that we have a myInt integer variable with the current value of 2.

In order to increment myInt, we first need to read its current value and then add 1 to it. In a single-threaded world, the two increments would happen in a strict sequence—we will read the initial value 2, add 1 to it, set the new value back to the variable, and then repeat the sequence. After the two increments, myInt holds the value 4.

In a multithreaded environment, we will run into potential timing issues. It is possible that two threads trying to increment the variable would both read the same initial value 2, add 1 to it, and set the result (in both cases, 3) back to the variable:

int myInt = 2;
...
public class MyThread extends Thread {

    public void run() {
         super.run();
         myInt++;
   }
}
...
Thread t1 = new MyThread();
Thread t2 = new MyThread();
t1.start();
t2.start();

Both threads behaved correctly in their localized view of the world, but in terms of the overall program, we will clearly have a correctness problem; 2 + 2 should not equal 3! This kind of timing issue is known as a race condition.

A common solution to correctness problems, such as race conditions, is mutual exclusion—preventing multiple threads from accessing certain resources at the same time. Typically, this is achieved by ensuring that threads acquire an exclusive lock before reading or updating shared data.

To achieve this correctness, we can make use of the synchronized construct to solve the correctness issue on the following piece of code:

Object lock = new Object();
public class MyThread extends Thread {
    public void run() {
        super.run();
        synchronized(lock) {
            myInt++;
        }
    }
}

In the preceding code, we used the intrinsic lock available in each Java object to create a mutually exclusive scope of code that will enforce that the increment sentence will work properly and will not suffer from correctness issues as explained previously. When one of the threads gets access to the protected scope, it is said that the thread acquired the lock, and after the thread gets out of the protected scope, it releases the lock that could be acquired by another thread.

Another way to create mutually exclusive scopes is to create a method with a synchronized method:

int myInt = 2;
synchronized void increment(){
    myInt++;
}
...
public class IncrementThread extends Thread {
    public void run() {
        super.run();
        increment();
    }
}

The synchronized method will use the object-intrinsic lock, where myInt is defined to create a mutually exclusive zone so IncrementThread, incrementing myInt through the increment(), will prevent any thread interference and memory consistency errors.

Liveness can be thought of as the ability of the application to do useful work and make progress towards goals. Liveness problems tend to be an unfortunate side effect of the solution to the correctness problems.

Both properties should be achieved in a proper concurrent program, notwithstanding the correctness is concerned with making progress in a program preventing a deadlock, livelock, or starvation from happening, and the correctness is concerned with making consistent and correct results.

By locking access to data or system resources, it is possible to create bottlenecks where many threads are contending to access a single lock, leading to potentially significant delays.

Worse, where multiple locks are used, it is possible to create a situation where no thread can make progress because each requires exclusive access to a lock that another thread currently owns—a situation known as a deadlock.

Thread coordination is an important topic in concurrent programming, especially when we want to perform the following tasks:

When we want to coordinate thread efforts to achieve a goal, we should try to avoid waiting or polling mechanisms that keep the CPU busy while we wait for an event in another thread.

The following example shows us a small loop where we will continuously occupy the CPU while we wait for a certain state change to happen:

while(!readyToProcess) {
  // do nothing .. busy waiting wastes processor time.
}

To overcome the coordination issue, and to implement our own constructs, we should use some low-level signals or messaging mechanisms to communicate between threads and coordinate the interaction.

In Java, every object has the wait(), notify(), and notifyAll()methods that provide low-level mechanisms to send thread signals between a group of threads and put a thread in a waiting state until a condition is met.

This mechanism, also known as monitor or guard, is a design pattern commonly used in another languages and it ensures that only one thread can enter a given section of code at any given time with an ability to wait until a condition happens.

This design pattern, in comparison with our previous example, delivers a better and efficient CPU-cycle management while waiting for any particular situation to happen on another thread, and is generally used in situations where we need to coordinate work between different lines of execution.

In the following code example, we are going to explain how to use this construct to create a basic multithreaded Logger with 10 threads that will wait in the monitor section until a message is pushed (condition) by any other thread in the application.

The Logger, which is responsible for logging on to the output, has a queue with a maximum of 20 positions to store the new logging text messages:

public class Logger {
    LinkedList<String> queue = new LinkedList<String>();
    private final int MAX_QUEUE_SIZE = 20;
    private final int MAX_THREAD_COUNT = 10;

In the next code, we will create a Runnable unit of work that runs indefinitely and retrieves a message from the queue to print the message on the Android log.

After that, we will create and start 10 threads that are going to execute the Runnable unit of work task:

public void start() {
    // Creates the Loop as a Runnable
    Runnable task = new Runnable() {
        @Override
        public void run() {
            while(true) {
                String message = pullMessage();
                Log.d(Thread.currentThread().
                         getName(),message);
		     // Do another processing
             }
         }
     };
    // Create a Group of Threads for processing
    for(int i=0; i< MAX_THREAD_COUNT; i++){
         new Thread(task).start();
    }
 }

The pullMessage(), which is a synchorized method, runs a mutual exclusion and puts the thread in the waiting state when it reaches the wait() method. All the created threads will stay in this state until another thread calls notifyAll():

// Pulls a message from the queue
// Only returns when a new message is retrieves
// from the queue.
private synchronized String pullMessage(){
    while (queue.isEmpty()) {
        try {
            wait();
        } catch (InterruptedException e) { ... }
    }
    return queue.pop();
}
// Push a new message to the tail of the queue if
// the queue has available positions
public synchronized void pushMessage(String logMsg) {
    if ( queue.size()< MAX_QUEUE_SIZE ) {
        queue.push(logMsg);      
        notifyAll();
    }
}

When any thread is in the waiting state, it releases the lock temporarily and gives a chance to another thread to enter the mutual exclusion to push new messages or enter into the wait state.

In the following snippet, we will first create the Logger instance and then we will call the start method to start the working threads and we will push 10 messages into a queue of work to be processed.

When the pushMessage()method is invoked, a new logging message is inserted at the end of the queue and notifiyAll() is invoked to notify all the available threads.

As the pullMessage() method runs in a mutual-exclusion (synchronized) zone, only one thread will wake up and return from the pull method. Once pullMessage() returns, the logging message is printed:

Logger logger =new Logger();
logger.start();
for ( int i=0; i< 10 ; i++) {
    ...
    logger.pushMessage(date+" : "+"Log Message #"+i);
}

In the following console output, we have an example of the output that this code will generate and the logging messages are processed by any available threads in an ordered manner:

D/Thread-108(23915): <Date>: Log Message #0
D/Thread-109(23915): ...: Log Message #1
D/Thread-110(23915): ...: Log Message #2
D/Thread-111(23915): ...: Log Message #3

This kind of low-level construct can also be used to control shared resources (polling) to manage background execution (parallelism) and control thread pools.

Other Java concurrent constructs provided by java.util.concurrent, which are also available on Android SDK are as follows:

Some Android-specific constructs use these classes as basic building blocks to implement their concurrent behavior, although they could be used by a developer to build custom concurrent constructs to solve a specific use case.

public interface Executor {
  void execute(Runnable command);
}

public class MyRunnable implements Runnable {
    public void run() {
        Log.d("Generic", "Running From Thread " +
              Thread.currentThread().getId());   
	 // Your Long Running Computation Task
    }
}
public void startWorking(){
    Executor executor = Executors.newFixedThreadPool(5);
    for ( int i=0; i < 20; i++ ) {
        executor.execute(new MyRunnable());
    }
}

The Activity building block has a tight connection with a presentation layer because it's the entity that manages the UI view over a defined tree of fragments and views that display information and respond to user interactions.

Android applications are typically composed of one or more subclasses of android.app.Activity. An Activity instance has a very well-defined lifecycle that the system manages through the execution of lifecycle method callbacks, all of which are executed on the main thread.

To keep the application responsive and reactive, and the activity transition smooth, the developer should understand the nature of each Activity lifecycle callback.

The most important callbacks on the Activity lifecycle are as follows:

An Activity instance that is completed should be eligible for garbage collection, but background threads that refer to Activity or part of its view hierarchy can prevent garbage collection and create a memory leak.

Similarly, it is easy to waste CPU cycles (and battery life) by continuing to do background work when the result can never be displayed as Activity is completed.

Finally, the Android platform is free at any time to kill processes that are not the user's current focus. This means that if we have long-running operations to complete, we need some way of letting the system know not to kill our process yet.

All of this complicates the do-not-block–the-main-thread rule as we need to worry about canceling background work in a timely fashion or decoupling it from the Activity lifecycle where appropriate.

The other Android-specific problem lies not in what you can do with the UI thread, but in what you cannot do.

This is because the user interface toolkit is not thread-safe, that is, accessing it from multiple threads may cause correctness problems. In fact, the user interface toolkit protects itself from potential problems by actively denying access to user interface components from threads other than the one that originally created these components.

If the system detects this, it will instantly notify the application by throwing CalledFromWrongThreadException.

The final challenge then lies in safely synchronizing background threads with the main thread so that the main thread can update the user interface with the results of the background work.

If the developer has access to an Activity instance, the runOnUiThread instance method can be used to update the UI from a background thread.

The method accepts a Runnable object like the one used to create an execution task for a thread:

public final void runOnUiThread (Runnable)

In the following example, we are going to use this facility to publish the result from a synonym search that was processed by a background thread.

To accomplish the goal during the OnCreate activity callback, we will set up onClickListener to run searchTask on a created thread:

// Get the Views references
Button search = (Button) findViewById(R.id.searchBut);
final EditText word = (EditText) findViewById(R.id.wordEt);

// When the User clicks on the search button 
// it searches for a synonym
search.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        // Runnable that Searchs for the synonym and
        // and updates the UI.
        Runnable searchTask = new Runnable() {
            @Override
            public void run() {
                // Retrieves the synonym for the word
                String result = searchSynomim(
                   word.getText().toString());
                // Runs the Runnable SetSynonymResult
                // to publish the result on the UI Thread
                runOnUiThread(new SetSynonymResult(result));
            }
        };
        // Executes the search synonym an independent thread
        Thread thread = new Thread(searchTask);
        Thread.start();
    }
});

When the user clicks on the Search button, we will create a Runnable anonymous class that searches for the word typed in R.id.wordEt EditText and starts the thread to execute Runnable.

When the search completes, we will create an instance of Runnable SetSynonymResult to publish the result back on the synonym TextView over the UI thread:

class SetSynonymResult implements Runnable {
    final String synonym;

    SetSynonymResult(String synonym){
      this.synonym = synonym;
    }
    public void run() {
      TextView tv = (TextView)findViewById(R.id.synonymTv);
      tv.setText(this.synonym);
    }
};

This technique is sometime not the most convenient one, especially when we don't have access to an Activity instance; therefore, in the following chapters, we are going to discuss simpler and cleaner techniques to update the UI from a background computing task.

These are the Android entities that run in the background, which usually perform tasks in the name application that does not require any user interaction.

Service, by default, runs in the main thread of the application process. It does not create its own thread, so if your Service is going to do any blocking operation, such as downloading an image, play a video, or access a network API, the user should design a strategy to offload the time of the work from the main thread into another thread.

As Service could have its own concurrent strategy, it should also take into account that, like Activity, it should update the UI over the main thread, so a strategy to post back the results from the background into the main loop is imperative.

In the Android services domain, the way the service is started distinguishes the nature of Service into the following two groups:

When we implement a started service, any application component is able to start it when it invokes the startService(Intent) method. Once the system receives startService(Intent) and the service is not yet started, the system calls onCreate() and then onStartCommand() with the arguments encapsulated on an Intent object. If the Service already exists, only onStartCommand() is invoked.

The callbacks used by a started service are as follows:

// Called every time a component starts the Service
// The service arguments are passed over the intent
int onStartCommand(Intent intent, int flags, int startId)

// Used to initialize your Service resources
void onCreate()

// Used to release your Service resources
void onDestroy()

In the onStartCommand() callback, once a long computing task is required to handle the service request, a handover to the background threads should be explicitly implemented and coordinated in order to avoid an undesired ANR:

int onStartCommand (Intent intent, int flags, int startId){
    // Hand over the request processing to your
    // background tasks
...
}

When the service is done, and it needs to publish results to the UI, a proper technique to communicate with the main thread should be used.

A bound service generally used when a strong interaction between an Android component and a service is required.

When the service runs on the same process, the interaction between the Android component (client) and the bound service (server) is always provided by a Binder class returned on onBind(). With the Binder instance on hand, the client has access to the service's public methods, so when any component invokes the bound service public methods, the component should be aware of the following:

This building block is a component that subscribes to system and application events and is notified when these events occur on the system. The broadcast receivers are defined statically in the application manifest or dynamically via the Context.registerReceiver().

The broadcast received is activated though the onReceive()callback and this method runs on the main thread, blocking another Android component from running if we try to execute time-consuming tasks.

Once onReceive() finishes, the system considers the object inactive and can release the resources attached to this instance and recycle the whole object. This behavior has a tremendous impact on what we can do inside, because if we hand over some processing to a concurrent thread, the resources that belong to BroadcastReceiver might be recycled and are no longer available, or in an extreme case, the process could be killed if there were no important components running on it.

The good news is that the Android platform provides specific constructs to address the concurrency general issues and to solve the specific problems presented by Android.

There are constructs that allow us to defer tasks to run later on the main thread, communicate easily between cooperating threads, and issue work to the managed pools of worker threads and reintegrate the results back in the main thread.

There are solutions to the constraints of the Activity lifecycle, both for medium-term operations that closely involve the user interface and longer-term work that must be completed even if the user leaves the application.

While some of these constructs were only introduced with newer releases of the Android platform, they are available through the support libraries, and with a few exceptions, the examples in this book target devices that run API level 8 (Android 2.2) and higher versions.