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Home > Programming > Concurrency > Learning Concurrency in Kotlin
Learning Concurrency in Kotlin
Learning Concurrency in Kotlin

Learning Concurrency in Kotlin: Build highly efficient and scalable applications

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Learning Concurrency in Kotlin

Coroutines in Action

It's time to start coding. In this chapter, we will go through the process of enabling support for coroutines in an Android project using Android Studio, and we will use coroutines to solve a common scenario for mobile apps: calling a REST service and displaying some of the response without blocking the UI thread.

Topics that will be covered in this chapter include:

  • Android Studio configuration for Kotlin and projects using coroutines
  • Android's UI thread
  • REST calls in a background thread using coroutines
  • Coroutine builders async() and launch()
  • Introduction to coroutine dispatchers

Downloading and installing Android Studio

The first step for this chapter is to install Android Studio. For this, you can head to https://developer.android.com/studio/index.html and download the installer for your platform.

Kotlin support was added to Android Studio in version 3.0. If you already have Android Studio installed, please check that the version is at least 3.0. It's recommended that you update to the newest stable version of Android Studio if you have an earlier version.
Downloading Android Studio from the official web page

Once you have finished the download, follow the steps on the official guide at https://developer.android.com/studio/install.html to complete the installation. By default, the instructions are going to be according to the operating system detected by the web page; if you wish to see the instructions for a different OS, please select the relevant...

Creating a Kotlin project

When you first open Android Studio, you will be presented with a wizard. To start, select Start a new Android Studio project,as shown in the following screenshot:

Android Studio wizard

Now you will be asked to define the properties of the project. Please make sure that the Include Kotlin support option is enabled:

Setting the properties of a new Android project with Kotlin support

The next step is to select the Android version that you are targeting. The default options are good for this step:

Selecting the target SDKs

Then, select the option to have just an empty activity, and leave its configuration as default:

Selecting an empty activity

Now you will be prompted to select the name for the class and the layout of the activity. Please write MainActivity as the name of the activity, while setting activity_main as the name of the layout:

Configuring...

Adding support for coroutines

Now that the project has been created, it's time to add coroutines support. To do this, open the Project section and double-click in build.gradle for the project. It is located inside the Gradle Scripts section:

The correct build.gradle file says Project: RssReader inside parenthesis

Inside this document, you will add a new variable to centralize the coroutines version. In a new line below ext.kotlin.version, add ext.coroutines_version, as shown here, for example:

buildscript {
  ext.kotlin_version = '1.2.50'
  ext.coroutines_version = '0.23.3'
  ...
}
Every time you modify the build.gradle files, Android Studio will ask for the configuration to be synchronized again. Until the synchronization is made, the changes will not be applied.

Now you need to add the dependencies for coroutines. Append the dependency configuration in...

Android's UI thread

As mentioned in the first chapter, Android applications have a thread that is dedicated to updating the UI and also to listening to user interactions and processing the events generated by the user – like the user clicking on a menu.

Let's review the basics of Android's UI thread to guarantee that we separate the work between the UI and background threads in the best way possible.

CalledFromWrongThreadException

Android will throw a CalledFromWrongThreadException whenever a thread that didn't create a view hierarchy tries to update its views. In practical terms, this exception will happen whenever a thread other than the UI thread updates a view. This is because the UI thread is...

Creating a thread

Kotlin has simplified thread creation so that it's a straightforward process and can be done easily. For now, creating a single thread will be enough, but in future chapters we will also create thread pools in order to run both CPU-bound and I/O-bound operations efficiently.

CoroutineDispatcher

It's important to understand that in Kotlin, while you can create threads and thread pools easily, you don't access or control them directly. What you do is create a CoroutineDispatcher, which is basically an orchestrator that distributes coroutines among threads based on availability, load, and configuration.

In our current case, for example, we will create a CoroutineDispatcher that has only one thread...

Adding networking permissions

Android requires applications to explicitly request permissions in order to access many features. This is done in order to present the user with an option to deny specific permissions, and prevent applications from doing something different from what the user expects.

Since we will be doing network requests, we will need to add the internet permission to the manifest of the application. Let's locate the AndroidManifest.xml file in the app/src/main directory, and edit it:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="co.starcarr.rssreader">
  <uses-permission android:name="android.permission.INTERNET" />
  <application
  ...
</manifest>
The name Internet is not accurate. This permission is needed in order to do any network request regardless of it actually reaching the...

Creating a coroutine to call a service

Now it's a good time to add a call to a service. To start with something simple, we will use Java's DocumentBuilder to call an RSS feed for us. First, we will add a variable to hold the DocumentBuilderFactory below where we put the dispatcher:

private val dispatcher = newSingleThreadContext(name = "ServiceCall")
private val factory = DocumentBuilderFactory.newInstance()

The second step is to create a function that will do the actual call:

private fun fetchRssHeadlines(): List<String> {
  val builder = factory.newDocumentBuilder()
  val xml = builder.parse("https://www.npr.org/rss/rss.php?id=1001")
  return emptyList()
}

Notice how the function is, for now, returning an empty list of strings after calling the feed. The idea is to implement this function so that it returns the headlines of the given feed. But...

Adding UI elements

Now we can start adding some UI elements to do some testing. First, let's update the layout of MainActivity, which is located in res/layout/activity_main.xml. For now, just adding a ProgressBar will do, so let's replace the contents of the ConstraintLayout so that a ProgressBar is located in the middle of the screen:

<android.support.constraint.ConstraintLayout ...>
    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
&lt...

Displaying the amount of news that were processed

Let's put a TextView in our layout and display the amount of news that were processed from the feed. Notice that the TextView will be located below the progress bar because of the property app:layout_constraintBottom_toBottomOf:

<android.support.constraint.ConstraintLayout ...>
    <ProgressBar ...>
    <TextView
        android:id="@+id/newsCount"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        app:layout_constraintTop_toBottomOf="@id/progressBar"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent" />

</android.support.constraint.ConstraintLayout>

In order to display the amount of news, we will obtain...

Using a UI dispatcher

In the same way that we used a CoroutineDispatcher to run a thread in the background, we can now use one to preform operations in the main thread.

Platform-specific UI libraries

Given that there are many types of GUI applications for the JVM, Kotlin has separated some platform-specific coroutine functionality into libraries.

  • kotlinx-coroutines-android
  • kotlinx-coroutines-javafx
  • kotlinx-coroutines-swing

Notice that all these platforms have the same UI model that we talked about before, in which only the UI thread can create and update the views. So, these tiny libraries are simply a CoroutineDispatcher that is implemented to confine coroutines to the UI thread.

The Android library also adds support for...

Creating an asynchronous function to hold the request... or not

Currently, a big part of our code to request and display the number of news is inside the onCreate() function. This is less than optimal, not only because it's mixed with the creation of the activity, but also because it prevents reusing this code. For example, if there were to be a refresh button, we would need to reuse all the code of the coroutine.

When considering the separation of this coroutine into its own function, there are many possible approaches. Here, we will cover the most common ones.

A synchronous function wrapped in an asynchronous caller

The first approach is quite simple. We can create a loadNews() function that directly invokes fetchRssHeadlines...

Summary

This chapter covered many interesting topics of practical concurrency in Kotlin. Let's do a recap of the key topics.

  • Android applications will throw NetworkOnMainThreadException if a network request is done on the UI thread.
  • Android applications can only update the UI on the UI thread, and trying to do it from a different thread will produce a CalledFromWrongThreadException.
  • Network requests have to be done in a background thread. The information has to be sent to the UI thread for the views to be updated.
  • A CoroutineDispatcher can be used to enforce a coroutine to run in a specific thread, or group of threads.
  • One or many coroutines can be run in a thread by using launch or async.
  • launch should be used in fire-and-forget scenarios, meaning cases where we aren't expecting the coroutine to return something.
  • async should be used when the coroutine will produce...
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Key benefits

  • Understand Kotlin’s unique approach to multithreading
  • Learn how to write concurrent non-blocking code with the help of practical examples
  • Improve the overall execution speed in multiprocessor and multicore systems

Description

Kotlin is a modern and statically typed programming language with support for concurrency. Complete with detailed explanations of essential concepts, practical examples and self-assessment questions, Learning Concurrency in Kotlin addresses the unique challenges in design and implementation of concurrent code. This practical guide will help you to build distributed and scalable applications using Kotlin. Beginning with an introduction to Kotlin's coroutines, you’ll learn how to write concurrent code and understand the fundamental concepts needed to write multithreaded software in Kotlin. You'll explore how to communicate between and synchronize your threads and coroutines to write collaborative asynchronous applications. You'll also learn how to handle errors and exceptions, as well as how to work with a multicore processor to run several programs in parallel. In addition to this, you’ll delve into how coroutines work with each other. Finally, you’ll be able to build an Android application such as an RSS reader by putting your knowledge into practice. By the end of this book, you’ll have learned techniques and skills to write optimized code and multithread applications.

Who is this book for?

If you’re a Kotlin or Android developer interested in learning how to write concurrent code to enhance the performance of your applications, then this is the book for you. Basic programming knowledge of Java or Kotlin will help you understand the concepts covered in this book.

What you will learn

  • Build secure applications by testing your concurrent code
  • Implement sequential and asynchronous suspending functions
  • Create suspending data sources that can be resumed on demand
  • Explore best practices for error handling
  • Use channels to communicate between coroutines
  • Discover how coroutines help to build parallel applications
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Publication date : Jul 30, 2018
Length: 266 pages
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ISBN-13 : 9781788627160
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Concepts :
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Publication date : Jul 30, 2018
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ISBN-13 : 9781788627160
Vendor :
JetBrains
Category :
Programming
Languages :
Kotlin
Concepts :
Concurrency

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Table of Contents

10 Chapters
Hello, Concurrent World! Chevron down icon Chevron up icon
Hello, Concurrent World!
Processes, threads, and coroutines
Introduction to concurrency
Concurrency is not parallelism
CPU-bound and I/O-bound
Why concurrency is often feared
Concurrency in Kotlin
Concepts and terminology
Summary
Coroutines in Action Chevron down icon Chevron up icon
Coroutines in Action
Downloading and installing Android Studio
Creating a Kotlin project
Adding support for coroutines
Android's UI thread
Creating a thread
Adding networking permissions
Creating a coroutine to call a service
Adding UI elements
Displaying the amount of news that were processed
Using a UI dispatcher
Creating an asynchronous function to hold the request... or not
Summary
Life Cycle and Error Handling Chevron down icon Chevron up icon
Life Cycle and Error Handling
Job and Deferred
States move in one direction only
RSS – Reading from multiple feeds concurrently
Non-happy path – Unexpected crash
Summary
Suspending Functions and the Coroutine Context Chevron down icon Chevron up icon
Suspending Functions and the Coroutine Context
Improving the UI of the RSS Reader
Suspending functions
The coroutine context
More about contexts
Summary
Iterators, Sequences, and Producers Chevron down icon Chevron up icon
Iterators, Sequences, and Producers
Suspendable sequences and iterators
Iterators
Sequences
Suspending Fibonacci
Producers
Producers in action
Summary
Channels - Share Memory by Communicating Chevron down icon Chevron up icon
Channels - Share Memory by Communicating
Understanding channels
Types of channels and backpressure
Interacting with channels
Channels in action
Summary
Thread Confinement, Actors, and Mutexes Chevron down icon Chevron up icon
Thread Confinement, Actors, and Mutexes
Atomicity violation
Thread confinement
Actors
Mutual exclusions
Volatile variables
Atomic data structures
Actors in action
Summary
Testing and Debugging Concurrent Code Chevron down icon Chevron up icon
Testing and Debugging Concurrent Code
Testing concurrent code
Writing the tests
Debugging
Resiliency and stability
Summary
The Internals of Concurrency in Kotlin Chevron down icon Chevron up icon
The Internals of Concurrency in Kotlin
Continuation Passing Style
Context switching
Summary
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Cliente Amazon Sep 07, 2020
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Very good book, very much comprehensive! It covers a lot of topics and it is really simple to understand. Highly recommended!
Amazon Verified review Amazon
Dimitri K May 30, 2020
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
Lots of buzzwords like "blocking" but no clear explanation of principles. Examples for Android, have many unnecessary details, but no organization. Messy, verbose book, with lots of marketing buzzwords, but poor on content. I only give it 3 stars because there is no good one, at least I could not find, and official spec is also very poor.
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Albert Sep 25, 2019
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
This book is a great start but lacks depth. In some cases, the introduction and summary are longer than the content. I believe that this book provides a good bridge to concurrency in Kotlin but refer to other books for a more in-depth dive in concurrency.
Amazon Verified review Amazon
Ahmed Shakil Sep 08, 2019
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Honestly I was not expecting this book to explain Coroutines in such an easy and simple way! Author is very experienced in Coroutines and knows how to transfer that knowledge through a book.I request the author to please update the book with new language features and final version of Coroutines release.
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Andrew E. Olson Aug 29, 2019
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
The coverage is pretty in depth and addressing the relevancy of Kotlin on both Android AND desktop/server is a good approach for breadth. That said, since this was written, Kotlin has moved on so that coroutines have since stabilized which puts the project setup instructions out of date and much of the code out of date. And as of Kotlin 1.3, coroutines have been declared a stable API making a potential new edition of this not only up-to-date, but LASTINGLY. Furthermore, Kotlin-native is now a thing and coverage on concurrency there to compliment the android/desktop breadth would take it another step along with some mention of how coroutines apply to the JavaScript target. So, in short, this is a wonderful book, but it is stuck in yesterday for the time being. And the subject is exploding with relevancy as of the new stability in 1.3 so I would be excited for a new edition. (as a note, I gave the android project with an OLD coroutines version a stab so that I could use the supplied code verbatim, hoping gradle could be coerced to work with the very old kotlin compiler + kotlinx libraries. Well, Android Studio flat out refused ancient Kotlin which is understandable since an IDE front-end must know how to parse a language, but command line gradle also wouldn't cooperate and a modest attempt to use old gradle versions also stumped me with tripwires. I hesitate to flatten a box to go old ubuntu + old android studio because it'll just be more work to learn now irrelevant info. Book really needs a 1.3 bump.
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About the author

Profile icon Castiblanco Torres
Castiblanco Torres
Miguel Angel Castiblanco Torres is a software engineer living in the United States. He works as a full-stack technical leader and software designer at Globant, where he has led many successful projects for a Forbes' Top Ten World's Most Valuable Brand and Top Five Regarded Company. Passionate about what's next, Miguel was an early adopter of Kotlin, writing about Kotlin's concurrency primitives from the first beta release of coroutines. He always keeps an eye on the new and upcoming features of Kotlin.
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Economy: Delivery to most addresses in the U.K. within 7-9 business days.
Shipments are not trackable

Premium: Trackable delivery to most addresses in the U.K. within 3-4 business days!
Add one extra business day for deliveries to Northern Ireland and Scottish Highlands and islands

EU:

Premium: Trackable delivery to most EU destinations within 4-9 business days.

Australia:

Economy: Can deliver to P. O. Boxes and private residences.
Trackable service with delivery to addresses in Australia only.
Delivery time ranges from 7-9 business days for VIC and 8-10 business days for Interstate metro
Delivery time is up to 15 business days for remote areas of WA, NT & QLD.

Premium: Delivery to addresses in Australia only
Trackable delivery to most P. O. Boxes and private residences in Australia within 4-5 days based on the distance to a destination following dispatch.

India:

Premium: Delivery to most Indian addresses within 5-6 business days

Rest of the World:

Premium: Countries in the American continent: Trackable delivery to most countries within 4-7 business days

Asia:

Premium: Delivery to most Asian addresses within 5-9 business days

Disclaimer:
All orders received before 5 PM U.K time would start printing from the next business day. So the estimated delivery times start from the next day as well. Orders received after 5 PM U.K time (in our internal systems) on a business day or anytime on the weekend will begin printing the second to next business day. For example, an order placed at 11 AM today will begin printing tomorrow, whereas an order placed at 9 PM tonight will begin printing the day after tomorrow.


Unfortunately, due to several restrictions, we are unable to ship to the following countries:

  1. Afghanistan
  2. American Samoa
  3. Belarus
  4. Brunei Darussalam
  5. Central African Republic
  6. The Democratic Republic of Congo
  7. Eritrea
  8. Guinea-bissau
  9. Iran
  10. Lebanon
  11. Libiya Arab Jamahriya
  12. Somalia
  13. Sudan
  14. Russian Federation
  15. Syrian Arab Republic
  16. Ukraine
  17. Venezuela
What is custom duty/charge? Chevron down icon Chevron up icon

Customs duty are charges levied on goods when they cross international borders. It is a tax that is imposed on imported goods. These duties are charged by special authorities and bodies created by local governments and are meant to protect local industries, economies, and businesses.

Do I have to pay customs charges for the print book order? Chevron down icon Chevron up icon

The orders shipped to the countries that are listed under EU27 will not bear custom charges. They are paid by Packt as part of the order.

List of EU27 countries: www.gov.uk/eu-eea:

A custom duty or localized taxes may be applicable on the shipment and would be charged by the recipient country outside of the EU27 which should be paid by the customer and these duties are not included in the shipping charges been charged on the order.

How do I know my custom duty charges? Chevron down icon Chevron up icon

The amount of duty payable varies greatly depending on the imported goods, the country of origin and several other factors like the total invoice amount or dimensions like weight, and other such criteria applicable in your country.

For example:

  • If you live in Mexico, and the declared value of your ordered items is over $ 50, for you to receive a package, you will have to pay additional import tax of 19% which will be $ 9.50 to the courier service.
  • Whereas if you live in Turkey, and the declared value of your ordered items is over € 22, for you to receive a package, you will have to pay additional import tax of 18% which will be € 3.96 to the courier service.
How can I cancel my order? Chevron down icon Chevron up icon

Cancellation Policy for Published Printed Books:

You can cancel any order within 1 hour of placing the order. Simply contact customercare@packt.com with your order details or payment transaction id. If your order has already started the shipment process, we will do our best to stop it. However, if it is already on the way to you then when you receive it, you can contact us at customercare@packt.com using the returns and refund process.

Please understand that Packt Publishing cannot provide refunds or cancel any order except for the cases described in our Return Policy (i.e. Packt Publishing agrees to replace your printed book because it arrives damaged or material defect in book), Packt Publishing will not accept returns.

What is your returns and refunds policy? Chevron down icon Chevron up icon

Return Policy:

We want you to be happy with your purchase from Packtpub.com. We will not hassle you with returning print books to us. If the print book you receive from us is incorrect, damaged, doesn't work or is unacceptably late, please contact Customer Relations Team on customercare@packt.com with the order number and issue details as explained below:

  1. If you ordered (eBook, Video or Print Book) incorrectly or accidentally, please contact Customer Relations Team on customercare@packt.com within one hour of placing the order and we will replace/refund you the item cost.
  2. Sadly, if your eBook or Video file is faulty or a fault occurs during the eBook or Video being made available to you, i.e. during download then you should contact Customer Relations Team within 14 days of purchase on customercare@packt.com who will be able to resolve this issue for you.
  3. You will have a choice of replacement or refund of the problem items.(damaged, defective or incorrect)
  4. Once Customer Care Team confirms that you will be refunded, you should receive the refund within 10 to 12 working days.
  5. If you are only requesting a refund of one book from a multiple order, then we will refund you the appropriate single item.
  6. Where the items were shipped under a free shipping offer, there will be no shipping costs to refund.

On the off chance your printed book arrives damaged, with book material defect, contact our Customer Relation Team on customercare@packt.com within 14 days of receipt of the book with appropriate evidence of damage and we will work with you to secure a replacement copy, if necessary. Please note that each printed book you order from us is individually made by Packt's professional book-printing partner which is on a print-on-demand basis.

What tax is charged? Chevron down icon Chevron up icon

Currently, no tax is charged on the purchase of any print book (subject to change based on the laws and regulations). A localized VAT fee is charged only to our European and UK customers on eBooks, Video and subscriptions that they buy. GST is charged to Indian customers for eBooks and video purchases.

What payment methods can I use? Chevron down icon Chevron up icon

You can pay with the following card types:

  1. Visa Debit
  2. Visa Credit
  3. MasterCard
  4. PayPal
What is the delivery time and cost of print books? Chevron down icon Chevron up icon

Shipping Details

USA:

'

Economy: Delivery to most addresses in the US within 10-15 business days

Premium: Trackable Delivery to most addresses in the US within 3-8 business days

UK:

Economy: Delivery to most addresses in the U.K. within 7-9 business days.
Shipments are not trackable

Premium: Trackable delivery to most addresses in the U.K. within 3-4 business days!
Add one extra business day for deliveries to Northern Ireland and Scottish Highlands and islands

EU:

Premium: Trackable delivery to most EU destinations within 4-9 business days.

Australia:

Economy: Can deliver to P. O. Boxes and private residences.
Trackable service with delivery to addresses in Australia only.
Delivery time ranges from 7-9 business days for VIC and 8-10 business days for Interstate metro
Delivery time is up to 15 business days for remote areas of WA, NT & QLD.

Premium: Delivery to addresses in Australia only
Trackable delivery to most P. O. Boxes and private residences in Australia within 4-5 days based on the distance to a destination following dispatch.

India:

Premium: Delivery to most Indian addresses within 5-6 business days

Rest of the World:

Premium: Countries in the American continent: Trackable delivery to most countries within 4-7 business days

Asia:

Premium: Delivery to most Asian addresses within 5-9 business days

Disclaimer:
All orders received before 5 PM U.K time would start printing from the next business day. So the estimated delivery times start from the next day as well. Orders received after 5 PM U.K time (in our internal systems) on a business day or anytime on the weekend will begin printing the second to next business day. For example, an order placed at 11 AM today will begin printing tomorrow, whereas an order placed at 9 PM tonight will begin printing the day after tomorrow.


Unfortunately, due to several restrictions, we are unable to ship to the following countries:

  1. Afghanistan
  2. American Samoa
  3. Belarus
  4. Brunei Darussalam
  5. Central African Republic
  6. The Democratic Republic of Congo
  7. Eritrea
  8. Guinea-bissau
  9. Iran
  10. Lebanon
  11. Libiya Arab Jamahriya
  12. Somalia
  13. Sudan
  14. Russian Federation
  15. Syrian Arab Republic
  16. Ukraine
  17. Venezuela