Java 9 concurrency cookbook 2nd edition

He is the author of the book, Java 7 Concurrency Cookbook and Mastering Concurrency Programming with Java 8 by Packt... The mechanisms are as follows: Basic thread management Thread sync

Java 9 Concurrency Cookbook

Java 9 Concurrency Cookbook

Second Edition

Copyright © 2017 Packt Publishing

All rights reserved No part of this book may be reproduced, stored in a retrieval system, ortransmitted in any form or by any means, without the prior written permission of thepublisher, except in the case of brief quotations embedded in critical articles or reviews.Every effort has been made in the preparation of this book to ensure the accuracy of theinformation presented However, the information contained in this book is sold withoutwarranty, either express or implied Neither the author, nor Packt Publishing, and itsdealers and distributors will be held liable for any damages caused or alleged to be causeddirectly or indirectly by this book

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First published: October 2012

Second edition: April 2017

Tejal Daruwale Soni

Content Development Editor

About the Author

Javier Fernández González is a software architect with almost 15 years of experience

in Java technologies He has worked as a teacher, researcher, programmer, analyst, andwriter, and he now works as an architect in all types of projects related to Java, especiallyJ2EE As a teacher, has taken over 1,000 hours of training in basic Java, J2EE, and the Strutsframework As a researcher, he has worked in the field of information retrieval, developingapplications for processing large amounts of data in Java, and has participated as a

coauthor in several journal articles and conference presentations Recently, he worked ondeveloping J2EE web applications for various clients from different sectors (public

administration, insurance, healthcare, transportation, and so on) Currently, he works as a

software architect He is the author of the book, Java 7 Concurrency Cookbook and Mastering

Concurrency Programming with Java 8 by Packt.

About the Reviewer

Piotr Bzdyl is focused on Java concurrency topics, including other JVM languages and theirlibraries, aimed at helping in creating highly concurrent applications (async IO, non-

blocking APIs, Scala, Akka, and Clojure) He has been helping teams with JVM tuning andtroubleshooting

He has also created a training course for Java concurrency topics, covering core JDK

multithreading concepts as well as those from external libraries and languages (actors, STM,parallel collections, and functional languages)

You can connect with Piotr on LinkedIn at h t t p s ://w w w l i n k e d i n c o m /i n /p i o t r b z d y l

and on GitHub at h t t p s ://g i t h u b c o m /p b z d y l You can follow him on Stack Overflow

at h t t p ://s t a c k o v e r f l o w c o m /c v /p i o t r e k b z d y l

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To Nuria, Paula, and Pelayo, for you infinite love and patience

Processing uncontrolled exceptions in a thread 32

There's more 117

Implementing the ThreadFactory interface to generate custom threads

When you work with a computer, you can do several things at once You can listen to musicwhile you edit a document in a word processor and read your e-mails This can be donebecause your operating system allows the concurrency of tasks Concurrent programming

is about the elements and mechanisms a platform offers to have multiple tasks or programsrunning at once and communicating with each other, to exchange data or to synchronizewith each other Java is a concurrent platform, and it offers a lot of classes to execute

concurrent tasks inside a Java program With each version, Java increases the functionalitiesoffered to programmers to facilitate the development of concurrent programs This bookcovers the most important and useful mechanisms included in version 9 of the Java

concurrency API, so you will be able to use them directly in your applications The

mechanisms are as follows:

Basic thread management

Thread synchronization mechanisms

Thread creation and management delegation with executors

Fork/Join framework to enhance the performance of your application

Parallel streams to process big sets of data in a parallel way, including the newJava 9 reactive streams

Data structures for concurrent programs

Adapting the default behavior of some concurrency classes to your needs

Testing Java concurrency applications

What this book covers

Chapter 1, Thread Management, teaches you how to make basic operations with threads The

creation, execution, and status management of threads are explained through basic

examples

Chapter 2, Basic Thread Synchronization, covers how to use low-level Java mechanisms to

synchronize code Locks and the synchronized keyword are explained in detail

Chapter 3, Thread Synchronization Utilities, teaches how to use the high-level utilities of Java

to manage the synchronization between threads in Java It includes an explanation of how

to use the Phaser class to synchronize tasks divided into phases

Chapter 4, Thread Executors, explores the delegation of thread management to executors.

They allow running, managing, and getting the results of concurrent tasks

Chapter 5, Fork/Join Framework, covers the use of the Fork/Join framework It’s a special

kind of executor oriented to execute tasks that will be divided into smaller ones using thedivide and conquer technique

Chapter 6, Parallel and Reactive Streams, teaches you how to create streams and use all its

intermediate and terminal operations to process big collections of data in a parallel andfunctional way Streams were introduced in Java 8 Java 9 has included some new interfaces

to implement reactive streams

Chapter 7, Concurrent Collections, explains how to use some concurrent data structures

provided by the Java language These data structures must be used in concurrent programs

to avoid the use of synchronized blocks of code in their implementation

Chapter 8, Customizing Concurrency Classes, teaches you how to adapt some of the most

useful classes of the Java concurrency API to your needs

Chapter 9, Testing Concurrent Applications, covers how to obtain information about the

status of some of the most useful structures of the Java 7 concurrency API You will alsolearn how to use some free tools to debug concurrent applications, such as the Eclipse,NetBeans IDE, or FindBugs applications to detect possible bugs in your applications

Chapter 10, Additional Information, explores the notions of synchronization, the executor,

the Fork/Join framework, concurrent data structures, and the monitoring of concurrentobjects, which were not included in the respective chapters

Chapter 11, Concurrent Programming Design, provides some tips that every programmer

should consider when they develop a concurrent application

What you need for this book

To follow this book, you need some basic knowledge of the Java programming language.You should know how to use an IDE, such as Eclipse or NetBeans, but this is not a

necessary prerequisite

Who this book is for

If you are a Java developer interested in enhancing your knowledge of concurrent

programming and multithreading further, as well as discovering the new concurrency

features of Java 8 and Java 9, then the Java 9 Concurrency Cookbook is for you You should

already be comfortable with general Java development practices, and a basic grasp ofthreads would be an advantage

Sections

In this book, you will find several headings that appear frequently (Getting ready, How to

do it, How it works, There's more, and See also)

To give clear instructions on how to complete a recipe, we use these sections as follows:

Getting ready

This section tells you what to expect in the recipe, and describes how to set up any software

or any preliminary settings required for the recipe

In this book, you will find a number of text styles that distinguish between different kinds

of information Here are some examples of these styles and an explanation of their meaning.Code words in text, database table names, folder names, filenames, file extensions,

pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "The onethat executes the main() method."

A block of code is set as follows:

Thread task=new PrimeGenerator();

task.start();

New terms and important words are shown in bold Words that you see on the screen, for

example, in menus or dialog boxes, appear in the text like this: "Create a new project by

clicking on the New Project option under the File menu"

Warnings or important notes appear in a box like this

Tips and tricks appear like this

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Thread Management

In this chapter, we will cover the following topics:

Creating, running, and setting the characteristics of a thread

Interrupting a thread

Controlling the interruption of a thread

Sleeping and resuming a thread

Waiting for the finalization of a thread

Creating and running a daemon thread

Processing uncontrolled exceptions in a thread

Using thread local variables

Grouping threads and processing uncontrolled exceptions in a group of threadsCreating threads through a factory

Introduction

In the computer world, when we talk about concurrency, we refer to a series of independentand unrelated tasks that run simultaneously on a computer This simultaneity can be real ifthe computer has more than one processor or a multi-core processor, or it can be apparent ifthe computer has only one core processor

All modern operating systems allow the execution of concurrent tasks You can read your mails while listening to music or reading news on a web page We can say this is process-level concurrency But inside a process, we can also have various simultaneous tasks.

e-Concurrent tasks that run inside a process are called threads Another concept related to

concurrency is parallelism There are different definitions and relations with the

concurrency concept Some authors talk about concurrency when you execute your

application with multiple threads in a single-core processor With this, you can see whenyour program execution is apparent They talk about parallelism when you execute yourapplication with multiple threads in a multi-core processor or in a computer with more thanone processor, so this case is real as well Other authors talk about concurrency when thethreads of an application are executed without a predefined order, and they discuss

parallelism when all these threads are executed in an ordered way

This chapter presents a number of recipes that will show you how to perform basic

operations with threads, using the Java 9 API You will see how to create and run threads in

a Java program, how to control their execution, process exceptions thrown by them, andhow to group some threads to manipulate them as a unit

Creating, running, and setting the

characteristics of a thread

In this recipe, we will learn how to do basic operations over a thread using the Java API Aswith every element in the Java language, threads are objects We have two ways of creating

a thread in Java:

Extending the Thread class and overriding the run() method

Building a class that implements the Runnable interface and the run() methodand then creating an object of the Thread class by passing the Runnable object as

a parameter this is the preferred approach and it gives you more flexibility

In this recipe, we will use the second approach to create threads Then, we will learn how tochange some attributes of the threads The Thread class saves some information attributesthat can help us identify a thread, know its status, or control its priority These attributesare:

ID: This attribute stores a unique identifier for each thread.

Name: This attribute stores the name of the thread.

Priority: This attribute stores the priority of the Thread objects In Java 9, threads

can have priority between 1 and 10, where 1 is the lowest priority and 10 is thehighest It's not recommended that you change the priority of the threads It'sonly a hint to the underlying operating system and it doesn't guarantee anything,but it's a possibility that you can use if you want

Status: This attribute stores the status of a thread In Java, a thread can be present

in one of the six states defined in the Thread.State enumeration: NEW,

RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, or TERMINATED The following

is a list specifying what each of these states means:

NEW: The thread has been created and it has not yet startedRUNNABLE: The thread is being executed in the JVMBLOCKED: The thread is blocked and it is waiting for a monitorWAITING: The thread is waiting for another thread

TIMED_WAITING: The thread is waiting for another thread with aspecified waiting time

TERMINATED: The thread has finished its execution

In this recipe, we will implement an example that will create and run 10 threads that wouldcalculate the prime numbers within the first 20,000 numbers

Getting ready

The example for this recipe has been implemented using the Eclipse IDE If you use Eclipse

or a different IDE, such as NetBeans, open it and create a new Java project

How to do it

Follow these steps to implement the example:

Create a class named Calculator that implements the Runnable interface:1

public class Calculator implements Runnable {

Implement the run() method This method will execute the instructions of the2

thread we are creating, so this method will calculate the prime numbers withinthe first 20000 numbers:

Then, implement the auxiliar isPrime() method This method determines

3

whether a number is a prime number or not:

private boolean isPrime(long number) {

Now implement the main class of the application Create a class named Main that4.

contains the main() method:

public class Main {

public static void main(String[] args) {

First, write some information regarding the values of the maximum, minimum,5

and default priority of the threads:

Thread threads[];

Thread.State status[];

threads = new Thread[10];

status = new Thread.State[10];

for (int i = 0; i < 10; i++) {

threads[i] = new Thread(new Calculator());

try (FileWriter file = new FileWriter(".\\data\\log.txt");

PrintWriter pw = new PrintWriter(file);) {

for (int i = 0; i < 10; i++) {

pw.println("Main : Status of Thread " + i + " : " +

threads[i].getState());

status[i] = threads[i].getState();

for the finalization of a thread recipe of this chapter, we can use the join() method

to wait for this to happen In this case, we want to write information about thethreads when their statuses change, so we can't use this method We use thisblock of code:

boolean finish = false;

for (int i = 0; i < 10; i++) {

finish = finish && (threads[i].getState() ==

pw.printf("Main : Priority: %d\n", thread.getPriority());

pw.printf("Main : Old State: %s\n", state);

pw.printf("Main : New State: %s\n", thread.getState());

pw.printf("Main : ************************************\n"); }

Run the program and see how the different threads work in parallel.

10

How it works

The following screenshot shows the console part of the output of the program We can seethat all the threads we have created run in parallel to do their respective jobs:

In this screenshot, you can see how threads are created and how the ones with an evennumber are executed first, as they have the highest priority, and the others executed later,

as they have minimum priority The following screenshot shows part of the output of thelog.txt file where we write information about the status of the threads:

Every Java program has at least one execution thread When you run the program, JVMruns the execution thread that calls the main() method of the program

When we call the start() method of a Thread object, we are creating another executionthread Our program will have as many execution threads as the number of calls made tothe start() method

The Thread class has attributes to store all of the information of a thread The OS scheduleruses the priority of threads to select the one that uses the CPU at each moment and

actualizes the status of every thread according to its situation

If you don't specify a name for a thread, JVM automatically assigns it one in this format:Thread-XX, where XX is a number You can't modify the ID or status of a thread TheThread class doesn't implement the setId() and setStatus() methods as these methodsintroduce modifications in the code

A Java program ends when all its threads finish (more specifically, when all its non-daemonthreads finish) If the initial thread (the one that executes the main() method) ends, the rest

of the threads will continue with their execution until they finish If one of the threads usesthe System.exit() instruction to end the execution of the program, all the threads willend their respective execution

Creating an object of the Thread class doesn't create a new execution thread Also, callingthe run() method of a class that implements the Runnable interface doesn't create a newexecution thread Only when you call the start() method, a new execution thread iscreated

There's more

As mentioned in the introduction of this recipe, there is another way of creating a newexecution thread You can implement a class that extends the Thread class and overridesthe run() method of this class Then, you can create an object of this class and call thestart() method to have a new execution thread

You can use the static method currentThread() of the Thread class to access the threadobject that is running the current object

You have to take into account that the setPriority() method can throw an

IllegalArgumentException exception if you try to establish priority that isn't between 1and 10

See also

The Creating threads through a factory recipe of this chapter

Java provides an interruption mechanism that indicates to a thread that you want to finish

it One peculiarity of this mechanism is that thread objects have to check whether they havebeen interrupted or not, and they can decide whether they respond to the finalizationrequest or not A thread object can ignore it and continue with its execution

In this recipe, we will develop a program that creates a thread and forces its finalizationafter 5 seconds, using the interruption mechanism

Getting ready

The example for this recipe has been implemented using the Eclipse IDE If you use Eclipse

or a different IDE, such as NetBeans, open it and create a new Java project

How to do it

Follow these steps to implement the example:

Create a class called PrimeGenerator that extends the Thread class:

1

public class PrimeGenerator extends Thread{

Override the run() method including a loop that will run indefinitely In this2

loop, process consecutive numbers beginning from one For each number,calculate whether it's a prime number; if yes, as in this case, write it to the