Home > Articles

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Concurrency Made Simple

The purpose of the discussion, up to this point, has been to reframe the idea of concurrency. Concurrency is not a way to make a program run faster. It is not a complex juggling trick that ninja coders use to keep multiple balls in the air at one time. On the contrary, it is the apparent sequential execution of a program that is the complex trick. Sequential execution is an illusion maintained by a cabal of compiler writers and hardware architects. Concurrency is simply the relaxation of a fabricated constraint.


In the developer’s environment, where time and order are rigid and implicit constraints, “concurrency” is just another word for “order unspecified”—the way things are everywhere else. A concurrent program is just one that announces to an already-concurrent world that its correctness does not depend on the ordering of events that occur in separate components. In a concurrent program, those separate, partially ordered components are called threads of execution, or just threads. Within a thread, instructions are still executed in rigidly sequential order. The order of execution of instructions in two separate threads, however, is completely unspecified.

In the early days of computing, the choice of threads as a model for concurrency was not obvious. Developers that needed out-of-order processing were forced to brew their own concurrency constructs. Both literature and code from the 1960s contain a wide variety of models for asynchronous execution.

Threads probably originated in the late 1960s, with IBM’s OS/360. They were called “tasks,” and were an OS-level service that saved developers the trouble of building their own concurrency abstraction. In 1991 Java, called Oak at the time, adopted the thread model and supported it in the language, even on operating systems that did not.

Even today, threads are not the only model for concurrency. Languages such as Erlang, Go, and Clojure, for instance, each use an entirely different model.

Introducing threads into the programming model does not present an intrinsic problem. Operating two cars in parallel causes no problems unless they both try to occupy the same space at the same time. Similarly, operating two threads that are completely independent is also perfectly safe. There are millions of programs, each concurrently running in its own thread of execution on millions of separate computers, at this very moment. Most of these programs don’t interact with each other in any way and their behavior is perfectly well defined. The problems only arise when threads need to share state and resources.

Atomic Execution

When multiple threads change state that is accessible to both, the results can easily be nondeterministic. Because there is no relationship between the order in which statements are executed, subtle changes in timing can change the result of running the program.

Consider the following code:


Just by inspection, it seems likely that the variable executionCount is meant to count the number of times that the function someTask is called. In a concurrent environment, however, this code, as it stands, does not have deterministic behavior because the ++ operation is not atomic—it is not a single, indivisible action. Table 1.1 demonstrates an execution sequence that fails to record one execution.

Table 1.1 Non-Atomic Execution

executionCount 1/2 4

Thread 1

Thread 2

read execution count (4)

read execution count (4)

increment (5)

increment (5)

store execution count (5)

store execution count (5)

call someTask

call someTask

Synchronization is the basic mechanism through which multiple Java threads share state in such a way that the result of the interaction is deterministic. In itself, synchronization is a simple idea: a critical section of code is protected by a mutual-exclusion lock or mutex. When a thread enters the critical section—that is, it begins executing instructions from it—it is said to seize the mutex. No other thread can enter the section until the first thread leaves.

The previous code becomes deterministic if only a single thread is allowed to execute the critical section at any given time:

synchronized(this) {

Synchronization is the crux of creating correct concurrent Java programs, and is the basis for a lot of things that are definitely not simple. Those things make up the content of the rest of this book.


There is one more thing, however, that is simple. Remember that the previous example is an abstraction! It is written in a computer language—Java, in this case—and is, therefore, related to the actual behavior of hardware only by the grace of compiler writers, JVM developers, and hardware architects. Those two Java statements translate into hundreds of microinstructions, many of them executed in parallel, over tens of hardware clock cycles. The illusion that there are two statements, happening in order, is no more than an illusion.

Maintaining the illusion is not something that near-the-metal developers do naturally. On the contrary, they find sequential programs naive, clumsy, and wasteful. They are only too happy to fix them by re-ordering instructions, executing multiple instructions in parallel, representing a single piece of program state as multiple copies, and so on. By doing so, they do their very best to make optimal use of the immense power of the multiple processors that comprise even the tiny devices we carry in our pockets.

In general, we’re glad to have them perform these optimizations. They make our programs run faster, on multiple hardware platforms, using tricks in which application developers are just not that interested. There is one important condition on this optimization, however: They must not break the illusion of sequentiality! In other words, compilers and hardware pipelines can reorder and parallelize all they want to optimize the code, as long as developers can’t tell that they did it.

In making a program concurrent, a developer clearly states that there is no sequential dependency between the states controlled by different threads. If there is no sequential dependency, a compiler should be free to perform all sorts of optimizations that would otherwise have been unsafe. Without an explicit ordering between events in different threads, the compiler is free to make changes in the execution sequence of one thread without any consideration of the statements in any other.

A correct concurrent program is one that abides by the contract for maintaining an illusion. Negotiation between application programmers and hardware developers produce a language, and that language is a contract. The application developers get their illusion of sequential execution, something about which they can reason. The hardware developers get a toolbox of clever tricks they can use to make programs run fast. In the middle is the contract.

In Java, that contract is called the memory model. On one side of the contract is the application programmer, reasoning about her program in a high-level language. On the other side of the contract are the compiler writers, virtual machine developers, and hardware architects, moving everything that isn’t explicitly forbidden. Developers who talk about hardware when discussing concurrency are missing the point. A correct concurrent program is not about hardware; it is about developers keeping their end of the contract.

Fortunately, in Java, the contract is easily stated. The following single sentence states it almost completely:

Whenever more than one thread accesses a given state variable, and one of them might write to it, they all must coordinate their access to it using synchronization.

  • (Göetz, et al. 2006)

A correct concurrent Java program is one that abides by this contract—no more, no less. Note in particular that whether a thread reads or writes mutable state does not affect its need for synchronization in any way.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020