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Java Concurrency in Practice

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A how-to companion to Doug Lea's "Concurrent Programming in Java", this book is the only authorative and practical guide to Java Concurrency

° Powerhouse author team with contributions from Doug Lea, Josh Bloch and David Holmes

° A practical, hands-on, example-driven guide for every working Java programmer

° Based on J2SE 5.0 which includes many new concurrency features that make concurrency development much more accesible (and necessary)


  • Copyright 2006
  • Dimensions: 7" x 9-1/4"
  • Pages: 432
  • Edition: 1st
  • Book
  • ISBN-10: 0-321-34960-1
  • ISBN-13: 978-0-321-34960-6

"I was fortunate indeed to have worked with a fantastic team on the design and implementation of the concurrency features added to the Java platform in Java 5.0 and Java 6. Now this same team provides the best explanation yet of these new features, and of concurrency in general. Concurrency is no longer a subject for advanced users only. Every Java developer should read this book."
--Martin Buchholz
JDK Concurrency Czar, Sun Microsystems

"For the past 30 years, computer performance has been driven by Moore's Law; from now on, it will be driven by Amdahl's Law. Writing code that effectively exploits multiple processors can be very challenging. Java Concurrency in Practice provides you with the concepts and techniques needed to write safe and scalable Java programs for today's--and tomorrow's--systems."
--Doron Rajwan
Research Scientist, Intel Corp

"This is the book you need if you're writing--or designing, or debugging, or maintaining, or contemplating--multithreaded Java programs. If you've ever had to synchronize a method and you weren't sure why, you owe it to yourself and your users to read this book, cover to cover."
--Ted Neward
Author of Effective Enterprise Java

"Brian addresses the fundamental issues and complexities of concurrency with uncommon clarity. This book is a must-read for anyone who uses threads and cares about performance."
--Kirk Pepperdine
CTO, JavaPerformanceTuning.com

"This book covers a very deep and subtle topic in a very clear and concise way, making it the perfect Java Concurrency reference manual. Each page is filled with the problems (and solutions!) that programmers struggle with every day. Effectively exploiting concurrency is becoming more and more important now that Moore's Law is delivering more cores but not faster cores, and this book will show you how to do it."
--Dr. Cliff Click
Senior Software Engineer, Azul Systems

"I have a strong interest in concurrency, and have probably written more thread deadlocks and made more synchronization mistakes than most programmers. Brian's book is the most readable on the topic of threading and concurrency in Java, and deals with this difficult subject with a wonderful hands-on approach. This is a book I am recommending to all my readers of The Java Specialists' Newsletter, because it is interesting, useful, and relevant to the problems facing Java developers today."
--Dr. Heinz Kabutz
The Java Specialists' Newsletter

"I've focused a career on simplifying simple problems, but this book ambitiously and effectively works to simplify a complex but critical subject: concurrency. Java Concurrency in Practice is revolutionary in its approach, smooth and easy in style, and timely in its delivery--it's destined to be a very important book."
--Bruce Tate
Author of Beyond Java

"Java Concurrency in Practice is an invaluable compilation of threading know-how for Java developers. I found reading this book intellectually exciting, in part because it is an excellent introduction to Java's concurrency API, but mostly because it captures in a thorough and accessible way expert knowledge on threading not easily found elsewhere."
--Bill Venners
Author of Inside the Java Virtual Machine

Threads are a fundamental part of the Java platform. As multicore processors become the norm, using concurrency effectively becomes essential for building high-performance applications. Java SE 5 and 6 are a huge step forward for the development of concurrent applications, with improvements to the Java Virtual Machine to support high-performance, highly scalable concurrent classes and a rich set of new concurrency building blocks. In Java Concurrency in Practice, the creators of these new facilities explain not only how they work and how to use them, but also the motivation and design patterns behind them.

However, developing, testing, and debugging multithreaded programs can still be very difficult; it is all too easy to create concurrent programs that appear to work, but fail when it matters most: in production, under heavy load. Java Concurrency in Practice arms readers with both the theoretical underpinnings and concrete techniques for building reliable, scalable, maintainable concurrent applications. Rather than simply offering an inventory of concurrency APIs and mechanisms, it provides design rules, patterns, and mental models that make it easier to build concurrent programs that are both correct and performant.

This book covers:

  • Basic concepts of concurrency and thread safety
  • Techniques for building and composing thread-safe classes
  • Using the concurrency building blocks in java.util.concurrent
  • Performance optimization dos and don'ts
  • Testing concurrent programs
  • Advanced topics such as atomic variables, nonblocking algorithms, and the Java Memory Model

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Table of Contents

Listings     xii
Preface     xvii

Chapter 1: Introduction     1

1.1  A (very) brief history of concurrency       1
1.2  Benefits of threads      3
1.3  Risks of threads       5
1.4  Threads are everywhere       9

Part I: Fundamentals     13

Chapter 2: Thread Safety     15

2.1  What is thread safety?      17
2.2  Atomicity     19
2.3  Locking     23
2.4  Guarding state with locks      27
2.5  Liveness and performance       29

Chapter 3: Sharing Objects     33

3.1  Visibility      33
3.2  Publication and escape       39
3.3  Thread confinement       42
3.4  Immutability       46
3.5  Safepublication       49

Chapter 4: Composing Objects     55

4.1  Designing a thread-safe class      55
4.2  Instance confinement      58
4.3  Delegating thread safety      62
4.4  Adding functionality to existing thread-safe classes       71
4.5  Documenting synchronization policies       74

Chapter 5: Building Blocks     79

5.1  Synchronized collections       79
5.2  Concurrent collections     84
5.3  Blocking queues and the producer-consumer pattern     87
5.4  Blocking and interruptible methods     92
5.5  Synchronizers     94
5.6  Building an efficient, scalable result cache      101

Part II: Structuring Concurrent Applications     111

Chapter 6: Task Execution     113

6.1  Executing tasks in threads      113
6.2  The Executor framework     117
6.3  Finding exploitable parallelism      123

Chapter 7: Cancellation and Shutdown     135

7.1  Task cancellation      135
7.2  Stopping a thread-based service       150
7.3  Handling abnormal thread termination       161
7.4  JVM shutdown      164

Chapter 8: Applying Thread Pools     167

8.1  Implicit couplings between tasks and execution policies     167
8.2  Sizing thread pools      170
8.3  Configuring ThreadPoolExecutor     171
8.4  Extending ThreadPoolExecutor     179
8.5  Parallelizing recursive algorithms     181

Chapter 9: GUI Applications     189

9.1  Why are GUIs single-threaded?      189
9.2  Short-running GUI tasks     192
9.3  Long-running GUI tasks     195
9.4  Shared data models     198
9.5  Other forms of single-threaded subsystems      202

Part III: Liveness, Performance, and Testing     203

Chapter 10: Avoiding Liveness Hazards     205

10.1  Deadlock     205
10.2  Avoiding and diagnosing deadlocks     215
10.3  Other liveness hazards      218

Chapter 11: Performance and Scalability     221

11.1  Thinking about performance      221
11.2  Amdahl's law     225
11.3  Costs introduced by threads     229
11.4  Reducing lock contention      232
11.5  Example: Comparing Map performance     242
11.6  Reducing context switch overhead      243

Chapter 12: Testing Concurrent Programs     247

12.1  Testing for correctness     248
12.2  Testing for performance      260
12.3  Avoiding performance testing pitfalls       266
12.4  Complementary testing approaches     270

Part IV: Advanced Topics     275

Chapter 13: Explicit Locks     277

13.1  Lock and ReentrantLock      277
13.2  Performance considerations      282
13.3  Fairness      283
13.4  Choosing between synchronized and ReentrantLock      285
13.5  Read-write locks     286

Chapter 14: Building Custom Synchronizers     291

14.1  Managing state dependence      291
14.2  Using condition queues      298
14.3  Explicit condition objects     306
14.4  Anatomy of a synchronizer     308
14.5  AbstractQueuedSynchronizer      311
14.6  AQS in java.util.concurrent synchronizer classes      314

Chapter15: Atomic Variables and Nonblocking Synchronization     319

15.1  Disadvantages of locking     319
15.2  Hardware support for concurrency      321
15.3  Atomic variable classes       324
15.4  Nonblocking algorithms      329

Chapter 16: The Java Memory Model     337

16.1  What is a memory model, and why would I want one?       337
16.2  Publication     344
16.3  Initialization safety     349

Appendix A: Annotations for Concurrency     353

A.1  Class annotations     353
A.2  Field andmethod annotations      353

Bibliography     355
Index     359


At this writing, multicore processors are just now becoming inexpensive enough for midrange desktop systems. Not coincidentally, many development teams are noticing more and more threading-related bug reports in their projects. In a recent post on the NetBeans developer site, one of the core maintainers observed that a single class had been patched over 14 times to fix threading-related problems. Dion Almaer, former editor of TheServerSide, recently blogged (after a painful debugging session that ultimately revealed a threading bug) that most Java programs are so rife with concurrency bugs that they work only "by accident". Indeed, developing, testing and debugging multithreaded programs can be extremely difficult because concurrency bugs do not manifest themselves predictably. And when they do surface, it is often at the worst possible time--in production, under heavy load.

One of the challenges of developing concurrent programs in Java is the mismatch between the concurrency features offered by the platform and how developers need to think about concurrency in their programs. The language provides low-level mechanisms such as synchronization and condition waits, but these mechanisms must be used consistently to implement application-level protocols or policies. Without such policies, it is all too easy to create programs that compile and appear to work but are nevertheless broken. Many otherwise excellent books on concurrency fall short of their goal by focusing excessively on low-level mechanisms and APIs rather than design-level policies and patterns.

Java 5.0 is a huge step forward for the development of concurrent applications in Java, providing new higher-level components and additional low-level mechanisms that make it easier for novices and experts alike to build concurrent applications. The authors are the primary members of the JCP Expert Group that created these facilities; in addition to describing their behavior and features, we present the underlying design patterns and anticipated usage scenarios that motivated their inclusion in the platform libraries. Our goal is to give readers a set of design rules and mental models that make it easier--and more fun--to build correct, performant concurrent classes and applications in Java.

We hope you enjoy Java Concurrency in Practice.

Brian Goetz
Williston, VT
March 2006

How to use this book

To address the abstraction mismatch between Java's low-level mechanisms and the necessary design-level policies, we present a simplified set of rules for writing concurrent programs. Experts may look at these rules and say "Hmm, that's not entirely true: class C is thread-safe even though it violates rule R." While it is possible to write correct programs that break our rules, doing so requires a deep understanding of the low-level details of the Java Memory Model, and we want developers to be able to write correct concurrent programs without having to master these details. Consistently following our simplified rules will produce correct and maintainable concurrent programs.

We assume the reader already has some familiarity with the basic mechanisms for concurrency in Java. Java Concurrency in Practice is not an introduction to concurrency--for that, see the threading chapter of any decent introductory volume, such as The Java Programming Language (Arnold et al., 2005). Nor is it an encyclopedic reference for All Things Concurrency--for that, see Concurrent Programming in Java (Lea, 2000). Rather, it offers practical design rules to assist developers in the difficult process of creating safe and performant concurrent classes. Where appropriate, we cross-reference relevant sections of The Java Programming Language, Concurrent Programming in Java, The Java Language Specification (Gosling et al., 2005), and Effective Java (Bloch, 2001) using the conventions JPL n.m, CPJ n.m, JLS n.m, and EJ Item n.

After the introduction (Chapter 1), the book is divided into four parts:

Fundamentals. Part I (Chapters 2-5) focuses on the basic concepts of concurrency and thread safety, and how to compose thread-safe classes out of the concurrent building blocks provided by the class library. A "cheat sheet" summarizing the most important of the rules presented in Part I appears on page 110.Chapters 2 (Thread Safety) and 3 (Sharing Objects) form the foundation for the book. Nearly all of the rules on avoiding concurrency hazards, constructing thread-safe classes, and verifying thread safety are here. Readers who prefer "practice" to "theory" may be tempted to skip ahead to Part II, but make sure to come back and read Chapters 2 and 3 before writing any concurrent code!

Chapter 4 (Composing Objects) covers techniques for composing thread-safe classes into larger thread-safe classes. Chapter 5 (Building Blocks) covers the concurrent building blocks--thread-safe collections and synchronizers--provided by the platform libraries.

Structuring Concurrent Applications. Part II (Chapters 6-9) describes how to exploit threads to improve the throughput or responsiveness of concurrent applications. Chapter 6 (Task Execution) covers identifying parallelizable tasks and executing them within the task-execution framework. Chapter 7 (Cancellation and Shutdown) deals with techniques for convincing tasks and threads to terminate before they would normally do so; how programs deal with cancellation and shutdown is often one of the factors that separates truly robust concurrent applications from those that merely work. Chapter 8 (Applying Thread Pools) addresses some of the more advanced features of the task-execution framework.

Chapter 9 (GUI Applications) focuses on techniques for improving responsivenessin single-threaded subsystems. Liveness, Performance, and Testing. Part III (Chapters 10-12) concerns itself with ensuring that concurrent programs actually do what you want them to do and do so with acceptable performance. Chapter 10 (Avoiding Liveness Hazards) describes how to avoid liveness failures that can prevent programs from making forward progress. Chapter 11 (Performance and Scalability) covers techniques for improving the performance and scalability of concurrent code. Chapter 12 (Testing Concurrent Programs) covers techniques for testing concurrent code for both correctness and performance.

Advanced Topics. Part IV (Chapters 13-16) covers topics that are likely to be of interest only to experienced developers: explicit locks, atomic variables, nonblocking algorithms, and developing custom synchronizers.

Code examples
While many of the general concepts in this book are applicable to versions of Java prior to Java 5.0 and even to non-Java environments, most of the code examples (and all the statements about the Java Memory Model) assume Java 5.0 or later. Some of the code examples may use library features added in Java 6.

The code examples have been compressed to reduce their size and to highlight the relevant portions. The full versions of the code examples, as well as supplementary examples and errata, are available from the book's website, http://www.javaconcurrencyinpractice.com.

The code examples are of three sorts: "good" examples, "not so good" examples, and "bad" examples. Good examples illustrate techniques that should be emulated.


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