Welcome!

Open Source Authors: Rex Morrow, Datical, Liz McMillan, Pat Romanski, Elizabeth White, Carmen Gonzalez

Related Topics: Java, XML, SOA & WOA, Open Source, AJAX & REA, Oracle

Java: Blog Post

A Painless Introduction to Java’s ThreadLocal Storage

Best practices for using another powerful class: ThreadLocal from java.lang

In my last blog, we explored WeakHashMap which is based by WeakReference in Java. Now let's look at some best practices for using another powerful class: ThreadLocal from java.lang, which is also implemented using WeakReference.

What is ThreadLocal? A simple example
As its name suggests, a single instance of ThreadLocal can store different values for each thread independently. Therefore, the value stored in a ThreadLocal instance is specific (local) to the current running Thread, any other code logic running on the same thread will see the same value, but not the values set on the same instance by other threads. (There are exceptions, like InhertiableThreadLocal, which inherits parent thread's values by default.)

Let's consider this example:

We have a TransactionManager class that provide static methods to:

  • Start a transaction with a generated ID
  • Store that ID as a static field and provide a transaction ID getter method to other code logic that needs to know the current transaction ID.

In a single threaded environment, TransactionManager can simply store the ID as a static field and return as is. However, this will certainly not work in a multiple-threaded environment. Imagine multiple threads are using TransactionManager - transaction IDs generated by each thread can overwrite each other as there is only one static instance of transaction ID. One may synchronize and block other transactions to avoid overwrites, but this would totally defeat the purpose of having multiple threads.

In order to solve this problem, ThreadLocal provides a very neat solution:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 public class TransactionManager { private static final ThreadLocal<String> context = new ThreadLocal<String>(); public static void startTransaction() { //logic to start a transaction //... context.set(generatedId); } public static String getTransactionId() { return context.get(); } public static void endTransaction() { //logic to end a transaction //... context.remove(); } }

Different thread that starts transactions via TransactionManager will get its own transaction ID stored in the context. Any logic within the same thread can call getTransactionId() later on to retrieve the value belongs/local to that Thread. So problem's solved!

The Internals of ThreadLocal and How it Works

Let's drill down a little bit into the ThreadLocal's internals. ThreadLocal is implemented by having a Map (a ThreadLocalMap) as field (with WeakReference entry) within each Thread instance. (There are actually 2 maps; the second one is used for InheritabeleThreadLocal, but let's not complicate the picture). The keys of those maps are the corresponding ThreadLocals themselves. Therefore, when a set/get is called on a ThreadLocal, it looks at the current thread, find the map, and look up the value with "this" ThreadLocal instance.

Still confused? I certainly am. Let's look at a real example.

  • Code running in Thread 1 calls set() on ThreadLocal instance "A" with value "123″
  • Code running in Thread 2 calls set() on ThreadLocal instance "A" with value "234″
  • Code running in Thread 1 calls set() on ThreadLocal instance "B" with value "345″

And this is the end result:

Thread 1 (the instance)'s field ThreadLocalMap (m1) has 2 entries:

KeyValue
ThreadLocal A "123″
ThreadLocal B "345″

Thread 2 (the instance)'s field ThreadLocalMap (m2) has 1 entry:

KeyValue
ThreadLocal A "234″

Now if some code logic in Thread 1 calls get() on ThreadLocal instance "A", the ThreadLocal logic will lookup the current Thread, which is instance Thread 1, then access the field ThreadLocalMap of that Thread instance, which is m1, it can then lookup the value by using m1.get(this), with "this" as ThreadLocal and the result is "123″

Now what to watch out for!
Did I hear weak reference for ThreadLocal entries? Does that mean I don't have to clean up? Well, it's not quite that simple.

First of all, the value object put into the ThreadLocal would not purge itself (garbage collected) if there are no more Strong references to it. Instead, the Weak reference is done on the thread instance, which means Java garbage collection would clean up the ThreadLocal map if the thread itself is not strongly referenced elsewhere.

So now the question is: when would the Thread object get garbage collected?

The answer is: it depends, but always assume the thread is long running. 2 common examples:

  • Servlets. The threads that handle servlet requests usually stay alive in the container for the lifetime of the server instance. Code logic that uses ThreadLocal might be referenced indirectly by servlets.
  • Thread pooling java.util.concurrent.Executors. Java encourages recycling threads!

A typical usage of Executor introduced in Java 1.5, if ThreadLocal maps are not cleaned up properly after a transaction is done, next TransactionProcessingTask might inherit values from another previous unrelated task!

1 2 ExecutorService service = Executors.newFixedThreadPool(10); service.submit(new TransactionProcessingTask());

Be careful with initialization of ThreadLocal, below is an implementation of a counter by Thread. Can you tell what is wrong in the below initialzation?

1 2 3 4 5 6 7 8 9 10 11 12 public class Counter { private static ThreadLocal<Integer> counter = new ThreadLocal<Integer>(); static { counter.set(0); } public int getCountInThread() { return counter.get(); } //.... }

The counter would not get initialized correctly! Though the counter is declared as static, it CANNOT be initialized by having a static initializer, as the initializer only runs once when the first thread references the Counter class. When the second thread comes in, it does not run counter.set(0) on that thread, therefore counter.get() returns null instead of 0! One solution is to sublcass ThreadLocal and override the initialValue() method to assign non-null initial value.

With these in mind, you can probably picture the consequences of not cleaning up after ourselves! An operation that runs on a recycled thread might inherit the values from previous operation on the same thread! Besides, it can also cause memory leaks as the instance stored in ThreadLocal will never get garbage collected if the thread is alive.

As a rule of thumb, always clean up/reset your threadlocal after you have finished your "unit of operation"! Even though the current code might be simple enough to bypass the cleanups, it might be adapted and integrated into servlets/thread pooling later on! After all, cleaning up responsibly is always appreciated both in the realms of programming and real life.

More Stories By Patson Luk

A Java developer who has spent the better part of the last decade working on financial services applications with companies from HSBC to Mobilearth and Parasoft, Patson is experienced in various aspects of computer systems, from large scale enterprise banking system to lightweight mobile payment solutions. He now leads Java instrumentation and tool development for the TraceView product at AppNeta. Patson's focus is on using java bytecode manipulation technologies to gain greater visibility into the full spectrum of Java based technologies. This includes higher level application frameworks from Spring and Struts to Webflow, AppServers from TomCat to JBoss, and Databases from MySQL to Oracle. He's also writes frequently on the AppNeta blog - www.appneta.com/blog

@ThingsExpo Stories

SUNNYVALE, Calif., Oct. 20, 2014 /PRNewswire/ -- Spansion Inc. (NYSE: CODE), a global leader in embedded systems, today added 96 new products to the Spansion® FM4 Family of flexible microcontrollers (MCUs). Based on the ARM® Cortex®-M4F core, the new MCUs boast a 200 MHz operating frequency and support a diverse set of on-chip peripherals for enhanced human machine interfaces (HMIs) and machine-to-machine (M2M) communications. The rich set of periphera...

WebRTC defines no default signaling protocol, causing fragmentation between WebRTC silos. SIP and XMPP provide possibilities, but come with considerable complexity and are not designed for use in a web environment. In his session at Internet of @ThingsExpo, Matthew Hodgson, technical co-founder of the Matrix.org, will discuss how Matrix is a new non-profit Open Source Project that defines both a new HTTP-based standard for VoIP & IM signaling and provides reference implementations.
SYS-CON Events announced today that Aria Systems, the recurring revenue expert, has been named "Bronze Sponsor" of SYS-CON's 15th International Cloud Expo®, which will take place on November 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Aria Systems helps leading businesses connect their customers with the products and services they love. Industry leaders like Pitney Bowes, Experian, AAA NCNU, VMware, HootSuite and many others choose Aria to power their recurring revenue business and deliver exceptional experiences to their customers.
The Internet of Things (IoT) is going to require a new way of thinking and of developing software for speed, security and innovation. This requires IT leaders to balance business as usual while anticipating for the next market and technology trends. Cloud provides the right IT asset portfolio to help today’s IT leaders manage the old and prepare for the new. Today the cloud conversation is evolving from private and public to hybrid. This session will provide use cases and insights to reinforce the value of the network in helping organizations to maximize their company’s cloud experience.
The Internet of Things (IoT) is making everything it touches smarter – smart devices, smart cars and smart cities. And lucky us, we’re just beginning to reap the benefits as we work toward a networked society. However, this technology-driven innovation is impacting more than just individuals. The IoT has an environmental impact as well, which brings us to the theme of this month’s #IoTuesday Twitter chat. The ability to remove inefficiencies through connected objects is driving change throughout every sector, including waste management. BigBelly Solar, located just outside of Boston, is trans...
SYS-CON Events announced today that Matrix.org has been named “Silver Sponsor” of Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Matrix is an ambitious new open standard for open, distributed, real-time communication over IP. It defines a new approach for interoperable Instant Messaging and VoIP based on pragmatic HTTP APIs and WebRTC, and provides open source reference implementations to showcase and bootstrap the new standard. Our focus is on simplicity, security, and supporting the fullest feature set.
Predicted by Gartner to add $1.9 trillion to the global economy by 2020, the Internet of Everything (IoE) is based on the idea that devices, systems and services will connect in simple, transparent ways, enabling seamless interactions among devices across brands and sectors. As this vision unfolds, it is clear that no single company can accomplish the level of interoperability required to support the horizontal aspects of the IoE. The AllSeen Alliance, announced in December 2013, was formed with the goal to advance IoE adoption and innovation in the connected home, healthcare, education, aut...
SYS-CON Events announced today that Red Hat, the world's leading provider of open source solutions, will exhibit at Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Red Hat is the world's leading provider of open source software solutions, using a community-powered approach to reliable and high-performing cloud, Linux, middleware, storage and virtualization technologies. Red Hat also offers award-winning support, training, and consulting services. As the connective hub in a global network of enterprises, partners, a...
The only place to be June 9-11 is Cloud Expo & @ThingsExpo 2015 East at the Javits Center in New York City. Join us there as delegates from all over the world come to listen to and engage with speakers & sponsors from the leading Cloud Computing, IoT & Big Data companies. Cloud Expo & @ThingsExpo are the leading events covering the booming market of Cloud Computing, IoT & Big Data for the enterprise. Speakers from all over the world will be hand-picked for their ability to explore the economic strategies that utility/cloud computing provides. Whether public, private, or in a hybrid form, clo...
Software AG helps organizations transform into Digital Enterprises, so they can differentiate from competitors and better engage customers, partners and employees. Using the Software AG Suite, companies can close the gap between business and IT to create digital systems of differentiation that drive front-line agility. We offer four on-ramps to the Digital Enterprise: alignment through collaborative process analysis; transformation through portfolio management; agility through process automation and integration; and visibility through intelligent business operations and big data.
The Transparent Cloud-computing Consortium (abbreviation: T-Cloud Consortium) will conduct research activities into changes in the computing model as a result of collaboration between "device" and "cloud" and the creation of new value and markets through organic data processing High speed and high quality networks, and dramatic improvements in computer processing capabilities, have greatly changed the nature of applications and made the storing and processing of data on the network commonplace.
Be Among the First 100 to Attend & Receive a Smart Beacon. The Physical Web is an open web project within the Chrome team at Google. Scott Jenson leads a team that is working to leverage the scalability and openness of the web to talk to smart devices. The Physical Web uses bluetooth low energy beacons to broadcast an URL wirelessly using an open protocol. Nearby devices can find all URLs in the room, rank them and let the user pick one from a list. Each device is, in effect, a gateway to a web page. This unlocks entirely new use cases so devices can offer tiny bits of information or simple i...
The Internet of Things (IoT) is going to require a new way of thinking and of developing software for speed, security and innovation. This requires IT leaders to balance business as usual while anticipating for the next market and technology trends. Cloud provides the right IT asset portfolio to help today’s IT leaders manage the old and prepare for the new. Today the cloud conversation is evolving from private and public to hybrid. This session will provide use cases and insights to reinforce the value of the network in helping organizations to maximize their company’s cloud experience.
Things are being built upon cloud foundations to transform organizations. This CEO Power Panel at 15th Cloud Expo, moderated by Roger Strukhoff, Cloud Expo and @ThingsExpo conference chair, will address the big issues involving these technologies and, more important, the results they will achieve. How important are public, private, and hybrid cloud to the enterprise? How does one define Big Data? And how is the IoT tying all this together?
TechCrunch reported that "Berlin-based relayr, maker of the WunderBar, an Internet of Things (IoT) hardware dev kit which resembles a chunky chocolate bar, has closed a $2.3 million seed round, from unnamed U.S. and Switzerland-based investors. The startup had previously raised a €250,000 friend and family round, and had been on track to close a €500,000 seed earlier this year — but received a higher funding offer from a different set of investors, which is the $2.3M round it’s reporting."
The Industrial Internet revolution is now underway, enabled by connected machines and billions of devices that communicate and collaborate. The massive amounts of Big Data requiring real-time analysis is flooding legacy IT systems and giving way to cloud environments that can handle the unpredictable workloads. Yet many barriers remain until we can fully realize the opportunities and benefits from the convergence of machines and devices with Big Data and the cloud, including interoperability, data security and privacy.
All major researchers estimate there will be tens of billions devices - computers, smartphones, tablets, and sensors - connected to the Internet by 2020. This number will continue to grow at a rapid pace for the next several decades. Over the summer Gartner released its much anticipated annual Hype Cycle report and the big news is that Internet of Things has now replaced Big Data as the most hyped technology. Indeed, we're hearing more and more about this fascinating new technological paradigm. Every other IT news item seems to be about IoT and its implications on the future of digital busines...
Cultural, regulatory, environmental, political and economic (CREPE) conditions over the past decade are creating cross-industry solution spaces that require processes and technologies from both the Internet of Things (IoT), and Data Management and Analytics (DMA). These solution spaces are evolving into Sensor Analytics Ecosystems (SAE) that represent significant new opportunities for organizations of all types. Public Utilities throughout the world, providing electricity, natural gas and water, are pursuing SmartGrid initiatives that represent one of the more mature examples of SAE. We have s...
The Internet of Things needs an entirely new security model, or does it? Can we save some old and tested controls for the latest emerging and different technology environments? In his session at Internet of @ThingsExpo, Davi Ottenheimer, EMC Senior Director of Trust, will review hands-on lessons with IoT devices and reveal privacy options and a new risk balance you might not expect.
IoT is still a vague buzzword for many people. In his session at Internet of @ThingsExpo, Mike Kavis, Vice President & Principal Cloud Architect at Cloud Technology Partners, will discuss the business value of IoT that goes far beyond the general public's perception that IoT is all about wearables and home consumer services. The presentation will also discuss how IoT is perceived by investors and how venture capitalist access this space. Other topics to discuss are barriers to success, what is new, what is old, and what the future may hold.