Welcome!

Government Cloud Authors: Elizabeth White, Liz McMillan, Gopala Krishna Behara, Raju Myadam, Kevin Jackson

Related Topics: Java IoT, Industrial IoT, Microservices Expo, Open Source Cloud, Machine Learning , Recurring Revenue

Java IoT: 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
Organizations planning enterprise data center consolidation and modernization projects are faced with a challenging, costly reality. Requirements to deploy modern, cloud-native applications simultaneously with traditional client/server applications are almost impossible to achieve with hardware-centric enterprise infrastructure. Compute and network infrastructure are fast moving down a software-defined path, but storage has been a laggard. Until now.
DXWorldEXPO LLC announced today that All in Mobile, a mobile app development company from Poland, will exhibit at the 22nd International CloudEXPO | DXWorldEXPO. All In Mobile is a mobile app development company from Poland. Since 2014, they maintain passion for developing mobile applications for enterprises and startups worldwide.
Digital Transformation is much more than a buzzword. The radical shift to digital mechanisms for almost every process is evident across all industries and verticals. This is often especially true in financial services, where the legacy environment is many times unable to keep up with the rapidly shifting demands of the consumer. The constant pressure to provide complete, omnichannel delivery of customer-facing solutions to meet both regulatory and customer demands is putting enormous pressure on...
The best way to leverage your CloudEXPO | DXWorldEXPO presence as a sponsor and exhibitor is to plan your news announcements around our events. The press covering CloudEXPO | DXWorldEXPO will have access to these releases and will amplify your news announcements. More than two dozen Cloud companies either set deals at our shows or have announced their mergers and acquisitions at CloudEXPO. Product announcements during our show provide your company with the most reach through our targeted audienc...
IoT is at the core or many Digital Transformation initiatives with the goal of re-inventing a company's business model. We all agree that collecting relevant IoT data will result in massive amounts of data needing to be stored. However, with the rapid development of IoT devices and ongoing business model transformation, we are not able to predict the volume and growth of IoT data. And with the lack of IoT history, traditional methods of IT and infrastructure planning based on the past do not app...
"Akvelon is a software development company and we also provide consultancy services to folks who are looking to scale or accelerate their engineering roadmaps," explained Jeremiah Mothersell, Marketing Manager at Akvelon, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
IoT is rapidly becoming mainstream as more and more investments are made into the platforms and technology. As this movement continues to expand and gain momentum it creates a massive wall of noise that can be difficult to sift through. Unfortunately, this inevitably makes IoT less approachable for people to get started with and can hamper efforts to integrate this key technology into your own portfolio. There are so many connected products already in place today with many hundreds more on the h...
More and more brands have jumped on the IoT bandwagon. We have an excess of wearables – activity trackers, smartwatches, smart glasses and sneakers, and more that track seemingly endless datapoints. However, most consumers have no idea what “IoT” means. Creating more wearables that track data shouldn't be the aim of brands; delivering meaningful, tangible relevance to their users should be. We're in a period in which the IoT pendulum is still swinging. Initially, it swung toward "smart for smart...
DXWorldEXPO LLC, the producer of the world's most influential technology conferences and trade shows has announced the 22nd International CloudEXPO | DXWorldEXPO "Early Bird Registration" is now open. Register for Full Conference "Gold Pass" ▸ Here (Expo Hall ▸ Here)
As data explodes in quantity, importance and from new sources, the need for managing and protecting data residing across physical, virtual, and cloud environments grow with it. Managing data includes protecting it, indexing and classifying it for true, long-term management, compliance and E-Discovery. Commvault can ensure this with a single pane of glass solution – whether in a private cloud, a Service Provider delivered public cloud or a hybrid cloud environment – across the heterogeneous enter...
The Jevons Paradox suggests that when technological advances increase efficiency of a resource, it results in an overall increase in consumption. Writing on the increased use of coal as a result of technological improvements, 19th-century economist William Stanley Jevons found that these improvements led to the development of new ways to utilize coal. In his session at 19th Cloud Expo, Mark Thiele, Chief Strategy Officer for Apcera, compared the Jevons Paradox to modern-day enterprise IT, examin...
DXWorldEXPO LLC announced today that ICC-USA, a computer systems integrator and server manufacturing company focused on developing products and product appliances, will exhibit at the 22nd International CloudEXPO | DXWorldEXPO. DXWordEXPO New York 2018, colocated with CloudEXPO New York 2018 will be held November 11-13, 2018, in New York City. ICC is a computer systems integrator and server manufacturing company focused on developing products and product appliances to meet a wide range of ...
Headquartered in Plainsboro, NJ, Synametrics Technologies has provided IT professionals and computer systems developers since 1997. Based on the success of their initial product offerings (WinSQL and DeltaCopy), the company continues to create and hone innovative products that help its customers get more from their computer applications, databases and infrastructure. To date, over one million users around the world have chosen Synametrics solutions to help power their accelerated business or per...
We are seeing a major migration of enterprises applications to the cloud. As cloud and business use of real time applications accelerate, legacy networks are no longer able to architecturally support cloud adoption and deliver the performance and security required by highly distributed enterprises. These outdated solutions have become more costly and complicated to implement, install, manage, and maintain.SD-WAN offers unlimited capabilities for accessing the benefits of the cloud and Internet. ...
Bill Schmarzo, author of "Big Data: Understanding How Data Powers Big Business" and "Big Data MBA: Driving Business Strategies with Data Science," is responsible for setting the strategy and defining the Big Data service offerings and capabilities for EMC Global Services Big Data Practice. As the CTO for the Big Data Practice, he is responsible for working with organizations to help them identify where and how to start their big data journeys. He's written several white papers, is an avid blogge...
Charles Araujo is an industry analyst, internationally recognized authority on the Digital Enterprise and author of The Quantum Age of IT: Why Everything You Know About IT is About to Change. As Principal Analyst with Intellyx, he writes, speaks and advises organizations on how to navigate through this time of disruption. He is also the founder of The Institute for Digital Transformation and a sought after keynote speaker. He has been a regular contributor to both InformationWeek and CIO Insight...
Join IBM November 1 at 21st Cloud Expo at the Santa Clara Convention Center in Santa Clara, CA, and learn how IBM Watson can bring cognitive services and AI to intelligent, unmanned systems. Cognitive analysis impacts today’s systems with unparalleled ability that were previously available only to manned, back-end operations. Thanks to cloud processing, IBM Watson can bring cognitive services and AI to intelligent, unmanned systems. Imagine a robot vacuum that becomes your personal assistant tha...
"MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
I think DevOps is now a rambunctious teenager - it's starting to get a mind of its own, wanting to get its own things but it still needs some adult supervision," explained Thomas Hooker, VP of marketing at CollabNet, in this SYS-CON.tv interview at DevOps Summit at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
Recently, WebRTC has a lot of eyes from market. The use cases of WebRTC are expanding - video chat, online education, online health care etc. Not only for human-to-human communication, but also IoT use cases such as machine to human use cases can be seen recently. One of the typical use-case is remote camera monitoring. With WebRTC, people can have interoperability and flexibility for deploying monitoring service. However, the benefit of WebRTC for IoT is not only its convenience and interopera...