Java IoT Authors: Elizabeth White, TJ Randall, Hollis Tibbetts, Pat Romanski, Carmen Gonzalez

Related Topics: Java IoT, Industrial IoT, Microservices Expo, Open Source Cloud, IoT User Interface, 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:

ThreadLocal A "123″
ThreadLocal B "345″

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

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
In the next five to ten years, millions, if not billions of things will become smarter. This smartness goes beyond connected things in our homes like the fridge, thermostat and fancy lighting, and into heavily regulated industries including aerospace, pharmaceutical/medical devices and energy. “Smartness” will embed itself within individual products that are part of our daily lives. We will engage with smart products - learning from them, informing them, and communicating with them. Smart produc...
SYS-CON Events announced today that 910Telecom will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Housed in the classic Denver Gas & Electric Building, 910 15th St., 910Telecom is a carrier-neutral telecom hotel located in the heart of Denver. Adjacent to CenturyLink, AT&T, and Denver Main, 910Telecom offers connectivity to all major carriers, Internet service providers, Internet backbones and ...
SYS-CON Events announced today that Coalfire will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Coalfire is the trusted leader in cybersecurity risk management and compliance services. Coalfire integrates advisory and technical assessments and recommendations to the corporate directors, executives, boards, and IT organizations for global brands and organizations in the technology, cloud, health...
SYS-CON Events announced today that Transparent Cloud Computing (T-Cloud) Consortium will exhibit at the 19th International Cloud Expo®, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. The Transparent Cloud Computing Consortium (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 proces...
The Internet of Things (IoT), in all its myriad manifestations, has great potential. Much of that potential comes from the evolving data management and analytic (DMA) technologies and processes that allow us to gain insight from all of the IoT data that can be generated and gathered. This potential may never be met as those data sets are tied to specific industry verticals and single markets, with no clear way to use IoT data and sensor analytics to fulfill the hype being given the IoT today.
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 @ThingsExpo, Matthew Hodgson, technical co-founder of the Matrix.org, discussed 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.
In his general session at 18th Cloud Expo, Lee Atchison, Principal Cloud Architect and Advocate at New Relic, discussed cloud as a ‘better data center’ and how it adds new capacity (faster) and improves application availability (redundancy). The cloud is a ‘Dynamic Tool for Dynamic Apps’ and resource allocation is an integral part of your application architecture, so use only the resources you need and allocate /de-allocate resources on the fly.
We're entering the post-smartphone era, where wearable gadgets from watches and fitness bands to glasses and health aids will power the next technological revolution. With mass adoption of wearable devices comes a new data ecosystem that must be protected. Wearables open new pathways that facilitate the tracking, sharing and storing of consumers’ personal health, location and daily activity data. Consumers have some idea of the data these devices capture, but most don’t realize how revealing and...
A completely new computing platform is on the horizon. They’re called Microservers by some, ARM Servers by others, and sometimes even ARM-based Servers. No matter what you call them, Microservers will have a huge impact on the data center and on server computing in general. Although few people are familiar with Microservers today, their impact will be felt very soon. This is a new category of computing platform that is available today and is predicted to have triple-digit growth rates for some ...
SYS-CON Events announced today that MathFreeOn will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. MathFreeOn is Software as a Service (SaaS) used in Engineering and Math education. Write scripts and solve math problems online. MathFreeOn provides online courses for beginners or amateurs who have difficulties in writing scripts. In accordance with various mathematical topics, there are more tha...
In past @ThingsExpo presentations, Joseph di Paolantonio has explored how various Internet of Things (IoT) and data management and analytics (DMA) solution spaces will come together as sensor analytics ecosystems. This year, in his session at @ThingsExpo, Joseph di Paolantonio from DataArchon, will be adding the numerous Transportation areas, from autonomous vehicles to “Uber for containers.” While IoT data in any one area of Transportation will have a huge impact in that area, combining sensor...
SYS-CON Events announced today that SoftNet Solutions will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. SoftNet Solutions specializes in Enterprise Solutions for Hadoop and Big Data. It offers customers the most open, robust, and value-conscious portfolio of solutions, services, and tools for the shortest route to success with Big Data. The unique differentiator is the ability to architect and ...
The best way to leverage your Cloud Expo presence as a sponsor and exhibitor is to plan your news announcements around our events. The press covering Cloud Expo and @ThingsExpo 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 Cloud Expo. Product announcements during our show provide your company with the most reach through our targeted audiences.
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 smar...
@ThingsExpo has been named the Top 5 Most Influential Internet of Things Brand by Onalytica in the ‘The Internet of Things Landscape 2015: Top 100 Individuals and Brands.' Onalytica analyzed Twitter conversations around the #IoT debate to uncover the most influential brands and individuals driving the conversation. Onalytica captured data from 56,224 users. The PageRank based methodology they use to extract influencers on a particular topic (tweets mentioning #InternetofThings or #IoT in this ...
SYS-CON Events announced today that Niagara Networks will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Niagara Networks offers the highest port-density systems, and the most complete Next-Generation Network Visibility systems including Network Packet Brokers, Bypass Switches, and Network TAPs.
In an era of historic innovation fueled by unprecedented access to data and technology, the low cost and risk of entering new markets has leveled the playing field for business. Today, any ambitious innovator can easily introduce a new application or product that can reinvent business models and transform the client experience. In their Day 2 Keynote at 19th Cloud Expo, Mercer Rowe, IBM Vice President of Strategic Alliances, and Raejeanne Skillern, Intel Vice President of Data Center Group and ...
Data is the fuel that drives the machine learning algorithmic engines and ultimately provides the business value. In his session at Cloud Expo, Ed Featherston, a director and senior enterprise architect at Collaborative Consulting, will discuss the key considerations around quality, volume, timeliness, and pedigree that must be dealt with in order to properly fuel that engine.
Virgil consists of an open-source encryption library, which implements Cryptographic Message Syntax (CMS) and Elliptic Curve Integrated Encryption Scheme (ECIES) (including RSA schema), a Key Management API, and a cloud-based Key Management Service (Virgil Keys). The Virgil Keys Service consists of a public key service and a private key escrow service. 

Fact is, enterprises have significant legacy voice infrastructure that’s costly to replace with pure IP solutions. How can we bring this analog infrastructure into our shiny new cloud applications? There are proven methods to bind both legacy voice applications and traditional PSTN audio into cloud-based applications and services at a carrier scale. Some of the most successful implementations leverage WebRTC, WebSockets, SIP and other open source technologies. In his session at @ThingsExpo, Da...