Java IoT Authors: Liz McMillan, TJ Randall, Hollis Tibbetts, Elizabeth White, Carmen Gonzalez

Related Topics: Java IoT, Weblogic, Linux Containers

Java IoT: Article

Java Basics: Introduction to Java Threads, Part 2

Internet Portals Like Yahoo, CNN, or Your Bank's Web Site Use Them

In the previous lesson www.sys-con.com/story/?storyid=46096&de=1 I've explained the basics of Java threads. This time we'll talk about using threads for creating a little more advanced programs.

I'm sure each of you have visited some of the major Internet portals like Yahoo, CNN or your bank's Web site. These portals usually display different types of information like News, Weather, Stock Market quotes, etc. Each of these info pieces appears on the screen instantaneously even though it's coming to the portal from different servers, i.e. the News server may be located in Washington and the stock market data come from New York (see Figure 1 below).

Let's say it takes 4 seconds to receive the news and 3 seconds to get the stock prices. If your program will do it in a sequence, it'll take you 7 seconds total, but why not do this in parallel and reduce the total time to 4 seconds? After all these servers have their own processors that can work in independently from each other! We are not going to discuss Web technologies here, but I'll show you how to spawn parallel processing using multi-threading, collect the returned data and display the results to the user in one shot.

Our program will consist of the following classes:

  • MyPortal that will spawn the threads and collect their returns in an ArrayList of strings. It'll print entire content of this array when all threads complete.
  • NewsServer that will run for 4 seconds and return a message "We have good and bad news";
  • StockServer that will run for 3 seconds and return a message "The stock market is on the rise!".
These threads do not contain any code that actually gets some news or market data. My goal is to show you how threads can communicate with other classes, and after this part works, it wont be difficult to replace the line that prints a static message with a method call that actually connects to the Internet and gets the data as it was explained in the lesson on getting data from the Internet:.

The class in Listing 1 creates and starts two threads (news and stocks) and goes to sleep for 10 seconds just to keep the program alive for a while. Please note that the class MyPortal also passes to each thread a reference to its instance so the threads know were to return the results. After each thread completes, it returns the result to MyPortal by calling its method submitResult(). Each of the resulting strings is being added to the ArrayList dataToDisplay, and when its size grows to two elements MyPortal prints the content of content the collection dataToDisplay. A little later I'll explain why such use of an ArrayList may not be the best solution for this example.

Listing 1. The source code of the class MyPortal

import java.util.ArrayList;
public class MyPortal {
	ArrayList dataToDisplay = new ArrayList();
    public static void main(String args[]){
    	MyPortal mp =new MyPortal();
    	// Spawn the threads and pass them the referennce
    	// to the instance of MyPortal
    	NewsServer myNews = new NewsServer(mp);
    	Thread newsThread = new Thread(myNews);

    	StockServer myStocks = new StockServer(mp);
    	Thread stockThread = new Thread(myStocks);

    	//Start the threads

    	try {
    		System.out.println("MyPortal is sleeping...!");
			Thread.sleep(10000); // wait for 10 sec 
		} catch (InterruptedException e) {

		System.out.println("Good bye!");

    // Add the data returned by a thread to collection
    public void submitResult(String data){

    	// Print the data if both threads have submitted the data
    	// (a buggy version)
    	if (dataToDisplay.size()==2){

The output of this program looks as follows:

MyPortal is sleeping...
[The stock market is on the rise!, We have good and bad news]
Good bye!

The first line will be printed almost immediately, the second line in 4 seconds and the third one in 10 seconds.

Listing 2. The source code of the class StockServer

public class StockServer implements Runnable {
    MyPortal papa;
    // Constructor
    StockServer(MyPortal parent){

    public void run() {
	// Sleep for 3 seconds to emulate some processing
	// and return a string with the market data to the parent
 	try {
		papa.submitResult("The stock market is on the rise!");
	} catch (InterruptedException e) {

Listing 3. The source code of the class NewsServer

public class NewsServer implements Runnable {
    MyPortal papa;

    // Constructor
    NewsServer(MyPortal parent){

	public void run() {
	// Sleep for 4 seconds to emulate some processing
	// and return a string with the news to the parent

		try {
			papa.submitResult("We have  good and bad news");
		} catch (InterruptedException e) {

The thread classes from Listing 2 and Listing 3 store the references to the parent class MyPortal in the variable papa. Each of the threads just sleeps for a specified number of seconds, wakes up and passes an appropriate text to papa.

Please note, that even on a single processor's machine the total execution time of our example is just a little more than 4 seconds. The reason is that our threads where "sleeping in parallel" and did not compete for the processor's time. But if you replace the sleeping part with a loop that performs some calculations, the timing will be different on a single processor machine: the program will run about 7 seconds. If you have a dual processor machine, you'll cut the processing time to 4 seconds again.

Thread Synchronization. A Race Condition.

When you write a multithreaded application you should consider possibility of a so-called race condition. This is a situation when you may get unpredictable results because multiple threads access a resource (i.e. a variable) at the same time. In our example two threads are calling the same method submitResult() which in turn accesses the variable dataToDisplay to add some data to it and check the size of this collection. Imagine that two or more threads finish their work at the same time. Let's look at a possible sequence of events:

  1. The NewsServer calls the method submitResult(). The size of dataToDisplay is 0.
  2. The StockServer calls the method submitResult() a split second later. The size of dataToDisplay is 0.
  3. The NewsServer grabs a zero-element dataToDisplay and starts adding its string there as a first element.
  4. The StockServer grabs a zero-element dataToDisplay (because the NewsServer has not finished adding its first the element yet) and starts adding its string there as a first element.
  5. After both threads are done, the dataToDisplay may wind up with having one element because the first thread's string has been overwritten by the second one. In this is the case, the size of the dataToDisplay will remain one and MyPortal will never print the news and stock data.
Since the probability of this situation is really small, your program may work properly for years and all of a sudden produce unexpected results. Bugs like this one are not easy to discover.

To avoid race conditions, the code that needs to access a "sensitive" variable must be locked (become unavailable for other threads) for the time when one thread works with it. When the first thread completes, the lock is released and another thread can get a hold of this variable/resource. You can arrange such locking either by using a Java keyword synchronized, or by using Java objects that are internally synchronized.

In our portal example, you can simply use the class Vector instead of ArrayList:

Vector dataToDisplay = new Vector();

Vector objects are internally synchronized in Java, and the second thread won't be able to add a string to the dataToDisplay collection until the first thread is done. Obviously, there is a price to pay for this convenience: synchronized objects are a little bit slower than non-synchronized ones.

The other solution is to put an explicit lock for a piece of code that must be completed without any interruption by other threads. For example, if you'll add the keyword synchronized to the signature of the method submitResult(), the second thread will not be able to call this method, if the first one is still executing it:

public synchronized void submitResult(String data){?}

You can also say that a lock is placed on the entire method submitResult().

You should try to minimize the locking time to avoid slowing down your programs. Java allows you to synchronize just a small portion of the code, which is more preferable than synchronizing an entire method.:

    public void submitResult(String data){
    	synchronized (this){

    	if (dataToDisplay.size()==2){

When a synchronized block is executed, the object in parenthesis is locked and cannot be used by any other thread until the lock is released.

Each Java thread has its own memory and the JVM copies there variables from the main program memory. The keyword synchronize means to synch up the content of the main and thread's portions of memory. This ensures that each thread works with the most current value of the resource (in our case its dataToDisplay).

If you spot a group of Java programmers in a bar, after a couple of beers they may start using some mysterious words: monitor and mutex.

A monitor is just a piece of a synchronized code. We can say that one of our threads can enter a monitor and safely modify the variable dataToDisplay. While the first thread is working, another thread(s) may start waiting for this monitor.

Mutex means mutually exclusive, and this term also refers to the fact that threads may take turns accessing some program variable(s).

In this lesson you've learned one of the ways of treating more than one thread as a group, but this is not the only way. Java has a class java.lang.ThreadGroup that allows you to create and start a group of threads, control the threads within the group and check which threads are still active. You may also consider the method join() of the class Thread if one thread needs to wait for completion of another.

Threads can communicate with other Java objects using special methods wait(), notify() and notifyAll(), but this is going to be a topic of another lesson. Meanwhile, you can read more about threads in the Java Tutorial over here: http://java.sun.com/docs/books/tutorial/essential/threads/

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Comments (3) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.

Most Recent Comments
Slava Pestov 02/18/05 07:57:05 PM EST

Yakov, your last threads example has a race condition.

Consider this:

thread 1 executes: synchronized (this){ dataToDisplay.add(data); }.

then thread 2 executes: synchronized (this){ dataToDisplay.add(data); }.

then thread 1 executes: if (dataToDisplay.size()==2){ System.out.println(dataToDisplay); }

then thread 2 executes: if (dataToDisplay.size()==2){ System.out.println(dataToDisplay); }

That last System.out.println(dataToDisplay); executes twice, which is not what you intended.

Yakov Fain 02/04/05 11:41:26 AM EST

Yes, J2EE spec does not recommend it, but if you do it right everything works fine. Here's how this could be done

To control threads in a J2EE container use a thread pool (it's a singleton) and get threads from there. If you use J2SE 5.0, use the package java.util.concurrent (in particular, ThreadPoolExecutor). In J2SE 1.4 and below use an excellent concurrent package created by Doug Lea.

Disclaimer: It's just my personal opinion based on my prior experience with a pretty serious financial application. But I do not recommend you to violate J2EE spec.

Feldhacker 02/04/05 08:35:41 AM EST

Is a J2EE version of this example available? Since J2EE forbids explicit thread management, how would this be done on a web server?

@ThingsExpo Stories
In the next forty months – just over three years – businesses will undergo extraordinary changes. The exponential growth of digitization and machine learning will see a step function change in how businesses create value, satisfy customers, and outperform their competition. In the next forty months companies will take the actions that will see them get to the next level of the game called Capitalism. Or they won’t – game over. The winners of today and tomorrow think differently, follow different...
“Media Sponsor” of SYS-CON's 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. CloudBerry Backup is a leading cross-platform cloud backup and disaster recovery solution integrated with major public cloud services, such as Amazon Web Services, Microsoft Azure and Google Cloud Platform.
According to Forrester Research, every business will become either a digital predator or digital prey by 2020. To avoid demise, organizations must rapidly create new sources of value in their end-to-end customer experiences. True digital predators also must break down information and process silos and extend digital transformation initiatives to empower employees with the digital resources needed to win, serve, and retain customers.
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 ...
You think you know what’s in your data. But do you? Most organizations are now aware of the business intelligence represented by their data. Data science stands to take this to a level you never thought of – literally. The techniques of data science, when used with the capabilities of Big Data technologies, can make connections you had not yet imagined, helping you discover new insights and ask new questions of your data. In his session at @ThingsExpo, Sarbjit Sarkaria, data science team lead ...
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 ...
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 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...
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.
Everyone knows that truly innovative companies learn as they go along, pushing boundaries in response to market changes and demands. What's more of a mystery is how to balance innovation on a fresh platform built from scratch with the legacy tech stack, product suite and customers that continue to serve as the business' foundation. In his General Session at 19th Cloud Expo, Michael Chambliss, Head of Engineering at ReadyTalk, will discuss why and how ReadyTalk diverted from healthy revenue an...
SYS-CON Media announced today that @WebRTCSummit Blog, the largest WebRTC resource in the world, has been launched. @WebRTCSummit Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @WebRTCSummit Blog can be bookmarked ▸ Here @WebRTCSummit conference site can be bookmarked ▸ Here
Intelligent machines are here. Robots, self-driving cars, drones, bots and many IoT devices are becoming smarter with Machine Learning. In her session at @ThingsExpo, Sudha Jamthe, CEO of IoTDisruptions.com, will discuss the next wave of business disruption at the junction of IoT and AI, impacting many industries and set to change our lives, work and world as we know it.
In today's uber-connected, consumer-centric, cloud-enabled, insights-driven, multi-device, global world, the focus of solutions has shifted from the product that is sold to the person who is buying the product or service. Enterprises have rebranded their business around the consumers of their products. The buyer is the person and the focus is not on the offering. The person is connected through multiple devices, wearables, at home, on the road, and in multiple locations, sometimes simultaneously...
Established in 1998, Calsoft is a leading software product engineering Services Company specializing in Storage, Networking, Virtualization and Cloud business verticals. Calsoft provides End-to-End Product Development, Quality Assurance Sustenance, Solution Engineering and Professional Services expertise to assist customers in achieving their product development and business goals. The company's deep domain knowledge of Storage, Virtualization, Networking and Cloud verticals helps in delivering ...
SYS-CON Events announced today that Hitrons 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. Hitrons Solutions Inc. is distributor in the North American market for unique products and services of small and medium-size businesses, including cloud services and solutions, SEO marketing platforms, and mobile applications.
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.
OnProcess Technology has announced it will be a featured speaker at @ThingsExpo, taking place November 1 - 3, 2016, in Santa Clara, California. Dan Gettens, OnProcess’ Chief Analytics Officer, will discuss how Internet of Things (IoT) data can be leveraged to predict product failures, improve uptime and slash costly inventory stock. @ThingsExpo is an annual gathering of IoT and cloud developers, practitioners and thought-leaders who exchange ideas and insights on topics ranging from Big Data in...
SYS-CON Events announced today that Enzu 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. Enzu’s mission is to be the leading provider of enterprise cloud solutions worldwide. Enzu enables online businesses to use its IT infrastructure to their competitive advantage. By offering a suite of proven hosting and management services, Enzu wants companies to focus on the core of their online busine...
The explosion of new web/cloud/IoT-based applications and the data they generate are transforming our world right before our eyes. In this rush to adopt these new technologies, organizations are often ignoring fundamental questions concerning who owns the data and failing to ask for permission to conduct invasive surveillance of their customers. Organizations that are not transparent about how their systems gather data telemetry without offering shared data ownership risk product rejection, regu...
The Open Connectivity Foundation (OCF), sponsor of the IoTivity open source project, and AllSeen Alliance, which provides the AllJoyn® open source IoT framework, today announced that the two organizations’ boards have approved a merger under the OCF name and bylaws. This merger will advance interoperability between connected devices from both groups, enabling the full operating potential of IoT and representing a significant step towards a connected ecosystem.