Welcome!

Linux Containers Authors: Pat Romanski, Yeshim Deniz, Elizabeth White, Liz McMillan, Derek Weeks

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
    	newsThread.start();
    	stockThread.start();

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

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

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

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

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){
       	papa=parent;
    }

    public void run() {
	// Sleep for 3 seconds to emulate some processing
	// and return a string with the market data to the parent
 	try {
		Thread.sleep(3000);
		papa.submitResult("The stock market is on the rise!");
	} catch (InterruptedException e) {
			e.printStackTrace();
	}
    }
}

Listing 3. The source code of the class NewsServer


public class NewsServer implements Runnable {
    MyPortal papa;

    // Constructor
    NewsServer(MyPortal parent){
       	papa=parent;
    }

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

		try {
			Thread.sleep(4000);
			papa.submitResult("We have  good and bad news");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

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){
    	  dataToDisplay.add(data);
    	}

    	if (dataToDisplay.size()==2){
        	System.out.println(dataToDisplay);
    	}
    }

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
Artificial intelligence, machine learning, neural networks. We’re in the midst of a wave of excitement around AI such as hasn’t been seen for a few decades. But those previous periods of inflated expectations led to troughs of disappointment. Will this time be different? Most likely. Applications of AI such as predictive analytics are already decreasing costs and improving reliability of industrial machinery. Furthermore, the funding and research going into AI now comes from a wide range of com...
Internet of @ThingsExpo, taking place October 31 - November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 21st Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago. All major researchers estimate there will be tens of billions devic...
We build IoT infrastructure products - when you have to integrate different devices, different systems and cloud you have to build an application to do that but we eliminate the need to build an application. Our products can integrate any device, any system, any cloud regardless of protocol," explained Peter Jung, Chief Product Officer at Pulzze Systems, in this SYS-CON.tv interview at @ThingsExpo, held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA
SYS-CON Events announced today that Ayehu will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on October 31 - November 2, 2017 at the Santa Clara Convention Center in Santa Clara California. Ayehu provides IT Process Automation & Orchestration solutions for IT and Security professionals to identify and resolve critical incidents and enable rapid containment, eradication, and recovery from cyber security breaches. Ayehu provides customers greater control over IT infras...
SYS-CON Events announced today that MobiDev, a client-oriented software development company, will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. MobiDev is a software company that develops and delivers turn-key mobile apps, websites, web services, and complex software systems for startups and enterprises. Since 2009 it has grown from a small group of passionate engineers and business...
SYS-CON Events announced today that GrapeUp, the leading provider of rapid product development at the speed of business, will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Grape Up is a software company, specialized in cloud native application development and professional services related to Cloud Foundry PaaS. With five expert teams that operate in various sectors of the market acr...
SYS-CON Events announced today that Enzu will exhibit at SYS-CON's 21st Int\ernational Cloud Expo®, which will take place October 31-November 2, 2017, 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 ...
New competitors, disruptive technologies, and growing expectations are pushing every business to both adopt and deliver new digital services. This ‘Digital Transformation’ demands rapid delivery and continuous iteration of new competitive services via multiple channels, which in turn demands new service delivery techniques – including DevOps. In this power panel at @DevOpsSummit 20th Cloud Expo, moderated by DevOps Conference Co-Chair Andi Mann, panelists examined how DevOps helps to meet the de...
SYS-CON Events announced today that Cloud Academy named "Bronze Sponsor" of 21st International Cloud Expo which will take place October 31 - November 2, 2017 at the Santa Clara Convention Center in Santa Clara, CA. Cloud Academy is the industry’s most innovative, vendor-neutral cloud technology training platform. Cloud Academy provides continuous learning solutions for individuals and enterprise teams for Amazon Web Services, Microsoft Azure, Google Cloud Platform, and the most popular cloud com...
SYS-CON Events announced today that IBM has been named “Diamond Sponsor” of SYS-CON's 21st Cloud Expo, which will take place on October 31 through November 2nd 2017 at the Santa Clara Convention Center in Santa Clara, California.
In his session at Cloud Expo, Alan Winters, an entertainment executive/TV producer turned serial entrepreneur, presented a success story of an entrepreneur who has both suffered through and benefited from offshore development across multiple businesses: The smart choice, or how to select the right offshore development partner Warning signs, or how to minimize chances of making the wrong choice Collaboration, or how to establish the most effective work processes Budget control, or how to ma...
With major technology companies and startups seriously embracing Cloud strategies, now is the perfect time to attend 21st Cloud Expo October 31 - November 2, 2017, at the Santa Clara Convention Center, CA, and June 12-14, 2018, at the Javits Center in New York City, NY, and learn what is going on, contribute to the discussions, and ensure that your enterprise is on the right path to Digital Transformation.
SYS-CON Events announced today that CA Technologies has been named "Platinum Sponsor" of SYS-CON's 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. CA Technologies helps customers succeed in a future where every business - from apparel to energy - is being rewritten by software. From planning to development to management to security, CA creates software that fuels transformation for companies in the applic...
Multiple data types are pouring into IoT deployments. Data is coming in small packages as well as enormous files and data streams of many sizes. Widespread use of mobile devices adds to the total. In this power panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists looked at the tools and environments that are being put to use in IoT deployments, as well as the team skills a modern enterprise IT shop needs to keep things running, get a handle on all this data, and deliver...
In his session at @ThingsExpo, Eric Lachapelle, CEO of the Professional Evaluation and Certification Board (PECB), provided an overview of various initiatives to certify the security of connected devices and future trends in ensuring public trust of IoT. Eric Lachapelle is the Chief Executive Officer of the Professional Evaluation and Certification Board (PECB), an international certification body. His role is to help companies and individuals to achieve professional, accredited and worldwide re...
Amazon started as an online bookseller 20 years ago. Since then, it has evolved into a technology juggernaut that has disrupted multiple markets and industries and touches many aspects of our lives. It is a relentless technology and business model innovator driving disruption throughout numerous ecosystems. Amazon’s AWS revenues alone are approaching $16B a year making it one of the largest IT companies in the world. With dominant offerings in Cloud, IoT, eCommerce, Big Data, AI, Digital Assista...
No hype cycles or predictions of zillions of things here. IoT is big. You get it. You know your business and have great ideas for a business transformation strategy. What comes next? Time to make it happen. In his session at @ThingsExpo, Jay Mason, Associate Partner at M&S Consulting, presented a step-by-step plan to develop your technology implementation strategy. He discussed the evaluation of communication standards and IoT messaging protocols, data analytics considerations, edge-to-cloud tec...
"When we talk about cloud without compromise what we're talking about is that when people think about 'I need the flexibility of the cloud' - it's the ability to create applications and run them in a cloud environment that's far more flexible,” explained Matthew Finnie, CTO of Interoute, in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
IoT solutions exploit operational data generated by Internet-connected smart “things” for the purpose of gaining operational insight and producing “better outcomes” (for example, create new business models, eliminate unscheduled maintenance, etc.). The explosive proliferation of IoT solutions will result in an exponential growth in the volume of IoT data, precipitating significant Information Governance issues: who owns the IoT data, what are the rights/duties of IoT solutions adopters towards t...
With the introduction of IoT and Smart Living in every aspect of our lives, one question has become relevant: What are the security implications? To answer this, first we have to look and explore the security models of the technologies that IoT is founded upon. In his session at @ThingsExpo, Nevi Kaja, a Research Engineer at Ford Motor Company, discussed some of the security challenges of the IoT infrastructure and related how these aspects impact Smart Living. The material was delivered interac...