Welcome!

Java Authors: Liz McMillan, Elizabeth White, AppDynamics Blog, Roger Strukhoff, Yeshim Deniz

Related Topics: Java, Weblogic, Linux

Java: 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 co-founder of two software companies: Farata Systems and SuranceBay. He authored several technical books and lots of articles on software development. Yakov is Java Champion (https://java-champions.java.net). He leads leads Princeton Java Users Group. Two of Yakov's books will go in print this year: "Enterprise Web Development" (O'Reilly) and "Java For Kids" (No Starch Press).

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
Code Halos - aka "digital fingerprints" - are the key organizing principle to understand a) how dumb things become smart and b) how to monetize this dynamic. In his session at @ThingsExpo, Robert Brown, AVP, Center for the Future of Work at Cognizant Technology Solutions, outlined research, analysis and recommendations from his recently published book on this phenomena on the way leading edge organizations like GE and Disney are unlocking the Internet of Things opportunity and what steps your organization should be taking to position itself for the next platform of digital competition.
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.
In their session at @ThingsExpo, Shyam Varan Nath, Principal Architect at GE, and Ibrahim Gokcen, who leads GE's advanced IoT analytics, focused on the Internet of Things / Industrial Internet and how to make it operational for business end-users. Learn about the challenges posed by machine and sensor data and how to marry it with enterprise data. They also discussed the tips and tricks to provide the Industrial Internet as an end-user consumable service using Big Data Analytics and Industrial Cloud.
SYS-CON Media announced that Splunk, a provider of the leading software platform for real-time Operational Intelligence, has launched an ad campaign on Big Data Journal. Splunk software and cloud services enable organizations to search, monitor, analyze and visualize machine-generated big data coming from websites, applications, servers, networks, sensors and mobile devices. The ads focus on delivering ROI - how improved uptime delivered $6M in annual ROI, improving customer operations by mining large volumes of unstructured data, and how data tracking delivers uptime when it matters most.
Today’s enterprise is being driven by disruptive competitive and human capital requirements to provide enterprise application access through not only desktops, but also mobile devices. To retrofit existing programs across all these devices using traditional programming methods is very costly and time consuming – often prohibitively so. In his session at @ThingsExpo, Jesse Shiah, CEO, President, and Co-Founder of AgilePoint Inc., discussed how you can create applications that run on all mobile devices as well as laptops and desktops using a visual drag-and-drop application – and eForms-buildi...
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, addressed the big issues involving these technologies and, more important, the results they will achieve. Rodney Rogers, chairman and CEO of Virtustream; Brendan O'Brien, co-founder of Aria Systems, Bart Copeland, president and CEO of ActiveState Software; Jim Cowie, chief scientist at Dyn; Dave Wagstaff, VP and chief architect at BSQUARE Corporation; Seth Proctor, CTO of NuoDB, Inc.; and Andris Gailitis, C...
SYS-CON Events announced today that CodeFutures, a leading supplier of database performance tools, has been named a “Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York, NY. CodeFutures is an independent software vendor focused on providing tools that deliver database performance tools that increase productivity during database development and increase database performance and scalability during production.
SYS-CON Events announced today that ActiveState, the leading independent Cloud Foundry and Docker-based PaaS provider, has been named “Silver Sponsor” of SYS-CON's DevOps Summit New York, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. ActiveState believes that enterprises gain a competitive advantage when they are able to quickly create, deploy and efficiently manage software solutions that immediately create business value, but they face many challenges that prevent them from doing so. The Company is uniquely positioned to help address these challenges thro...
IoT is still a vague buzzword for many people. In his session at @ThingsExpo, Mike Kavis, Vice President & Principal Cloud Architect at Cloud Technology Partners, discussed the business value of IoT that goes far beyond the general public's perception that IoT is all about wearables and home consumer services. He also discussed how IoT is perceived by investors and how venture capitalist access this space. Other topics discussed were barriers to success, what is new, what is old, and what the future may hold. Mike Kavis is Vice President & Principal Cloud Architect at Cloud Technology Pa...
Dale Kim is the Director of Industry Solutions at MapR. His background includes a variety of technical and management roles at information technology companies. While his experience includes work with relational databases, much of his career pertains to non-relational data in the areas of search, content management, and NoSQL, and includes senior roles in technical marketing, sales engineering, and support engineering. Dale holds an MBA from Santa Clara University, and a BA in Computer Science from the University of California, Berkeley.
SYS-CON Media announced that Cisco, a worldwide leader in IT that helps companies seize the opportunities of tomorrow, has launched a new ad campaign in Cloud Computing Journal. The ad campaign, a webcast titled 'Is Your Data Center Ready for the Application Economy?', focuses on the latest data center networking technologies, including SDN or ACI, and how customers are using SDN and ACI in their organizations to achieve business agility. The Cisco webcast is available on-demand.
The Internet of Things (IoT) promises to evolve the way the world does business; however, understanding how to apply it to your company can be a mystery. Most people struggle with understanding the potential business uses or tend to get caught up in the technology, resulting in solutions that fail to meet even minimum business goals. In his session at @ThingsExpo, Jesse Shiah, CEO / President / Co-Founder of AgilePoint Inc., showed what is needed to leverage the IoT to transform your business. He discussed opportunities and challenges ahead for the IoT from a market and technical point of vie...
"People are a lot more knowledgeable about APIs now. There are two types of people who work with APIs - IT people who want to use APIs for something internal and the product managers who want to do something outside APIs for people to connect to them," explained Roberto Medrano, Executive Vice President at SOA Software, in this SYS-CON.tv interview at Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
Performance is the intersection of power, agility, control, and choice. If you value performance, and more specifically consistent performance, you need to look beyond simple virtualized compute. Many factors need to be considered to create a truly performant environment. In his General Session at 15th Cloud Expo, Harold Hannon, Sr. Software Architect at SoftLayer, discussed how to take advantage of a multitude of compute options and platform features to make cloud the cornerstone of your online presence.
In this Women in Technology Power Panel at 15th Cloud Expo, moderated by Anne Plese, Senior Consultant, Cloud Product Marketing at Verizon Enterprise, Esmeralda Swartz, CMO at MetraTech; Evelyn de Souza, Data Privacy and Compliance Strategy Leader at Cisco Systems; Seema Jethani, Director of Product Management at Basho Technologies; Victoria Livschitz, CEO of Qubell Inc.; Anne Hungate, Senior Director of Software Quality at DIRECTV, discussed what path they took to find their spot within the technology industry and how do they see opportunities for other women in their area of expertise.
Almost everyone sees the potential of Internet of Things but how can businesses truly unlock that potential. The key will be in the ability to discover business insight in the midst of an ocean of Big Data generated from billions of embedded devices via Systems of Discover. Businesses will also need to ensure that they can sustain that insight by leveraging the cloud for global reach, scale and elasticity.
“The age of the Internet of Things is upon us,” stated Thomas Svensson, senior vice-president and general manager EMEA, ThingWorx, “and working with forward-thinking companies, such as Elisa, enables us to deploy our leading technology so that customers can profit from complete, end-to-end solutions.” ThingWorx, a PTC® (Nasdaq: PTC) business and Internet of Things (IoT) platform provider, announced on Monday that Elisa, Finnish provider of mobile and fixed broadband subscriptions, will deploy ThingWorx® platform technology to enable a new Elisa IoT service in Finland and Estonia.
Advanced Persistent Threats (APTs) are increasing at an unprecedented rate. The threat landscape of today is drastically different than just a few years ago. Attacks are much more organized and sophisticated. They are harder to detect and even harder to anticipate. In the foreseeable future it's going to get a whole lot harder. Everything you know today will change. Keeping up with this changing landscape is already a daunting task. Your organization needs to use the latest tools, methods and expertise to guard against those threats. But will that be enough? In the foreseeable future attacks w...
As enterprises move to all-IP networks and cloud-based applications, communications service providers (CSPs) – facing increased competition from over-the-top providers delivering content via the Internet and independently of CSPs – must be able to offer seamless cloud-based communication and collaboration solutions that can scale for small, midsize, and large enterprises, as well as public sector organizations, in order to keep and grow market share. The latest version of Oracle Communications Unified Communications Suite gives CSPs the capability to do just that. In addition, its integration ...
From telemedicine to smart cars, digital homes and industrial monitoring, the explosive growth of IoT has created exciting new business opportunities for real time calls and messaging. In his session at @ThingsExpo, Ivelin Ivanov, CEO and Co-Founder of Telestax, shared some of the new revenue sources that IoT created for Restcomm – the open source telephony platform from Telestax. Ivelin Ivanov is a technology entrepreneur who founded Mobicents, an Open Source VoIP Platform, to help create, deploy, and manage applications integrating voice, video and data. He is the co-founder of TeleStax, a...