Click here to close now.

Welcome!

Java Authors: Carmen Gonzalez, Liz McMillan, Yeshim Deniz, Pat Romanski, Elizabeth White

Related Topics: Java

Java: Article

Multi-Threading in Java

Multi-Threading in Java

Introduction
Multi-tasking is rapidly becoming a necessity in software development today. All major operating systems support some form of multi-tasking, and as costs come down it is becoming common for high end systems to incorporate multiple processors.

Multi-Tasking and Threads
At its most basic level, multi-tasking allows multiple programs to be run at the "same" time. The best way to visualize this is to think of each application as running on its own processor.

It would be quite inefficient for each application to have a dedicated processor. A major function of most modern operating systems is to make each application share access to processors by preempting one application to let another one run. Figure 1 illustrates the difference between processor sharing and non-sharing.

Just like programs can run concurrently, pieces of the same program can run concurrently. This ability is known as threading and it is what Java supports. Figure 2 illustrates how a program can be threaded.

Threads are becoming more popular because they are faster to set up, often require less memory and allow better encapsulation.

Commonly, it is the responsibility of the operating system to schedule and preempt each thread, just like it preempts each application. This usually leads to platform-specific methods of multi-threading.

Platform-Independent Threading
Most programming languages rely on operating system-specific calls to support multi-threading. For example: C/C++ programs in Unix often use fork() and Window 95/NT C/C++ programs often use CreateThread(). This can cause a lot of headaches when trying to port an application.

Since one of Java's goals is to "write once, run anywhere", the Java language specification contains support for threading. In theory, this allows multi-threaded programs to be run on any platform which supports Java without concern for how the Java Virtual Machine (JVM) actually implements the threading.

Most JVMs don't actually use native operating system threads to implement threading. They implement their own task scheduling and context switching algorithms within the JVM. This makes the JVM easier to port from one operating system to another. However, Sun is going to be releasing a JVM for the Sun Solaris SPARC which uses native threads to implement Java threads. Having the JVM use native threads can be a real benefit to Java applications because of gained responsiveness to other running processes. Best of all, a Java application doesn't have to do anything special to make use of benefits supplied by different implementations.

Introduction to Java Threading
Java is one of the few common-programming languages that actually supports threading in the language itself.

Java defines a Thread class and a Runnable interface that can be used to define a thread object. Take a look at the Basic Thread example that derives from the Thread class. It creates two threads that display messages asynchronously.

Run this example multiple times and under different JVMs and see how and when the messages are displayed.

Depending on the speed of your machine and the JVM you are using, it may appear that the threads are not being preempted. For example: All of the first thread's messages may be printed followed by all of the second thread's messages.

This usually happens when running on a fast computer or using Just-In-Time (JIT) Java. The reason for this is that one of the sample threads may actually finishing printing all of its messages before it is scheduled for preemption. Try making the threads take longer to finish by increasing MAX_INDEX to 100 (or more) and see what happens.

Runnable Interface
The Basic Thread example derives from the Thread class to create a thread object; However, a class may implement the Runnable interface instead.

Implementing the Runnable interface is useful when a class needs to be multi-threaded and also be derived from another class. Remember that Java supports only single inheritance.

To change the Basic Thread example to use a Runnable interface, change the CountThread definition to:

public class CountThread implements Runnable

Then, change the CountThread object declarations to:

CountThread countRunnable1 =
new CountThread( "Thread 1" );
CountThread countRunnable2 =
new CountThread( "Thread 2" ); Thread countThread1 =
new Thread( CountRunnable1 ); Thread countThread2 =
new Thread( countRunnable2 );

This works because the Thread class supports a special constructor that accepts a Runnable interface. Therefore, we can create a thread based on a Runnable interface. The only method that the interface defines is run().

start() and run()
By calling a Thread object's start() method a Java application tells the JVM to start a separate thread of execution. The JVM will only allow a Thread object to create a single thread of execution for the lifetime of the object. Subsequent calls to an object's start() method will be ignored if the thread associated with the object has terminated; otherwise, the JVM will cause the start() method to throw illegalThreadStateException.

Once the JVM sets up the separate thread of execution it will call the object's run() method from within the newly created thread. The run() method of a Java Thread is like the thread's "main" method. Once started by the JVM, the thread exists until the run() method terminates.

stop()
The stop() method is used to stop the execution of a thread before its run() method terminates.

However, the use of stop() is discouraged because it will not always stop a thread. The stop() method is a synchronized method and as such will not stop other synchronized method blocks. This means that a deadlocked thread can't be stopped, which isn't very useful. Synchronized methods are discussed in the section on monitors.

The best way to stop a thread is to let the run() method exit gracefully by using proper synchronization techniques like the ones that follow.

join()
Join is a simple synchronization mechanism that allows one thread to wait for another to finish. In the Basic Thread example, the main application waits for the threads that it started to finish. Note that the order in which threads are joined is not important.

Thread Synchronization
Writing multi-threaded applications usually involves much more than just starting and stopping threads. Usually some form of thread synchronization is required at key points in time. There are two main types of synchronization that Java supports: Monitors and Mutexes.

Monitors
The term monitor comes from the monolithic monitor (more commonly known today as a kernel) found in operating systems. A fundamental responsibility of an operating system is to protect system resources from unrestricted access, much like a monitor protects the internal data and methods of an object from unrestricted access by other threads.

Each Java object has a monitor and only one thread at a time has access to that monitor. When more than one thread wants access to an object's monitor, they must wait until that monitor is released. Notice that the Thread object itself has nothing to do with implementing monitors. Monitors are inherent in every Java object, and every Java object has its own independent monitor.

Java defines the keyword "synchronized" to gain access to an object's monitor. There are two ways to use synchronize: either by method or by block.

To allow only one thread at a time to access an object's method, use the "synchronized" keyword in the definition of the method.

public synchronized void sem_wait( String currentThreadName )
{
// Statements here are under protection of the object"s monitor.
// Each instance of the object will allow only one thread at a time access to the method.
}

To allow only one thread at a time access to a portion of an object's method, use the block form of synchronized within the method (see Listing 5).

Notice that the block form of "synchronized" takes an object as a parameter. The monitor associated with the given object is used to perform the synchronization.

Although we can specify individual methods and blocks to be synchronized, there is still only a single monitor per object. Once a thread enters a synchronized section of an object it has acquired that object's monitor. Since the object's monitor is now acquired, all other threads trying to acquire that monitor will have to wait until it is released. The monitor will be released when the one thread that entered the object's synchronized section leaves the section.

While a thread has acquired an object's monitor, it will immediately succeed in subsequent attempts to acquire that same object's monitor. This makes sense because the purpose of monitors is to allow only a single thread access to some section of code. Since the thread already has access to the monitor it is safe to let that thread execute the code. This is very useful because it means that an object's synchronized methods may call each other without fear of delay or deadlocking.

If for some reason a thread acquires an object's monitor and doesn't release it, the waiting threads will wait "forever". This is called a deadlock and can occur very easily. Deadlocks can be hard to find in code and may not show up under testing depending on the timing of the threads. It is recommended that a monitor be used to protect only what is absolutely necessary for correct behavior. Use the synchronized block mechanism to limit the scope of the monitor. In addition, don't call any methods within a synchronized block except the class Object methods (wait, notify,). This will greatly reduce the chances of deadlocks.

Remember that monitors are based on objects so be careful when using references to objects. Each reference to an object uses the monitor of the object being referenced.

One complexity of monitors is that static methods may also be synchronized. However, a static method is not associated with an object. To handle this, all static synchronized methods of a class share a single monitor that works independently of an object's monitor. When a synchronized method calls a static synchronized method, it must acquire another monitor (the one associated with all static synchronized methods of the class).

Mutexes
Mutexes are used when two or more threads can't interleave certain types of operations. Thus, one sequence must be completed before the other is started. The join() method, used in the Basic Thread example, was a simple fixed use mutex that waits for the thread being joined to stop.

The generic mutex methods: wait(), notify() and notifyAll() are available to all Java objects because they are declared in the Object Java class. These methods allow any thread to wait for any other thread to complete some activity. When the activity is complete, the thread notifies one (or all if notifyAll is called) waiting threads.

wait()
The thread that calls an object's wait() method will be suspended and any monitors the thread had acquired will be released. The thread will remain suspended until it is notified and the monitors needed by the thread can be reacquired.

In order for a thread to call an object's wait() method, it must own the object's monitor.

notify() and notifyAll()
A thread calls an object's notify() method when it wants to let a thread waiting on that object know that some activity has been completed. A waiting thread will be awakened and put back in the queue of running threads. However, the awakened thread still has to reacquire all monitors that it released when it called wait().

The thread that calls an object's notify() or notifyAll() method must have possession of the object's monitor.

When notifyAll() is called, all threads that are currently waiting on the object's monitor will be a awakened.

Semaphores
One of the problems with Java Mutexes is that notify will only wake up threads that are currently waiting. This can cause synchronization headaches because one must make sure that a thread waits before notify is called. In other words, the notification is lost when there is no one waiting. While not directly supported by Java, a semaphore can be emulated to solve this problem.

Just like mutexes, semaphores are used when two or more threads can't interleave certain types of operations. Thus, one sequence must be completed before the other is started. However, a semaphore contains more state information that allows it to overcome the limitations of Java mutexes.

We can emulate the most common form of semaphore, called a blocked-set semaphore, by using monitors and mutexes. The blocked-set semaphore has the following definition shown in Listing 4. A single thread awakens one suspended thread.

See the example Semaphore.java for an implementation of this semaphore.

Notice that we will only call notify() if we have first done a wait(), and we will remember when we called sem_signal() without wait() being called.

Putting it all Together
One of the classic concurrent programming problems is the producer/consumer problem. It involves two threads: one producer thread and one consumer thread. Take a look at the producer/consumer example.

The producer produces integer numbers and prints a message that states an integer was produced. The consumer consumes an integer number, supplied by the producer, and prints a message that states an integer was consumed. When the consumer receives the product (number) 0 it knows the producer is done and quits.

A small integer item buffer is used so that the producer can make multiple products without having to wait for the consumer to consume them.

The example just produces integer numbers to keep the example concise. However, the producer/consumer principle can be used to solve many real world problems. A producer could search for files and produce found filenames to a consumer window. A producer could do database queries that send results to a report window. There are countless concurrency problems that may be solved with this technique.

Conclusion
Java developers not only get a great object-oriented language, but also get a language that supports multi-threading. However, just like good object-oriented development requires a different way of thinking, good threaded programming requires a different way of thinking - with the rewards just as great.

Be creative and remember that all aspects of Java can be threaded to solve everyday problems: Windowing interfaces (AWT), saving and loading files (File I/O) and reusable components (Beansª).

References
M. Ben-Ari, "Principles of Concurrent and Distributed Programming", Prentice Hall, New York, 1990.
S. Oaks & H. Wong, "Java Threads", O'Reilly, MA 1997

More Stories By Tod Cunningham

Tod Cunningham is a software engineer with development experience in real-time multi-processing systems written in C/C++. as well as Java. He graduated from the University of Toledo in 1992 with a BS in Computer Science and Engineering.

Comments (0)

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.


@ThingsExpo Stories
The 4th International Internet of @ThingsExpo, co-located with the 17th International Cloud Expo - to be held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA - announces that its Call for Papers is open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
The worldwide cellular network will be the backbone of the future IoT, and the telecom industry is clamoring to get on board as more than just a data pipe. In his session at @ThingsExpo, Evan McGee, CTO of Ring Plus, Inc., discussed what service operators can offer that would benefit IoT entrepreneurs, inventors, and consumers. Evan McGee is the CTO of RingPlus, a leading innovative U.S. MVNO and wireless enabler. His focus is on combining web technologies with traditional telecom to create a new breed of unified communication that is easily accessible to the general consumer. With over a de...
Disruptive macro trends in technology are impacting and dramatically changing the "art of the possible" relative to supply chain management practices through the innovative use of IoT, cloud, machine learning and Big Data to enable connected ecosystems of engagement. Enterprise informatics can now move beyond point solutions that merely monitor the past and implement integrated enterprise fabrics that enable end-to-end supply chain visibility to improve customer service delivery and optimize supplier management. Learn about enterprise architecture strategies for designing connected systems tha...
Cloud is not a commodity. And no matter what you call it, computing doesn’t come out of the sky. It comes from physical hardware inside brick and mortar facilities connected by hundreds of miles of networking cable. And no two clouds are built the same way. SoftLayer gives you the highest performing cloud infrastructure available. One platform that takes data centers around the world that are full of the widest range of cloud computing options, and then integrates and automates everything. Join SoftLayer on June 9 at 16th Cloud Expo to learn about IBM Cloud's SoftLayer platform, explore se...
SYS-CON Media announced today that 9 out of 10 " most read" DevOps articles are published by @DevOpsSummit Blog. Launched in October 2014, @DevOpsSummit 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. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long development cycles that produce softw...
With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo in Silicon Valley. Learn what is going on, contribute to the discussions, and ensure that your enterprise is as "IoT-Ready" as it can be! Internet of @ThingsExpo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 17th 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 an...
15th Cloud Expo, which took place Nov. 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA, expanded the conference content of @ThingsExpo, Big Data Expo, and DevOps Summit to include two developer events. IBM held a Bluemix Developer Playground on November 5 and ElasticBox held a Hackathon on November 6. Both events took place on the expo floor. The Bluemix Developer Playground, for developers of all levels, highlighted the ease of use of Bluemix, its services and functionality and provide short-term introductory projects that developers can complete between sessions.
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...
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...
Grow your business with enterprise wearable apps using SAP Platforms and Google Glass. SAP and Google just launched the SAP and Google Glass Challenge, an opportunity for you to innovate and develop the best Enterprise Wearable App using SAP Platforms and Google Glass and gain valuable market exposure. In his session at @ThingsExpo, Brian McPhail, Senior Director of Business Development, ISVs & Digital Commerce at SAP, outlined the timeline of the SAP Google Glass Challenge and the opportunity for developers, start-ups, and companies of all sizes to engage with SAP today.
The 3rd International @ThingsExpo, co-located with the 16th International Cloud Expo – to be held June 9-11, 2015, at the Javits Center in New York City, NY – is now accepting Hackathon proposals. Hackathon sponsorship benefits include general brand exposure and increasing engagement with the developer ecosystem. At Cloud Expo 2014 Silicon Valley, IBM held the Bluemix Developer Playground on November 5 and ElasticBox held the DevOps Hackathon on November 6. Both events took place on the expo floor. The Bluemix Developer Playground, for developers of all levels, highlighted the ease of use of...
Enthusiasm for the Internet of Things has reached an all-time high. In 2013 alone, venture capitalists spent more than $1 billion dollars investing in the IoT space. With "smart" appliances and devices, IoT covers wearable smart devices, cloud services to hardware companies. Nest, a Google company, detects temperatures inside homes and automatically adjusts it by tracking its user's habit. These technologies are quickly developing and with it come challenges such as bridging infrastructure gaps, abiding by privacy concerns and making the concept a reality. These challenges can't be addressed w...
The industrial software market has treated data with the mentality of “collect everything now, worry about how to use it later.” We now find ourselves buried in data, with the pervasive connectivity of the (Industrial) Internet of Things only piling on more numbers. There’s too much data and not enough information. In his session at @ThingsExpo, Bob Gates, Global Marketing Director, GE’s Intelligent Platforms business, to discuss how realizing the power of IoT, software developers are now focused on understanding how industrial data can create intelligence for industrial operations. Imagine ...
SYS-CON Events announced today that Liaison Technologies, a leading provider of data management and integration cloud services and solutions, has been named "Silver 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. Liaison Technologies is a recognized market leader in providing cloud-enabled data integration and data management solutions to break down complex information barriers, enabling enterprises to make smarter decisions, faster.
The 17th International Cloud Expo has announced that its Call for Papers is open. 17th International Cloud Expo, to be held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, APM, APIs, Microservices, Security, Big Data, Internet of Things, DevOps and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit your speaking proposal today!
Hadoop as a Service (as offered by handful of niche vendors now) is a cloud computing solution that makes medium and large-scale data processing accessible, easy, fast and inexpensive. In his session at Big Data Expo, Kumar Ramamurthy, Vice President and Chief Technologist, EIM & Big Data, at Virtusa, will discuss how this is achieved by eliminating the operational challenges of running Hadoop, so one can focus on business growth. The fragmented Hadoop distribution world and various PaaS solutions that provide a Hadoop flavor either make choices for customers very flexible in the name of opti...
Cultural, regulatory, environmental, political and economic (CREPE) conditions over the past decade are creating cross-industry solution spaces that require processes and technologies from both the Internet of Things (IoT), and Data Management and Analytics (DMA). These solution spaces are evolving into Sensor Analytics Ecosystems (SAE) that represent significant new opportunities for organizations of all types. Public Utilities throughout the world, providing electricity, natural gas and water, are pursuing SmartGrid initiatives that represent one of the more mature examples of SAE. We have s...
The Internet of Things will put IT to its ultimate test by creating infinite new opportunities to digitize products and services, generate and analyze new data to improve customer satisfaction, and discover new ways to gain a competitive advantage across nearly every industry. In order to help corporate business units to capitalize on the rapidly evolving IoT opportunities, IT must stand up to a new set of challenges. In his session at @ThingsExpo, Jeff Kaplan, Managing Director of THINKstrategies, will examine why IT must finally fulfill its role in support of its SBUs or face a new round of...
Can call centers hang up the phones for good? Intuitive Solutions did. WebRTC enabled this contact center provider to eliminate antiquated telephony and desktop phone infrastructure with a pure web-based solution, allowing them to expand beyond brick-and-mortar confines to a home-based agent model. It also ensured scalability and better service for customers, including MUY! Companies, one of the country's largest franchise restaurant companies with 232 Pizza Hut locations. This is one example of WebRTC adoption today, but the potential is limitless when powered by IoT.
One of the biggest challenges when developing connected devices is identifying user value and delivering it through successful user experiences. In his session at Internet of @ThingsExpo, Mike Kuniavsky, Principal Scientist, Innovation Services at PARC, described an IoT-specific approach to user experience design that combines approaches from interaction design, industrial design and service design to create experiences that go beyond simple connected gadgets to create lasting, multi-device experiences grounded in people's real needs and desires.