|By Tod Cunningham||
|January 1, 1998 12:00 AM EST||
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.
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.
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.
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 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.
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.
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 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.
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.
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.
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).
M. Ben-Ari, "Principles of Concurrent and Distributed Programming", Prentice Hall, New York, 1990.
S. Oaks & H. Wong, "Java Threads", O'Reilly, MA 1997
For IoT to grow as quickly as analyst firms’ project, a lot is going to fall on developers to quickly bring applications to market. But the lack of a standard development platform threatens to slow growth and make application development more time consuming and costly, much like we’ve seen in the mobile space. In his session at @ThingsExpo, Mike Weiner, Product Manager of the Omega DevCloud with KORE Telematics Inc., discussed the evolving requirements for developers as IoT matures and conducted a live demonstration of how quickly application development can happen when the need to comply wit...
Aug. 2, 2015 11:15 AM EDT Reads: 344
SYS-CON Events announced today that HPM Networks will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. For 20 years, HPM Networks has been integrating technology solutions that solve complex business challenges. HPM Networks has designed solutions for both SMB and enterprise customers throughout the San Francisco Bay Area.
Aug. 1, 2015 04:45 PM EDT Reads: 496
The Internet of Everything (IoE) brings together people, process, data and things to make networked connections more relevant and valuable than ever before – transforming information into knowledge and knowledge into wisdom. IoE creates new capabilities, richer experiences, and unprecedented opportunities to improve business and government operations, decision making and mission support capabilities.
Aug. 1, 2015 10:00 AM EDT Reads: 331
Explosive growth in connected devices. Enormous amounts of data for collection and analysis. Critical use of data for split-second decision making and actionable information. All three are factors in making the Internet of Things a reality. Yet, any one factor would have an IT organization pondering its infrastructure strategy. How should your organization enhance its IT framework to enable an Internet of Things implementation? In his session at @ThingsExpo, James Kirkland, Red Hat's Chief Architect for the Internet of Things and Intelligent Systems, described how to revolutionize your archit...
Jul. 30, 2015 07:30 PM EDT Reads: 1,417
MuleSoft has announced the findings of its 2015 Connectivity Benchmark Report on the adoption and business impact of APIs. The findings suggest traditional businesses are quickly evolving into "composable enterprises" built out of hundreds of connected software services, applications and devices. Most are embracing the Internet of Things (IoT) and microservices technologies like Docker. A majority are integrating wearables, like smart watches, and more than half plan to generate revenue with APIs within the next year.
Jul. 30, 2015 02:30 PM EDT Reads: 131
Growth hacking is common for startups to make unheard-of progress in building their business. Career Hacks can help Geek Girls and those who support them (yes, that's you too, Dad!) to excel in this typically male-dominated world. Get ready to learn the facts: Is there a bias against women in the tech / developer communities? Why are women 50% of the workforce, but hold only 24% of the STEM or IT positions? Some beginnings of what to do about it! In her Opening Keynote at 16th Cloud Expo, Sandy Carter, IBM General Manager Cloud Ecosystem and Developers, and a Social Business Evangelist, d...
Jul. 30, 2015 12:00 PM EDT Reads: 2,073
In his keynote at 16th Cloud Expo, Rodney Rogers, CEO of Virtustream, discussed the evolution of the company from inception to its recent acquisition by EMC – including personal insights, lessons learned (and some WTF moments) along the way. Learn how Virtustream’s unique approach of combining the economics and elasticity of the consumer cloud model with proper performance, application automation and security into a platform became a breakout success with enterprise customers and a natural fit for the EMC Federation.
Jul. 30, 2015 09:00 AM EDT Reads: 2,173
The Internet of Things is not only adding billions of sensors and billions of terabytes to the Internet. It is also forcing a fundamental change in the way we envision Information Technology. For the first time, more data is being created by devices at the edge of the Internet rather than from centralized systems. What does this mean for today's IT professional? In this Power Panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists addressed this very serious issue of profound change in the industry.
Jul. 29, 2015 03:00 PM EDT Reads: 1,295
Discussions about cloud computing are evolving into discussions about enterprise IT in general. As enterprises increasingly migrate toward their own unique clouds, new issues such as the use of containers and microservices emerge to keep things interesting. In this Power Panel at 16th Cloud Expo, moderated by Conference Chair Roger Strukhoff, panelists addressed the state of cloud computing today, and what enterprise IT professionals need to know about how the latest topics and trends affect their organization.
Jul. 29, 2015 02:00 PM EDT Reads: 1,199
It is one thing to build single industrial IoT applications, but what will it take to build the Smart Cities and truly society-changing applications of the future? The technology won’t be the problem, it will be the number of parties that need to work together and be aligned in their motivation to succeed. In his session at @ThingsExpo, Jason Mondanaro, Director, Product Management at Metanga, discussed how you can plan to cooperate, partner, and form lasting all-star teams to change the world and it starts with business models and monetization strategies.
Jul. 28, 2015 04:30 PM EDT Reads: 1,775
Converging digital disruptions is creating a major sea change - Cisco calls this the Internet of Everything (IoE). IoE is the network connection of People, Process, Data and Things, fueled by Cloud, Mobile, Social, Analytics and Security, and it represents a $19Trillion value-at-stake over the next 10 years. In her keynote at @ThingsExpo, Manjula Talreja, VP of Cisco Consulting Services, discussed IoE and the enormous opportunities it provides to public and private firms alike. She will share what businesses must do to thrive in the IoE economy, citing examples from several industry sectors.
Jul. 28, 2015 11:00 AM EDT Reads: 2,049
There will be 150 billion connected devices by 2020. New digital businesses have already disrupted value chains across every industry. APIs are at the center of the digital business. You need to understand what assets you have that can be exposed digitally, what their digital value chain is, and how to create an effective business model around that value chain to compete in this economy. No enterprise can be complacent and not engage in the digital economy. Learn how to be the disruptor and not the disruptee.
Jul. 27, 2015 10:00 AM EDT Reads: 2,043
Akana has released Envision, an enhanced API analytics platform that helps enterprises mine critical insights across their digital eco-systems, understand their customers and partners and offer value-added personalized services. “In today’s digital economy, data-driven insights are proving to be a key differentiator for businesses. Understanding the data that is being tunneled through their APIs and how it can be used to optimize their business and operations is of paramount importance,” said Alistair Farquharson, CTO of Akana.
Jul. 27, 2015 09:00 AM EDT Reads: 333
Business as usual for IT is evolving into a "Make or Buy" decision on a service-by-service conversation with input from the LOBs. How does your organization move forward with cloud? In his general session at 16th Cloud Expo, Paul Maravei, Regional Sales Manager, Hybrid Cloud and Managed Services at Cisco, discusses how Cisco and its partners offer a market-leading portfolio and ecosystem of cloud infrastructure and application services that allow you to uniquely and securely combine cloud business applications and services across multiple cloud delivery models.
Jul. 27, 2015 08:00 AM EDT Reads: 1,910
The enterprise market will drive IoT device adoption over the next five years. In his session at @ThingsExpo, John Greenough, an analyst at BI Intelligence, division of Business Insider, analyzed how companies will adopt IoT products and the associated cost of adopting those products. John Greenough is the lead analyst covering the Internet of Things for BI Intelligence- Business Insider’s paid research service. Numerous IoT companies have cited his analysis of the IoT. Prior to joining BI Intelligence, he worked analyzing bank technology for Corporate Insight and The Clearing House Payment...
Jul. 26, 2015 09:00 PM EDT Reads: 1,584
"Optimal Design is a technology integration and product development firm that specializes in connecting devices to the cloud," stated Joe Wascow, Co-Founder & CMO of Optimal Design, in this SYS-CON.tv interview at @ThingsExpo, held June 9-11, 2015, at the Javits Center in New York City.
Jul. 25, 2015 02:00 PM EDT Reads: 402
SYS-CON Events announced today that CommVault has been named “Bronze Sponsor” of SYS-CON's 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. A singular vision – a belief in a better way to address current and future data management needs – guides CommVault in the development of Singular Information Management® solutions for high-performance data protection, universal availability and simplified management of data on complex storage networks. CommVault's exclusive single-platform architecture gives companies unp...
Jul. 25, 2015 01:00 PM EDT Reads: 1,969
Electric Cloud and Arynga have announced a product integration partnership that will bring Continuous Delivery solutions to the automotive Internet-of-Things (IoT) market. The joint solution will help automotive manufacturers, OEMs and system integrators adopt DevOps automation and Continuous Delivery practices that reduce software build and release cycle times within the complex and specific parameters of embedded and IoT software systems.
Jul. 25, 2015 12:15 PM EDT Reads: 483
"ciqada is a combined platform of hardware modules and server products that lets people take their existing devices or new devices and lets them be accessible over the Internet for their users," noted Geoff Engelstein of ciqada, a division of Mars International, in this SYS-CON.tv interview at @ThingsExpo, held June 9-11, 2015, at the Javits Center in New York City.
Jul. 25, 2015 12:00 PM EDT Reads: 1,548
Internet of Things is moving from being a hype to a reality. Experts estimate that internet connected cars will grow to 152 million, while over 100 million internet connected wireless light bulbs and lamps will be operational by 2020. These and many other intriguing statistics highlight the importance of Internet powered devices and how market penetration is going to multiply many times over in the next few years.
Jul. 25, 2015 09:00 AM EDT Reads: 1,499