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Multi-Threading in Java

Multi-Threading in Java

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.

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.

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.

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

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.

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