Internet Programming with Java Course
1.3. Multithreading and Thread Synchronization
Understanding Multithreading
All the sample programs you developed in the preceding chapters have had only a single thread of execution. Each program proceeded sequentially, one instruction after another, until it completed its processing and terminated.
Multithreaded programs are similar to the single-threaded programs that you have been studying. They differ only in the fact that they support more than one concurrent thread of execution-that is, they are able to simultaneously execute multiple sequences of instructions. Each instruction sequence has its own unique flow of control that is independent of all others. These independently executed instruction sequences are known as threads.
If your computer has only a single CPU, you might be wondering how it can execute more than one thread at the same time. In single-processor systems, only a single thread of execution occurs at a given instant. The CPU quickly switches back and forth between several threads to create the illusion that the threads are executing at the same time. Single-processor systems support logical concurrency, not physical concurrency. Logical concurrency is the characteristic exhibited when multiple threads execute with separate, independent flows of control. On multiprocessor systems, several threads do, in fact, execute at the same time, and physical concurrency is achieved. The important feature of multithreaded programs is that they support logical concurrency, not whether physical concurrency is actually achieved.
Many programming languages support multiprogramming. Multiprogramming is the logically concurrent execution of multiple programs. For example, a program can request that the operating system execute programs A, B, and C by having it spawn a separate process for each program. These programs can run in parallel, depending upon the multiprogramming features supported by the underlying operating system. Multithreading differs from multiprogramming in that multithreading provides concurrency within the context of a single process and multiprogramming provides concurrency between processes. Threads are not complete processes in and of themselves. They are a separate flow of control that occurs within a process. Figure 8.1 illustrates the difference between multithreading and multiprogramming.
f8-1.gif){kind=link}
Figure 8.1 : Multithreading versus multiprogramming.
An executing program is generally associated with a single process. The advantage of multithreading is that concurrency can be used within a process to provide multiple simultaneous services to the user. Multithreading also requires less processing overhead than multiprogramming because concurrent threads are able to share common resources more easily. Multiple executing programs tend to duplicate resources and share data as the result of more time-consuming interprocess communication.
How Java Supports Multithreading
Java's multithreading support is centered around the java.lang.Thread class. The Thread class provides the capability to create objects of class Thread, each with its own separate flow of control. The Thread class encapsulates the data and methods associated with separate threads of execution and allows multithreading to be integrated within the object-oriented framework.
Java provides two approaches to creating threads. In the first approach, you create a subclass of class Thread and override the run() method to provide an entry point into the thread's execution. When you create an instance of your Thread subclass, you invoke its start() method to cause the thread to execute as an independent sequence of instructions. The start() method is inherited from the Thread class. It initializes the Thread object using your operating system's multithreading capabilities and invokes the run() method. You learn how to create threads using this approach in the next section.
The approach to creating threads identified in the previous paragraph is very simple and straightforward. However, it has the drawback of requiring your Thread objects to be under the Thread class in the class hierarchy. In some cases, as you'll see when you study applets in Part VI, "Programming the Web with Applets and Scripts," this requirement can be somewhat limiting.
Java's other approach to creating threads does not limit the location of your Thread objects within the class hierarchy. In this approach, your class implements the java.lang.Runnable interface. The Runnable interface consists of a single method, the run() method, which must be overridden by your class. The run() method provides an entry point into your thread's execution. In order to run an object of your class as an independent thread, you pass it as an argument to a constructor of class Thread. You learn how to create threads using this approach later in this chapter in the section titled "Implementing Runnable."
Creating Subclasses of Thread
In this section, you create your first multithreaded program by creating a subclass of Thread and then creating, initializing, and starting two Thread objects from your class. The threads will execute concurrently and display Java is hot, aromatic, and invigorating. to the console window.
The source code of the ThreadTest1 program.
class ThreadTest1
{
public static void main(String args[])
{
MyThread thread1 = new MyThread("thread1: ");
MyThread thread2 = new MyThread("thread2: ");
thread1.start();
thread2.start();
boolean thread1IsAlive = true;
boolean thread2IsAlive = true;
do {
if (thread1IsAlive && !thread1.isAlive()) {
thread1IsAlive = false;
System.out.println("Thread 1 is dead.");
}
if (thread2IsAlive && !thread2.isAlive()) {
thread2IsAlive = false;
System.out.println("Thread 2 is dead.");
}
} while(thread1IsAlive || thread2IsAlive);
}
}
class MyThread extends Thread
{
static String message[] =
{ "Java", "is", "hot,", "aromatic,", "and", "invigorating."};
public MyThread(String id)
{
super(id);
}
public void run()
{
String name = getName();
for (int i=0;i<message.length;++i) {
randomWait();
System.out.println(name + message[i]);
}
}
void randomWait()
{
try {
sleep((long)(3000*Math.random()));
} catch (InterruptedException x) {
System.out.println("Interrupted!");
}
}
}
This program creates two threads of execution, thread1 and thread2, from the MyThread class. It then starts both threads and executes a do statement that waits for the threads to die. The threads display the Java is hot, aromatic, and invigorating. message word by word, while waiting a short, random amount of time between each word. Because both threads share the console window, the program's output identifies which threads were able to write to the console at various times during the program's execution.
Run ThreadTest1 to get an idea of the output that it produces. Each time you run the program you might get a different program display. This is because the program uses a random number generator to determine how long each thread should wait before displaying its output. Look at the following output:
C:\java\jdg\ch08>java ThreadTest1
thread1: Java
thread2: Java
thread2: is
thread2: hot,
thread2: aromatic,
thread1: is
thread1: hot,
thread2: and
thread1: aromatic,
thread1: and
thread2: invigorating.
Thread 2 is dead.
thread1: invigorating.
Thread 1 is dead.
This output shows that thread1 executed first and displayed Java to the console window. It then waited to execute while thread2 displayed Java, is, hot,, and aromatic,. After that, thread2 waited while thread1 continued its execution. thread1 displayed is and then hot,. At this point, thread2 took over again. thread2 displayed and and then went back into waiting. thread1 then displayed aromatic, and and. thread2 finished its execution by displaying invigorating.. Having completed its execution, thread2 died, leaving thread1 as the only executing task. thread1 displayed invigorating. and then completed its execution.
The ThreadTest1 class consists of a single main() method. This method begins by creating thread1 and thread2 as new objects of class MyThread. It then starts both threads using the start() method. At this point, main() enters a do loop that continues until both thread1 and thread2 are no longer alive. The loop monitors the execution of the two threads and displays a message when it has detected the death of each thread. It uses the isAlive() method of the Thread class to tell when a thread has died. The thread1IsAlive and thread2IsAlive variables are used to ensure that a thread's obituary is only displayed once.
The MyThread class extends class Thread. It declares a statically initialized array, named message[], that contains the message to be displayed by each thread. It has a single constructor that invokes the Thread class constructor via super(). It contains two access methods: run() and randomWait(). The run() method is required. It uses the getName() method of class Thread to get the name of the currently executing thread. It then prints each word of the output display message while waiting a random length of time between each print. The randomWait() method invokes the sleep() method within a try statement. The sleep() method is another method inherited from class Thread. It causes the currently executing task to "go to sleep" or wait until a randomly specified number of milliseconds has transpired. Because the sleep() method throws the InterruptedException when its sleep is interrupted (how grouchy!), the exception is caught and handled by the randomWait() method. The exception is handled by displaying the fact that an interruption has occurred to the console window.
Implementing Runnable
In the previous section, you created a multithreaded program by creating the MyThread subclass of Thread. In this section, you create a program with similar behavior, but you create your threads as objects of the class MyClass, which is not a subclass of Thread. MyClass will implement the Runnable interface and objects of MyClass will be executed as threads by passing them as arguments to the Thread constructor.
The ThreadTest2 program's source code is shown in Listing 8.2. Enter it into the ThreadTest2.java file and compile it.
The source code of the ThreadTest2 program.
class ThreadTest2
{
public static void main(String args[])
{
Thread thread1 = new Thread(new MyClass("thread1: "));
Thread thread2 = new Thread(new MyClass("thread2: "));
thread1.start();
thread2.start();
boolean thread1IsAlive = true;
boolean thread2IsAlive = true;
do {
if (thread1IsAlive && !thread1.isAlive()) {
thread1IsAlive = false;
System.out.println("Thread 1 is dead.");
}
if (thread2IsAlive && !thread2.isAlive()) {
thread2IsAlive = false;
System.out.println("Thread 2 is dead.");
}
} while(thread1IsAlive || thread2IsAlive);
}
}
class MyClass implements Runnable
{
static String message[] =
{ "Java", "is", "hot,", "aromatic,", "and", "invigorating."};
String name;
public MyClass(String id)
{
name = id;
}
public void run()
{
for(int i=0;i<message.length;++i) {
randomWait();
System.out.println(name+message[i]);
}
}
void randomWait()
{
try {
Thread.currentThread().sleep((long)(3000*Math.random()));
} catch (InterruptedException x) {
System.out.println("Interrupted!");
}
}
}
The ThreadTest2 program is very similar to ThreadTest1. It differs only in the way that the threads are created. You should run ThreadTest2 a few times to examine its output. Here are the results of a sample run I made on my computer:
C:\java\jdg\ch08>java ThreadTest2
thread2: Java
thread1: Java
thread2: is
thread2: hot,
thread1: is
thread2: aromatic,
thread1: hot,
thread1: aromatic,
thread1: and
thread2: and
thread1: invigorating.
Thread 1 is dead.
thread2: invigorating.
Thread 2 is dead.
These results show thread2 beginning its output before thread1. It does not mean that thread2 began executing before thread1. Thread1 executed first, but went to sleep before generating any output. Thread2 then executed and started its output display before going to sleep. You can follow these results on your own to analyze how thread1 and thread2 switched back and forth during their execution to display their results to the console window.
The main() method of ThreadTest2 differs from that of ThreadTest1 in the way that it creates thread1 and thread2. ThreadTest1 created the threads as new instances of the MyThread class. ThreadTest2 was not able to create the threads directly, because MyClass is not a subclass of Thread. Instead, ThreadTest2 first created instances of MyClass and then passed them to the Thread() constructor, creating instances of class Thread. The Thread() constructor used by ThreadTest2 takes as its argument any class that implements the Runnable interface. This is an example of the flexibility and multiple-inheritance features provided by Java interfaces. The rest of the ThreadTest2 main() method is the same as that of ThreadTest1.
MyClass is declared as implementing the Runnable interface. This is a simple interface to implement; it only requires that you implement the run() method. MyClass declares the name variable to hold the names of MyClass objects that are created. In the first example, the MyThread class did not need to do this because a thread-naming capability was provided by Thread and inherited by MyThread. MyClass contains a simple constructor that initializes the name variable.
The run() methods of ThreadTest2 and ThreadTest1 are nearly identical, differing only with respect to the name issue. This is also true of the randomWait() method. In ThreadTest2, the randomWait() method must use the currentThread() method of class Thread to acquire a reference to an instance of the current thread in order to invoke its sleep() method.
Because these two examples are so similar, you might be wondering why you would pick one approach to creating a class over another. The advantage of using the Runnable interface is that your class does not need to extend the Thread class. This will be very helpful feature when you start using multithreading in applets in Part VI of this book. The only disadvantages to this approach are ones of convenience. You have to do a little more work to create your threads and to access their methods.
Thread States
You have now learned how to declare, create, initialize, start, and run Java threads. The ThreadTest1 and ThreadTest2 programs also introduced you to the concept of a thread's death. Threads transition through several states from the time they are created until the time of their death. This section reviews these states.
A thread is created by creating a new object of class Thread or of one of its subclasses. When a thread is first created, it does not exist as an independently executing set of instructions. Instead, it is a template from which an executing thread will be created. It first executes as a thread when it is started using the start() method and run via the run() method. Before a thread is started it is said to be in the new thread state. After a thread is started, it is in the runnable state. When a class is in the runnable state, it may be executing or temporarily waiting to share processing resources with other threads. A runnable thread enters an extended wait state when one of its methods is invoked that causes it to drop from the runnable state into a not runnable state. In the not runnable state, a thread is not just waiting for its share of processing resources, but is blocked waiting for the occurrence of an event that will send it back to the runnable state.
For example, the sleep() method was invoked in the ThreadTest1 and ThreadTest2 programs to cause a thread to wait for a short period of time so that the other thread could execute. The sleep() method causes a thread to enter the not runnable state until the specified time has expired. A thread may also enter the not runnable state while it is waiting for I/O to be completed, or as the result of the invocation of other methods.
A thread leaves the not runnable state and returns to the runnable state when the event that it is waiting for has occurred. For example, a sleeping thread must wait for its specified sleep time to occur. A thread that is waiting on I/O must wait for the I/O operation to be completed.
A thread may transition from the new thread, runnable, or not runnable state to the dead state when its stop() method is invoked or the thread's execution is completed. When a thread enters the dead state, it's a goner. It can't be revived and returned to any other state.
Thread Priority and Scheduling
From an abstract or a logical perspective, multiple threads execute as concurrent sequences of instructions. This may be physically true for multiprocessor systems, under certain conditions. However, in the general case, multiple threads do not always physically execute at the same time. Instead, the threads share execution time with each other based on the availability of the system's CPU (or CPUs).
The approach used to determining which threads should execute at a given time is referred to as scheduling. Scheduling is performed by the Java runtime system. It schedules threads based on their priority. The highest-priority thread that is in the runnable state is the thread that is run at any given instant. The highest-priority thread continues to run until it enters the death state, enters the not runnable state, or has its priority lowered, or when a higher-priority thread becomes runnable.
A thread's priority is an integer value between MIN_PRIORITY and MAX_PRIORITY. These constants are defined in the Thread class. In Java 1.0, MIN_PRIORITY is 1 and MAX_PRIORITY is 10. A thread's priority is set when it is created. It is set to the same priority as the thread that created it. The default priority of a thread is NORM_PRIORITY and is equal to 5. The priority of a thread can be changed using the setPriority() method.
Java's approach to scheduling is referred to as preemptive scheduling. When a thread of higher priority becomes runnable, it preempts threads of lower priority and is immediately executed in their place. If two or more higher-priority threads become runnable, the Java scheduler alternates between them when allocating execution time.
Synchronization
There are many situations in which multiple threads must share access to common objects. For example, all of the programs in this chapter have illustrated the effects of multithreading by having multiple executing threads write to the Java console, a common shared object. These examples have not required any coordination or synchronization in the way the threads access the console window: Whatever thread was currently executing was able to write to the console window. No coordination between concurrent threads was required.
There are times when you might want to coordinate access to shared resources. For example, in a database system, you might not want one thread to be updating a database record while another thread is trying to read it. Java enables you to coordinate the actions of multiple threads using synchronized methods and synchronized statements.
An object for which access is to be coordinated is accessed through the use of synchronized methods. These methods are declared with the synchronized keyword. Only one synchronized method can be invoked for an object at a given point in time. This keeps synchronized methods in multiple threads from conflicting with each other.
All classes and objects are associated with a unique monitor. The monitor is used to control the way in which synchronized methods are allowed to access the class or object. When a synchronized method is invoked for a given object, it is said to acquire the monitor for that object. No other synchronized method may be invoked for that object until the monitor is released. A monitor is automatically released when the method completes its execution and returns. A monitor may also be released when a synchronized method executes certain methods, such as wait(). The thread associated with the currently executing synchronized method becomes not runnable until the wait condition is satisfied and no other method has acquired the object's monitor.
The following example shows how synchronized methods and object monitors are used to coordinate access to a common object by multiple threads. This example adapts the ThreadTest1 program for use with synchronized methods, as shown in Listing 8.3.
The source code of the ThreadSynchronization program.
class ThreadSynchronization
{
public static void main(String args[])
{
MyThread thread1 = new MyThread("thread1: ");
MyThread thread2 = new MyThread("thread2: ");
thread1.start();
thread2.start();
boolean thread1IsAlive = true;
boolean thread2IsAlive = true;
do {
if (thread1IsAlive && !thread1.isAlive()) {
thread1IsAlive = false;
System.out.println("Thread 1 is dead.");
}
if (thread2IsAlive && !thread2.isAlive()) {
thread2IsAlive = false;
System.out.println("Thread 2 is dead.");
}
} while(thread1IsAlive || thread2IsAlive);
}
}
class MyThread extends Thread
{
static String message[] =
{ "Java", "is", "hot,", "aromatic,", "and", "invigorating."};
public MyThread(String id)
{
super(id);
}
public void run()
{
SynchronizedOutput.displayList(getName(),message);
}
void randomWait()
{
try {
sleep((long)(3000*Math.random()));
} catch (InterruptedException x) {
System.out.println("Interrupted!");
}
}
}
class SynchronizedOutput
{
public static synchronized void displayList(String name,String list[])
{
for(int i=0;i<list.length;++i) {
MyThread t = (MyThread) Thread.currentThread();
t.randomWait();
System.out.println(name+list[i]);
}
}
}
Compile and run the program before going on with its analysis. You might be surprised at the results that you've obtained. Here are the results of an example run on my system:
C:\java\jdg\ch08>java
ThreadSynchronization
thread1: Java
thread1: is
thread1: hot,
thread1: aromatic,
thread1: and
thread1: invigorating.
Thread 1 is dead.
thread2: Java
thread2: is
thread2: hot,
thread2: aromatic,
thread2: and
thread2: invigorating.
Thread 2 is dead.
Now edit ThreadSynchronization.java and delete the synchronized keyword in the declaration of the displayList() method of class SynchronizedOutput. Save ThreadSynchronization.java, recompile it, and rerun it with the new change in place. You may now get output similar to this:
C:\java\jdg\ch08>java
ThreadSynchronization
thread2: Java
thread1: Java
thread1: is
thread2: is
thread2: hot,
thread2: aromatic,
thread1: hot,
thread2: and
thread2: invigorating.
Thread 2 is dead.
thread1: aromatic,
thread1: and
thread1: invigorating.
Thread 1 is dead.
The difference in the program's output should give you a feel for the effects of synchronization upon multithreaded program execution. Let's analyze the program and explain these results.
The ThreadSynchronization class is essentially the same as the ThreadTest1 class. The only difference is the class name.
The MyThread class was modified slightly to allow for the use of the SynchronizedOutput class. Instead of the output being displayed in the run() method, as in ThreadTest1, the run() method simply invokes the displayList() method of the SynchronizedOutput class. It is important to understand that the displayList() method is static and applies to the SynchronizedOutput class as a whole, not to any particular instance of the class. The method displays the Java is hot, aromatic, and invigorating. message in the same manner as it was displayed in the previous examples of this chapter. It invokes randomWait() to wait a random amount of time before displaying each word in the message. The displayList() method uses the currentThread() method of class Thread to reference the current thread in order to invoke randomWait().
What difference, then, does the fact that displayList() is synchronized have on the program's execution? When displayList() is not synchronized, it may be invoked by one thread, say thread1, display some output, and wait while thread2 executes. When thread2 executes, it too invokes displayList() to display some output. Two separate invocations of displayList(), one for thread1 and the other for thread2, execute concurrently. This explains the mixed output display.
When the synchronized keyword is used, thread1 invokes displayList(), acquires a monitor for the SynchronizedOutput class (because displayList() is a static method), and displayList() proceeds with the output display for thread1. Because thread1 acquired a monitor for the SynchronizedOutput class, thread2 must wait until the monitor is released before it is able to invoke displayList() to display its output. This explains why one task's output is completed before the other's.
Daemon Threads
Java borrows the notion of a daemon thread from the UNIX daemon process. A daemon thread is a thread that executes in the background and provides services to other threads. It typically executes a continuous loop of instructions that wait for a service request, perform the service, and wait for the next service request. Daemon threads continue to execute until there are no more threads for which services can be provided. At this time, the daemon threads die and the Java interpreter terminates its execution. Any thread can be changed to a daemon thread using the setDaemon() method.
Thread Groups
Thread groups are objects that consist of a collection of threads. Every thread is a member of a unique thread group. Thread groups are used to invoke methods that apply to all threads in the group. For example, a thread group can be used to start or stop all threads in a group, to change their priorities, or to change them to daemon threads.
A thread is entered into a thread group when it is created. After the thread enters a thread group, it remains a member of the group throughout its existence. A thread can never become a member of another group.
Threads are entered into a group using Thread constructors that take a ThreadGroup parameter. These constructors are described in the Thread class API documentation. If a thread's group is not specified in its constructor, as is the usual case, the thread is entered into the same group as the thread that created it. The default thread group for a newly executing Java application is the main group.
A Thread's Life
A Thread continues to execute until one of the following things happens:
- It returns from its target run() method
- It's interrupted by an uncaught exception
- Its stop() method is called
So what happens if the run() method for a thread never terminates, and the application that started the thread never calls its stop() method? The answer is that the thread lives on, even after the application that created it has finished. This means we have to be aware of how our threads eventually terminate, or an application can end up leaving orphaned threads that unnecessarily consume resources.
In many cases, what we really want is to create background threads that do simple, periodic tasks in an application. The setDaemon() method can be used to mark a Thread as a daemon thread that should be killed and discarded when no other application threads remain. Normally, the Java interpreter continues to run until all threads have completed. But when daemon threads are the only threads still alive, the interpreter will exit.
Here's a devilish example of using daemon threads:
class Devil extends Thread {
Devil() {
setDaemon( true );
start();
}
public void run() {
// Perform evil tasks
...
}
}
In the above example, the Devil thread sets its daemon status when it is created. If any Devil threads remain when our application is otherwise complete, Java kills them for us. We don't have to worry about cleaning them up.
Daemon threads are primarily useful in standalone Java applications and in the implementation of the Java system itself, but not in applets. Since an applet runs inside of another Java application, any daemon threads it creates will continue to live until the controlling application exits--probably not the desired effect.
More about Thread Synchronization
Every thread has a life of its own. Normally, a thread goes about its business without any regard for what other threads in the application are doing. Threads may be time-sliced, which means they can run in arbitrary spurts and bursts as directed by the operating system. On a multiprocessor system, it is even possible for many different threads to be running simultaneously on different CPUs. This section is about coordinating the activities of two or more threads, so they can work together and not collide in their use of the same address space.
Java provides a few simple structures for synchronizing the activities of threads. They are all based on the concept of monitors, a widely used synchronization scheme developed by C.A.R. Hoare. You don't have to know the details about how monitors work to be able to use them, but it may help you to have a picture in mind.
A monitor is essentially a lock. The lock is attached to a resource that many threads may need to access, but that should be accessed by only one thread at a time. It's not unlike a public restroom at a gas station. If the resource is not being used, the thread can acquire the lock and access the resource. By the same token, if the restroom is unlocked, you can enter and lock the door. When the thread is done, it relinquishes the lock, just as you unlock the door and leave it open for the next person. However, if another thread already has the lock for the resource, all other threads have to wait until the current thread finishes and releases the lock, just as if the restroom is locked when you arrive, you have to wait until the current occupant is done and unlocks the door.
Fortunately, Java makes the process of synchronizing access to resources quite easy. The language handles setting up and acquiring locks; all you have to do is specify which resources require locks.
Serializing Methods
The most common need for synchronization among threads in Java is to serialize their access to some resource, namely an object. In other words, synchronization makes sure only one thread at a time can perform certain activities that manipulate an object. In Java, every object has a lock associated with it. To be more specific, every class and every instance of a class has its own lock. The synchronized keyword marks places where a thread must acquire the lock before proceeding.
For example, say we implemented a SpeechSynthesizer class that contains a say() method. We don't want multiple threads calling say() at the same time or we wouldn't be able to understand anything being said. So we mark the say() method as synchronized, which means that a thread has to acquire the lock on the SpeechSynthesizer object before it can speak:
class SpeechSynthesizer
{
synchronized void say( String words ) {
// Speak
}
}
Because say() is an instance method, a thread has to acquire the lock on the particular SpeechSynthesizer instance it is using before it can invoke the say() method. When say() has completed, it gives up the lock, which allows the next waiting thread to acquire the lock and run the method. Note that it doesn't matter whether the thread is owned by the SpeechSynthesizer itself or some other object; every thread has to acquire the same lock, that of the SpeechSynthesizer instance. If say() were a class (static) method instead of an instance method, we could still mark it as synchronized. But in this case as there is no instance object involved, the lock would be on the class object itself.
Often, you want to synchronize multiple methods of the same class, so that only one of the methods modifies or examines parts of the class at a time. All static synchronized methods in a class use the same class object lock. By the same token, all instance methods in a class use the same instance object lock. In this way, Java can guarantee that only one of a set of synchronized methods is running at a time. For example, a SpreadSheet class might contain a number of instance variables that represent cell values, as well as some methods that manipulate the cells in a row:
class SpreadSheet
{
int cellA1, cellA2, cellA3;
synchronized int sumRow()
{
return cellA1 + cellA2 + cellA3;
}
synchronized void setRow( int a1, int a2, int a3 )
{
cellA1 = a1;
cellA2 = a2;
cellA3 = a3;
}
...
}
In this example, both methods setRow() and sumRow() access the cell values. You can see that problems might arise if one thread were changing the values of the variables in setRow() at the same moment another thread was reading the values in sumRow(). To prevent this, we have marked both methods as synchronized. When threads are synchronized, only one will be run at a time. If a thread is in the middle of executing setRow() when another thread calls sumRow(), the second thread waits until the first one is done executing setRow() before it gets to run sumRow(). This synchronization allows us to preserve the consistency of the SpreadSheet. And the best part is that all of this locking and waiting is handled by Java; it's transparent to the programmer.
In addition to synchronizing entire methods, the synchronized keyword can be used in a special construct to guard arbitrary blocks of code. In this form it also takes an explicit argument that specifies the object for which it is to acquire a lock:
synchronized ( myObject ) {
// Functionality that needs to be synced
...
}
The code block above can appear in any method. When it is reached, the thread has to acquire the lock on myObject before proceeding. In this way, we can have methods (or parts of methods) in different classes synchronized the same as methods in the same class.
A synchronized method is, therefore, equivalent to a method with its statements synchronized on the current object. Thus:
synchronized void myMethod () { ... }
is equivalent to:
void myMethod ()
{
synchronized ( this )
{
...
}
}
Using wait( ) and notify( )
With the synchronized keyword, we can serialize the execution of complete methods and blocks of code. The wait() and notify() methods of the Object class extend this capability. Every object in Java is a subclass of Object, so every object inherits these methods. By using wait() and notify(), a thread can give up its hold on a lock at an arbitrary point, and then wait for another thread to give it back before continuing. All of the coordinated activity still happens inside of synchronized blocks, and still only one thread is executing at a given time.
By executing wait() from a synchronized block, a thread gives up its hold on the lock and goes to sleep. A thread might do this if it needs to wait for something to happen in another part of the application, as you'll see shortly. Later, when the necessary event happens, the thread that is running it calls notify() from a block synchronized on the same object. Now the first thread wakes up and begins trying to acquire the lock again.
When the first thread manages to reacquire the lock, it continues from the point it left off. However, the thread that waited may not get the lock immediately (or perhaps ever). It depends on when the second thread eventually releases the lock, and which thread manages to snag it next. Note also, that the first thread won't wake up from the wait() unless another thread calls notify(). There is an overloaded version of wait(), however, that allows us to specify a timeout period. If another thread doesn't call notify() in the specified period, the waiting thread automatically wakes up.
Let's look at a simple scenario to see what's going on. In the following example, we'll assume there are three threads--one waiting to execute each of the three synchronized methods of the MyThing class. We'll call them the waiter, notifier, and related threads, respectively. Here's a code fragment to illustrate:
class MyThing
{
synchronized void waiterMethod()
{
// Do some stuff
// Now we need to wait for notifier to do something
wait();
// Continue where we left off
}
synchronized void notifierMethod()
{
// Do some stuff
// Notify waiter that we've done it
notify();
// Do more things
}
synchronized void relatedMethod()
{
// Do some related stuff
}
}
Let's assume waiter gets through the gate first and begins executing waiterMethod(). The two other threads are initially blocked, trying to acquire the lock for the MyThing object. When waiter executes the wait() method, it relinquishes its hold on the lock and goes to sleep. Now there are now two viable threads waiting for the lock. Which thread gets it depends on several factors, including chance and the priorities of the threads. (We'll discuss thread scheduling in the next section.)
Let's say that notifier is the next thread to acquire the lock, so it begins to run. waiter continues to sleep and related languishes, waiting for its turn. When notifier executes the call to notify(), Java prods the waiter thread, effectively telling it something has changed. waiter then wakes up and rejoins related in vying for the MyThing lock. Note that it doesn't actually receive the lock; it just changes from saying "leave me alone" to "I want the lock."
At this point, notifier still owns the lock and continues to hold it until it leaves its synchronized method (or perhaps executes a wait() itself ). When it finally completes, the other two methods get to fight over the lock. waiter would like to continue executing waiterMethod() from the point it left off, while unrelated, which has been patient, would like to get started. We'll let you choose your own ending for the story.
For each call to notify(), Java wakes up just one method that is asleep in a wait() call. If there are multiple threads waiting, Java picks the first thread on a first-in, first-out basis. The Object class also provides a notifyAll() call to wake up all waiting threads. In most cases, you'll probably want to use notifyAll() rather than notify(). Keep in mind that notify() really means "Hey, something related to this object has changed. The condition you are waiting for may have changed, so check it again." In general, there is no reason to assume only one thread at a time is interested in the change or able to act upon it. Different threads might look upon whatever has changed in different ways.
Often, our waiter thread is waiting for a particular condition to change and we will want to sit in a loop like the following:
...
while ( condition != true )
wait();
...
Other synchronized threads call notify() or notifyAll() when they have modified the environment so that waiter can check the condition again. This is the civilized alternative to polling and sleeping, as you'll see the following example.
The Message Passer (Producer/Consumer Problem)
Now we'll illustrate a classic interaction between two threads: a Producer and a Consumer. A producer thread creates messages and places them into a queue, while a consumer reads them out and displays them. To be realistic, we'll give the queue a maximum depth. And to make things really interesting, we'll have our consumer thread be lazy and run much slower than the producer. This means that Producer occasionally has to stop and wait for Consumer to catch up. The example below shows the Producer and Consumer classes.
import java.util.Vector;
class Producer extends Thread {
static final int MAXQUEUE = 5;
private Vector messages = new Vector();
public void run() {
try {
while ( true ) {
putMessage();
sleep( 1000 );
}
}
catch( InterruptedException e ) { }
}
private synchronized void putMessage()
throws InterruptedException
{
while ( messages.size() == MAXQUEUE )
wait();
messages.addElement( new java.util.Date().toString() );
notify();
}
// Called by Consumer
public synchronized String getMessage()
throws InterruptedException
{
notify();
while ( messages.size() == 0 )
wait();
String message = (String)messages.firstElement();
messages.removeElement( message );
return message;
}
}
class Consumer extends Thread {
Producer producer;
Consumer(Producer p) {
producer = p;
}
public void run() {
try {
while ( true ) {
String message = producer.getMessage();
System.out.println("Got message: " + message);
sleep( 2000 );
}
}
catch( InterruptedException e ) { }
}
public static void main(String args[]) {
Producer producer = new Producer();
producer.start();
new Consumer( producer ).start();
}
}
For convenience, we have included a main() method that runs the complete example in the Consumer class. It creates a Consumer that is tied to a Producer and starts the two classes. You can run the example as follows:
% java Consumer
The output is the time-stamp messages created by the Producer:
Got message: Sun Dec 19 03:35:55 CST 1996 Got message: Sun Dec 19 03:35:56 CST 1996 Got message: Sun Dec 19 03:35:57 CST 1996 ...
The time stamps initially show a spacing of one second, although they appear every two seconds. Our Producer runs faster than our Consumer. Producer would like to generate a new message every second, while Consumer gets around to reading and displaying a message only every two seconds. Can you see how long it will take the message queue to fill up? What will happen when it does?
Let's look at the code. We are using a few new tools here. Producer and Consumer are subclasses of Thread. It would have been a better design decision to have Producer and Consumer implement the Runnable interface, but we took the slightly easier path and subclassed Thread. You should find it fairly simple to use the other technique; you might try it as an exercise.
The Producer and Consumer classes pass messages through an instance of a java.util.Vector object. We haven't discussed the Vector class yet, but you can think of this one as a queue where we add and remove elements in first-in, first-out order.
The important activity is in the synchronized methods: putMessage() and getMessage(). Although one of the methods is used by the Producer thread and the other by the Consumer thread, they both live in the Producer class because they have to be synchronized on the same object to work together. Here they both implicitly use the Producer object's lock. If the queue is empty, the Consumer blocks in a call in the Producer, waiting for another message.
Another design option would implement the getMessage() method in the Consumer class and use a synchronized code block to explicitly synchronize on the Producer object. In either case, synchronizing on the Producer is important because it allows us to have multiple Consumer objects that feed on the same Producer.
putMessage()'s job is to add a new message to the queue. It can't do this if the queue is already full, so it first checks the number of elements in messages. If there is room, it stuffs in another time stamp. If the queue is at its limit however, putMessage() has to wait until there's space. In this situation, putMessage() executes a wait() and relies on the consumer to call notify() to wake it up after a message has been read. Here we have putMessage() testing the condition in a loop. In this simple example, the test probably isn't necessary; we could assume that when putMessage() wakes up, there is a free spot. However, this test is another example of good programming practice. Before it finishes, putMessage() calls notify() itself to prod any Consumer that might be waiting on an empty queue.
getMessage() retrieves a message for the Consumer. It enters a loop like the Producer's, waiting for the queue to have at least one element before proceeding. If the queue is empty, it executes a wait() and expects the producer to call notify() when more items are available. Notice that getMessage() makes its own unconditional call to notify(). This is a somewhat lazy way of keeping the Producer on its toes, so that the queue should generally be full. Alternatively, getMessage() might test to see if the queue had fallen below a low water mark before waking up the producer.
Now let's add another Consumer to the scenario, just to make things really interesting. Most of the necessary changes are in the Consumer class; the example below shows the code for the modified class.
class Consumer extends Thread
{
Producer producer;
String name;
Consumer(String name, Producer producer)
{
this.producer = producer;
this.name = name;
}
public void run()
{
try {
while ( true ) {
String message = producer.getMessage();
System.out.println(name + " got message: " + message);
sleep( 2000 );
}
} catch( InterruptedException e ) {
}
}
public static void main(String args[])
{
Producer producer = new Producer();
producer.start();
// Start two this time
new Consumer( "One", producer ).start();
new Consumer( "Two", producer ).start();
}
}
The Consumer constructor now takes a string name, to identify each consumer. The run() method uses this name in the call to println() to identify which consumer received the message.
The only modification to make in the Producer code is to change the call to notify() in putMessage() to a call to notifyAll(). Now, instead of the consumer and producer playing tag with the queue, we can have many players waiting on the condition of the queue to change. We might have a number of consumers waiting for a message, or we might have the producer waiting for a consumer to take a message. Whenever the condition of the queue changes, we prod all of the waiting methods to reevaluate the situation by calling notifyAll(). Note, however, that we don't need to change the call to notify() in getMessage(). If a Consumer thread is waiting for a message to appear in the queue, it's not possible for the Producer to be simultaneously waiting because the queue is full.
Here is some sample output when there are two consumers running, as in the main() method shown above:
One got message: Wed Mar 20 20:00:01 CST 1996 Two got message: Wed Mar 20 20:00:02 CST 1996 One got message: Wed Mar 20 20:00:03 CST 1996 Two got message: Wed Mar 20 20:00:04 CST 1996 One got message: Wed Mar 20 20:00:05 CST 1996 Two got message: Wed Mar 20 20:00:06 CST 1996 One got message: Wed Mar 20 20:00:07 CST 1996 Two got message: Wed Mar 20 20:00:08 CST 1996 ...
We see nice, orderly alternation between the two consumers, as a result of the calls to sleep() in the various methods. Interesting things would happen, however, if we were to remove all of the calls to sleep() and let things run at full speed. The threads would compete and their behavior would depend on whether or not the system is using time slicing. On a time-sliced system, there should be a fairly random distribution between the two consumers, while on a nontime-sliced system, a single consumer could monopolize the messages. And since you're probably wondering about time slicing, let's talk about thread priority and scheduling.