846 Multithreading Chapter 15 38 public PrintThread( String name ) 39 { 40 super( name ); 41 42 // sleep between 0 and 5 seconds 43 sleepTime = (int) ( Math.random() * 5000 ); 44 45 // display name and sleepTime 46 System.err.println( 47 “Name: ” + getName() + “; sleep: ” + sleepTime ); 48 } 49 50 // control thread’s execution 51 public void run() 52 { 53 // put thread to sleep for a random interval 54 try { 55 System.err.println( getName() + ” going to sleep” ); 56 57 // put thread to sleep 58 Thread.sleep( sleepTime ); 59 } 60 61 // if thread interrupted during sleep, catch exception 62 // and display error message 63 catch ( InterruptedException interruptedException ) { 64 System.err.println( interruptedException.toString() ); 65 } 66 67 // print thread name 68 System.err.println( getName() + ” done sleeping” ); 69 } 70 71 } // end class PrintThread Name: thread1; sleep: 3593 Name: thread2; sleep: 2653 Name: thread3; sleep: 4465 Name: thread4; sleep: 1318 Starting threads Threads started thread1 going to sleep thread2 going to sleep thread3 going to sleep thread4 going to sleep thread4 done sleeping thread2 done sleeping thread1 done sleeping thread3 done sleeping Fig. 15.3Multiple threads printing at random intervals (part 2 of 3). Fig. 15.3
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Chapter 15 Multithreading 845 run method. Variable sleepTime stores a random integer value chosen when the program creates a PrintThreadobject. Each PrintThreadobject sleeps for the amount of time specified by its sleepTime, then outputs its name. The PrintThread constructor (lines 38 48) initializes sleepTime to a random integer between 0 and 4999 (0 to 4.999 seconds). Then, the constructor outputs the name of the thread and the value of sleepTime to show the values for the particular Print- Thread being constructed. The name of each thread is specified as a String argument to the PrintThread constructor and is passed to the superclass constructor at line 40. [Note: It is possible to allow class Thread to choose a name for your thread by using the Threadclass s default constructor. 1 // Fig. 15.3: ThreadTester.java 2 // Show multiple threads printing at different intervals. 3 4 public class ThreadTester { 5 6 // create and start threads 7 public static void main( String args[] ) 8 { 9 PrintThread thread1, thread2, thread3, thread4; 10 11 // create four PrintThread objects 12 thread1 = new PrintThread( “thread1″ ); 13 thread2 = new PrintThread( “thread2″ ); 14 thread3 = new PrintThread( “thread3″ ); 15 thread4 = new PrintThread( “thread4″ ); 16 17 System.err.println( “nStarting threads” ); 18 19 // start executing PrintThreads 20 thread1.start(); 21 thread2.start(); 22 thread3.start(); 23 thread4.start(); 24 25 System.err.println( “Threads startedn” ); 26 } 27 28 } // end class ThreadTester 29 30 // Each object of this class picks a random sleep interval. 31 // When a PrintThread executes, it prints its name, sleeps, 32 // prints its name again and terminates. 33 class PrintThread extends Thread { 34 private int sleepTime; 35 36 // PrintThread constructor assigns name to thread 37 // by calling superclass Thread constructor Fig. 15.3Multiple threads printing at random intervals (part 1 of 3). Fig. 15.3
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844 Multithreading Chapter 15 Ready threads Priority 10 A B Priority 9 Priority 8 C Priority 7 D E F Priority 6 G Priority 5 Priority 4 Priority 3 H I Priority 2 Priority 1 J K Fig. 15.2Java thread priority scheduling. 15. The application of Fig. 15.3 demonstrates basic threading techniques, including creation of a class derived from Thread, construction of a Threadand using the Thread class sleepmethod. Each thread of execution we create in the program displays its name after sleeping for a random amount of time between 0 and 5 seconds. You will see that the mainmethod (i.e., the main thread of execution) terminates before the application terminates. The program consists of two classes ThreadTester(lines 4 28) and Print- Thread(lines 33 71). Class PrintThreadinherits from Thread, so that each object of the class can execute in parallel. The class consists of instance variable sleepTime, a constructor and a
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Chapter 15 Multithreading 843 thread gets interrupted by a higher priority thread) before that thread s peers get a chance to execute. With timeslicing, each thread receives a brief burst of processor time called a quantum during which that thread can execute. At the completion of the quantum, even if that thread has not finished executing, the operating system takes the processor away from that thread and gives it to the next thread of equal priority (if one is available). The job of the Java scheduler is to keep the highest priority thread running at all times, and if timeslicing is available, to ensure that several equally high-priority threads each execute for a quantum in round-robin fashion (i.e., these threads can be timesliced). Figure 15.2 illustrates Java s multilevel priority queue for threads. In the figure, threads A and B each execute for a quantum in round-robin fashion until both threads complete execution. Next, thread C runs to completion. Then, threads D, E and F each execute for a quantum in round-robin fashion until they all complete execution. This process continues until all threads run to completion. Note that new higher-priority threads could postpone possibly indefinitely the execution of lower priority threads. Such indefinite postponement often is referred to more colorfully as starvation. A thread s priority can be adjusted with method setPriority, which takes an int argument. If the argument is not in the range 1 through 10, setPriority throws an IllegalArgumentException. Method getPriority returns the thread s priority. A thread can call the yield method to give other threads a chance to execute. Actually, whenever a higher priority thread becomes ready, the operating system preempts the current thread. So, a thread cannot yield to a higher priority thread, because the first thread would have been preempted when the higher priority thread became ready. Similarly, yield always allows the highest priority-ready thread to run, so if only lower priority threads are ready at the time of a yield call, the current thread will be the highest priority thread and will continue executing. Therefore, a thread yields to give threads of an equal priority a chance to run. On a timesliced system this is unnecessary, because threads of equal priority will each execute for their quantum (or until they lose the processor for some other reason), and other threads of equal priority will execute in round- robin fashion. Thus yield is appropriate for nontimesliced systems in which a thread would ordinarily run to completion before another thread of equal priority would have an opportunity to run. Performance Tip 15.3 On nontimesliced systems, cooperating threads of equal priority should periodically call yield to enable their peers to proceed smoothly. Portability Tip 15.2 Java applets and applications should be programmed to work on all Java platforms to realize Java s goal of true portability. When designing applets and applications that use threads, you must consider the threading capabilities of all the platforms on which the applets and applications will execute. A thread executes unless it dies, it becomes blocked by the operating system for input/ output (or some other reason), it calls sleep, it calls wait, it calls yield, it is preempted by a thread of higher priority or its quantum expires. A thread with a higher priority than the running thread can become ready (and hence preempt the running thread) if a sleeping thread finishes sleeping, if I/O completes for a thread waiting for that I/O or if either notifyor notifyAll is called on a thread that has called wait.
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842 Multithreading Chapter 15 born start ready running waiting sleeping dead blocked dispatch (assign a processor) quantum expiration issueI/Orequestsleepwait I/Ocompletionnotify complete ornotifyAll yield interrupt sleep interval expires Fig. 15.1State diagram showing the Life cycle of a thread. 15. When a running thread calls wait, the thread enters a waiting state for the particular object on which wait was called. One thread in the waiting state for a particular object becomes ready on a call to notify issued by another thread associated with that object. Every thread in the waiting state for a given object becomes ready on a call to notifyAll by another thread associated with that object. The wait, notify and notifyAll methods will be discussed in more depth shortly, when we consider monitors. A thread enters the dead state when its run method either completes or throws an uncaught exception. 15.4 Thread Priorities and Thread Scheduling Every Java applet or application is multithreaded. Every Java thread has a priority in the range Thread.MIN_PRIORITY (a constant of 1) and Thread.MAX_PRIORITY (a constant of 10). By default, each thread is given priority Thread.NORM_PRIORITY (a constant of 5). Each new thread inherits the priority of the thread that creates it. Some Java platforms support a concept called timeslicing and some do not. Without timeslicing, each thread in a set of equal-priority threads runs to completion (unless the thread leaves the running state and enters the waiting, sleeping or blocked state, or the
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Chapter 15 Multithreading 841 caller immediately. The caller then executes concurrently with the launched thread. The start method throws an IllegalThreadStateException if the thread it is trying to start has already been started. The static method sleep is called with an argument specifying how long the currently executing thread should sleep (in milliseconds); while a thread sleeps, it does not contend for the processor, so other threads can execute. This can give lower priority threads a chance to run. The interrupt method is called to interrupt a thread. The static method interrupted returns true if the current thread has been interrupted and false otherwise. A program can invoke a specific thread s isInterrupted method to determine whether that thread has been interrupted. Method isAlive returns trueif start has been called for a given thread and the thread is not dead (i.e., its controlling run method has not completed execution). Method setName sets a Thread s name. Method getName returns the name of the Thread. Method toString returns a String consisting of the name of the thread, the priority of the thread and the thread s ThreadGroup (discussed in Section 15.11). The static method currentThread returns a reference to the currently executing Thread. Method join waits for the Thread to which the message is sent to die before the calling Thread can proceed; no argument or an argument of 0 milliseconds to method join indicates that the current Thread will wait forever for the target Thread to die before the calling Threadproceeds. Such waiting can be dangerous; it can lead to two particularly serious problems called deadlock and indefinite postponement. We will discuss these momentarily. Testing and Debugging Tip 15.2 Method dumpStack is useful for debugging multithreaded applications. A program calls static method dumpStack to print a method-call stack trace for the current Thread. 15.3 Thread States: Life Cycle of a Thread At any time, a thread is said to be in one of several thread states (illustrated in Fig. 15.1). Let us say that a thread that was just created is in the born state. The thread remains in this state until the program calls the thread s start method, which causes the thread to enter the ready state (also known as the runnable state). The highest priority ready thread enters the running state (i.e., the thread begins executing), when the system assigns a processor to the thread. A thread enters the dead state when its run method completes or terminates for any reason a dead thread eventually will be disposed of by the system. One common way for a running thread to enter the blocked state is when the thread issues an input/output request. In this case, a blocked thread becomes ready when the I/O for which it is waiting completes. A blocked thread cannot use a processor even if one is available. When the program calls method sleep in a running thread, that thread enters the sleeping state. A sleeping thread becomes ready after the designated sleep time expires. A sleeping thread cannot use a processor even if one is available. If the program calls method interrupt on a sleeping thread, that thread exits the sleeping state and becomes ready to execute.
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840 Multithreading Chapter 15 book, moving the book you are reading closer so you can see it, pushing books you are not reading aside, and amidst all this chaos, trying to comprehend the content of the books! Performance Tip 15.2 A problem with single-threaded applications is that lengthy activities must complete before other activities can begin. In a multithreaded application, threads can share a processor (or set of processors), so that multiple tasks are performed in parallel. Although Java is perhaps the world s most portable programming language, certain portions of the language are nevertheless platform dependent. In particular, there are differences among the first three Java platforms implemented, namely the Solaris implementation and the Win32 implementations (i.e., Windows-based implementations for Windows 95 and Windows NT). The Solaris Java platform runs a thread of a given priority to completion or until a higher priority thread becomes ready. At that point preemption occurs (i.e., the processor is given to the higher priority thread while the previously running thread must wait). In the 32-bit Java implementations for Windows 95 and Windows NT, threads are timesliced. This means that each thread is given a limited amount of time (called a time quantum) to execute on a processor, and when that time expires the thread is made to wait while all other threads of equal priority get their chances to use their quantum in round- robin fashion. Then the original thread resumes execution. Thus, on Windows 95 and Windows NT, a running thread can be preempted by a thread of equal priority; whereas, on the Solaris implementation, a running Java thread can only be preempted by a higher priority thread. Future Solaris Java systems are expected to perform timeslicing as well. Portability Tip 15.1 Java multithreading is platform dependent. Thus, a multithreaded application could behave differently on different Java implementations. 15.2 Class Thread: An Overview of the Thread Methods In this section, we overview the various thread-related methods in the Java API. We use many of these methods in live-code examples throughout the chapter. The reader should refer to the Java API directly for more details on using each method, especially the exceptions thrown by each method. Class Thread (package java.lang) has several constructors. The constructor public Thread( String threadName ) constructs a Thread object whose name is threadName. The constructor public Thread() constructs a Thread whose name is “Thread-” concatenated with a number, like Thread-1, Thread-2, and so on. The code that does the real work of a thread is placed in its run method. The run method can be overridden in a subclass of Thread or it may be implemented in a Runnable object; Runnable is an important Java interface that we study in Section 15.10. A program launches a thread s execution by calling the thread s start method, which, in turn, calls method run. After start launches the thread, start returns to its
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Chapter 15 Multithreading 839 [Note: On many computer platforms, C and C++ programs can perform multithreading by using system specific code libraries.] Software Engineering Observation 15.1 Unlike many languages that do not have built-in multithreading (such as C and C++) and must therefore make calls to operating system multithreading primitives, Java includes multithreading primitives as part of the language itself (actually in classes Thread, ThreadGroup, ThreadLocal and ThreadDeath of the java.lang package). This encourages the use of multithreading among a larger part of the applications-programming community. We will discuss many applications of concurrent programming. When programs download large files such as audio clips or video clips from the World Wide Web, we do not want to wait until an entire clip is downloaded before starting the playback. So we can put multiple threads to work: One that downloads a clip and another that plays the clip so that these activities, or tasks, may proceed concurrently. To avoid choppy playback, we will coordinate the threads so that the player thread does not begin until there is a sufficient amount of the clip in memory to keep the player thread busy. Another example of multithreading is Java s automatic garbage collection. In C and C++, the programmer is responsible for reclaiming dynamically allocated memory. Java provides a garbage collector thread that reclaims dynamically allocated memory that the program no longer needs. Testing and Debugging Tip 15.1 In C and C++, programmers must provide explicit statements that reclaim dynamically allocated memory. When memory is not reclaimed (because a programmer forgets to do so, because of a logic error or because an exception diverts program control), this results in an all-too-common error called a memory leak that can eventually exhaust the supply of free memory and may cause premature program termination. Java s automatic garbage collection eliminates the vast majority of memory leaks, that is, those that are due to orphaned (unreferenced) objects. Java s garbage collector runs as a low-priority thread. When Java determines that there are no longer any references to an object, it marks the object for eventual garbage collection. The garbage-collector thread runs when processor time is available and when there are no higher priority runnable threads. However, the garbage collector will run immediately when the system is out of memory. Performance Tip 15.1 Setting an object reference to null marks that object for eventual garbage collection (if there are no other references to the object). This can help conserve memory in a system in which a local variable that refers to an object does not go out of scope because the method in which it appears executes for a lengthy period. Writing multithreaded programs can be tricky. Although the human mind can perform many functions concurrently, humans find it difficult to jump between parallel trains of thought. To see why multithreading can be difficult to program and understand, try the following experiment: open three books to page 1. Now try reading the books concurrently. Read a few words from the first book, then read a few words from the second book, then read a few words from the third book, then loop back and read the next few words from the first book, and so on. After a brief time, you will appreciate the challenges of multithreading: switching between books, reading briefly, remembering your place in each
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838 Multithreading Chapter 15 Outline 15.1 Introduction 15.2 Class Thread: An Overview of the Thread Methods 15.3 Thread States: Life Cycle of a Thread 15.4 Thread Priorities and Thread Scheduling 15.5 Thread Synchronization 15.6 Producer/Consumer Relationship without Thread Synchronization 15.7 Producer/Consumer Relationship with Thread Synchronization 15.8 Producer/Consumer Relationship: The Circular Buffer 15.9 Daemon Threads 15.10 Runnable Interface 15.11 Thread Groups 15.12 (Optional Case Study) Thinking About Objects: Multithreading 15.13 (Optional) Discovering Design Patterns: Concurrent Design Patterns Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises 15.1 Introduction It would be nice if we could do one thing at a time and do it well, but that is simply not how the world works. The human body performs a great variety of operations in parallel, or as we will say throughout this chapter, concurrently. Respiration, blood circulation and digestion, for example, can occur concurrently. All of the senses seeing, touching, smelling, tasting and hearing can all occur concurrently. An automobile can be accelerating, turning, air conditioning and playing music concurrently. Computers, too, perform operations concurrently. It is common today for desktop personal computers to be compiling a program, printing a file and receiving e-mail messages over a network concurrently. Concurrency is important in our lives. Ironically, though, most programming languages do not enable programmers to specify concurrent activities. Rather, programming languages generally provide only a simple set of control structures that enable programmers to perform one action at a time then proceed to the next action after the previous one is finished. The kind of concurrency that computers perform today normally is implemented as operating systems primitives available only to highly experienced systems programmers. The Ada programming language developed by the United States Department of Defense made concurrency primitives widely available to defense contractors building command and control systems. But Ada has not been widely used in universities and commercial industry. Java is unique among popular general-purpose programming languages in that it makes concurrency primitives available to the applications programmer. The programmer specifies that applications contain threads of execution, each thread designating a portion of a program that may execute concurrently with other threads. This capability, called multithreading, gives the Java programmer powerful capabilities not available in C and C++, the languages on which Java is based. C and C++ are called single-threaded languages.
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15 Multithreading Objectives To understand the notion of multithreading. To appreciate how multithreading can improve performance. To understand how to create, manage and destroy threads. To understand the life cycle of a thread. To study several examples of thread synchronization. To understand thread priorities and scheduling. To understand daemon threads and thread groups. The spider s touch, how exquisitely fine! Feels at each thread, and lives along the line. Alexander Pope A person with one watch knows what time it is; a person with two watches is never sure. Proverb Conversation is but carving! Give no more to every guest, Than he s able to digest. Jonathan Swift Learn to labor and to wait. Henry Wadsworth Longfellow The most general definition of beauty Multeity in Unity. Samuel Taylor Coleridge
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