java pdf 417 reader EXERCISE 9-1 Creating a Thread and Putting It to Sleep in Java

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EXERCISE 9-1 Creating a Thread and Putting It to Sleep
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In this exercise we will create a simple counting thread. It will count to 100, pausing one second between each number. Also, in keeping with the counting theme, it will output a string every ten numbers.
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1. Create a class and extend the Thread class. As an option, you can implement the Runnable interface. 2. Override the run() method of Thread. This is where the code will go that will output the numbers. 3. Create a for loop that will loop 100 times. Use the modulo operation to check whether there are any remainder numbers when divided by 10. 4. Use the static method Thread.sleep() to pause. The long number represents milliseconds.
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To understand yield(), you must understand the concept of thread priorities. Threads always run with some priority, represented usually as a number between 1 and 10 (although in some cases the range is less than 10). The scheduler in most JVMs uses preemptive, priority-based scheduling. This does not mean that all JVMs use time slicing. The JVM specification does not require a VM to implement a time-slicing scheduler, where each thread is allocated a fair amount of time and then sent back to runnable to give another thread a chance. Although many JVMs do use time slicing, another may use a scheduler that lets one thread stay running until the thread completes its run() method.
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In most JVMs, however, the scheduler does use thread priorities in one important way: If a thread enters the runnable state, and it has a higher priority than any of the threads in the pool and higher than the currently running thread, the lower-priority running thread usually will be bumped back to runnable and the highest-priority thread will be chosen to run. In other words, at any given time the currently running thread usually will not have a priority that is lower than any of the threads in the pool. The running thread will be of equal or greater priority than the highest priority threads in the pool. This is as close to a guarantee about scheduling as you ll get from the JVM specification, so you must never rely on thread priorities to guarantee correct behavior of your program.
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Don't rely on thread priorities when designing your multithreaded application. Because thread-scheduling priority behavior is not guaranteed, use thread priorities as a way to improve the efficiency of your program, but just be sure your program doesn't depend on that behavior for correctness.
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What is also not guaranteed is the behavior when threads in the pool are of equal priority, or when the currently running thread has the same priority as threads in the pool. All priorities being equal, a JVM implementation of the scheduler is free to do just about anything it likes. That means a scheduler might do one of the following (among other things):
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Pick a thread to run, and keep it there until it blocks or completes its
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run() method.
Time slice the threads in the pool to give everyone an equal opportunity to run.
Setting a Thread s Priority A thread gets a default priority that is the priority of the thread of execution that creates it. For example, in the code
public class TestThreads { public static void main (String [] args) { MyThread t = new MyThread(); } }
the thread referenced by t will have the same priority as the main thread, since the main thread is executing the code that creates the MyThread instance.
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You can also set a thread s priority directly by calling the setPriority() method on a Thread instance as follows:
FooRunnable r = new FooRunnable(); Thread t = new Thread(r); t.setPriority(8); t.start();
Priorities are set using a positive integer, usually between 1 and 10, and the JVM will never change a thread s priority. However, the values 1 through 10 are not guaranteed, so if you have, say, ten threads each with a different priority, and the current application is running in a JVM that allocates a range of only five priorities, then two or more threads might be mapped to one priority. The default priority is 5. The Thread class has three constants (static final variables) that define the range of thread priorities:
Thread.MIN_PRIORITY (1) Thread.NORM_PRIORITY (5) Thread.MAX_PRIORITY (10)
So what does the static Thread.yield() have to do with all this Not that much, in practice. What yield() is supposed to do is make the currently running thread head back to runnable to allow other threads of the same priority to get their turn. So the intention is to use yield() to promote graceful turn-taking among equal-priority threads. In reality, though, the yield() method isn t guaranteed to do what it claims, and even if yield() does cause a thread to step out of running and back to runnable, there s no guarantee the yielding thread won t just be chosen again over all the others! So while yield() might and often does make a running thread give up its slot to another runnable thread of the same priority, there s no guarantee.
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