JAVA-THREAD-POOL | THREAD-POOLING IN JAVA

Java-Thread-Pool stellt eine Gruppe von Arbeitsthreads dar, die auf den Job warten und viele Male wiederverwendet werden.

Im Falle eines Thread-Pools wird eine Gruppe von Threads fester Größe erstellt. Ein Thread aus dem Thread-Pool wird herausgezogen und vom Dienstanbieter einem Job zugewiesen. Nach Abschluss des Jobs ist der Thread wieder im Thread-Pool enthalten.

Thread-Pool-Methoden

newFixedThreadPool(int s): Die Methode erstellt einen Thread-Pool der festen Größe s.

newCachedThreadPool(): Die Methode erstellt einen neuen Thread-Pool, der die neuen Threads bei Bedarf erstellt, aber weiterhin die zuvor erstellten Threads verwendet, wenn diese zur Verwendung verfügbar sind.

newSingleThreadExecutor(): Die Methode erstellt einen neuen Thread.

Vorteil des Java Thread Pools

Bessere Leistung Das spart Zeit, da kein neuer Thread erstellt werden muss.

primitive Datentypen in Java

Nutzung in Echtzeit

Es wird in Servlet und JSP verwendet, wo der Container einen Thread-Pool erstellt, um die Anfrage zu verarbeiten.

Beispiel für einen Java-Thread-Pool

Sehen wir uns ein einfaches Beispiel des Java-Thread-Pools mit ExecutorService und Executors an.

Voreingenommenheit und Varianz

Datei: WorkerThread.java

 import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; class WorkerThread implements Runnable { private String message; public WorkerThread(String s){ this.message=s; } public void run() { System.out.println(Thread.currentThread().getName()+&apos; (Start) message = &apos;+message); processmessage();//call processmessage method that sleeps the thread for 2 seconds System.out.println(Thread.currentThread().getName()+&apos; (End)&apos;);//prints thread name } private void processmessage() { try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } } }

Datei: TestThreadPool.java

 public class TestThreadPool { public static void main(String[] args) { ExecutorService executor = Executors.newFixedThreadPool(5);//creating a pool of 5 threads for (int i = 0; i <10; i++) { runnable worker="new" workerthread('' + i); executor.execute(worker); calling execute method of executorservice } executor.shutdown(); while (!executor.isterminated()) system.out.println('finished all threads'); < pre> <p> <strong>Output:</strong> </p> <pre>pool-1-thread-1 (Start) message = 0 pool-1-thread-2 (Start) message = 1 pool-1-thread-3 (Start) message = 2 pool-1-thread-5 (Start) message = 4 pool-1-thread-4 (Start) message = 3 pool-1-thread-2 (End) pool-1-thread-2 (Start) message = 5 pool-1-thread-1 (End) pool-1-thread-1 (Start) message = 6 pool-1-thread-3 (End) pool-1-thread-3 (Start) message = 7 pool-1-thread-4 (End) pool-1-thread-4 (Start) message = 8 pool-1-thread-5 (End) pool-1-thread-5 (Start) message = 9 pool-1-thread-2 (End) pool-1-thread-1 (End) pool-1-thread-4 (End) pool-1-thread-3 (End) pool-1-thread-5 (End) Finished all threads </pre> download this example <h2>Thread Pool Example: 2</h2> <p>Let&apos;s see another example of the thread pool.</p> <p> <strong>FileName:</strong> ThreadPoolExample.java</p> <pre> // important import statements import java.util.Date; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.text.SimpleDateFormat; class Tasks implements Runnable { private String taskName; // constructor of the class Tasks public Tasks(String str) { // initializing the field taskName taskName = str; } // Printing the task name and then sleeps for 1 sec // The complete process is getting repeated five times public void run() { try { for (int j = 0; j <= 5; j++) { if (j="=" 0) date dt="new" date(); simpledateformat sdf="new" simpledateformat('hh : mm ss'); prints the initialization time for every task system.out.println('initialization name: '+ taskname + '=" + sdf.format(dt)); } else { Date dt = new Date(); SimpleDateFormat sdf = new SimpleDateFormat(" hh execution system.out.println('time of is complete.'); } catch(interruptedexception ie) ie.printstacktrace(); public class threadpoolexample maximum number threads in thread pool static final int max_th="3;" main method void main(string argvs[]) creating five new tasks runnable rb1="new" tasks('task 1'); rb2="new" 2'); rb3="new" 3'); rb4="new" 4'); rb5="new" 5'); a with size fixed executorservice pl="Executors.newFixedThreadPool(MAX_TH);" passes objects to execute (step 3) pl.execute(rb1); pl.execute(rb2); pl.execute(rb3); pl.execute(rb4); pl.execute(rb5); shutdown pl.shutdown(); < pre> <p> <strong>Output:</strong> </p> <pre> Initialization time for the task name: task 1 = 06 : 13 : 02 Initialization time for the task name: task 2 = 06 : 13 : 02 Initialization time for the task name: task 3 = 06 : 13 : 02 Time of execution for the task name: task 1 = 06 : 13 : 04 Time of execution for the task name: task 2 = 06 : 13 : 04 Time of execution for the task name: task 3 = 06 : 13 : 04 Time of execution for the task name: task 1 = 06 : 13 : 05 Time of execution for the task name: task 2 = 06 : 13 : 05 Time of execution for the task name: task 3 = 06 : 13 : 05 Time of execution for the task name: task 1 = 06 : 13 : 06 Time of execution for the task name: task 2 = 06 : 13 : 06 Time of execution for the task name: task 3 = 06 : 13 : 06 Time of execution for the task name: task 1 = 06 : 13 : 07 Time of execution for the task name: task 2 = 06 : 13 : 07 Time of execution for the task name: task 3 = 06 : 13 : 07 Time of execution for the task name: task 1 = 06 : 13 : 08 Time of execution for the task name: task 2 = 06 : 13 : 08 Time of execution for the task name: task 3 = 06 : 13 : 08 task 2 is complete. Initialization time for the task name: task 4 = 06 : 13 : 09 task 1 is complete. Initialization time for the task name: task 5 = 06 : 13 : 09 task 3 is complete. Time of execution for the task name: task 4 = 06 : 13 : 10 Time of execution for the task name: task 5 = 06 : 13 : 10 Time of execution for the task name: task 4 = 06 : 13 : 11 Time of execution for the task name: task 5 = 06 : 13 : 11 Time of execution for the task name: task 4 = 06 : 13 : 12 Time of execution for the task name: task 5 = 06 : 13 : 12 Time of execution for the task name: task 4 = 06 : 13 : 13 Time of execution for the task name: task 5 = 06 : 13 : 13 Time of execution for the task name: task 4 = 06 : 13 : 14 Time of execution for the task name: task 5 = 06 : 13 : 14 task 4 is complete. task 5 is complete. </pre> <p> <strong>Explanation:</strong> It is evident by looking at the output of the program that tasks 4 and 5 are executed only when the thread has an idle thread. Until then, the extra tasks are put in the queue.</p> <p>The takeaway from the above example is when one wants to execute 50 tasks but is not willing to create 50 threads. In such a case, one can create a pool of 10 threads. Thus, 10 out of 50 tasks are assigned, and the rest are put in the queue. Whenever any thread out of 10 threads becomes idle, it picks up the 11<sup>th </sup>task. The other pending tasks are treated the same way.</p> <h2>Risks involved in Thread Pools</h2> <p>The following are the risk involved in the thread pools.</p> <p> <strong>Deadlock:</strong> It is a known fact that deadlock can come in any program that involves multithreading, and a thread pool introduces another scenario of deadlock. Consider a scenario where all the threads that are executing are waiting for the results from the threads that are blocked and waiting in the queue because of the non-availability of threads for the execution.</p> <p> <strong>Thread Leakage:</strong> Leakage of threads occurs when a thread is being removed from the pool to execute a task but is not returning to it after the completion of the task. For example, when a thread throws the exception and the pool class is not able to catch this exception, then the thread exits and reduces the thread pool size by 1. If the same thing repeats a number of times, then there are fair chances that the pool will become empty, and hence, there are no threads available in the pool for executing other requests.</p> <p> <strong>Resource Thrashing:</strong> A lot of time is wasted in context switching among threads when the size of the thread pool is very large. Whenever there are more threads than the optimal number may cause the starvation problem, and it leads to resource thrashing.</p> <h2>Points to Remember</h2> <p>Do not queue the tasks that are concurrently waiting for the results obtained from the other tasks. It may lead to a deadlock situation, as explained above.</p> <p>Care must be taken whenever threads are used for the operation that is long-lived. It may result in the waiting of thread forever and will finally lead to the leakage of the resource.</p> <p>In the end, the thread pool has to be ended explicitly. If it does not happen, then the program continues to execute, and it never ends. Invoke the shutdown() method on the thread pool to terminate the executor. Note that if someone tries to send another task to the executor after shutdown, it will throw a RejectedExecutionException.</p> <p>One needs to understand the tasks to effectively tune the thread pool. If the given tasks are contrasting, then one should look for pools for executing different varieties of tasks so that one can properly tune them.</p> <p>To reduce the probability of running JVM out of memory, one can control the maximum threads that can run in JVM. The thread pool cannot create new threads after it has reached the maximum limit.</p> <p>A thread pool can use the same used thread if the thread has finished its execution. Thus, the time and resources used for the creation of a new thread are saved.</p> <h2>Tuning the Thread Pool</h2> <p>The accurate size of a thread pool is decided by the number of available processors and the type of tasks the threads have to execute. If a system has the P processors that have only got the computation type processes, then the maximum size of the thread pool of P or P + 1 achieves the maximum efficiency. However, the tasks may have to wait for I/O, and in such a scenario, one has to take into consideration the ratio of the waiting time (W) and the service time (S) for the request; resulting in the maximum size of the pool P * (1 + W / S) for the maximum efficiency.</p> <h2>Conclusion</h2> <p>A thread pool is a very handy tool for organizing applications, especially on the server-side. Concept-wise, a thread pool is very easy to comprehend. However, one may have to look at a lot of issues when dealing with a thread pool. It is because the thread pool comes with some risks involved it (risks are discussed above).</p> <hr></=></pre></10;>

Laden Sie dieses Beispiel herunter

Beispiel für einen Thread-Pool: 2

Sehen wir uns ein weiteres Beispiel des Thread-Pools an.

Dateiname: ThreadPoolExample.java

 // important import statements import java.util.Date; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.text.SimpleDateFormat; class Tasks implements Runnable { private String taskName; // constructor of the class Tasks public Tasks(String str) { // initializing the field taskName taskName = str; } // Printing the task name and then sleeps for 1 sec // The complete process is getting repeated five times public void run() { try { for (int j = 0; j <= 5; j++) { if (j="=" 0) date dt="new" date(); simpledateformat sdf="new" simpledateformat(\'hh : mm ss\'); prints the initialization time for every task system.out.println(\'initialization name: \'+ taskname + \'=" + sdf.format(dt)); } else { Date dt = new Date(); SimpleDateFormat sdf = new SimpleDateFormat(" hh execution system.out.println(\'time of is complete.\'); } catch(interruptedexception ie) ie.printstacktrace(); public class threadpoolexample maximum number threads in thread pool static final int max_th="3;" main method void main(string argvs[]) creating five new tasks runnable rb1="new" tasks(\'task 1\'); rb2="new" 2\'); rb3="new" 3\'); rb4="new" 4\'); rb5="new" 5\'); a with size fixed executorservice pl="Executors.newFixedThreadPool(MAX_TH);" passes objects to execute (step 3) pl.execute(rb1); pl.execute(rb2); pl.execute(rb3); pl.execute(rb4); pl.execute(rb5); shutdown pl.shutdown(); < pre> <p> <strong>Output:</strong> </p> <pre> Initialization time for the task name: task 1 = 06 : 13 : 02 Initialization time for the task name: task 2 = 06 : 13 : 02 Initialization time for the task name: task 3 = 06 : 13 : 02 Time of execution for the task name: task 1 = 06 : 13 : 04 Time of execution for the task name: task 2 = 06 : 13 : 04 Time of execution for the task name: task 3 = 06 : 13 : 04 Time of execution for the task name: task 1 = 06 : 13 : 05 Time of execution for the task name: task 2 = 06 : 13 : 05 Time of execution for the task name: task 3 = 06 : 13 : 05 Time of execution for the task name: task 1 = 06 : 13 : 06 Time of execution for the task name: task 2 = 06 : 13 : 06 Time of execution for the task name: task 3 = 06 : 13 : 06 Time of execution for the task name: task 1 = 06 : 13 : 07 Time of execution for the task name: task 2 = 06 : 13 : 07 Time of execution for the task name: task 3 = 06 : 13 : 07 Time of execution for the task name: task 1 = 06 : 13 : 08 Time of execution for the task name: task 2 = 06 : 13 : 08 Time of execution for the task name: task 3 = 06 : 13 : 08 task 2 is complete. Initialization time for the task name: task 4 = 06 : 13 : 09 task 1 is complete. Initialization time for the task name: task 5 = 06 : 13 : 09 task 3 is complete. Time of execution for the task name: task 4 = 06 : 13 : 10 Time of execution for the task name: task 5 = 06 : 13 : 10 Time of execution for the task name: task 4 = 06 : 13 : 11 Time of execution for the task name: task 5 = 06 : 13 : 11 Time of execution for the task name: task 4 = 06 : 13 : 12 Time of execution for the task name: task 5 = 06 : 13 : 12 Time of execution for the task name: task 4 = 06 : 13 : 13 Time of execution for the task name: task 5 = 06 : 13 : 13 Time of execution for the task name: task 4 = 06 : 13 : 14 Time of execution for the task name: task 5 = 06 : 13 : 14 task 4 is complete. task 5 is complete. </pre> <p> <strong>Explanation:</strong> It is evident by looking at the output of the program that tasks 4 and 5 are executed only when the thread has an idle thread. Until then, the extra tasks are put in the queue.</p> <p>The takeaway from the above example is when one wants to execute 50 tasks but is not willing to create 50 threads. In such a case, one can create a pool of 10 threads. Thus, 10 out of 50 tasks are assigned, and the rest are put in the queue. Whenever any thread out of 10 threads becomes idle, it picks up the 11<sup>th </sup>task. The other pending tasks are treated the same way.</p> <h2>Risks involved in Thread Pools</h2> <p>The following are the risk involved in the thread pools.</p> <p> <strong>Deadlock:</strong> It is a known fact that deadlock can come in any program that involves multithreading, and a thread pool introduces another scenario of deadlock. Consider a scenario where all the threads that are executing are waiting for the results from the threads that are blocked and waiting in the queue because of the non-availability of threads for the execution.</p> <p> <strong>Thread Leakage:</strong> Leakage of threads occurs when a thread is being removed from the pool to execute a task but is not returning to it after the completion of the task. For example, when a thread throws the exception and the pool class is not able to catch this exception, then the thread exits and reduces the thread pool size by 1. If the same thing repeats a number of times, then there are fair chances that the pool will become empty, and hence, there are no threads available in the pool for executing other requests.</p> <p> <strong>Resource Thrashing:</strong> A lot of time is wasted in context switching among threads when the size of the thread pool is very large. Whenever there are more threads than the optimal number may cause the starvation problem, and it leads to resource thrashing.</p> <h2>Points to Remember</h2> <p>Do not queue the tasks that are concurrently waiting for the results obtained from the other tasks. It may lead to a deadlock situation, as explained above.</p> <p>Care must be taken whenever threads are used for the operation that is long-lived. It may result in the waiting of thread forever and will finally lead to the leakage of the resource.</p> <p>In the end, the thread pool has to be ended explicitly. If it does not happen, then the program continues to execute, and it never ends. Invoke the shutdown() method on the thread pool to terminate the executor. Note that if someone tries to send another task to the executor after shutdown, it will throw a RejectedExecutionException.</p> <p>One needs to understand the tasks to effectively tune the thread pool. If the given tasks are contrasting, then one should look for pools for executing different varieties of tasks so that one can properly tune them.</p> <p>To reduce the probability of running JVM out of memory, one can control the maximum threads that can run in JVM. The thread pool cannot create new threads after it has reached the maximum limit.</p> <p>A thread pool can use the same used thread if the thread has finished its execution. Thus, the time and resources used for the creation of a new thread are saved.</p> <h2>Tuning the Thread Pool</h2> <p>The accurate size of a thread pool is decided by the number of available processors and the type of tasks the threads have to execute. If a system has the P processors that have only got the computation type processes, then the maximum size of the thread pool of P or P + 1 achieves the maximum efficiency. However, the tasks may have to wait for I/O, and in such a scenario, one has to take into consideration the ratio of the waiting time (W) and the service time (S) for the request; resulting in the maximum size of the pool P * (1 + W / S) for the maximum efficiency.</p> <h2>Conclusion</h2> <p>A thread pool is a very handy tool for organizing applications, especially on the server-side. Concept-wise, a thread pool is very easy to comprehend. However, one may have to look at a lot of issues when dealing with a thread pool. It is because the thread pool comes with some risks involved it (risks are discussed above).</p> <hr></=>

Erläuterung: Wenn man sich die Ausgabe des Programms ansieht, wird deutlich, dass die Aufgaben 4 und 5 nur dann ausgeführt werden, wenn der Thread einen inaktiven Thread hat. Bis dahin werden die zusätzlichen Aufgaben in die Warteschlange gestellt.

Wie groß ist dieser Monitor?

Die Schlussfolgerung aus dem obigen Beispiel ist, dass man 50 Aufgaben ausführen möchte, aber nicht bereit ist, 50 Threads zu erstellen. In einem solchen Fall kann man einen Pool von 10 Threads erstellen. Somit werden 10 von 50 Aufgaben zugewiesen und der Rest in die Warteschlange gestellt. Immer wenn einer der zehn Threads inaktiv wird, übernimmt er den 11^ThAufgabe. Die anderen anstehenden Aufgaben werden genauso behandelt.

Mit Thread-Pools verbundene Risiken

Im Folgenden sind die mit den Thread-Pools verbundenen Risiken aufgeführt.

Sackgasse: Es ist eine bekannte Tatsache, dass es in jedem Programm, das Multithreading beinhaltet, zu einem Deadlock kommen kann, und ein Thread-Pool führt zu einem weiteren Deadlock-Szenario. Stellen Sie sich ein Szenario vor, in dem alle Threads, die ausgeführt werden, auf die Ergebnisse der Threads warten, die blockiert sind und in der Warteschlange warten, weil keine Threads für die Ausführung verfügbar sind.

Thread-Leckage: Ein Thread-Leck tritt auf, wenn ein Thread aus dem Pool entfernt wird, um eine Aufgabe auszuführen, aber nach Abschluss der Aufgabe nicht dorthin zurückkehrt. Wenn beispielsweise ein Thread die Ausnahme auslöst und die Poolklasse diese Ausnahme nicht abfangen kann, wird der Thread beendet und die Thread-Poolgröße um 1 reduziert. Wenn sich dasselbe mehrmals wiederholt, besteht eine gute Chance, dass dies der Fall ist Der Pool wird leer und daher sind im Pool keine Threads für die Ausführung anderer Anforderungen verfügbar.

Ressourcenvernichtung: Wenn der Thread-Pool sehr groß ist, wird beim Kontextwechsel zwischen Threads viel Zeit verschwendet. Immer wenn mehr Threads als die optimale Anzahl vorhanden sind, kann es zu einem Hungerproblem kommen, das zu einer Überlastung der Ressourcen führt.

Punkte, die man sich merken sollte

Stellen Sie die Aufgaben nicht in die Warteschlange, die gleichzeitig auf die Ergebnisse der anderen Aufgaben warten. Es kann, wie oben erläutert, zu einer Deadlock-Situation führen.

Bei der Verwendung von Threads für einen langlebigen Vorgang ist Vorsicht geboten. Dies kann dazu führen, dass der Thread ewig wartet und schließlich die Ressource verloren geht.

Am Ende muss der Thread-Pool explizit beendet werden. Geschieht dies nicht, wird das Programm weiter ausgeführt und nie beendet. Rufen Sie die Methode „shutdown()“ im Thread-Pool auf, um den Executor zu beenden. Beachten Sie, dass jemand, der nach dem Herunterfahren versucht, eine weitere Aufgabe an den Executor zu senden, eine RejectedExecutionException auslöst.

Man muss die Aufgaben verstehen, um den Thread-Pool effektiv zu optimieren. Wenn die gegebenen Aufgaben gegensätzlich sind, sollte man nach Pools für die Ausführung verschiedener Arten von Aufgaben suchen, damit man sie richtig abstimmen kann.

Listenmethoden Java

Um die Wahrscheinlichkeit zu verringern, dass JVM nicht über genügend Arbeitsspeicher verfügt, kann man die maximale Anzahl an Threads steuern, die in JVM ausgeführt werden können. Der Thread-Pool kann keine neuen Threads erstellen, nachdem er die maximale Grenze erreicht hat.

Ein Thread-Pool kann denselben verwendeten Thread verwenden, wenn der Thread seine Ausführung abgeschlossen hat. Dadurch werden Zeit und Ressourcen gespart, die für die Erstellung eines neuen Threads erforderlich sind.

Optimieren des Thread-Pools

Die genaue Größe eines Thread-Pools wird durch die Anzahl der verfügbaren Prozessoren und die Art der Aufgaben bestimmt, die die Threads ausführen müssen. Verfügt ein System über P Prozessoren, die nur über Berechnungstypprozesse verfügen, dann erreicht die maximale Größe des Thread-Pools von P oder P + 1 die maximale Effizienz. Allerdings müssen die Aufgaben möglicherweise auf E/A warten, und in einem solchen Szenario muss das Verhältnis der Wartezeit (W) und der Servicezeit (S) für die Anforderung berücksichtigt werden; Daraus ergibt sich die maximale Größe des Pools P * (1 + W / S) für die maximale Effizienz.

Abschluss

Ein Thread-Pool ist ein sehr praktisches Werkzeug zum Organisieren von Anwendungen, insbesondere auf der Serverseite. Vom Konzept her ist ein Thread-Pool sehr einfach zu verstehen. Beim Umgang mit einem Thread-Pool müssen jedoch möglicherweise viele Probleme berücksichtigt werden. Dies liegt daran, dass der Thread-Pool einige Risiken mit sich bringt (Risiken werden oben besprochen).