Java pavedienu baseins apzīmē darbinieku pavedienu grupu, kas gaida darbu un tiek izmantotas daudzas reizes.
Vītņu kopas gadījumā tiek izveidota fiksēta izmēra pavedienu grupa. Pavediens no pavedienu kopas tiek izvilkts un pakalpojumu sniedzējs piešķir tam darbu. Pēc darba pabeigšanas pavediens atkal tiek ievietots pavedienu baseinā.
Vītņu baseina metodes
newFixedThreadPool(int s): Metode izveido vītņu kopu ar fiksētu izmēru s.
newCachedThreadPool(): Šī metode izveido jaunu pavedienu kopu, kas vajadzības gadījumā izveido jaunus pavedienus, taču joprojām izmantos iepriekš izveidoto pavedienu, kad vien tie būs pieejami.
newSingleThreadExecutor(): Metode izveido jaunu pavedienu.
dhl nozīme
Java pavedienu baseina priekšrocības
Labāka veiktspēja Tas ietaupa laiku, jo nav nepieciešams izveidot jaunu pavedienu.
Reāllaika lietošana
To izmanto Servlet un JSP, kur konteiners izveido pavedienu pūlu, lai apstrādātu pieprasījumu.
Java pavedienu baseina piemērs
Apskatīsim vienkāršu Java pavedienu pūla piemēru, izmantojot ExecutorService un Executors.
kā java izsaukt metodi
Fails: 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()+' (Start) message = '+message); processmessage();//call processmessage method that sleeps the thread for 2 seconds System.out.println(Thread.currentThread().getName()+' (End)');//prints thread name } private void processmessage() { try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } } } Fails: 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'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;>lejupielādējiet šo piemēruVītņu kopas piemērs: 2
Apskatīsim vēl vienu pavedienu kopas piemēru.
git add --all
Faila nosaukums: 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></=> Paskaidrojums: Aplūkojot programmas izvadi, ir redzams, ka 4. un 5. uzdevums tiek izpildīts tikai tad, ja pavedienam ir dīkstāves pavediens. Līdz tam papildu uzdevumi tiek likti rindā.
Iepriekšminētā piemēra izņēmums ir tad, kad kāds vēlas izpildīt 50 uzdevumus, bet nevēlas izveidot 50 pavedienus. Šādā gadījumā var izveidot 10 pavedienu kopu. Tādējādi tiek piešķirti 10 no 50 uzdevumiem, bet pārējie tiek ievietoti rindā. Ikreiz, kad kāds no 10 pavedieniem kļūst neaktīvs, tas uztver 11thuzdevums. Pārējie nepabeigtie uzdevumi tiek apstrādāti tāpat.
Ar pavedienu grupām saistītie riski
Tālāk ir norādīts risks, kas saistīts ar pavedienu kopumiem.
Strupceļš: Ir zināms fakts, ka strupceļš var nonākt jebkurā programmā, kas ietver daudzpavedienu izmantošanu, un pavedienu pūls ievieš vēl vienu strupceļa scenāriju. Apsveriet scenāriju, kurā visi izpildāmie pavedieni gaida rezultātus no pavedieniem, kas ir bloķēti un gaida rindā, jo pavedieni nav pieejami izpildei.
Vītnes noplūde: Pavedienu noplūde rodas, ja pavediens tiek noņemts no pūla, lai izpildītu uzdevumu, bet pēc uzdevuma pabeigšanas tajā neatgriežas. Piemēram, ja pavediens izdara izņēmumu un pūla klase nespēj uztvert šo izņēmumu, pavediens iziet un samazina pavedienu kopas lielumu par 1. Ja tas pats atkārtojas vairākas reizes, pastāv liela iespēja, ka pūls kļūs tukšs, un tāpēc pūlā nav pieejami pavedieni citu pieprasījumu izpildei.
Resursu sagraušana: Daudz laika tiek tērēts kontekstā, pārslēdzoties starp pavedieniem, ja pavedienu kopas lielums ir ļoti liels. Ikreiz, kad pavedienu ir vairāk nekā optimālais skaits, var rasties bada problēma, un tas noved pie resursu sagraušanas.
Punkti, kas jāatceras
Nelieciet rindā uzdevumus, kas vienlaikus gaida rezultātus, kas iegūti no citiem uzdevumiem. Tas var izraisīt strupceļa situāciju, kā paskaidrots iepriekš.
Jāievēro piesardzība vienmēr, kad tiek izmantoti pavedieni ilgstošai darbībai. Tas var izraisīt pavedienu uz visiem laikiem un beidzot izraisīt resursa noplūdi.
pirmās kārtas loģika
Galu galā pavedienu kopums ir skaidri jāizbeidz. Ja tas nenotiek, programma turpina izpildīt, un tā nekad nebeidzas. Izsauciet shutdown() metodi pavedienu pūlā, lai pārtrauktu izpildītāju. Ņemiet vērā: ja kāds mēģina nosūtīt izpildītājam citu uzdevumu pēc izslēgšanas, tas iemetīs RejectedExecutionException.
Lai efektīvi noregulētu pavedienu kopu, ir jāsaprot uzdevumi. Ja dotie uzdevumi ir kontrastējoši, tad jāmeklē pūli dažādu uzdevumu izpildei, lai varētu tos pareizi noskaņot.
sql concat
Lai samazinātu varbūtību, ka JVM darbināšanai pietrūks atmiņas, var kontrolēt maksimālo pavedienu skaitu, ko var palaist JVM. Pavedienu kopums nevar izveidot jaunus pavedienus pēc tam, kad ir sasniegts maksimālais ierobežojums.
Pavedienu pūls var izmantot to pašu izmantoto pavedienu, ja pavediens ir beidzis izpildi. Tādējādi tiek ietaupīts laiks un resursi, kas tiek izmantoti jauna pavediena izveidei.
Vītņu kopas noregulēšana
Precīzu pavedienu kopas lielumu nosaka pieejamo procesoru skaits un to uzdevumu veids, kas pavedieniem ir jāizpilda. Ja sistēmā ir P procesori, kuriem ir tikai skaitļošanas tipa procesi, tad maksimālais pavedienu kopas izmērs P vai P + 1 sasniedz maksimālo efektivitāti. Tomēr uzdevumiem var būt jāgaida I/O, un šādā scenārijā ir jāņem vērā pieprasījuma gaidīšanas laika (W) un apkalpošanas laika (S) attiecība; kā rezultātā tiek sasniegts maksimālais baseina izmērs P * (1 + W / S) maksimālai efektivitātei.
Secinājums
Pavedienu kopums ir ļoti ērts rīks lietojumprogrammu organizēšanai, īpaši servera pusē. Koncepcijas ziņā pavedienu baseins ir ļoti viegli uztverams. Tomēr, strādājot ar pavedienu kopu, var nākties aplūkot daudzas problēmas. Tas ir tāpēc, ka pavedienu kopums ir saistīts ar dažiem riskiem (riski ir apspriesti iepriekš).