ThreadLocal, ThreadLocalRandom

Ing pawulangan iki, kita bakal ngomong umum babagan nggarap kelas java.lang.ThreadLocal<> lan carane nggunakake ing lingkungan multithreaded.

Kelas ThreadLocal digunakake kanggo nyimpen variabel . A fitur khusu saka kelas iki sing nyimpen salinan sawijining kapisah saka nilai kanggo saben thread nggunakake.

Delving luwih jero menyang operasi saka kelas, kita bisa mbayangno Peta sing peta Utas kanggo nilai, saka thread saiki njupuk Nilai cocok nalika perlu kanggo nggunakake.

Konstruktor kelas ThreadLocal

Metode

entuk() & set()

Ayo nulis conto ing ngendi kita nggawe rong counter. Pisanan, variabel biasa, bakal kanggo ngetung jumlah benang. Kapindho kita bakal mbungkus ing ThreadLocal . Lan kita bakal weruh carane padha bisa bebarengan. Pisanan, ayo nulis kelas ThreadDemo sing entuk warisan Runnable lan ngemot data lan metode run () sing paling penting . Kita uga bakal nambah cara kanggo nampilake counter ing layar:

class ThreadDemo implements Runnable {

    int counter;
    ThreadLocal<Integer> threadLocalCounter = new ThreadLocal<>();

    public void run() {
        counter++;

        if(threadLocalCounter.get() != null) {
            threadLocalCounter.set(threadLocalCounter.get() + 1);
        } else {
            threadLocalCounter.set(0);
        }
        printCounters();
    }

    public void printCounters(){
        System.out.println("Counter: " + counter);
        System.out.println("threadLocalCounter: " + threadLocalCounter.get());
    }
}

Kanthi saben roto saka kelas kita, kita nambahnglawanvariabel nelpon cara njaluk () kanggo njupuk data saka variabel ThreadLocal . Yen thread anyar ora duwe data, banjur kita bakal nyetel menyang 0. Yen ana data, kita bakal nambah siji. Lan ayo nulis metode utama :

public static void main(String[] args) {
    ThreadDemo threadDemo = new ThreadDemo();

    Thread t1 = new Thread(threadDemo);
    Thread t2 = new Thread(threadDemo);
    Thread t3 = new Thread(threadDemo);

    t1.start();
    t2.start();
    t3.start();

}

Mlaku kelas kita, kita weruh sing variabel ThreadLocal tetep padha preduli saka thread sing ngakses, nanging nomer Utas mundak akeh.

mbusak ()

Kanggo mangerteni carane cara mbusak , kita mung bakal ngganti kode ing kelas ThreadDemo :

if(threadLocalCounter.get() != null) {
      threadLocalCounter.set(threadLocalCounter.get() + 1);
  } else {
      if (counter % 2 == 0) {
          threadLocalCounter.remove();
      } else {
          threadLocalCounter.set(0);
      }
  }

Ing kode iki, yen thread counter iku nomer malah, banjur kita bakal nelpon mbusak () cara ing variabel ThreadLocal . asil:

Lan ing kene kita bisa ndeleng manawa variabel ThreadLocal ing utas kapindho null .

ThreadLocal.withInitial()

Cara iki nggawe variabel thread-lokal.

Implementasi kelas ThreadDemo :

class ThreadDemo implements Runnable {

    int counter;
    ThreadLocal<Integer> threadLocalCounter = ThreadLocal.withInitial(() -> 1);

    public void run() {
        counter++;
        printCounters();
    }

    public void printCounters(){
        System.out.println("Counter: " + counter);
        System.out.println("threadLocalCounter: " + threadLocalCounter.get());
    }
}

Lan kita bisa ndeleng asil kode kita:

Napa kita kudu nggunakake variabel kasebut?

ThreadLocal menehi abstraksi liwat variabel lokal ing hubungan kanggo thread eksekusi java.lang.Thread .

Variabel ThreadLocal beda karo sing biasa amarga saben thread duwe conto variabel sing diinisialisasi dhewe, sing diakses liwat metode get () lan set () .

Saben utas, kayata conto kelas Utas , duwe peta variabel ThreadLocal sing ana gandhengane. Tombol peta minangka referensi kanggo obyek ThreadLocal , lan nilai kasebut minangka referensi kanggo variabel ThreadLocal "angsal" .

Apa kelas Random ora cocog kanggo ngasilake nomer acak ing aplikasi multithreaded?

Kita nggunakake kelas Random kanggo njaluk nomer acak. Nanging apa bisa uga ing lingkungan multithreaded? Bener, ora. Acak ora cocok kanggo lingkungan multithreaded, amarga nalika sawetara Utas ngakses kelas ing wektu sing padha, kinerja nandhang sangsara.

Kanggo ngatasi masalah iki, JDK 7 ngenalaken kelas java.util.concurrent.ThreadLocalRandom kanggo generate nomer acak ing lingkungan multithreaded. Iku kasusun saka rong kelas: ThreadLocal lan Random .

Nomer acak sing ditampa dening siji utas ora gumantung saka benang liyane, nanging java.util.Random nyedhiyakake nomer acak global. Uga, ora kaya Random , ThreadLocalRandom ora ndhukung seeding eksplisit. Nanging, overrides cara setSeed () dipun warisaken saka Random , supaya tansah unsupportedOperationException nalika disebut.

Ayo goleki metode kelas ThreadLocalRandom :

Entuk nomer acak nggunakake ThreadLocalRandom.current()

ThreadLocalRandom minangka kombinasi saka kelas ThreadLocal lan Random . Iki entuk kinerja sing luwih apik ing lingkungan multithreaded kanthi mung ngindhari akses bebarengan menyang kelas Random .

Ayo ngleksanakake conto sing nglibatake pirang-pirang utas lan deleng aplikasi kita karo kelas ThreadLocalRandom :

import java.util.concurrent.ThreadLocalRandom;

class RandomNumbers extends Thread {

    public void run() {
        try {
            int bound = 100;
            int result = ThreadLocalRandom.current().nextInt(bound);
            System.out.println("Thread " + Thread.currentThread().getId() + " generated " + result);
        }
        catch (Exception e) {
            System.out.println("Exception");
        }
    }

    public static void main(String[] args) {
        long startTime = System.currentTimeMillis();

				for (int i = 0; i < 10; i++) {
            RandomNumbers randomNumbers = new RandomNumbers();
            randomNumbers.start();
        }

        long endTime = System.currentTimeMillis();

        System.out.println("Time taken: " + (endTime - startTime));
    }
}

Hasil program kami:

Lan saiki ayo ngganti kelas RandomNumbers lan gunakake Random ing:

int result = new Random().nextInt(bound);

Kanggo nyimpulake lan mbaleni maneh beda antarane kelas Random lan ThreadLocalRandom :