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Java 多線程有序執(zhí)行的幾種方法總結(jié)
同事無(wú)意間提出了這個(gè)問(wèn)題,親自實(shí)踐了兩種方法。當(dāng)然肯定還會(huì)有更多更好的方法。
方法一
import java.util.concurrent.atomic.AtomicInteger; public class OrderedThread1 { static AtomicInteger count = new AtomicInteger(0); public static void main(String[] args) throws InterruptedException { Task task1 = new Task(count, 0); Task task2 = new Task(count, 1); Task task3 = new Task(count, 2); Thread thread1 = new Thread(task1); Thread thread2 = new Thread(task2); Thread thread3 = new Thread(task3); thread1.setDaemon(true); thread2.setDaemon(true); thread3.setDaemon(true); thread1.start(); thread2.start(); thread3.start(); Thread.sleep(1 * 1000); } } class Task implements Runnable { private AtomicInteger count; private int order; public Task(AtomicInteger count, int order) { this.count = count; this.order = order; } @Override public void run() { while (true) { if (count.get() % 3 == order) { System.out.println(Thread.currentThread().getName() + " ===== "+ order); count.incrementAndGet(); } } } }
這種方法應(yīng)該是比較常見(jiàn)的解決方案。利用原子遞增控制線程準(zhǔn)入順序。
方法二
public class OrderedThread2 { static Holder holder = new Holder(); public static void main(String[] args) throws InterruptedException { Task1 task1 = new Task1(holder, 0); Task1 task2 = new Task1(holder, 1); Task1 task3 = new Task1(holder, 2); Thread thread1 = new Thread(task1); Thread thread2 = new Thread(task2); Thread thread3 = new Thread(task3); thread1.setDaemon(true); thread2.setDaemon(true); thread3.setDaemon(true); thread1.start(); thread2.start(); thread3.start(); Thread.sleep(1 * 1000); } } class Task1 implements Runnable { Holder holder; int order; public Task1(Holder holder, int order) { this.holder = holder; this.order = order; } @Override public void run() { while (true) { if (holder.count % 3 == order) { System.out.println(Thread.currentThread().getName() + " ===== "+ order); holder.count ++; } } // int i = 0; // while(i ++ < 10000){ // holder.count ++; // } } } class Holder { volatile int count = 0; }
方法二使用了volatile關(guān)鍵字。讓每個(gè)線程都能拿到最新的count的值,當(dāng)其中一個(gè)線程執(zhí)行++操作后,其他兩個(gè)線程就會(huì)拿到最新的值,并檢查是否符合準(zhǔn)入條件。
ps:volatile不是線程安全的。而且兩者沒(méi)有任何關(guān)系。volatile變量不在用戶線程保存副本,因此對(duì)所有線程都能提供最新的值。但試想,如果多個(gè)線程同時(shí)并發(fā)更新這個(gè)變量,其結(jié)果也是顯而易見(jiàn)的,最后一次的更新會(huì)覆蓋前面所有更新,導(dǎo)致線程不安全。在方法二中,一次只有一個(gè)線程滿足準(zhǔn)入條件,因此不存在對(duì)變量的并發(fā)更新。volatile的值是最新的與線程安全完全是不相干的,所以不要誤用volatile實(shí)現(xiàn)并發(fā)控制。
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