Java多線程之線程虛假喚醒

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本文目錄提綱

問題：兩個線程對一個初始值為零的變量操作，實現一個線程加一，另一個線程減一，來十次。

問題：四個線程對一個初始值為零的變量操作，實現兩個線程加一，另外兩個線程減一，來十次。

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1. 兩個線程對一個初始值為零的變量操作，實現一個線程加一，另一個線程減一，來十次。

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代碼實現：

class ShareData{private int number = 0;public synchronized void increment() throws InterruptedException { // 判斷if (number!=0){this.wait();} // 干活++number;System.out.println(Thread.currentThread().getName()+"\t"+number); // 通知喚醒this.notifyAll();}public synchronized void decrement() throws InterruptedException {if (number==0){this.wait();}--number;System.out.println(Thread.currentThread().getName()+"\t"+number);this.notifyAll();} }public class ThreadDemo2 {public static void main(String[] args) {ShareData sd = new ShareData();new Thread(()->{for (int i = 1; i <=10 ; i++) {try {sd.increment();} catch (InterruptedException e) {e.printStackTrace();}} },"A").start();new Thread(()->{for (int i = 1; i <=10 ; i++) {try {sd.decrement();} catch (InterruptedException e) {e.printStackTrace();}}},"B").start();} }

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編譯結果：

多次測試，是沒有問題的。

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2. 問題：四個線程對一個初始值為零的變量操作，實現兩個線程加一，另外兩個線程減一，來十次。

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于是把上面的代碼，線程弄多兩個，運行發現出現問題。編譯結果如下：

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通過閱讀文檔，發現：

As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:

synchronized (obj) {while (<condition does not hold>)obj.wait();... // Perform action appropriate to condition}

即中斷和偽喚醒是可能的，并且這個方法應該在while循環中使用

畫圖：

將上面代碼if換成while再執行，發現沒有問題，代碼如下：

class ShareData {private int number = 0;public synchronized void increment() throws InterruptedException { // 判斷while (number != 0) {this.wait();} // 干活++number;System.out.println(Thread.currentThread().getName() + "\t" + number); // 通知喚醒this.notifyAll();}public synchronized void decrement() throws InterruptedException {while (number == 0) {this.wait();}--number;System.out.println(Thread.currentThread().getName() + "\t" + number);this.notifyAll();} }public class ThreadDemo2 {public static void main(String[] args) {ShareData sd = new ShareData();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(200);sd.increment();} catch (InterruptedException e) {e.printStackTrace();}}}, "A").start();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(300); //模擬執行其他代碼sd.decrement();} catch (InterruptedException e) {e.printStackTrace();}}}, "B").start();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(400);sd.increment();} catch (InterruptedException e) {e.printStackTrace();}}}, "C").start();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(500);sd.decrement();} catch (InterruptedException e) {e.printStackTrace();}}}, "D").start();} }

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編譯結果：

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使用Lock優化

前置知識：

Condition factors out the Object monitor methods (wait, notify and notifyAll) into distinct objects to give the effect of having multiple wait-sets per object, by combining them with the use of arbitrary Lock implementations. Where a Lock replaces the use ofsynchronized methods and statements, a Condition replaces the use of the Object monitor methods.

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優化代碼：

import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;class ShareData {private int number = 0;private Lock lock = new ReentrantLock();private Condition condition = lock.newCondition();public synchronized void increment() throws InterruptedException{lock.lock();try{while (number != 0) { // this.wait();condition.await();} // 干活++number;System.out.println(Thread.currentThread().getName() + "\t" + number); // 通知喚醒 // this.notifyAll();condition.signalAll();}catch (Exception e){e.printStackTrace();}finally {lock.unlock();}}public synchronized void decrement() throws InterruptedException{lock.lock();try{while (number == 0) { // this.wait();condition.await();} // 干活--number;System.out.println(Thread.currentThread().getName() + "\t" + number); // 通知喚醒 // this.notifyAll();condition.signalAll();}catch (Exception e){e.printStackTrace();}finally {lock.unlock();}}public class ThreadDemo2 {public static void main(String[] args) {ShareData sd = new ShareData();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(200);sd.increment();} catch (InterruptedException e) {e.printStackTrace();}}}, "A").start();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(300);sd.decrement();} catch (InterruptedException e) {e.printStackTrace();}}}, "B").start();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(400);sd.increment();} catch (InterruptedException e) {e.printStackTrace();}}}, "C").start();new Thread(() -> {for (int i = 1; i <= 10; i++) {try {Thread.sleep(500);sd.decrement();} catch (InterruptedException e) {e.printStackTrace();}}}, "D").start();} }

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