說實話,第一遍看這個概念並沒有看懂,第二遍硬磕了一下,發現原來不理解的原因是**if()和while()方法給忽視了**
- if(){…},如果說程序在判斷完滿足if條件之後,那麼就會進入大括號裏面,即使在裏面等待一段時間之後,我們也是會直接出來
- while(){…}如果說滿足了條件,那麼就會進入大括號裏面,在裏面呆了一段時間,要想出來那得先去while判斷一下,如果滿足,還是不能出來。
即if進去要口令,出來不要口令
while進去和出來都要口令
那麼我們來看具體的代碼:如果線程1在執行dowait方法釋放鎖之後進入等待狀態,即myMonitorObject.wait();那麼如果另一個線程2執行donotify方法去喚醒線程1被線程3截胡了,那麼等線程2去獲得鎖-》執行操作,使得wasSignalled爲false之後,最後如果釋放鎖被線程1獲得到了,如果是if,那麼就直接向下執行,這就是假喚醒,有可能使我們程序出現問題,但是如果是while的話,因爲它喚醒還要再看一下wasSignalled是否爲true,因爲是false,所以不會被喚醒。
public class MyWaitNotify3{
MonitorObject myMonitorObject = new MonitorObject();
boolean wasSignalled = false;
public void doWait(){
synchronized(myMonitorObject){
if(!wasSignalled){
try{
myMonitorObject.wait();
} catch(InterruptedException e){...}
}
//clear signal and continue running.
wasSignalled = false;
}
}
public void doNotify(){
synchronized(myMonitorObject){
wasSignalled = true;
myMonitorObject.notify();
}
}
}
來看一則具體的例子,在生產者和消費者模型中
public class Test {
private final Object lock = new Object();
private int product = 0;
//如果沒有產品,在lock對象上等待喚醒,如果有產品,消費.
private Runnable consumer = () -> {
System.out.println(Thread.currentThread().getName() + " prepare consume");
synchronized (lock) {
if (product <= 0) {//替換爲while解決線程虛假喚醒問題
try {
System.out.println(Thread.currentThread().getName() + " wait");
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " wakeup");
}
product--;
System.out.println(Thread.currentThread().getName() + " consumed product:" + product);
if (product < 0) {
System.err.println(Thread.currentThread().getName() + " spurious lock happend, product: " + product);
}
}
};
//生產一個產品然後喚醒一個在lock對象上等待的consumer
private Runnable producer = () -> {
System.out.println(Thread.currentThread().getName() + " prepare produce");
synchronized (lock) {
product += 1;
System.out.println(Thread.currentThread().getName() + "produced product: " + product);
lock.notify();
}
};
public void producerAndConsumer() {
// 啓動2個consumer,1個producer
Thread c1 = new Thread(consumer);
Thread c2 = new Thread(consumer);
Thread p = new Thread(producer);
c1.start();
c2.start();
p.start();
}
public static void main(String[] args) {
//運行100次,以便觸發異常現象
for (int i = 0; i < 100; i++) {
new Test().producerAndConsumer();
}
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.exit(0);
}
}