6.可见性关键字（volidate）
可见性关键字（volidate）： 如果对java内存模型了解较清楚的话，我们知道每个线程都会被分配一个线程栈。
线程栈里存的是对象的引用，但当前cache缓存机制，可能会把数据拷贝。
就是，命中缓存，去数据是从cache中获取，而不是从本地内存读取。
不加关键字实例：
package com.xm.thread.t_19_01_27;
import java.util.concurrent.TimeUnit;
public class VolatileDemo implements Runnable{Boolean state = false;
volatile int count = 0;
@Override
public void run() {
if(state==true || count<10) {
count ++;
System.out.println("state="+state+";
count="+count);
} else {
System.out.println("state="+state+";
count="+count);
}
}public static void main(String[] args) throws InterruptedException {
VolatileDemo demo = new VolatileDemo();
for(int i=0;
i<100;
i++) {
new Thread(demo).start();
}TimeUnit.MILLISECONDS.sleep(10);
demo.state = true;
TimeUnit.MILLISECONDS.sleep(10);
demo.state = false;
TimeUnit.MILLISECONDS.sleep(10);
demo.state = true;
}
}
运行结果：
state=false; count=2加关键字实例：
state=false; count=2
state=false; count=3
state=false; count=2
state=false; count=4
state=false; count=5
state=false; count=6
state=false; count=7
state=false; count=8
state=false; count=9
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=true; count=19
state=true; count=18
state=true; count=17
state=true; count=16
state=true; count=15
state=true; count=15
state=true; count=15
state=true; count=13
state=true; count=12
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
package com.xm.thread.t_19_01_27;
import java.util.concurrent.TimeUnit;
public class VolatileDemo implements Runnable{volatile Boolean state = false;
volatile int count = 0;
@Override
public void run() {
if(state==true || count<10) {
count ++;
System.out.println("state="+state+";
count="+count);
} else {
System.out.println("state="+state+";
count="+count);
}
}public static void main(String[] args) throws InterruptedException {
VolatileDemo demo = new VolatileDemo();
for(int i=0;
i<100;
i++) {
new Thread(demo).start();
}TimeUnit.MILLISECONDS.sleep(10);
demo.state = true;
TimeUnit.MILLISECONDS.sleep(10);
demo.state = false;
}
}
运行结果：
state=false; count=1结果分析： volidate就是保证每次读数据都会从内存中读取，但只是保证多线程内共享资源的可见性。
state=false; count=2
state=false; count=3
state=false; count=4
state=false; count=5
state=false; count=6
state=false; count=7
state=false; count=8
state=false; count=9
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=true; count=12
state=false; count=10
state=true; count=14
state=true; count=17
state=true; count=18
state=true; count=21
state=true; count=25
state=true; count=28
state=false; count=10
state=true; count=31
state=true; count=32
state=true; count=33
state=true; count=35
state=true; count=29
state=true; count=28
state=true; count=26
state=true; count=24
state=true; count=23
state=true; count=22
state=true; count=20
state=true; count=19
state=true; count=17
state=true; count=15
state=true; count=13
state=true; count=11
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=false; count=10
state=true; count=42
state=true; count=41
state=true; count=41
state=true; count=40
state=true; count=39
state=true; count=39
state=true; count=39
state=true; count=34
state=true; count=31
可见性，只是保证取出来的数据是当前内存中放的数据，但无法保证数据一定写入正确。
【6.可见性关键字（volidate）】转载于:https://www.cnblogs.com/TimerHotel/p/thread06.html
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