Java 为什么不使用volatile时CPU时间会不同?
代码1:在这段代码中,我使用了Java 为什么不使用volatile时CPU时间会不同?,java,multithreading,Java,Multithreading,代码1:在这段代码中,我使用了System.out.println()获取由其他线程更改的值,而不是使用volatile package edu.seu.juc.vol; import edu.seu.juc.annotation.ThreadNotSafe; import java.lang.management.ManagementFactory; /** * @author: zs.sun * Create at: 2020/5/24 10:31 * @Package: edu
System.out.println()
获取由其他线程更改的值,而不是使用volatile
package edu.seu.juc.vol;
import edu.seu.juc.annotation.ThreadNotSafe;
import java.lang.management.ManagementFactory;
/**
* @author: zs.sun
* Create at: 2020/5/24 10:31
* @Package: edu.seu.juc.vol
* @ProjectName: af-study
* @Description:
*/
@ThreadNotSafe
public class TestVolatile01 {
public static void main(String[] args) {
Long startTime = System.currentTimeMillis();
ThreadDemo td = new ThreadDemo();
new Thread(td).start();
while (true) {
if (td.isFlag()) {
System.out.println("--------------------------");
break;
}
System.out.println(); // to see the value changed by other thread
}
System.out.println("total: " + (System.currentTimeMillis() - startTime));
System.out.println("CPU time: " + ManagementFactory.getThreadMXBean().getThreadCpuTime(Thread.currentThread().getId())
/ (1000 * 1000));
}
private static class ThreadDemo implements Runnable {
private boolean flag = false;
@Override
public void run() {
try {
Thread.sleep(2000);
flag = true;
System.out.println("flag = " + isFlag());
} catch (Exception e) {
e.printStackTrace();
}
}
public boolean isFlag() {
return flag;
}
public void setFlag(boolean flag) {
this.flag = flag;
}
}
}
Result1
...(some blank lines)
flag = true
--------------------------
total: 2010
CPU time: 875
代码2:在此代码中,我使用volatile
获取其他线程更改的值
package edu.seu.juc.vol;
import edu.seu.juc.annotation.ThreadSafe;
import java.lang.management.ManagementFactory;
/**
* @author: zs.sun
* Create at: 2020/5/24 10:31
* @Package: edu.seu.juc.vol
* @ProjectName: af-study
* @Description:
*/
@ThreadSafe
public class TestVolatile02 {
public static void main(String[] args) {
Long startTime = System.currentTimeMillis();
ThreadDemo td = new ThreadDemo();
new Thread(td).start();
while (true) {
if (td.isFlag()) {
System.out.println("--------------------------");
break;
}
}
System.out.println("total: " + (System.currentTimeMillis() - startTime));
System.out.println("CPU time: " + ManagementFactory.getThreadMXBean().getThreadCpuTime(Thread.currentThread().getId())
/ (1000 * 1000));
}
private static class ThreadDemo implements Runnable {
private volatile boolean flag = false;
@Override
public void run() {
try {
Thread.sleep(2000);
flag = true;
System.out.println("flag = " + isFlag());
} catch (Exception e) {
e.printStackTrace();
}
}
public boolean isFlag() {
return flag;
}
public void setFlag(boolean flag) {
this.flag = flag;
}
}
}
结果2
--------------------------
total: 2005
flag = true
CPU time: 1968
有人能告诉我为什么CPU时间不同吗?
在我看来,两个代码中的主线程都保持CPU的时间,但是<强>代码1 <强>保持绝对更少的CPU时间。代码1中的< P>,在OS内核中花费的时间来打印空白行(并且可能滚动终端窗口),时间不在java线程中使用,所以在打印时不计算所有CPU时间。
代码2中的循环是一个纯CPU循环,因此所有的时间都由Java线程花费。感谢您的回答。我可以得到在操作系统内核中花费的时间吗?有API吗?@JosanSun不是来自Java,没有。