我正在尝试从此处尝试多线程和以下示例:https://docs.oracle.com/javase/specs/jls/se10/html/jls-8.html#jls-8.3.1.4
我已经在下面发布了我的代码。您能否帮助我了解为什么发生“ if (x < y) {
”而不是“ if (y > x) {
”的数据竞争?
我正在使用openjdk-14.0.1:
Linux void-MS-7678 5.4.0-29-generic#33-Ubuntu SMP Wed Apr 29 14:32:27 UTC 2020 x86_64 x86_64 x86_64 GNU / Linux
代码:
public class Main {
public static void main(String[] args) {
DataRace dr = new DataRace();
Thread t1 = new Thread(()-> {
for (int i = 0; i < 100_000; i++) {
dr.increment();
}
});
Thread t2 = new Thread(()-> {
for (int i = 0; i < 100_000; i++) {
dr.check();
}
});
t1.start();
t2.start();
}
private static class DataRace {
private volatile int x = 0, y = 0;
public void increment() {
x++;
y++;
}
public void check() {
// System.out.println("x=" + x + " y="+ y); // - NO ISSUES
// if (y > x) { - NO ISSUES
// if (x < y) { - ISSUES
if (x < y) {
System.out.println("DataRace detected: x < y");
}
}
}
}
输出:
/home/void/.jdks/openjdk-14.0.1/bin/java -javaagent:/home/void/Development/idea-IC-183.4588.61/lib/idea_rt.jar=46411:/home/void/Development/idea-IC-183.4588.61/bin -Dfile.encoding=UTF-8 -classpath /home/void/Development/multithreading/out/production/classes Main
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
Process finished with exit code 0
比较if (x < y) {
不是原子的。
t2
加载x
进行比较t2
停止工作t1
递增x
和y
t1
停止t2
开始t2
加载y
进行比较x
是旧值,y
是新值,递增,x < y
是true
。这里是如何使用synchronized
解决该问题的示例:
class Main {
public static void main(String[] args) {
DataRace dr = new DataRace();
Thread t1 = new Thread(()-> {
for (int i = 0; i < 100_000; i++) {
dr.increment();
}
});
Thread t2 = new Thread(()-> {
for (int i = 0; i < 100_000; i++) {
dr.check();
}
});
t1.start();
t2.start();
}
private static class DataRace {
private volatile int x = 0, y = 0;
public synchronized void increment() {
x++;
y++;
}
public void check() {
// System.out.println("x=" + x + " y="+ y); // - NO ISSUES
// if (y > x) { - NO ISSUES
// if (x < y) { - ISSUES
boolean xSmallerY = false;
synchronized (this) {
xSmallerY = x < y;
}
if (xSmallerY) {
System.out.println("DataRace detected: x < y");
}
}
}
}