Java-多线程状态获取异常！

[coffee_cold]

!!!线程状态转换问题！
问题简述：
下面的代码中，B线程被设计为Daemon线程，用来不断获取A线程的运行状态并输出。奇怪的事情出现了！
B线程有一定可能获取到A线程的状态为Blocked！！！这是为什么？好奇，求大佬解答，万分感谢!!!
(大佬可以直接编译并运行下面的代码，多运行几次就会发现我所述的现象。）



import static java.lang.System.in;
import static java.lang.System.out;
import java.util.Scanner;
import java.text.DecimalFormat;
public class Temp{
public static void main(String... args) throws InterruptedException{
//通过Thread调用实现了runnable接口的类所生产的实例
Runnable runnerA = new Runnable(){
public void run(){
int i=10;
while(i-->0){
out.println("runnerA's step:"+i+"\t#"+Thread.currentThread().getState());
}
}
};
Thread threadA = new Thread(runnerA);
//直接继承Thread类
Thread threadB = new Thread(){
public void run(){
int i=200;
while(i-->0){
out.println("runnerB's step:"+i);
out.println("threadB中观察threadA的状态为:"+threadA.getState());
}
}
};
//类似于直接继承Thread类，但方式是通过lambda表达式来设计
new Thread(()->{
int i=20;
while(i-->0) out.println("runnerC's step:"+i);
}).start();
out.println("启动threadA前:"+threadA.getState());
threadA.start();
out.println("启动threadA后:"+threadA.getState());
threadB.setDaemon(true);
threadB.start();
//Thread.sleep(100);
}
}

[coffee_cold]

输出示例： ... 启动threadA前:NEW runnerC's step:19 runnerC's step:18 启动threadA后:RUNNABLE runnerA's step:9 #RUNNABLE runnerC's step:17 runnerA's step:8 #RUNNABLE runnerC's step:16 runnerA's step:7 #RUNNABLE runnerC's step:15 runnerA's step:6 #RUNNABLE runnerC's step:14 runnerA's step:5 #RUNNABLE runnerC's step:13 runnerB's step:199 runnerA's step:4 #RUNNABLE threadB中观察threadA的状态为:RUNNABLE runnerC's step:12 runnerB's step:198 threadB中观察threadA的状态为:BLOCKED runnerA's step:3 #RUNNABLE runnerB's step:197 runnerC's step:11 threadB中观察threadA的状态为:BLOCKED runnerB's step:196 runnerA's step:2 #RUNNABLE runnerA's step:1 #RUNNABLE threadB中观察threadA的状态为:RUNNABLE runnerB's step:195 runnerC's step:10 threadB中观察threadA的状态为:BLOCKED runnerB's step:194 runnerA's step:0 #RUNNABLE threadB中观察threadA的状态为:RUNNABLE runnerC's step:9 runnerB's step:193 runnerC's step:8 threadB中观察threadA的状态为:TERMINATED runnerB's step:192 runnerC's step:7 runnerC's step:6 threadB中观察threadA的状态为:TERMINATED runnerB's step:191 runnerC's step:5 runnerC's step:4 threadB中观察threadA的状态为:TERMINATED runnerC's step:3 runnerB's step:190 runnerC's step:2 threadB中观察threadA的状态为:TERMINATED runnerC's step:1 runnerB's step:189 runnerC's step:0 threadB中观察threadA的状态为:TERMINATED

[coffee_cold]

个人觉得，是不是JVM中线程在中止之前会跳转到Blocked状态，随后才是跳转到TERMINATED 。仅是个人猜测。

[oh_Maxy]

多个线程同时在跑，会有CPU切换。当前正在运行的线程，要让出CPU时，就是阻塞状态的。

[coffee_cold]

结贴：洗完澡后智商上线，java.lang.System.out的println()方法是线程同步（线程安全）的，blocked状态就是因这个而来。

[coffee_cold]

多谢老哥解答 我找到问题所在了：System.out.println方法是线程安全的