Daemon线程

public class ThreadA {
    public static void main(String[] args) {
        System.out.println("ThreadA执行：");
        
        Thread threadB = new Thread(new Runnable() {
            public void run() {
                try {
                    System.out.println("ThreadB开始：");
                    for (int i = 0; i < 5; i++ ) {
                        Thread.sleep(1000);
                        System.out.println("ThreadB执行..");
                    }
                    System.out.println("ThreadB即将结束..");
                }
                catch(InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
        );
        threadB.start();
                threadB.setDaemon(true);//设置为Daemon线程
        System.out.println("ThreadA执行..");
    }
}
C:\Users\Administrator\Desktop\classes>java ThreadA
ThreadA执行：
ThreadB开始：
Exception in thread "main" java.lang.IllegalThreadStateException
        at java.lang.Thread.setDaemon(Thread.java:1275)
        at ThreadA.main(ThreadA.java:121)
ThreadB执行..
ThreadB执行..
ThreadB执行..
ThreadB执行..
ThreadB执行..
ThreadB即将结束..
为什么没有执行System.out.println("ThreadA执行..");这个语句，求指导