tkinter join() 界面卡住

import sys
import time
import threading
from tkinter import *
from tkinter.messagebox import showinfo

class Demo:

def __init__(self, master):
Button(master, text='execute', command=self.a).pack()

def a(self):
t = threading.Thread(target=self.b, args=(123, ))
t.start()
t.join()
print('over')

def b(self, val):
showinfo('xx', 'xxxx')
print(val)
sys.stdout.flush()

root = Tk()
display = Demo(root)
root.mainloop()


请问为什么用了 join()之后点击界面就卡住了 不用 tkinter 一切都正常