帮我看看这段代码为什么运行不出来import threading import time class Library(threading.Thread): def __init__(self): threading.Thread.__init__(self) self.book_num=0 def borrow(self): global book_count global lock while True: lock = threading.Lock() time.sleep(1) if book_count>0: lock.acquire() time.sleep(0.1) book_count-=1 # book_count=book_count-1 self.book_num+=1 lock.release() print(f"借走{self.book_num}本,现有图书{book_count}本") else: break def back(self): global book_count global lock while True: lock = threading.Lock() time.sleep(3) # time.sleep(0.1) lock.acquire() book_count += 1 # book_count=book_count+1 lock.release() self.book_num -= 1 print(f"归还{self.book_num}本,现有图书{book_count}本") book_count=100 result1=Library() result2=Library() result1.start() result2.start() result1.join()

这段代码有几个问题：

1. 在 borrow 和 back 方法里面，每次 while 循环都会重新创建一个锁对象。应该在外部创建好锁对象，然后将锁对象传递给 borrow 和 back 方法。

2. 在 borrow 方法里面，调用了 lock.acquire() 方法获得锁，但是在程序执行完毕后没有调用 lock.release() 方法释放锁。这会导致程序出现死锁。

3. 在 back 方法里面，self.book_num 减去 1 的代码应该在 lock.release() 方法之前执行，否则会导致程序出现竞争条件。

下面是修改后的代码：

import threading 
import time 

class Library(threading.Thread):
    def __init__(self, lock):
        threading.Thread.__init__(self)
        self.book_num = 0
        self.lock = lock
    
    def borrow(self):
        global book_count
        while True:
            time.sleep(1)
            if book_count > 0:
                self.lock.acquire()
                time.sleep(0.1)
                book_count -= 1
                self.book_num += 1
                print(f"借走{self.book_num}本,现有图书{book_count}本")
                self.lock.release()
            else:
                break
    
    def back(self):
        global book_count
        while True:
            time.sleep(3)
            self.lock.acquire()
            self.book_num -= 1
            book_count += 1
            print(f"归还{self.book_num}本,现有图书{book_count}本")
            self.lock.release()

book_count = 100
lock = threading.Lock()
result1 = Library(lock)
result2 = Library(lock)
result1.start()
result2.start()
result1.join()