和Lock很像,信号量对象内部维护一个倒计算器,每次调用acquire()都会 -1 , 当acquire()时,计算器为0就会阻塞请求线程,直到其它线程对信号量 release()后,计数器 +1,恢复阻塞线程

name	implication
Semaphore(value=1)	构造方法,value<0,抛ValueError异常
acquire(blocking=True,timeout=None)	获取Semaphore,计数器 -1,获取成功返回True
release()	释放信号量,计数器+1

 

计数器永远>=0

 

import threading,logging,time
FORMAT='%(asctime)-15s\t [ %(threadName)s %(thread)8d ] %(message)s'
logging.basicConfig(level=logging.ERROR,format=FORMAT)

def w(barrier:threading.Semaphore):
    logging.error('in sub thread')
    logging.error(s.acquire())
    logging.error('sub thread over')

s=threading.Semaphore(3)

logging.error(s.acquire())
logging.error(s.acquire())
logging.error(s.acquire())
# logging.error(s.acquire())
threading.Thread(target=w,args=(s,)).start()
threading.Event().wait(2)
logging.error(s.acquire(blocking=False))
logging.error(s.acquire(timeout=3))
logging.error(s.release())

 

连接池:

连接池应该有容量,有一个工厂方法可以获取连接,能够把不用的连接返回

import threading,logging,time
FORMAT='%(asctime)-15s\t [ %(threadName)s %(thread)8d ] %(message)s'
logging.basicConfig(level=logging.ERROR,format=FORMAT)

class Conn:
    def __init__(self,name):
        self.name=name

class Pool:
    def __init__(self,num,C:Conn):
        self.num=num
        self.__pool=[C('conn-{}'.format(b)) for b in range(num)]
        self.semaphore=threading.Semaphore(num)

    def acquire(self):
        # if len(self.__pool)>0:
        #     return self.__pool.pop()
        # else:
        #     return None
        self.semaphore.acquire()
        return self.__pool.pop()

    def release(self,conn:Conn):
        self.__pool.append(conn)
        self.semaphore.release()

pool=Pool(3,Conn)
pool.acquire()
pool.acquire()
pool.acquire()

 

class Conn:
    def __init__(self,name):
        self.name=name

class Pool:
    def __init__(self,num:int):
        self.num=num
        self.__pool=[self.__connect('conn-{}'.format(b)) for b in range(self.num)]
    def __connect(self,name):
        return Conn(name)
    
    def acquire(self):
        if len(self.__pool)>0:
            return self.__pool.pop()
        else:
            pass
    
    def release(self,conn:Conn):
        self.__pool.append(conn)
    
    
        

上面acquire()方法,在多线程的时候,有线程安全问题

假设池中正好有一个链接,有可能多个线程判断池的长度是>0的,当一个线程拿走了连接对象,其他线程再来pop就会抛异常?

• 加锁,在读写的地方加锁
• 使用信号量Semaphore