单例模式-4中模式

基于类实现

1,基于类实现

import threading
import time

class Sigle(object):
    lock=threading.Lock()
    def __init__(self):
        pass
    @classmethod
    def get_sigle(cls,*args,**kwargs):
        if not hasattr(Sigle,'instance'):
            with Sigle.lock:　　　　　　　　#加上线程锁,一次性只能进一次　　如果只是单线程则不需要
                if not hasattr(Sigle,'instance'):
                    Sigle.instance=Sigle(*args,**kwargs)
        return Sigle.instance

def func():　　　　
    l=Sigle.get_sigle()
    print(l)
for i in range(10):
    t=threading.Thread(target=func,)　　　　#多线程一起进入
    t.start()

2,类基于__new__方法实现,在实例化之前创建一个对象

import time
import threading
class Singleton(object):
    _instance_lock = threading.Lock()
    def __init__(self):
        pass
    def __new__(cls, *args, **kwargs):
        if not hasattr(Singleton, "_instance"):
            with Singleton._instance_lock:
                if not hasattr(Singleton, "_instance"):
                    Singleton._instance = object.__new__(cls, *args, **kwargs)
        return Singleton._instance

def task(arg):
    obj = Singleton()
    print(obj)

for i in range(10):
    t = threading.Thread(target=task,args=[i,])
    t.start()

3,类基于metaclass创建单例模式

import threading
class Sigle(type):
    lock=threading.Lock()
    def __call__(cls, *args, **kwargs):
        if not hasattr(cls,'instence'):
            with Sigle.lock:
                if not hasattr(cls,'instence'):
                    cls.instence=super(Sigle,cls).__call__(*args,**kwargs)
        return cls.instence
class Foo(metaclass=Sigle):　　#metaclass 指定该类的元类,也就是以后他继承是Sigle类,而不是object.
    def __init__(self,name):
        self.name=name
obj=Foo('yuan')

4,数据库连接池单例模式

创建数据连接池

from DBUtils.PooledDB import PooledDB
import pymysql
import threading

class Sigle(object):
    lock=threading.Lock()
    def __init__(self):
        self.pool=PooledDB(
            creator=pymysql,  # 使用链接数据库的模块
            maxconnections=6,  # 连接池允许的最大连接数，0和None表示不限制连接数
            mincached=2,  # 初始化时，链接池中至少创建的空闲的链接，0表示不创建

            maxcached=5,  # 链接池中最多闲置的链接，0和None不限制
            maxshared=3,
            # 链接池中最多共享的链接数量，0和None表示全部共享。PS: 无用，因为pymysql和MySQLdb等模块的 threadsafety都为1，所有值无论设置为多少，_maxcached永远为0，所以永远是所有链接都共享。
            blocking=True,  # 连接池中如果没有可用连接后，是否阻塞等待。True，等待；False，不等待然后报错
            maxusage=None,  # 一个链接最多被重复使用的次数，None表示无限制
            setsession=[],  # 开始会话前执行的命令列表。如：["set datestyle to ...", "set time zone ..."]
            ping=0,
            # ping MySQL服务端，检查是否服务可用。# 如：0 = None = never, 1 = default = whenever it is requested, 2 = when a cursor is created, 4 = when a query is executed, 7 = always
            host='127.0.0.1',
            port=3306,
            user='root',
            password='',
            database='flask',
            charset='utf8'
        )

    def __new__(cls, *args, **kwargs):
        if not hasattr(Sigle,'instance'):
            with Sigle.lock:
                if not hasattr(Sigle,'instance'):
                    Sigle.instance=object.__new__(cls,*args,**kwargs)
        return Sigle.instance

    def connect(self):
        return self.pool.connection()

调用数据连接池

import pool
import pymysql
def run():
    poo=pool.Sigle()
    conn=poo.connect()
    cursor=conn.cursor()
    cursor.execute('select * from user')
    cur_list=cursor.fetchall()
    print(cur_list)
    conn.close()
if __name__ == '__main__':
    run()