3月16日学习内容整理：单例模式的实现方法

一、单例模式的四种实现方法

1、基于文件（模块）导入实现

模块或者文件导入一次就不会被再次导入了

让模块再次导入的方法：

import db
print(db.obj)
import importlib
importlib.reload(db)

 

2、基于类方法classsmethod，但是会改变调用方式，并且并发时就不是单例模式了所以得加锁

import threading
import time

class Foo(object):
    instance = None
    lock = threading.Lock()

    def __init__(self):
        self.a1 = 1
        self.a2 = 2
        import time
        import random
        time.sleep(2)

    @classmethod
    def get_instance(cls,*args,**kwargs):
        if not cls.instance:
            with cls.lock:
                if not cls.instance:
                    obj = cls(*args,**kwargs)
                    cls.instance = obj
                return cls.instance
        return cls.instance
def task():
    obj = Foo.get_instance()
    print(obj)

import threading
for i in range(5):
    t = threading.Thread(target=task,)
    t.start()

time.sleep(10)
Foo.get_instance()

 

3、基于_ _new_ _方法，不会改变调用方式，但是让要加锁

import threading
import time
class Foo(object):
    instance = None
    lock = threading.Lock()

    def __init__(self):
        self.a1 = 1
        self.a2 = 2
        import time
        import random
        time.sleep(2)

    def __new__(cls, *args, **kwargs):
        if not cls.instance:
            with cls.lock:
                if not cls.instance:
                    obj = super(Foo,cls).__new__(cls,*args, **kwargs)
                    cls.instance = obj
                return cls.instance
        return cls.instance


def task():
    obj = Foo()
    print(obj)


for i in range(5):
    t = threading.Thread(target=task,)
    t.start()

 

4、基于metaclass

import threading

lock = threading.Lock()

class Singleton(type):
    def __call__(cls, *args, **kwargs):
        if not hasattr(cls, 'instance'):
            with lock:
                if not hasattr(cls,'instance'):
                    obj = cls.__new__(cls,*args, **kwargs)
                    obj.__init__(*args, **kwargs)
                    setattr(cls,'instance',obj)
                return getattr(cls,'instance')
        return getattr(cls, 'instance')


class Foo(object,metaclass=Singleton):
    def __init__(self):
        self.name = 'alex'

obj1 = Foo()
obj2 = Foo()

print(obj1,obj2)