并发编程(3)扩展—单例模式

之前写一个类，每次执行 类() 都会实例化一个类的对象。

class Foo:
    pass

obj1 = Foo()

obj2 = Foo()
print(obj1,obj2)

以后开发会遇到单例模式，每次实例化类的对象时，都是最开始创建的那个对象，不再重复创建对象。

• 简单的实现单例模式

  class Singleton:
      instance = None
  
      def __init__(self, name):
          self.name = name
          
      def __new__(cls, *args, **kwargs):
          # 返回空对象
          if cls.instance:
              return cls.instance
          cls.instance = object.__new__(cls)
          return cls.instance
  
  obj1 = Singleton('alex')
  obj2 = Singleton('SB')
  
  print(obj1,obj2)
  

• 多线程执行单例模式，有BUG

  import threading
  import time
  
  
  class Singleton:
      instance = None
  
      def __init__(self, name):
          self.name = name
  
      def __new__(cls, *args, **kwargs):
          if cls.instance:
              return cls.instance
          time.sleep(0.1)
          cls.instance = object.__new__(cls)
          return cls.instance
  
  
  def task():
      obj = Singleton('x')
      print(obj)
  
  
  for i in range(10):
      t = threading.Thread(target=task)
      t.start()
  
  

• 加锁解决BUG

  import threading
  import time
  class Singleton:
      instance = None
      lock = threading.RLock()
  
      def __init__(self, name):
          self.name = name
          
      def __new__(cls, *args, **kwargs):
          with cls.lock:
              if cls.instance:
                  return cls.instance
              time.sleep(0.1)
              cls.instance = object.__new__(cls)
          return cls.instance
  
  def task():
      obj = Singleton('x')
      print(obj)
  
  for i in range(10):
      t = threading.Thread(target=task)
      t.start()
  

• 加判断，提升性能

  import threading
  import time
  class Singleton:
      instance = None
      lock = threading.RLock()
  
      def __init__(self, name):
          self.name = name
          
      def __new__(cls, *args, **kwargs):
  
          if cls.instance:
              return cls.instance
          with cls.lock:
              if cls.instance:
                  return cls.instance
              time.sleep(0.1)
              cls.instance = object.__new__(cls)
          return cls.instance
  
  def task():
      obj = Singleton('x')
      print(obj)
  
  for i in range(10):
      t = threading.Thread(target=task)
      t.start()
  
  # 执行1000行代码
  
  data = Singleton('asdfasdf')
  print(data)