-------------------------------------------------java 方式----------------------------------------------------------------------------------

package algurimProject;
public class SingleClass {  

//懒汉模式，线程不安全
    public static class Singleton{
        private static Singleton singleton = null;
        private Singleton() {

        }
        public static Singleton getSingleton() {
            if(singleton ==null) {
                singleton = new Singleton();
            }
            return singleton;
        }   
    }

    //饿汉模式，线程安全
    public static class Singleton2{
        private static Singleton2 singleton2 = new Singleton2();
        private Singleton2() {

        }
        public static Singleton2 getSingleton2(){
            return singleton2;
        }
    }


    // 懒汉模式，线程安全模式
    public static class Singleton3{
        private static Singleton3 singleton3 = null;
        private Singleton3() {

        }
        public static synchronized Singleton3 getSingleton3() {
            if(singleton3 ==null) {
                singleton3 = new Singleton3();
            }
            return singleton3;
        }
    }


    /** 
     * 静态内部类，使用双重校验锁，线程安全【推荐】 
     */  
    public static class Singleton4 {  
        private volatile static Singleton4 instance = null;  

        private Singleton4() {  

        }  

        public static Singleton4 getInstance() {  
            if (instance == null) {  
                synchronized (Singleton4.class) {  
                    if (instance == null) {  
                        instance = new Singleton4();  
                    }  
                }  
            }  

            return instance;  
        }  
    }  


    /** 
     * 单例模式，使用静态内部类，线程安全【推荐】 
     */  
    public static class Singleton5 {  
        private final static class SingletonHolder {  
            private static final Singleton5 INSTANCE = new Singleton5();  
        }  

        private Singleton5() {  

        }  

        public static Singleton5 getInstance() {  
            return SingletonHolder.INSTANCE;  
        }  
    }  
}

-------------------------------------------------python 方式----------------------------------------------------------------------------------

# 用__new__实现
class SingleClass(object):
    __instance = None
    __first_init = None

    def __new__(cls, age, name):
        if not cls.__instance:
            SingleClass.__instance = object.__new__(cls)
        return cls.__instance

    def __init__(self, age, name):
        if not self.__first_init:
            self.age = age
            self.name = name
            SingleClass.__first_init = True

a = SingleClass(21, "jatrix")
b = SingleClass(2, "jatrix")

print(id(a))
print(id(b))
print(a.age)
print(b.age)

a.age = 33
print(b.age)

# 使用注解的方式
def singleton(cls, *args, **kwargs):
    instance = {}

    def __singleton():
        if cls not in instance:
            instance[cls] = cls
        return instance[cls]

    return __singleton


@singleton
class MyClass:
    kind = "type"

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


@singleton
class MyAnotherClass:
    name = "another"

    def __init__(self, age):
        self.age = age


one = MyClass()
two = MyClass()
print(id(one))
print(id(two))


another_one = MyAnotherClass()
another_two = MyAnotherClass()
print(id(another_one))
print(id(another_two))