设计模式-单例模式的8种实现

Posted 2022-12-12 百里东君~

//1、饿汉式（静态变量）
class Singleton1 
    //构造器私有化
    private Singleton1() 
    

    private final static Singleton1 instance = new Singleton1();

    public static Singleton1 getInstance() 
        return instance;
    


//2、饿汉式（静态代码块）
class Singleton2 
    private Singleton2() 
    

    private static Singleton2 instance;

    //静态代码块
    static 
        instance = new Singleton2();
    

    public static Singleton2 getInstance() 
        return instance;
    


//3、懒汉式（线程不安全）
class Singleton3 
    private Singleton3() 
    

    private static Singleton3 instance;

    //线程不安全
    public static Singleton3 getInstance() 
        if (instance == null) 
            instance = new Singleton3();
        
        return instance;
    


//4、懒汉式（线程安全）
class Singleton4 
    private Singleton4() 
    

    private static Singleton4 instance;

    //加了synchronized，线程安全,效率低
    public static synchronized Singleton4 getInstance() 
        if (instance == null) 
            instance = new Singleton4();
        
        return instance;
    


//5、懒汉式（线程安全，不良改进版）
class Singleton5 
    private Singleton5() 
    

    private static Singleton5 instance;

    //有问题的改进版，高并发时候可能存在多个线程都同时进入到if中，而产生多个实例
    public static Singleton5 getInstance() 
        if (instance == null) 
            synchronized (Singleton5.class) 
                instance = new Singleton5();
            
        
        return instance;
    


//6、懒汉式（线程安全，双重检查，推荐版）
class Singleton6 
    private Singleton6() 
    

    private static volatile Singleton6 instance;

    public static Singleton6 getInstance() 
        if (instance == null) 
            synchronized (Singleton6.class) 
                if (instance == null) 
                    instance = new Singleton6();
                
            
        
        return instance;
    


//7、懒汉式（静态内部类，推荐使用）
class Singleton7 
    private Singleton7() 
    

    private static class SingletonInstance 
        private static final Singleton7 INSTANCE = new Singleton7();
    

    //利用类加载机制保证初始化实例时只有一个线程，线程安全
    //利用静态内部类特点实现延迟加载，效率高
    public static Singleton7 getInstance() 
        return SingletonInstance.INSTANCE;
    


//8、通过枚举实现,推荐使用
enum SingletonEnum
    INSTANCE;
    public void helloWord()
        System.out.println("hello java!");