【大厂面试题系列】：请写出Java中线程安全的单例模式【设计模式】

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第一种：双重检查

public class Singleton {
    
    

    private volatile static Singleton singleton;

    private Singleton() {
    
    

    }

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

第二种：静态内部类

public class Singleton {
    
    


    private Singleton() {
    
    

    }

    public static class SingletonHolder {
    
    
        private static final Singleton singleton = new Singleton();
    }

    public static Singleton getInstance() {
    
    
        return SingletonHolder.singleton;
    }
}

第三种：枚举

public enum  Singleton {
    
    
    INSTANCE;

    Singleton() {
    
    
    }

}

创建枚举默认就是线程安全的，并且还能防止反序列化导致重新创建新的对象。

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完善使用enum枚举实现单例模式

public class SingletonFactory {
    
    

    private enum Singleton {
    
    
        singleProtect;

        private product instance;

        private Singleton() {
    
    
            instance = new product();
        }

        public product getInstance() {
    
    
            return instance;
        }

    }

    public static product getInstance() {
    
    
        return Singleton.singleProtect.getInstance();
    }
}

//需要实现单例的类
class product {
    
    
    public product(){
    
    }
}

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第四种：懒汉式（加锁）

public class Singleton {
    
    

    private static Singleton singleton; 
    
    private Singleton() {
    
    
    }
    
    public static synchronized Singleton getInstance() {
    
    
        if (singleton == null)
            singleton = new Singleton();
        return singleton;
    }

}

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