3.设计模式之单例模式

单例模式：确保一个类只有一个实例，并提供一个全局访问点

1. 饿汉式单例

public class HungrySingleton {
    
    
    private static HungrySingleton instance = new HungrySingleton();

    private HungrySingleton() {
    
    };

    public static HungrySingleton getInstance() {
    
    
        return instance;
    }
}

2. 懒汉式单例

public class LazySingleton {
    
    
    private static LazySingleton instance;

    private LazySingleton() {
    
    };

    public static synchronized LazySingleton getInstance() {
    
    
        if (null == instance) {
    
    
            instance = new LazySingleton();
        }
        return instance;
    }
}

3. 双重检查锁实现

public class DoubleCheckedSingleton {
    
    
    private volatile static DoubleCheckedSingleton instance;

    private DoubleCheckedSingleton() {
    
    };

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

4. 静态内部类实现

public class Singleton {
    
    

    private Singleton() {
    
    };

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

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

5. 枚举方式实现

public class EnumSingleton {
    
    

    public enum ResourceInstance {
    
    
        INSTANCE;
        private Resource resource;

        ResourceInstance() {
    
    
            this.resource = new Resource();
        }

        public Resource getResource() {
    
    
            return this.resource;
        }
    }
}

class Resource {
    
    }