单例设计模式

单例设计模式

单例模式：让对象独一无二

• 优点：内存中之后一个实例，减少内存开支
• 缺点：单例模式一般没有接口，扩展很困难

1. 饿汉单例

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

2. 懒汉单例

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

3. 双重检查单例

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