创建型设计模式-单例模式

Java中单例模式是一种广泛使用的设计模式。

确保某一个类只有一个实例，而且自行实例化并向整个系统提供这个实例。

1. 饿汉式

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

2. 懒汉式

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

3. 双重检查锁定

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

4. 静态内部类

   class Singleton{
   private Singleton(){}
   private static class SingletonHolder{
   private static final Singleton INSTANCE = new Singleton();
   }
   public static Singleton getInstance(){
   return SingletonHolder.INSTANCE;
   }
   }