创建单例的几种方式

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class Singleton1{   //双重锁
   private static Singleton1 singleton;
   private Singleton1() {}
   public static Singleton1 getInstance() {
       if (singleton==null) {
           synchronized (Singleton1.class) {
               if (singleton==null) {
                   singleton=new Singleton1();
               }
           }
       }
       return singleton;
   }
}
class Singleton2{ //饿汉模式
   private static Singleton2 singleton=new Singleton2();
   private Singleton2() {}
   public static Singleton2 getInstance(){
       return singleton;
   }
}
class Singleton3{ //懒汉模式
   private static Singleton3 singleton;
   private Singleton3(){}
   public static Singleton3 getInstance(){
       if (singleton==null) {
           singleton=new Singleton3();
       }
       return singleton;
   }
}
enum Singleton4{ //枚举
   INSTANCE;
   public void method(){
       System.out.println("枚举");
   }
}

class Singleton5{ //静态内部类
   private static class SingletonHolder{
       private final static Singleton5 singleton=new Singleton5();
   }
   private Singleton5(){}
   public static Singleton5 getInstance(){
       return SingletonHolder.singleton;
   }
}
class Singleton6{ //登记模式可以继承的单例
   private static HashMap<String, Singleton6> registry;
   private static final String KEY_STRING="SINGLETONKEY";
   static{

       Singleton6 singleton=new Singleton6();
       registry.put("SINGLETONKEY", singleton);

   }
   protected Singleton6(){}
   public static Singleton6 getInstacne(String key){
       if (key==null) {
           key=KEY_STRING;
       }
       if(registry.get(key)==null){
           try {
               registry.put(key, (Singleton6) Class.forName(key).newInstance());
           } catch (InstantiationException | IllegalAccessException
                   | ClassNotFoundException e) {
               e.printStackTrace();
           }
       }
       return registry.get(key);
   }
}
class Singleton6Child extends Singleton6{
   protected Singleton6Child(){

   }
   public static Singleton6Child getInstacne(){
       return (Singleton6Child) Singleton6.getInstacne(null);
   }
}

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