単例モード2種類の書き方

//     
class  Singleton{
           private static Singleton instance;
           private static byte[] lock = new byte[0];
           private  Singleton(){
           }
           
           public static Singleton getInstance(){
           
                     if(instance==null){
                                synchronized(lock){
                                         if(instance==null){
                                                instance =new Singleton();
                                         }
                                
                                }
                     
                     }
           return  instance;
           }
}

二番目の書き方

class Singleton{
        private static Singleton instance;
        
        private static ReentrantLock lock =new ReentrantLock();
        private Singleton(){}
        
        public static Singleton getInstance(){
                if(instance ==null){
                      lock.lock();
                      if(instance==null){
                             instance =new Singleton();
                      }
                      lock.unlock();
                }
                return instance;
        }


}