싱글톤 (DCL)

Doble Checking Locking 싱글톤 예제

package com.frjufvjn.netty.websocket;
 
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
 
/**
 * @author PJW
 * @description 채널 그룹 관리 맵 싱글톤 객체 DCL (Double Checking Locking)
 */
public class UserChannelMap {
    private UserChannelMap() {}
    
    private volatile static ConcurrentMap<String, Set<Object>> UserChannelMap;
 
    public static synchronized ConcurrentMap<String, Set<Object>> getInsatance() {
        if(UserChannelMap == null) {
            synchronized(UserChannelMap.class) {
                if(UserChannelMap == null) {
                    UserChannelMap = new ConcurrentHashMap<String, Set<Object>>();
                }
            }
        }
 
        return UserChannelMap;
    }
    
    
}
 

Lazy Holder 

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