Java基础知识

public class ApplicationCache{
 
   private Map<String, Object> attributeMap;
   // volatile so that JVM out-of-order writes do not happen
   private static volatile ApplicationCache instance;
 
   public static ApplicationCache getInstance(){
      // Checked once
      if(instance == null){
         // Synchronized on Class level lock
         synchronized(ApplicationCache.class){
            // Checked again
            if(instance == null){
               instance == new ApplicationCache();
            }
         }
      }
      return instance;
   }
 
   private ApplicationCache(){
      attributeMap = createCache(); // Initialize the cache
   }
}