在singleton中对synchronized的使用

public class DBConnPool{	
	private static DBConnPool instance = null;
	public synchronized static DBConnPool getInstance() {
		if (instance == null) {
			try {
				String modbase = System.getenv("DM_HOME");
				modbase = modbase != null ? modbase : ".";
				String dbcfg = modbase + File.separator + "conf" + File.separator + "dbcfg.xml";

				instance = new DBConnPool(dbcfg);
			} catch (Exception e) {
				LOG.fatal("create connection pool instance failed", e);
			}
		}
		return instance;
	}

}

public class Singleton{
private static volatile Singleton _instance;

public static Singleton getInstance(){

   if(_instance == null){
            synchronized(Singleton.class){
              if(_instance == null)
              _instance = new Singleton();
            }

   }
   return _instance;

}