简单单例模式的实现，用JVM保证线程安全

Java-Singleton.java

import java.net.UnknownHostException;
 
import com.mongodb.DB;
import com.mongodb.MongoClient;
import com.qingcong.mobile.constant.AppConfig;
 
public class MongoHelper {
 
  	private MongoClient mongoClient;
  	private DB db;
 
	/**
	 * 私有化构造方法，单例模式实现
	 */
	private MongoHelper() {
		try {
			mongoClient = new MongoClient(AppConfig.DB_IP, AppConfig.DB_PORT);
			db = mongoClient.getDB(AppConfig.DB_NAME);
		} catch (UnknownHostException e) {
			e.printStackTrace();
		}
	}
 
	private static class MongoHelperHolder {
		/**
		 * 静态成员，用JVM来保证线程安全
		 */
		private static MongoHelper mongoHelperImpl = new MongoHelper();
	}
 
	/**
	 * 获得MongoHelper的实例
	 * 
	 * @return
	 */
	public static MongoHelper getInstance() {
		return MongoHelperHolder.mongoHelperImpl;
	}
 
	public DB getDb() {
		return db;
	}
 
}