单例模式(Mongo对象的创建)

单例模式:

　　饿汉式单例

//饿汉式单例类.在类初始化时，已经自行实例化
public class Singleton1 {
    //私有的默认构造子
    private Singleton1() {}
    //已经自行实例化
    private static final Singleton1 single = new Singleton1();
    //静态工厂方法
    public static Singleton1 getInstance() {
        return single;
    }
}

　　懒汉式单例

//懒汉式单例类.在第一次调用的时候实例化
public class Singleton2 {
    //私有的默认构造子
    private Singleton2() {}
    //注意，这里没有final
    private static Singleton2 single=null;
    //静态工厂方法
    public synchronized  static Singleton2 getInstance() {
         if (single == null) {
             single = new Singleton2();
         }
        return single;
    }
}  

Mongo对象创建的一个简单单例：

　　在单例类中可以使它的属性也成为单例例如本例子中的 wMongodb 在私有的构造函数中初始化该属性即可实现 属性单例化

 public class MongoDB {

     private Mongo wMongodb;

     private static MongoDB instance = null;

     private static synchronized MongoDB GetInstance()
     {
         if(!isInstanceAlive())
         {
             instance = new MongoDB();
         }
         return instance;
     }

     /**
      * 判断数据库是否处于连接状态中
      * @return true:连接中<br/>
      *        false:已断开
      */
     private static boolean isInstanceAlive() {
         boolean retBool = false;
         try {
             // 尝试访问一次数据库
             DBCollection col = instance.wMongodb.getDB("olacloud").getCollection("noexist");
             col.find().count();
             retBool = true;
         } catch (Exception e) {
             try {
                 instance.wMongodb.close();
             } catch (Exception ex) {}
         }
         return retBool;
     }

     private MongoDB()
     {
         try {
             MongoClientOptions.Builder voicedbBuilder = MongoClientOptions.builder();
             voicedbBuilder.connectTimeout(3000);
             voicedbBuilder.socketTimeout(6000);
             voicedbBuilder.autoConnectRetry(true);
             voicedbBuilder.connectionsPerHost(5);
             voicedbBuilder.readPreference(ReadPreference.secondaryPreferred());
             voicedbBuilder.socketKeepAlive(true);
             MongoClientOptions voicedbOptions = voicedbBuilder.build();

 //            wMongodb = new MongoClient(new ServerAddress("172.16.10.15", 27020),voicedbOptions);
             wMongodb = new MongoClient(new ServerAddress("wmongo.olavoice.com", 27020),voicedbOptions);

             DB db = wMongodb.getDB("olacloud");
 //            DB db = wMongodb.getDB("olacloud_internal");
             db.authenticate("olacloud", "olacloud".toCharArray());
             db.setWriteConcern(WriteConcern.SAFE);
         } catch (Exception e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }

     }

     /**
      * 获取数据库连接
      * @return 已经连接的数据库
      */
     public static DB getDB() {
         DB db = MongoDB.GetInstance().wMongodb.getDB("olacloud");
 //        DB db = MongoDB.GetInstance().wMongodb.getDB("olacloud_internal");
         return db;
     }

     /**
      * 获取table的连接
      * @param tableName table名
      * @return table连接
      */
     public static DBCollection getDBCollection(String tableName) {
         DB db = MongoDB.getDB();
         DBCollection col = db.getCollection(tableName);
         return col;
     }

     /**
      * 关闭当前的Mongodb连接
      */
     public static void close() {
         if (instance != null) {
             instance.wMongodb.close();
         }
     }
 }