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();
        }
    }
}