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

 1 public class MongoDB {
 2 
 3     private Mongo wMongodb;
 4     
 5     private static MongoDB instance = null;
 6     
 7     private static synchronized MongoDB GetInstance()
 8     {
 9         if(!isInstanceAlive())
10         {
11             instance = new MongoDB();
12         }
13         return instance;
14     }
15 
16     /**
17      * 判断数据库是否处于连接状态中
18      * @return true:连接中<br/>
19      *        false:已断开
20      */
21     private static boolean isInstanceAlive() {
22         boolean retBool = false;
23         try {
24             // 尝试访问一次数据库
25             DBCollection col = instance.wMongodb.getDB("olacloud").getCollection("noexist");
26             col.find().count();
27             retBool = true;
28         } catch (Exception e) {
29             try {
30                 instance.wMongodb.close();
31             } catch (Exception ex) {}
32         }
33         return retBool;
34     }
35     
36     private MongoDB()
37     {
38         try {
39             MongoClientOptions.Builder voicedbBuilder = MongoClientOptions.builder();
40             voicedbBuilder.connectTimeout(3000);
41             voicedbBuilder.socketTimeout(6000);
42             voicedbBuilder.autoConnectRetry(true);
43             voicedbBuilder.connectionsPerHost(5);
44             voicedbBuilder.readPreference(ReadPreference.secondaryPreferred());
45             voicedbBuilder.socketKeepAlive(true);
46             MongoClientOptions voicedbOptions = voicedbBuilder.build();
47 
48 //            wMongodb = new MongoClient(new ServerAddress("172.16.10.15", 27020),voicedbOptions);
49             wMongodb = new MongoClient(new ServerAddress("wmongo.olavoice.com", 27020),voicedbOptions);
50             
51             DB db = wMongodb.getDB("olacloud");
52 //            DB db = wMongodb.getDB("olacloud_internal");
53             db.authenticate("olacloud", "olacloud".toCharArray());
54             db.setWriteConcern(WriteConcern.SAFE);
55         } catch (Exception e) {
56             // TODO Auto-generated catch block
57             e.printStackTrace();
58         }
59         
60     }
61 
62     /**
63      * 获取数据库连接
64      * @return 已经连接的数据库
65      */
66     public static DB getDB() {
67         DB db = MongoDB.GetInstance().wMongodb.getDB("olacloud");
68 //        DB db = MongoDB.GetInstance().wMongodb.getDB("olacloud_internal");
69         return db;
70     }
71 
72     /**
73      * 获取table的连接
74      * @param tableName table名
75      * @return table连接
76      */
77     public static DBCollection getDBCollection(String tableName) {
78         DB db = MongoDB.getDB();
79         DBCollection col = db.getCollection(tableName);
80         return col;
81     }
82     
83     /**
84      * 关闭当前的Mongodb连接
85      */
86     public static void close() {
87         if (instance != null) {
88             instance.wMongodb.close();
89         }
90     }
91 }