单例模式的理解

 1 package yanbober.github.io;
 2 //饿汉模式
 3 class DBManager {
 4     private static DBManager mInstance = new DBManager();
 5 
 6     private DBManager() {}
 7 
 8     public static DBManager getInstance() {
 9         return mInstance;
10     }
11 
12     public boolean update() {
13         System.out.println("update dbase success!");
14         return true;
15     }
16 }
17 
18 public class Main {
19     public static void main(String[] args) {
20         DBManager dbManager = DBManager.getInstance();
21         dbManager.update();
22     }
23 }

package yanbober.github.io;
//懒汉模式（线程安全高并发低性能）
class DBManager {
    privatestatic DBManager mInstance;

    privateDBManager() {}

    publicstaticsynchronized DBManager getInstance() {
        if (null == mInstance) {
            mInstance = new DBManager();
        }
        return mInstance;
    }

    publicbooleanupdate() {
        System.out.println("update dbase success!");
        returntrue;
    }
}

publicclass Main {publicstaticvoidmain(String[] args) {
        DBManager dbManager = DBManager.getInstance();
        dbManager.update();
    }
}

 1 package yanbober.github.io;
 2 //懒汉模式(相对比较靠谱模式)
 3 class DBManager {
 4     private static volatile DBManager mInstance;
 5 
 6     private DBManager() {}
 7 
 8     public static DBManager getInstance() {
 9         if (null == mInstance) {
10             synchronized(DBManager.class) {
11                 if (null == mInstance) {
12                     mInstance = new DBManager();
13                 }
14             }
15         }
16         return mInstance;
17     }
18 
19     public boolean update() {
20         System.out.println("update dbase success!");
21         return true;
22     }
23 }
24 
25 public class Main {
26     public static void main(String[] args) {
27         DBManager dbManager = DBManager.getInstance();
28         dbManager.update();
29     }
30 }