用枚举强化singleton属性

使用枚举实现单例的好处

• 代码简洁易于阅读
• 能有效防止序列化或者反射攻击

1.创建一个连接池类(例如数据库连接池),这种功能的类,我们一般都希望它是一个单例的类

public class ConnectionPool {
    public void getConnection(){
        System.out.println(this.hashCode() + ": get a connection");
    }
}

2.新建一个枚举类,用来管理连接池类

public enum  PoolManager {
    INSTANCE;
    private ConnectionPool connectionPool = new ConnectionPool();
    public ConnectionPool getConnectionPool(){
        return this.connectionPool;
    }
}

3.测试单线程

public class App {
    public static void main(String[] args) {
        PoolManager.INSTANCE.getConnectionPool().getConnection();
        PoolManager.INSTANCE.getConnectionPool().getConnection();
        PoolManager.INSTANCE.getConnectionPool().getConnection();

        new ConnectionPool().getConnection();
        new ConnectionPool().getConnection();
        new ConnectionPool().getConnection();
    }
}

result : 
    2061475679: get a connection
    2061475679: get a connection
    2061475679: get a connection
    
    140435067: get a connection
    1450495309: get a connection
    1670782018: get a connection

• PoolManager管理的ConnectionPool的hashcode是一致的
• new 出来的ConnectionPool hashcode不一致,并不是同一个对象

4.测试多线程环境

  public static void main(String[] args) {
       for (int i = 0; i < 10000; i++){
           new Thread(new Runnable() {
               @Override
               public void run() {
                   PoolManager.INSTANCE.getConnectionPool().getConnection();
               }
           }).start();
       }
    }
    
resut :
    1184554318: get a connection
    1184554318: get a connection
    1184554318: get a connection
    1184554318: get a connection
    1184554318: get a connection
    ....
    ....

• 多线程环境下ConnectionPool hashcode也是一致的