为什么在单例类中必须有私有构造函数 [英] Why is it mandatory to have private Constructor inside a Singleton class

问题描述

这是我获取数据库连接的单例类.

This is my singleton class for obtaining the database connection.

我在这里有一个问题:为什么在单例类中必须有一个私有构造函数(因为在我的整个应用程序中，我只调用了一次这个类)并且因为一个类的一个实例可以使用静态方法来实现?

I have a question here: why it compulsory to have a private constructor inside a singleton class (as throughout my entire application I am calling this class only once) and as one instance of a class can be achieved using the static method?

这个私有构造函数可以避免吗，还是必须的?

Can this private constructor can be avoided, or is it mantadatory?

 public class ConnPoolFactory {
        private static DataSource dataSource;
        private static Connection connection;

        private ConnPoolFactory() {
            System.out.println(" ConnPoolFactory cons is called ");
        }

        public static synchronized Connection getConnection() throws SQLException {

            try {

                if (connection == null) {
                    Context initContext = new InitialContext();
                    Context envContext = (Context) initContext
                            .lookup("java:/comp/env");
                    dataSource = (DataSource) envContext.lookup("jdbc/Naresh");
                    connection = dataSource.getConnection();
                } else {
                    return connection;
                }

            } catch (NamingException e) {
                e.printStackTrace();
            }

            return connection;

        }
    }

推荐答案

否则每个人都可以创建你的类的一个实例，这样它就不再是一个单例了.根据定义，单例只能存在一个实例.

Otherwise everyone can create an instance of your class, so it's not a singleton any more. For a singleton by definition there can exist only one instance.

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