单例的数据库连接类：

import com.alibaba.druid.pool.DruidDataSourceFactory;
import javax.sql.DataSource;
import java.io.FileInputStream;
import java.sql.Connection;
import java.util.Properties;public class Singleton {// 创建单例对象private static Singleton singleton;// 创建数据库获取连接的对象private static Connection singletonConnection = null;// 将构造方法私有化,防止外部newpublic Singleton() throws Exception {singletonConnection = Singleton.getConnection();}private static Connection getConnection() throws Exception {Properties prop = new Properties();prop.load(new FileInputStream("src\\main\\resources\\jdbc.properties"));DataSource dataSource = DruidDataSourceFactory.createDataSource(prop);Connection singletonConnection = dataSource.getConnection();return singletonConnection;}//  对外提供实例化的方法public static Connection getInstance() throws Exception {if (singleton == null) {singleton = new Singleton();}return singletonConnection;}
}

测试类：

package com.gy.config;import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;public class test {public static void main(String[] args) throws Exception {Connection instance = Singleton.getInstance();Connection instance1 = Singleton.getInstance();Connection instance2 = Singleton.getInstance();System.out.println(instance);System.out.println(instance1);System.out.println(instance2);System.out.println("============================");Statement statement = instance.createStatement();String sql="select * from user";ResultSet resultSet = statement.executeQuery(sql);List<User> users=new ArrayList<>();while (resultSet.next()){String name = resultSet.getString(1);String password = resultSet.getString(2);int id = resultSet.getInt(3);User user = new User(name, password, id);users.add(user);}System.out.println(users);}
}

运行结果：（成功返回单例对象）