<dependency><groupId>com.mchange</groupId><artifactId>c3p0</artifactId><version>0.9.5.2</version></dependency><dependency><groupId>com.mchange</groupId><artifactId>mchange-commons-java</artifactId><version>0.2.15</version></dependency>    

<?xml version="1.0" encoding="UTF-8"?>
<c3p0-config><default-config><property name="driverClass">com.mysql.cj.jdbc.Driver</property><property name="jdbcUrl">jdbc:mysql://localhost:3306/first?&amp;serverTimezone=UTC</property><property name="user">root</property><property name="password">root</property><property name="acquireIncrement">5</property><property name="initialPoolSize">10</property><property name="minPoolSize">5</property><property name="maxPoolSize">20</property></default-config><named-config name="mysql"><property name="driverClass">com.mysql.cj.jdbc.Driver</property><property name="jdbcUrl">jdbc:mysql://localhost:3306/first?&amp;serverTimezone=UTC</property><property name="user">root</property><property name="password">root</property><property name="acquireIncrement">5</property><property name="initialPoolSize">10</property><property name="minPoolSize">5</property><property name="maxPoolSize">20</property></named-config><named-config name="oracle"><property name="driverClass">oracle.jdbc.driver.OracleDriver</property><property name="jdbcUrl">jdbc:oracle:thin:@//localhost:1521/事例名...</property><property name="user">用户名</property><property name="password">密码</property><property name="acquireIncrement">5</property><property name="initialPoolSize">10</property><property name="minPoolSize">5</property><property name="maxPoolSize">20</property></named-config>
</c3p0-config>

public class DBUtils {private static ComboPooledDataSource comboPooledDataSource = null;static {//它会自动寻找配置文件，节点为mysql的数据库【如果没有指定，就使用默认的！】comboPooledDataSource = new ComboPooledDataSource("mysql");}public static DataSource getDataSource() {return comboPooledDataSource ;}public static Connection getConnection() {try {return comboPooledDataSource.getConnection();} catch (SQLException e) {e.printStackTrace();throw new RuntimeException("数据库初始化失败了！");}}}

CREATE TABLE user (id       VARCHAR(20) PRIMARY KEY,username VARCHAR(20) UNIQUE NOT NULL,password VARCHAR(20)        NOT NULL,email    VARCHAR(20),birthday DATE);

public class UserImplDataBase implements UserDao {public User find(String username, String password) {return null;}public void register(User user) {}}

@Overridepublic User find(String username, String password) {QueryRunner queryRunner = new QueryRunner(DBUtils.getDataSource());String sql = "SELECT * FROM user WHERE username=? AND password=?";try {User user = (User) queryRunner.query(sql, new BeanHandler(User.class), new Object[]{username, password});return user;} catch (SQLException e) {e.printStackTrace();throw new RuntimeException("登陆失败了！");}}@Overridepublic void register(User user) {QueryRunner queryRunner = new QueryRunner(DBUtils.getDataSource());String sql = "INSERT INTO user (id, username, password, email,birthday) VALUES (?,?,?,?,?);";String id = user.getId();String username = user.getUsername();String password = user.getPassword();String email = user.getEmail();Date date = user.getBirthday();try {queryRunner.update(sql, new Object[]{id, username, password, email,date});} catch (SQLException e) {e.printStackTrace();throw new RuntimeException("注册失败了");}}
}

    UserDao userDao = new UserImplXML();//或者UserDao userDao= new UserImplDataBase();

public class DaoFactory {private DaoFactory() {}private static final DaoFactory DAO_FACTORY = new DaoFactory();//暴露公开方法获取工厂对象public static DaoFactory newInstance() {return DAO_FACTORY;}}

#class需要的是完整的对象名称(包括包)
userClass=dao.UserImplDataBase
#userClass=dao.UserImplXML

　　private static UserDao userDao = null;private DaoFactory() {try {//读取配置文件的信息InputStream inputStream = DaoFactory.class.getClassLoader().getResourceAsStream("UserDao.properties");Properties properties = new Properties();properties.load(inputStream);String userClass = properties.getProperty("userClass");//利用反射机制创建相对应的对象userDao = (UserDao) Class.forName(userClass).newInstance();} catch (IOException e) {e.printStackTrace();throw new RuntimeException("读取文件失败了！");} catch (IllegalAccessException e) {e.printStackTrace();throw new RuntimeException("反射失败了！");} catch (InstantiationException e) {e.printStackTrace();throw new RuntimeException("反射失败了！");} catch (ClassNotFoundException e) {e.printStackTrace();throw new RuntimeException("反射失败了！");}}public static UserDao createUserDao() {return userDao;}

　　UserDao userDao = DaoFactory.newInstance().createUserDao();