1.查看配置文件的路径、名称是否写错

在一个web项目中，配置文件放在src目录下
而在maven项目中，我们的配置文件时放在resources目录下，而不是src

还要注意配置文件的写法c3p0-config.xml ，这个必须这么写。

2.配置文件的写法

<c3p0-config>
    <default-config>
        <property name="driverClass">com.mysql.jdbc.Driver</property>
        <property name="jdbcUrl">jdbc:mysql://192.168.10.129:3306/smis_0213_xfw</property>
        <property name="user">root</property>
        <property name="password">password</property>
        <property name="initialPoolSize">10</property>
        <property name="maxPoolSize">20</property>
        <property name="minPoolSize">10</property>
        <property name="acquireIncrement">10</property>
        <property name="checkoutTimeout">3000</property>
        <property name="maxStatements">100</property>
        <property name="autoCommitOnClose">true</property>
        <property name="maxStatementsPerConnection">20</property>
        <property name="numHelperThreads">8</property>
        <property name="acquireRetryAttempts">10</property>
        <property name="acquireRetryDelay">3000</property>
        <property name="testConnectionOnCheckin">true</property>
        <property name="preferredTestQuery">SELECT 1</property>
        <property name="idleConnectionTestPeriod">6000</property>
        <property name="maxIdleTime">7000</property>
        <property name="maxAdministrativeTaskTime">300</property>
    </default-config>
</c3p0-config>

3. 配置文件不能用，可以试试set方法

    private static final ComboPooledDataSource DATA_SOURCE =
            new ComboPooledDataSource();

    public static ComboPooledDataSource getDataSource() {
        try {
            DATA_SOURCE.setDriverClass("com.mysql.jdbc.Driver");
            DATA_SOURCE.setJdbcUrl("jdbc:mysql://192.168.10.129:3306/smis_0213_xfw");
            DATA_SOURCE.setMaxPoolSize(20);
            DATA_SOURCE.setUser("root");
            DATA_SOURCE.setPassword("password");
        } catch (PropertyVetoException e) {
            e.printStackTrace();
        }
        return DATA_SOURCE;
    }

4. 看看连接池对象中有没有配置文件的参数

我的同学的eclipse的web项目的配置文件都放在src下，它们不需要传递参数就能加载到配置文件。而我用的是maven项目，放在resources下，我观察源码时发现它是可以传参数的：

没错，这个参数就是传递配置文件的名称的

    private static final ComboPooledDataSource DATA_SOURCE =
            new ComboPooledDataSource("c3p0-config.xml");

我把参数加进去，发现代码就能跑动了。

一、导入jar包

下载：Maven
c3p0-0.9.1.2.jar

二、配置文件

c3p0详细配置（c3p0-config.xml）

c3p0-config.xml名称必须一模一样
<?xml version=“1.0” encoding=“UTF-8”?>必须在文件的第一行
中文编码，&符号要写成&amp;

<?xml version="1.0" encoding="UTF-8"?>
<c3p0-config>
   <default-config>
     <property name="jdbcUrl">jdbc:mysql://localhost:3308/shopping?useUnicode=true&amp;characterEncoding=UTF-8</property>
     <property name="driverClass">com.mysql.cj.jdbc.Driver</property>
     <property name="user">root</property>
     <property name="password">123456</property>
<!--       初始化大小-->
     <property name="initialPoolSize">3</property>
<!--  最多有多少个连接-->
     <property name="maxPoolSize">6</property>
<!--       最少几个连接-->
       <property name="mPoolSize">2</property>
<!--   最大空闲数-->
     <property name="maxIdleTime">1000</property>
<!--    每次最多可以执行多少个批处理语句-->
       <property name="maxStatements">50</property>
   </default-config>
  <named-config name="demo">
    <property name="user">root</property>
    <property name="password">123456</property>
    <property name="driverClass">com.mysql.cj.jdbc.Driver</property>
    <property name="jdbcUrl">jdbc:mysql://localhost:3308/shopping?useUnicode=true&amp;characterEncoding=UTF-8</property>
    <property name="initialPoolSize">10</property>
    <property name="maxIdleTime">30</property>
    <property name="maxPoolSize">100</property>
    <property name="minPoolSize">10</property>
  </named-config>

</c3p0-config>

三、 写C3p0Utils工具类（具有初始化连接池，获得连接的方法）

// 新建数据库连接池，注意这里的参数必须同c3p0-config.xml中的named-config标签中的name的值相同
// 如果不写，则默认调用default-config

package com.itqf.utils;

import com.mchange.v2.c3p0.ComboPooledDataSource;
import java.sql.Connection;
import java.sql.SQLException;
public class C3p0Utils {
    private static ComboPooledDataSource dataSource = null;

    static {
        dataSource = new ComboPooledDataSource();
    }

    public static ComboPooledDataSource getDataSource() {
        return dataSource;
    }

    public static Connection getConnection() throws SQLException {
        return dataSource.getConnection();
    }
}

四、C3p0Utils在Dao层使用

package com.itqf.dao.impl;

import com.itqf.dao.UserDao;
import com.itqf.entity.User;
import com.itqf.utils.C3p0Utils;
import com.mchange.v2.c3p0.ComboPooledDataSource;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanHandler;

import java.sql.Connection;
import java.sql.SQLException;

/*
 * 用户模块数据库访问的实现类*/
public class UserDaoImpl implements UserDao {
    private QueryRunner queryRunner = new QueryRunner(C3p0Utils.getDataSource());
    @Override
    public User selectUserByUname(String username) {
        User user = null;
        String sql = "select u_id as uid,u_name as uname,u_password as upassword" +
                "u_email as uemail,u_sex as usex,u_status as status ,u_code as ucode" +
                "u_role as urole from user where u_name = ?";
        try {
            user = queryRunner.query(sql, new BeanHandler<User>(User.class), username);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return user;
    }
}

名字要命名为c3p0-config.xml

以下为常用项，具体的查看文档

<c3p0-config>
  <default-config>
    <property name="driverClass">com.mysql.jdbc.Driver</property>
    <property name="jdbcUrl">jdbc:mysql://localhost/std</property>
    <property name="user">root</property>
    <property name="password">root</property>
    
    <property name="initialPoolSize">10</property>
    <property name="maxIdleTime">30</property>
    <property name="maxPoolSize">100</property>
    <property name="minPoolSize">10</property>
    <property name="maxStatements">200</property>
  </default-config>

  <!-- This app is massive! -->
  <named-config name="intergalactoApp"> 
    <property name="acquireIncrement">50</property>
    <property name="initialPoolSize">100</property>
    <property name="minPoolSize">50</property>
    <property name="maxPoolSize">1000</property>

    <!-- intergalactoApp adopts a different approach to configuring statement caching -->
    <property name="maxStatements">0</property> 
    <property name="maxStatementsPerConnection">5</property>

    <!-- he's important, but there's only one of him -->
    <user-overrides user="master-of-the-universe"> 
      <property name="acquireIncrement">1</property>
      <property name="initialPoolSize">1</property>
      <property name="minPoolSize">1</property>
      <property name="maxPoolSize">5</property>
      <property name="maxStatementsPerConnection">50</property>
    </user-overrides>
  </named-config>
</c3p0-config>

通过xml的方式还可以配置其他的，当我们要连接oracle数据库的时候，配置
named-config name=“oracle”

然后在创建C3p0的数据库连接池对象时传构造函数的参数进去，就可以读到oracle的配置信息了

c3p0的中文配置信息：
https://www.cnblogs.com/tanhao/p/7053102.html