尝试了所有评论中的方法，依然连接不上！！！

1.修改jdbc.properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/seckill?useUnicode=true&characterEncoding=uft8&serverTimezone=UTC&useSSL=false
jdbc.user=root
jdbc.password=***

同时修改spring-dao.xml；另将超时时间改为0，无限等待

<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
    <property name="driverClass" value="${jdbc.driver}"/>
    <property name="jdbcUrl" value="${jdbc.url}"/>
    <property name="user" value="${jdbc.user}"/>
    <property name="password" value="${jdbc.password}"/>

    <!-- c3p0连接池的私有属性 -->
    <!-- 数据库最大连接对象 默认:15 -->
    <property name="maxPoolSize" value="30"/>
    <!-- 数据库最小连接对象 默认:3 -->
    <property name="minPoolSize" value="10"/>
    <!-- 关闭连接后不自动commit 默认:false -->
    <property name="autoCommitOnClose" value="false"/>
    <!-- 获取连接超时时间 默认:0,表示无限等待-->
    <property name="checkoutTimeout" value="0"/>
    <!-- 当获取连接失败时重试次数 -->
    <property name="acquireRetryAttempts" value="2"/>
</bean>

3.修改pom.xml中各项配置版本

<dependency>
  <groupId>mysql</groupId>
  <artifactId>mysql-connector-java</artifactId>
  <version>8.0.11</version>
  <scope>runtime</scope>
</dependency>
<dependency>
  <groupId>com.mchange</groupId>
  <artifactId>c3p0</artifactId>
  <version>0.9.5.2</version>
</dependency>

依然报错

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException: 

### Error querying database.  Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database!

### The error may exist in file [F:\seckill\target\classes\mapper\SeckillDao.xml]

### The error may involve org.seckill.dao.SeckillDao.queryById

### The error occurred while executing a query

### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database!

已经无力了，求大神解决！！！