maven工程添加了log4j和mysql connector依赖之后，为什么报了找不驱动的错？

log4j:ERROR Failed to load driver
java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

 pom.xml文件:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>test</groupId>
    <artifactId>test</artifactId>
    <version>0.1-SNAPSHOT</version>
<dependencies>
    <!-- https://mvnrepository.com/artifact/log4j/log4j -->
    <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.4</version>
    </dependency>
    <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.40</version>
    </dependency>
</dependencies>
</project>

log4j.properties:

log4j.rootLogger=DEBUG,stdout,DATABASE
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d [%t] %p %c - %m%n

log4j.appender.DATABASE=org.apache.log4j.jdbc.JDBCAppender
log4j.appender.DATABASE.URL=jdbc:mysql://localhost:3306/test  
log4j.appender.DATABASE.driver=com.mysql.jdbc.Driver  
log4j.appender.DATABASE.user=root  
log4j.appender.DATABASE.password=123456 
log4j.appender.DATABASE.layout=org.apache.log4j.PatternLayout
log4j.appender.DATABASE.sql=INSERT INTO log4j (log_date, log_level, location, message) VALUES ('%d{ISO8601}', '%p', '%C,%L', '%m')

项目右键。maven菜单 有个update project试一试