Mybatis源码报错java.lang.ClassNotFoundException: Cannot find class: com.mysql.jdbc.Driver

在阅读mysql源码的时候，下载下来源码，写了一个简单demo，运行，报错

java.sql.SQLException: Error setting driver on UnpooledDataSource. Cause: java.lang.ClassNotFoundException: Cannot find class: com.mysql.jdbc.Driver
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:153)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:145)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:76)
at org.apache.ibatis.demo.MybatisMain.main(MybatisMain.java:36)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

 

网上查了一下，大部分千篇一律都是mysql-connector-java-x.x.x-bin.jar的问题，但是不论我引入什么版本的包，问题依然存在。

经过一步步调试，发现加载com.mysql.jdbc.Driver类的地方是org.apache.ibatis.io.ClassLoaderWapper的如下方法

Class<?> classForName(String name, ClassLoader[] classLoader) throws ClassNotFoundException {
  for (ClassLoader cl : classLoader) {
    if (null != cl) {
      try {
        return Class.forName(name, true, cl);
      } catch (ClassNotFoundException e) {
        // we'll ignore this until all classloaders fail to locate the class
      }
    }
  }
  throw new ClassNotFoundException("Cannot find class: " + name);
}

 

但是为什么加载不了呢？

于是，在main方法中测试一下

public class MybatisMain {
  public static void main(String[] args) throws IOException
  {
    try {
      Class.forName("com.mysql.jdbc.Driver", true, Thread.currentThread().getContextClassLoader());
    } catch (ClassNotFoundException e) {
      e.printStackTrace();
    }
  }
}

 

结果，仍然加载不了。

结论：

大家看pom文件

<dependency>
  <groupId>mysql</groupId>
  <artifactId>mysql-connector-java</artifactId>
  <version>8.0.24</version>
  <scope>test</scope>
</dependency>

 

注意到pom文件中<scope>test</scope>，问题就出在这，注释掉这一行，问题解决。