在用Spring使用连接mysql数据库时出现如下错误：
Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception isjava.sql.SQLException: Cannot load JDBC driver class '${driver}'
错误详细信息如下：
Exception in thread "main" org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException: ### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Cannot load JDBC driver class '${driver}'### The error may exist in file [/Users/guowei/EclipseProjects/workspace/FrameworkMaven/target/classes/com/guowei/maven/framework/mapping/UserMapper.xml]### The error may involve com.guowei.maven.framework.dao.UserMapper.getUserById### The error occurred while executing a query### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Cannot load JDBC driver class '${driver}'at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:76)at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:399)at com.sun.proxy.$Proxy7.selectOne(Unknown Source)at org.mybatis.spring.SqlSessionTemplate.selectOne(SqlSessionTemplate.java:165)at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:69)at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:53)at com.sun.proxy.$Proxy8.getUserById(Unknown Source)at com.guowei.maven.framework.servlet.Test.main(Test.java:18)Caused by: org.apache.ibatis.exceptions.PersistenceException: ### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Cannot load JDBC driver class '${driver}'### The error may exist in file [/Users/guowei/EclipseProjects/workspace/FrameworkMaven/target/classes/com/guowei/maven/framework/mapping/UserMapper.xml]### The error may involve com.guowei.maven.framework.dao.UserMapper.getUserById### The error occurred while executing a query### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Cannot load JDBC driver class '${driver}'at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:122)at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:113)at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:73)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:497)at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:386)原因是因为配置了applicationContext配置文件配置了default-autowire="byName"属性，这样会导致提前初始化一些类，这个时候，PropertyPlaceholderConfigurer还没来得及替换定义中的变量，导致把表达式当作字符串复制了。
解决方案就是删掉这个配置属性即可。
参考资料：
http://songjianyong.iteye.com/blog/1663170
http://www.oschina.net/question/188964_32305