1、问题描述
org.springframework.context.annotation.ConfigurationClassPostProcessor enhanceConfigurationClasses
Cannot enhance @Configuration bean definition 'mybatisConfiguration' since its singleton instance has been created too early.
The typical cause is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor return type: Consider declaring such methods as 'static'.大致意思就是不能对加了
@Configuration注解的MybatisConfiguration进行增强代理，因为这个类过早实例化了。
2、解决方法
检查一下在
@Configuraion注解的类中加了@Bean注解的方法，看是否有类是BeanDefinitionRegistryPostProcessor的子类，将该方法改为static修饰的静态方法即可。作者这边是用了MapperScannerConfigurer，后面将这个方法去掉，改为@MapperScan注解也能解决。
3、简单分析
若
@Bean注解的非静态方法返回的类为BeanDefinitionRegistryPostProcessor的子类，则会导致其所在的类过早实例化。如下mapperScannerConfigurer()方法。
package com.example.demo.config;
import com.alibaba.druid.pool.DruidDataSource;
import org.apache.ibatis.logging.jdk14.Jdk14LoggingImpl;
import org.mybatis.spring.SqlSessionFactoryBean;
import org.mybatis.spring.annotation.MapperScan;
import org.mybatis.spring.mapper.MapperScannerConfigurer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.ClassPathResource;
import javax.sql.DataSource;
//@MapperScan(value = {"com.example.demo.mapper"})
@Configuration
public class MybatisConfiguration {
@Bean
public DataSource dataSource() {
DruidDataSource dataSource = new DruidDataSource();
dataSource.setUrl("jdbc:mysql://localhost:3306/test?serverTimezone=UTC");
dataSource.setDriverClassName("com.mysql.cj.jdbc.Driver");
dataSource.setUsername("root");
dataSource.setPassword("123456");
return dataSource;
}
@Bean
public SqlSessionFactoryBean sqlSessionFactoryBean() {
SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
sqlSessionFactoryBean.setDataSource(dataSource());
sqlSessionFactoryBean.setMapperLocations(new ClassPathResource("mapper/UserMapper.xml"));
org.apache.ibatis.session.Configuration configuration = new org.apache.ibatis.session.Configuration();
configuration.setLogImpl(Jdk14LoggingImpl.class);
sqlSessionFactoryBean.setConfiguration(configuration);
return sqlSessionFactoryBean;
}
// MapperScannerConfigurer 类是 BeanDefinitionRegistryPostProcessor 的子类
// 在对后置处理器 MapperScannerConfigurer 进行实例化时，会导致 MybatisConfiguration 也实例化
// 如下图
@Bean
public MapperScannerConfigurer mapperScannerConfigurer() {
MapperScannerConfigurer mapperScannerConfigurer = new MapperScannerConfigurer();
mapperScannerConfigurer.setBasePackage("com.example.demo.mapper");
return mapperScannerConfigurer;
}
}在
org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors中实例化后置处理器MapperScannerConfigurer
实例化后置处理器
MapperScannerConfigurer时，会先创建MybatisConfiguration。但时在这个时候创建，它的Class还未被代理。
打印日志时，此时
Spring的生命周期还未到Bean实例化的阶段，但容器中singletonObjects已经存在MybatisConfiguration
如下图，
MybatisConfiguration的Class已经被代理了，那么从这里开始，MybatisConfiguration只要等到实例化时就可创建为代理类，但是它是单例的，前面实例化一次，不会再次实例化了，也就不能被增强代理了。所以才会提示"Cannot enhance @Configuration bean definition 'mybatisConfiguration' since its singleton instance has been created too early."。
