关于@Value注解,注意事项

笔者07年毕业,到现在已经参加工作有10年了,从事JAVA开发满打满算也就五年的时间,至于这空白的五年算是不务正业吧,现在又回来做开发主要还是因为对程序猿这份职业的偏执!今年10月份刚换的公司,用的技术都是笔者之前从未接触过的,说实话,在大公司里呆久了,很多技术都落后了,也不思进取,什么spring boot,jms云云,咱不懂,只能从头开始,毕竟基础还在那,学习起来也快!今天要讲的是@Value这个注解,环境也是从零搭建,不用spring boot,从最原始的spring + springmvc + mybatis开始,每个框架都是找的官方教程,纯英文,读起来确实吃力,但原汁原味,最有说服力!废话不多说,直接上代码:

例如：

@Configuration
@PropertySource("classpath:test.properties")
public class DataSourceConfig {
@Value("${test}")
private String test2;-->读取

//使用DBCP数据连接池
@Bean
public DataSource dbcpDataSource(DataSourceProperties dataSourceProperties) {
System.out.println(test2);--->打印值
BasicDataSource basicDataSource = new BasicDataSource();
basicDataSource.setUrl(dataSourceProperties.getUrl());
basicDataSource.setUsername(dataSourceProperties.getUsername());
basicDataSource.setPassword(dataSourceProperties.getPassword());
basicDataSource.setDriverClassName(dataSourceProperties.getDriverClassName());
basicDataSource.setTimeBetweenEvictionRunsMillis(dataSourceProperties.getTimeBetweenEvictionRunsMillis());
basicDataSource.setMinEvictableIdleTimeMillis(dataSourceProperties.getMinEvictableIdleTimeMillis());
return basicDataSource;
}

@Bean
public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
sqlSessionFactoryBean.setDataSource(dataSource);
PathMatchingResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
sqlSessionFactoryBean.setMapperLocations(resolver.getResources("classpath*:spittr/mapper/sqlMaps/**/*.xml"));
sqlSessionFactoryBean.setTypeAliasesPackage("spittr.entity");
return sqlSessionFactoryBean.getObject();
}

@Bean
public SqlSession sqlSession(SqlSessionFactory sqlSessionFactory) {
SqlSessionTemplate sqlSessionTemplate = new SqlSessionTemplate(sqlSessionFactory, ExecutorType.BATCH);
return sqlSessionTemplate;
}

@Bean
public DataSourceTransactionManager transactionManager(DataSource dataSource) {
DataSourceTransactionManager dataSourceTransactionManager = new DataSourceTransactionManager(dataSource);
return dataSourceTransactionManager;
}

@Bean
public  MapperScannerConfigurer mapperScannerConfigurer() {
MapperScannerConfigurer mapperScannerConfigurer = new MapperScannerConfigurer();
mapperScannerConfigurer.setBasePackage("spittr.mapper");
return mapperScannerConfigurer;
}
}

classpath中读取test.properties文件,通过@Value注入到test2这个变量中,但不知道为什么,就是null. 用过spring的都知道,spring有两个上下文,其中一个是contextloader创建的,还有一个就是DispatcherServlet创建的,这个DataSourceConfig 就是由contextloader创建的上下文管理的,DispatcherServlet创建的上下文,你无论在哪里用@Value都能取到值,唯独在contextloader创建的上下文中就是取不到值,google,baidu,bing,全都没找到答案,网友的回答都是片面的,而且还是不知道从哪复制的答案,全都千篇一律. 实在没办法只能自己硬着头皮上,log4j日志调成debug级别,一行一行的看.找到下面warn的语句:

WARN [RMI TCP Connection(3)-127.0.0.1] - Cannot enhance @Configuration bean definition ‘dataSourceConfig’ 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’.

继续跟踪,找出BeanDefinitionRegistryPostProcessor接口的具体实现,MapperScannerConfigurer,这个是mybatis-spring包中的类,在上面的配置类中,把方法调成static,重启服务器,问题解决!

问题总结

读万卷书,行万里路,但更重要的是要自己去实践,而不是众人云亦云,多思考多探索,方能成功!