java - Spring 静态方法

Question

我正在学习Spring，正在看的书有如下代码行

@Bean
public static PropertySourcesPlaceholderConfigurer placeholderConfigurer() {
    return new PropertySourcesPlaceholderConfigurer();
}

为什么需要static关键字，在静态方法上声明@Bean注解是否正常，如果是，为什么？

Answer 1

PropertySourcesPlaceholderConfigurer 对象是一个 BeanFactoryPostProcessor，因此，根据 Spring documentation :

Special consideration must be taken for @Bean methods that return Spring BeanFactoryPostProcessor (BFPP) types. Because BFPP objects must be instantiated very early in the container lifecycle, they can interfere with processing of annotations such as @Autowired, @Value, and @PostConstruct within @Configuration classes. To avoid these lifecycle issues, mark BFPP-returning @Bean methods as static.

By marking this method as static, it can be invoked without causing instantiation of its declaring @Configuration class, thus avoiding the above-mentioned lifecycle conflicts. Note however that static @Bean methods will not be enhanced for scoping and AOP semantics as mentioned above. This works out in BFPP cases, as they are not typically referenced by other @Bean methods. As a reminder, a WARN-level log message will be issued for any non-static @Bean methods having a return type assignable to BeanFactoryPostProcessor.