spring学习笔记（一）@ConfigurationProperties注解

结论: 这个注解主要是为了将配置文件中的属性映射到实体类上，并且支持嵌套映射。

代码说明：

@ConfigurationProperties(prefix = "person")
@Data
public class Person {
    String name;
    Integer age;
    //Dog dog;
}

@SpringBootApplication
@EnableConfigurationProperties({Person.class})
public class CustomApplication implements ApplicationRunner,ApplicationContextAware {

    ApplicationContext applicationContext;

    public static void main(String[] args) {
        SpringApplication.run(CustomApplication.class, args);
    }
    
    /*
    *这个地方也可以直接注入Person
    * @autowired
    * private Person person;
    *
    */
    @Override
    public void run(ApplicationArguments args) throws Exception {
        Person bean = applicationContext.getBean(Person.class);
        System.out.println(bean);
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}

# 这是我的application.yml文件
person: 
  name: zhangsan
  age: 12

启动chen程序后控制台打印：Person(name=zhangsan, age=12)

证明：配置中的值被映射到了实体类中。

嵌套映射：

   新增实体类：

@Data
public class Dog {
    private String name;
}

同时将personle类中的注解打开，我这里就不贴重复的代码了，yml文件中新增如下

person: 
  name: zhangsan
  age: 12
  dog: 
    name: wangwang

运行程序结果如下：

Person(name=zhangsan, age=12, dog=Dog(name=wangwang))

证明嵌套映射没有问题。

之前一直不明白为什么@ConfigurationProperties跟@EnableConfigurationProperties需要同时使用。跟踪@EnableConfigurationProperties源码发现如下代码：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
// 导入了EnableConfigurationPropertiesImportSelector,我们继续跟踪这个类
@Import({EnableConfigurationPropertiesImportSelector.class})
public @interface EnableConfigurationProperties {
    Class<?>[] value() default {};
}

public static class ConfigurationPropertiesBeanRegistrar implements ImportBeanDefinitionRegistrar {
        public ConfigurationPropertiesBeanRegistrar() {
        }

        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
            this.getTypes(metadata).forEach((type) -> {
                this.register(registry, (ConfigurableListableBeanFactory)registry, type);
            });
        }
        ........

其实就是将我们在@EnableConfigurationProperties({Person.class})定义的class注册到spring容器中，对于我们的案例来说，就是将person类注册到容器中。为了验证我的想法，我将@EnableConfigurationProperties注解注释，并给person类型添加了@component注解

代码如下：

@ConfigurationProperties(prefix = "person")
@Data
@Component // 新增注解
public class Person {
    String name;
    Integer age;
    Dog dog;
}


@SpringBootApplication
//@EnableConfigurationProperties({Person.class}) 去除这个注解
public class CustomApplication implements ApplicationRunner,ApplicationContextAware {

    ApplicationContext applicationContext;

    public static void main(String[] args) {
        SpringApplication.run(CustomApplication.class, args);
    }

    @Autowired
    Person person;

    @Override
    public void run(ApplicationArguments args) throws Exception {
        Person bean = applicationContext.getBean(Person.class);
        System.out.println(person);
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}

程序照样正常运行，验证正确~