springboot——@EnableConfigurationProperties是如何起作用

前言

用springboot开发的过程中，我们会用到@ConfigurationProperties注解，主要是用来把properties或者yml配置文件转化为bean来使用的，而@EnableConfigurationProperties注解的作用是@ConfigurationProperties注解生效。
如果只配置@ConfigurationProperties注解，在IOC容器中是获取不到properties配置文件转化的bean的，当然在@ConfigurationProperties加入注解的类上加@Component也可以使交于springboot管理。

举个栗子

第一步：创建一个类TestConfigurationProperties

@ConfigurationProperties(prefix = "properties")
public class TestConfigurationProperties {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

注意：得加上set和get方法
第二步：创建TestAutoConfiguration类

@Configuration
@EnableConfigurationProperties(TestConfigurationProperties.class)
public class TestAutoConfiguration {

    private TestConfigurationProperties testConfigurationProperties;

    public TestAutoConfiguration(TestConfigurationProperties testConfigurationProperties) {
        this.testConfigurationProperties = testConfigurationProperties;
    }

    @Bean
    public User user(){
        User user = new User();
        user.setName(testConfigurationProperties.getName());
        return user;
    }
}

注意：得创建一个有参构造方法
第三步：配置文件加入属性

properties.name=test
第四步：跑一下，打印出User这个类

@RestController
@RequestMapping("/api/test")
@Slf4j
public class TestController {
    @Autowired
    TestConfigurationProperties testConfigurationProperties;

    @Autowired
    User user;

    @RequestMapping(value = "/testConfigurationProperties")
    public String testConfigurationProperties() {
        log.info("test testConfigurationProperties.............{}", testConfigurationProperties.getName());
        log.info("user:{}", user);
        return "SUCCESS";
    }
}

控制台输出：

2019-04-21/16:11:36.638||||||||^_|[http-nio-8088-exec-1] INFO com.stone.zplxjj.controller.TestController 37 - test testConfigurationProperties.............test
2019-04-21/16:11:36.639||||||||^_|[http-nio-8088-exec-1] INFO com.stone.zplxjj.controller.TestController 38 - user:User(id=null, name=test)
@EnableConfigurationProperties是怎么加载的
通过查看@EnableConfigurationProperties的注解：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(EnableConfigurationPropertiesImportSelector.class)
public @interface EnableConfigurationProperties {

    /**
     * Convenient way to quickly register {@link ConfigurationProperties} annotated beans
     * with Spring. Standard Spring Beans will also be scanned regardless of this value.
     * @return {@link ConfigurationProperties} annotated beans to register
     */
    Class<?>[] value() default {};

}

通过分析自动配置可以知道，肯定是这个类EnableConfigurationPropertiesImportSelector起的作用：

private static final String[] IMPORTS = {
            ConfigurationPropertiesBeanRegistrar.class.getName(),
            ConfigurationPropertiesBindingPostProcessorRegistrar.class.getName() };

    @Override
    public String[] selectImports(AnnotationMetadata metadata) {
        return IMPORTS;
    }

selectImports方法返回了这两个类：ConfigurationPropertiesBeanRegistrar和ConfigurationPropertiesBindingPostProcessorRegistrar，是何时加载的，我们只需要看这个类ConfigurationPropertiesBeanRegistrar即可：

public static class ConfigurationPropertiesBeanRegistrar
            implements ImportBeanDefinitionRegistrar {

        @Override
        public void registerBeanDefinitions(AnnotationMetadata metadata,
                BeanDefinitionRegistry registry) {
            getTypes(metadata).forEach((type) -> register(registry,
                    (ConfigurableListableBeanFactory) registry, type));
        }
        //找到加入这个注解@EnableConfigurationProperties里面的value值，其实就是类class
        private List<Class<?>> getTypes(AnnotationMetadata metadata) {
            MultiValueMap<String, Object> attributes = metadata
                    .getAllAnnotationAttributes(
                            EnableConfigurationProperties.class.getName(), false);
            return collectClasses((attributes != null) ? attributes.get("value")
                    : Collections.emptyList());
        }

        private List<Class<?>> collectClasses(List<?> values) {
            return values.stream().flatMap((value) -> Arrays.stream((Object[]) value))
                    .map((o) -> (Class<?>) o).filter((type) -> void.class != type)
                    .collect(Collectors.toList());
        }
      //注册方法：根据找到的类名name和type，将加入注解@ConfigurationProperties的类加入spring容器里面
        private void register(BeanDefinitionRegistry registry,
                ConfigurableListableBeanFactory beanFactory, Class<?> type) {
            String name = getName(type);
            if (!containsBeanDefinition(beanFactory, name)) {
                registerBeanDefinition(registry, name, type);
            }
        }
    //找到加入注解@ConfigurationProperties的类的名称，加入一定格式的拼接
        private String getName(Class<?> type) {
            ConfigurationProperties annotation = AnnotationUtils.findAnnotation(type,
                    ConfigurationProperties.class);
            String prefix = (annotation != null) ? annotation.prefix() : "";
            return (StringUtils.hasText(prefix) ? prefix + "-" + type.getName()
                    : type.getName());
        }

        private boolean containsBeanDefinition(
                ConfigurableListableBeanFactory beanFactory, String name) {
            if (beanFactory.containsBeanDefinition(name)) {
                return true;
            }
            BeanFactory parent = beanFactory.getParentBeanFactory();
            if (parent instanceof ConfigurableListableBeanFactory) {
                return containsBeanDefinition((ConfigurableListableBeanFactory) parent,
                        name);
            }
            return false;
        }

        private void registerBeanDefinition(BeanDefinitionRegistry registry, String name,
                Class<?> type) {
            assertHasAnnotation(type);
            GenericBeanDefinition definition = new GenericBeanDefinition();
            definition.setBeanClass(type);
            registry.registerBeanDefinition(name, definition);
        }

        private void assertHasAnnotation(Class<?> type) {
            Assert.notNull(
                    AnnotationUtils.findAnnotation(type, ConfigurationProperties.class),
                    () -> "No " + ConfigurationProperties.class.getSimpleName()
                            + " annotation found on  '" + type.getName() + "'.");
        }

    }

结语

另外还有这个类：ConfigurationPropertiesBindingPostProcessorRegistrar，刚刚没有分析，看了下源码，其实他做的事情就是将配置文件当中的属性值赋予到加了@ConfigurationProperties的注解的类的属性上，具体就不分析了，有兴趣自己可以阅读，入口知道了，就简单了