springboot(四)——@EnableConfigurationProperties是如何起做用的你知道嗎

前言

用springboot開發的過程當中，咱們會用到@ConfigurationProperties註解，主要是用來把properties或者yml配置文件轉化爲bean來使用的，而@EnableConfigurationProperties註解的做用是@ConfigurationProperties註解生效。 若是隻配置@ConfigurationProperties註解，在IOC容器中是獲取不到properties配置文件轉化的bean的，固然在@ConfigurationProperties加入註解的類上加@Component也可使交於springboot管理。

舉個栗子

第一步:建立一個類TestConfigurationProperties

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}
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注意:得加上set和get方法
第二步:建立TestAutoConfiguration類

@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;
    }
}
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注意:得建立一個有參構造方法
第三步:配置文件加入屬性

properties.name=test
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第四步:跑一下，打印出User這個類

@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";
    }
}
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控制檯輸出：

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)
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@EnableConfigurationProperties是怎麼加載的

經過查看@EnableConfigurationProperties的註解：

@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 {};

}
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經過分析自動配置能夠知道，確定是這個類EnableConfigurationPropertiesImportSelector起的做用：

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

	@Override
	public String[] selectImports(AnnotationMetadata metadata) {
		return IMPORTS;
	}
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selectImports方法返回了這兩個類：ConfigurationPropertiesBeanRegistrar和ConfigurationPropertiesBindingPostProcessorRegistrar，是什麼時候加載的，咱們只須要看這個類ConfigurationPropertiesBeanRegistrar便可：

public static class ConfigurationPropertiesBeanRegistrar implements ImportBeanDefinitionRegistrar {

		@Override
		public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
			getTypes(metadata).forEach((type) -> register(registry,
					(ConfigurableListableBeanFactory) registry, type));
		}
        //找到加入這個註解@EnableConfigurationProperties裏面的value值，其實就是類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());
		}
      //註冊方法：根據找到的類名name和type，將加入註解@ConfigurationProperties的類加入spring容器裏面
		private void register(BeanDefinitionRegistry registry, ConfigurableListableBeanFactory beanFactory, Class<?> type) {
			String name = getName(type);
			if (!containsBeanDefinition(beanFactory, name)) {
				registerBeanDefinition(registry, name, type);
			}
		}
    //找到加入註解@ConfigurationProperties的類的名稱，加入必定格式的拼接
		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() + "'.");
		}

	}
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結語

另外還有這個類：ConfigurationPropertiesBindingPostProcessorRegistrar，剛剛沒有分析，看了下源碼，其實他作的事情就是將配置文件當中的屬性值賦予到加了@ConfigurationProperties的註解的類的屬性上，具體就不分析了，有興趣本身能夠閱讀，入口知道了，就簡單了

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