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;
    }
}

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;
    }
}

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

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";
    }
}

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)

@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 {};

}

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;
    }

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() + "'.");
        }

    }

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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