SpringBoot条件注解@Conditional详细解析_开发_开发者

一、条件注解@Conditional

@Conditional是Spring4.0提供的一个用于条件装配的注解，其定义了一个Condition的数组，只有当数组所有的条件都满足的时候，组件才会被导入容器。

/**
 * Indicates that a component is only eligible for registration when all
 * {@linkplain #value specified conditions} match.
 *
 * <p>A <em>condition</em> is any state that can be determined programmatically
 * before the bean definition is due to be registered (see {@link Condition} for details).
 *
 * <p>The {@code @Conditional} annotation may be used in any of the following ways:
 * <ul>
 * <li>as a type-level annotation on any class directly or indirectly annotated with
 * {@code @Component}, including {@link Configuration @Configuration} classes</li>
 * <li>as a meta-annotation, for the purpose of composing custom stereotype
 * annotations</li>
 * <li>as a method-level annotation on any {@link Bean @Bean} method</li>
 * </ul>
 *
 * <p>If a {@code @Configuration} class is marked with {@code @Conditional},
 * all of the {@code @Bean} methods, {@link Import @Import} annotations, and
 * {@link ComponentScan @ComponentScan} annotations associated with that
 * class will be subject to the conditions.
 *
 * <p><strong>NOTE</strong>: Inheritance of {@code @Conditional} annotations
 * is not supported; any conditions from superclasses or from overridden
 * methods will not be considered. In order to enforce these semantics,
 * {@code @Conditional} itself is not declared as
 * {@link Java.lang.annotation.Inherited @Inherited}; furthermore, any
 * custom <em>composed annotation</em> that is meta-annotated with
 * {@code @Conditional} must not be declared as {@code @Inherited}.
 *
 * @author Phillip Webb
 * @author Sam Brannen
 * @since 4.0
 * @see Condition
 */
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
	/**
	 * All {@link Condition}s that must {@linkplain Condition#matches match}
	 * in order for the component to be registered.
	 */
	Class<? extends Condition>[] value();
}

@Conditional注解可以有两种使用方法：

类型级别，任意直接或者间接标注了@Conponent注解的类或者注解，比如@Configuration或者@Profile
方法级别，任意标注了@Bean注解的方法

如果一个@Configuration类标注了@Conditional，那么这个类所有的@Bean方法，@ComponentScan和@Import的结果都受@Conditional注解的条件约束。特别要注意的是：@Conditional是不支持继承的，任何父类的条件注解或者方法继承的条件注解都不会生效。为了强化这些语义，@Conditional本身并没有标注@Inherited。另外，任何使用了@Conditional注解的组合注解都不能声明为@Inherited。

二、条件判断接口Condition

@Conditional注解依赖于Condition接口，该接口提供真正的条件判断逻辑。

/**
 * A single {@code condition} that must be {@linkplain #matches matched} in order
 * for a component to be registered.
 *
 * <p>Conditions are checked immediately before the bean-definition is due to be
 * registered and are free to veto registration based on any criteria that can
 * be determined at that point.
 *
 * <p>Conditions must follow the same restrictions as {@link BeanFactoryPostProcessor}
 * and take care to never interact with bean instances. For more fine-grained control
 * of conditions that interact with {@code @Configuration} beans consider the
 * {@link ConfigurationCondition} interface.
 *
 * @author Phillip Webb
 * @since 4.0
 * @see ConfigurationCondition
 * @see Conditional
 * @see ConditionContext
 */
public interface Condition {
	/**
	 * Determine if the condition matches.
	 * @param context the condition context
	 * @param metadata metadata of the {@link org.springframework.core.type.AnnotationMetadata class}
	 * or {@link org.springframework.core.type.MethodMetadata method} being checked.
	 * @return {@code true} if the condition matches and the component can be registered
	 * or {@code false} to veto registration.
	 */
	boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

Condition接口传递两个参数ConditionContext和AnnotatedTypeMetadata，在Condition实现类中可以直接使用这两个参数，获取环境、容器、类等相关信息。

1. ConditionContext

/**
 * Context information for use by {@link Condition}s.
 *
 * @author Phillip Webb
 * @since 4.0
 */
public interface ConditionContext {
	/**
	 * Return the {@link BeanDefinitionRegistry} that will hold the bean definition
	 * should the condition match or {@code null} if the registry is not available.
	 * @return the registry or {@code null}
	 */
	BeanDefinitionRegistry getRegistry();
	/**
	 * Return the {@link ConfigurableListableBeanFactory} that will hold the bean
	 * definition should the condition match or {@code null} if the bean factory
	 * is not available.
	 * @return the bean factory or {@code null}
	 */
	ConfigurableListableBeanFactory getBeanFactory();
	/**
	 * Return the {@link Environment} for which the current application is running
	 * or {@code null} if no environment is available.
	 * @return the environment or {@code null}
	 */
	Environment getEnvironment();
	/**
	 * Return the {@link ResourceLoader} currently being used or {@code null}
	 * if the resource loader cannot be obtained.
	 * @return a resource loader or {@code null}
	 */
	ResourceLoader getResourceLoader();
	/**
	 * Return the {@link ClassLoader} that should be used to load additional
	 * classes or {@code null} if the default classloader should be used.
	 * @return the class loader or {@code null}
	 */
	ClassLoader getClassLoader();
}

CondtitionContext可以获取到BeanDefinitionRegistry、ConfigurableListableBeanFactory、Environment、ResourceLoader、ClassLoader这些环境相关的信息。

2. AnnotatedTypeMetadata

public interface AnnotatedTypeMetadata {

    // 根据“全类名”判断是否被指定 直接注解或元注解 标注
    booleandroidan isAnnotated(String annotationName);
    
    // 根据”全类名“获取所有注解属性（包括元注解）
    @Nullable
    Map<String, Object> getAnnotationAttributes(String annotationName);
    
    @Nullable
    // 同上，但是第二个参数传 true 时会把属性中对应值为 Class 的值
    // 转为 字符串，避免需要预先加载对应 Class
    Map<String, Object> getAnnotationAttributes(String annotationName, boolean classValuesAsString);
    
    @Nullable
    // 同上，MultiValueMap 是一个 key 可以对应多个 value 的变种 map
    MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName);
    @Nullable
    MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName, boolean classValuesAsString);

}

顶层接口，可被注解标注类型（类、方法）元数据的抽象，提供了两个核心方法：

根据全类名判断是否被指定注解标注
根据全类名返回指定注解的属性集合（包括元注解）

三、@Conditional如何被解析，Condition方法何时调用？

@Conditional和Condition的相关逻辑是在类ConditionEvaLuator#中实现的。

class ConditionEvaluator {
	private final ConditionContextImpl context;
	/**
	 * Create a new {@link ConditionEvaluator} instance.
	 */
	public ConditionEvaluator(BeanDefinitionRegistry registry, Environment environment, ResourceLoader resourceLoader) {
		this.context = new ConditionContextImpl(registry, environment, resourceLoader);
	}
	/**
	 * Determine if an item should be skipped based on {@code @Conditional} annotations.
	 * The {@link ConfigurationPhase} will be deduced from the type of item (i.e. a
	 * {@code @Configuration} class will be {@link ConfigurationPhase#PARSE_CONFIGURATION})
	 * @param metadata the meta data
	 * @return if the item should be skipped
	 */
	public boolean shouldSkip(AnnotatedTypeMetadata metadata) {
		return shouldSkip(metadata, null);
	}
	/**
	 * Determine if an item should be skipped based on {@code @Conditional} annotations.
	 * @param metadata the meta data
	 * @param phase the phase of the call
	 * @return if the item should be skipped
	 */
	public boolean shouldSkip(AnnotatedTypeMetadata metadata, ConfigurationPhase phase) {
		if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
			return false;
		}
		if (phase == null) {
			if (metadata instanceof AnnotationMetadata &&
					ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
				return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
			}
			return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
		}
		List<Condition> conditions = new ArrayList<Condition>();
		for (String[] conditionClasses : getConditionClasses(metadata)) {
			for (String conditionClass : conditionClasses) {
				Condition condition = getCondition(conditionClass, this.context.getClassLoader());
				conditions.add(condition);
			}
		}
		AnnotationAwareOrderComparator.sort(conditions);
		for (Condition condition : conditions) {
			ConfigurationPhase requiredPhase = null;
			if (condition instanceof ConfigurationCondition) {
				requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
			}
			if (requiredPhase == null || requiredPhase == phase) {
				if (!condition.matches(this.context, metadata)) {
					return true;
				}
			}
		}
		return false;
	}
	@SuppressWarnings("unchecked")
	private List<String[]> getConditionClasses(AnnotatedTypeMetadata metadata) {
		MultiValueMap<String, Object> attributes = metadata.getAllAnnotationAttributes(Conditional.class.getName(), true);
		Object values = (attributes != null ? attributes.get("value") : null);
		return (List<String[]>) (values != null ? values : Collections.emptyList());
	}
	private Condition getCondition(String conditionClassName, ClassLoader classloader) {
		Class<?> conditionClass = ClassUtils.resolveClassName(conditionClassName, classloader);
		return (Condition) BeanUtils.instantiateClass(conditionClass);
	}
	/**
	 * Implementation of a {@link ConditionContext}.
	 */
	private static class ConditionContextImpl implements ConditionContext {
		private final BeanDefinitionRegistry registry;
		private final ConfigurableListableBeanFactory beanFactory;
		private final Environment environment;
		private final ResourceLoader resourceLoader;
		public ConditionContextImpl(BeanDefinitionRegistry registry, Environment environment, ResourceLoader resourceLoader) {
			this.registry = registry;
			this.beanFactory = deduceBeanFactory(registry);
			this.environment = (environment != null ? environment : deduceEnvironment(registry));
			this.resourceLoader = (resourceLoader != null ? resourceLoader : deduceResourceLoader(registry));
		}
		private ConfigurableListableBeanFactory deduceBeanFactory(BeanDefinitionRegistry source) {
			if (source instanceof ConfigurableListableBeanFactory) {
				return (ConfigurableListableBeanFactory) source;
			}
			if (source instanceof ConfigurableApplicationContext) {
				return (((ConfigurableApplicationContext) source).getBeanFactory());
			}
			return null;
		}
		private Environment deduceEnvironment(BeanDefinitionRegistry source) {
			if (source instanceof EnvironmentCapable) {
				return ((EnvironmentCapable) source).getEnvironment();
			}
			return null;
		}
		private ResourceLoader deduceResourceLoader(BeanDefinitionRegistry source) {
			if (source instanceof ResourceLoader) {
				return (ResourceLoader) source;
			}
			return null;
		}
		@Override
		public BeajavascriptnDefinitiohttp://www.devze.comnRegistry getRegistry() {
			return this.registry;
		}
		@Override
		public ConfigurableListableBeanFactory getBeanFactory() {
			return this.beanFactory;
		}
		@Override
		public Environment getEnvironment() {
			return this.environment;
		}
		@Override
		public ResourceLoader getResourceLoader() {
			return this.resourceLoader;
		}
		@Override
		public ClassLoader getClassLoader() {
			if (this.resourceLoader != null) {
				return this.resourceLoader.getClassLoader();
			}
			if (this.beanFactory != null) {
				return this.beanFactory.getBeanClassLoader();
			}
			return null;
		}
	}
}

而该类根据构造方法的调用点，可知以下几个类会使用到。

SpringBoot条件注解@Conditional详细解析

AnnotatedBeanDefinitionReader 注解标注时候
ClassPathScanningCandidateComponentProvider注解扫描时候
ConfigurationClassBeanDefinitionReader、ConfigurationClassParser(ConfigurationClassPostProcessor) 解析Chttp://www.devze.comonfiguration注解的过程中

四、典型应用 @Profile

@Profile就是典型地基于@Conditional的扩展，其条件逻辑封装在ProfileCondition中

class ProfileCondition implements Condition {
	@Override
	public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
		if (context.getEnvironment() != null) {
			MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName());
			if (attrs != null) {
				for (Object value : attrs.get("value")) {
					if (context.getEnvironment().acceptsProfiles(((String[]) valuejs))) {
						return true;
					}
				}
				return false;
			}
		}
		return true;
	}
}

Springboot中的应用

SpringBoot条件注解@Conditional详细解析