Spring_总结_04_高级配置(二)_条件注解@Conditional
一、前言
本文承接上一节：Spring_总结_04_高级配置(一)之Profile
在上一节，我们了解到 Profile 为不同环境下使用不同的配置提供了支持，那么Profile到底是如何实现的呢？其实Profile正是通过条件注解来实现的。
条件注解的应用场景举例：
（1）希望一个或多个 bean 只有在应用的类路径下包含特定的库时才创建
（2）希望某个bean只有当另外某个特定bean也声明了之后才创建
（3）希望只有某个特定的环境变量设置之后，才会创建某个bean
上述场景能让我们联想到springboot的自动化配置、Profile以及配置文件等。
二、概述
1.定义
@Conditionnal 能根据特定条件来控制Bean的创建行为。
2.用法
@Conditional注解可以用到带有@Bean注解的方法上，如果给定的条件计算结果为true,就会创建这个bean，否则，忽略这个bean。
3.用处
可以用来进行一些自动化配置，如上述三个应用场景。
三、使用实例
1.创建Condition实现类
Condition接口如下，只有一个 matches 方法，用来判断是否满足条件。
@FunctionalInterface 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 the annotated component's registration */ boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata); }
Condition实现类 MagicExistsCondition
public class MagicExistsCondition implements Condition { /** * 1.判断是否满足条件 * @param context * @param metadata * @return */ @Override public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { Environment environment = context.getEnvironment(); /** * 检查环境中是否存在magic属性，若存在则满足条件 */ return environment.containsProperty("magic"); } }
2.使用@Conditional注解
如以下代码，在装配Bean的时候，使用@conditional注解，以便只有在满足条件地情况下才创建此bean
/** * 条件化地创建Bean * 若满足MagicExistsCondition的条件，则创建此Bean,否则忽略此bean. * @return */ @Conditional(MagicExistsCondition.class) @Bean public MagicBean magicBean(){ return new MagicBean(); }
四、Condition接口分析
Condition接口如下，只有一个 matches 方法，用来判断是否满足条件。
@FunctionalInterface 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 the annotated component's registration */ boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata); }
在设置条件时，可用 ConditionContext 和 AnnotatedTypeMetadata 来进行条件地设置。
1.ConditionContext
源码如下：
/** * Context information for use by {@link Condition}s. * * @author Phillip Webb * @author Juergen Hoeller * @since 4.0 */ public interface ConditionContext { /** * Return the {@link BeanDefinitionRegistry} that will hold the bean definition * should the condition match. * @throws IllegalStateException if no registry is available (which is unusual: * only the case with a plain {@link ClassPathScanningCandidateComponentProvider}) */ 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 (or not downcastable to {@code ConfigurableListableBeanFactory}). */ @Nullable ConfigurableListableBeanFactory getBeanFactory(); /** * Return the {@link Environment} for which the current application is running. */ Environment getEnvironment(); /** * Return the {@link ResourceLoader} currently being used. */ ResourceLoader getResourceLoader(); /** * Return the {@link ClassLoader} that should be used to load additional classes * (only {@code null} if even the system ClassLoader isn't accessible). * @see org.springframework.util.ClassUtils#forName(String, ClassLoader) */ @Nullable ClassLoader getClassLoader(); }
作用如下：
| 1 |
getRegistry |
利用返回的 BeanDefinitionRegistry 来检查bean定义 |
| 2 |
getBeanFactory |
利用返回的 ConfigurableListableBeanFactory来检查bean是否存在，甚至探查bean的属性 |
| 3 |
getEnvironment |
利用返回的 Environment 检查环境变量是否存在以及它的值是什么 |
| 4 |
getResourceLoader |
读取并探查返回的 ResourceLoader 所加载的资源 |
| 5 |
getClassLoader |
借助返回的ClassLoader加载并检查类是否存在 |
2.AnnotatedTypeMetadata
源码如下：
/** * Defines access to the annotations of a specific type ({@link AnnotationMetadata class} * or {@link MethodMetadata method}), in a form that does not necessarily require the * class-loading. * * @author Juergen Hoeller * @author Mark Fisher * @author Mark Pollack * @author Chris Beams * @author Phillip Webb * @author Sam Brannen * @since 4.0 * @see AnnotationMetadata * @see MethodMetadata */ public interface AnnotatedTypeMetadata { /** * Determine whether the underlying element has an annotation or meta-annotation * of the given type defined. * <p>If this method returns {@code true}, then * {@link #getAnnotationAttributes} will return a non-null Map. * @param annotationName the fully qualified class name of the annotation * type to look for * @return whether a matching annotation is defined */ boolean isAnnotated(String annotationName); /** * Retrieve the attributes of the annotation of the given type, if any (i.e. if * defined on the underlying element, as direct annotation or meta-annotation), * also taking attribute overrides on composed annotations into account. * @param annotationName the fully qualified class name of the annotation * type to look for * @return a Map of attributes, with the attribute name as key (e.g. "value") * and the defined attribute value as Map value. This return value will be * {@code null} if no matching annotation is defined. */ @Nullable Map<String, Object> getAnnotationAttributes(String annotationName); /** * Retrieve the attributes of the annotation of the given type, if any (i.e. if * defined on the underlying element, as direct annotation or meta-annotation), * also taking attribute overrides on composed annotations into account. * @param annotationName the fully qualified class name of the annotation * type to look for * @param classValuesAsString whether to convert class references to String * class names for exposure as values in the returned Map, instead of Class * references which might potentially have to be loaded first * @return a Map of attributes, with the attribute name as key (e.g. "value") * and the defined attribute value as Map value. This return value will be * {@code null} if no matching annotation is defined. */ @Nullable Map<String, Object> getAnnotationAttributes(String annotationName, boolean classValuesAsString); /** * Retrieve all attributes of all annotations of the given type, if any (i.e. if * defined on the underlying element, as direct annotation or meta-annotation). * Note that this variant does <i>not</i> take attribute overrides into account. * @param annotationName the fully qualified class name of the annotation * type to look for * @return a MultiMap of attributes, with the attribute name as key (e.g. "value") * and a list of the defined attribute values as Map value. This return value will * be {@code null} if no matching annotation is defined. * @see #getAllAnnotationAttributes(String, boolean) */ @Nullable MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName); /** * Retrieve all attributes of all annotations of the given type, if any (i.e. if * defined on the underlying element, as direct annotation or meta-annotation). * Note that this variant does <i>not</i> take attribute overrides into account. * @param annotationName the fully qualified class name of the annotation * type to look for * @param classValuesAsString whether to convert class references to String * @return a MultiMap of attributes, with the attribute name as key (e.g. "value") * and a list of the defined attribute values as Map value. This return value will * be {@code null} if no matching annotation is defined. * @see #getAllAnnotationAttributes(String) */ @Nullable MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName, boolean classValuesAsString); }
通过 AnnotatedTypeMetadata 可以检查 @Bean 注解的方法上还有什么其他注解。
五、@Profile注解的实现原理
@Profile注解是基于@Conditional 和 condition 实现的
1.Profile注解类
@Target({ElementType.TYPE, ElementType.METHOD}) @Retention(RetentionPolicy.RUNTIME) @Documented @Conditional({ProfileCondition.class}) public @interface Profile { String[] value(); }
可以看到，@Profile 注解本身也使用了 @Conditional 注解，而Condition实现类为ProfileCondition
2.ProfileCondition
/** * {@link Condition} that matches based on the value of a {@link Profile @Profile} * annotation. * * @author Chris Beams * @author Phillip Webb * @author Juergen Hoeller * @since 4.0 */ class ProfileCondition implements Condition { @Override public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName()); if (attrs != null) { for (Object value : attrs.get("value")) { if (context.getEnvironment().acceptsProfiles((String[]) value)) { return true; } } return false; } return true; } }
可以看到， 此类主要是
（1）通过 AnnotatedTypeMetadata 获取到了 @Profile 的所有属性。
（2）然后检查value属性值，该属性包含了bean的profile名称
（3）根据 通过 ConditionContext 得到的Environment来检查 该profile是否处于激活状态【借助 acceptsProfiles方法】。
