conditional注解是Springboot starter的基石，自動裝配的時候會根據(jù)條件確定是否需要注入這個類。

含義：基于條件的注解。

作用：根據(jù)是否滿足某個特定條件來決定是否創(chuàng)建某個特定的Bean。

意義：Springboot實現(xiàn)自動配置的關(guān)鍵基礎(chǔ)能力。

@Conditional相關(guān)注解

@ConditionalOnBean:僅僅在當(dāng)前上下文中存在某個對象時，才會實例化一個Bean
@ConditionalOnClass:某個class位于類路徑上才會實例化一個Bean
@ConditionalOnExpression:當(dāng)表達(dá)式為true的時候，才會實例化一個Bean
@ConditionalOnMissingBean:僅僅在當(dāng)前上下文中不存在某個對象時，才會實例化一個Bean
@ConditionalOnMissingClass:某個class不位于類路徑上才會實例化一個Bean
@ConditionalOnNotWebApplication:不是web應(yīng)用

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配置文件有這個配置

spring.data.mongodb.uri=mongodb://paopaoedu:paopaoedu@localhost:27017/paopaoedu

判斷有這個配置才注入這個類?

package com.paopaoedu.springboot.condition;import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.stereotype.Component;@Component
@ConditionalOnProperty("spring.data.mongodb.uri")
public class ConditionalTest {
}

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測試文件

package com.paopaoedu.springboot;import com.paopaoedu.springboot.condition.ConditionalTest;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.BeansException;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.test.context.junit4.SpringRunner;@RunWith(SpringRunner.class)
@SpringBootTest(classes = {SpringBootLearningApplication.class})
//不用類似new ClassPathXmlApplicationContext()的方式，從已有的spring上下文取得已實例化的bean。通過ApplicationContextAware接口進(jìn)行實現(xiàn)。
public class SpringBootLearningApplicationTests implements ApplicationContextAware {private ApplicationContext applicationContext;@Overridepublic void setApplicationContext(ApplicationContext context) throws BeansException {applicationContext = context;}@Testpublic void testA() {System.out.println("testA>>>"+applicationContext.getBean(ConditionalTest.class));}}

關(guān)于ApplicationContextAware使用理解參考https://www.jianshu.com/p/4c0723615a52

可以看出這個ConditionalTest被注入了：

如果你去掉這個配置測試用例就會報錯找不到這個bean：

自定義conditional注解實現(xiàn)

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自定義注解引入condition接口實現(xiàn)類

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(MyCondition.class)
public @interface MyConditionAnnotation {String[] value() default {};
}

實現(xiàn)一個自定義注解

實現(xiàn)Condition接口重寫matches方法，符合條件返回true

public class MyCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {String[] properties = (String[])metadata.getAnnotationAttributes("com.paopaoedu.springboot.condition.MyConditionAnnotation").get("value");for (String property : properties) {if (StringUtils.isEmpty(context.getEnvironment().getProperty(property))) {return false;}}return true;}
}

引入conditional注解

@Component
@MyConditionAnnotation({"spring.redis.master.host", "spring.redis.follow.host"})
public class ConditionalTest2 {
}

配置文件

# Redis服務(wù)器地址
spring.redis.master.host=r-xxx1.redis.rds.aliyuncs.com
# Redis服務(wù)器連接端口
spring.redis.master.port=6379
# Redis服務(wù)器連接密碼（默認(rèn)為空）
spring.redis.master.password=# Redis服務(wù)器地址
spring.redis.follow.host=r-xxx2.redis.rds.aliyuncs.com
# Redis服務(wù)器連接端口
spring.redis.follow.port=6379
# Redis服務(wù)器連接密碼（默認(rèn)為空）
spring.redis.follow.password=

修改測試用例

@RunWith(SpringRunner.class)
@SpringBootTest(classes = {SpringBootLearningApplication.class})
//不用類似new ClassPathXmlApplicationContext()的方式，從已有的spring上下文取得已實例化的bean。通過ApplicationContextAware接口進(jìn)行實現(xiàn)。
public class SpringBootLearningApplicationTests implements ApplicationContextAware {private ApplicationContext applicationContext;@Overridepublic void setApplicationContext(ApplicationContext context) throws BeansException {applicationContext = context;}@Testpublic void testA() {System.out.println("testA>>>"+applicationContext.getBean(ConditionalTest.class));System.out.println("testA>>>"+applicationContext.getBean(ConditionalTest2.class));}}

輸出

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重要的調(diào)試斷點ClassPathScanningCandidateComponentProvider.scanCandidateComponents()

	private Set<BeanDefinition> scanCandidateComponents(String basePackage) {Set<BeanDefinition> candidates = new LinkedHashSet<>();try {String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +resolveBasePackage(basePackage) + '/' + this.resourcePattern;Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);boolean traceEnabled = logger.isTraceEnabled();boolean debugEnabled = logger.isDebugEnabled();for (Resource resource : resources) {if (traceEnabled) {logger.trace("Scanning " + resource);}if (resource.isReadable()) {try {MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);if (isCandidateComponent(metadataReader)) {ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);sbd.setResource(resource);sbd.setSource(resource);if (isCandidateComponent(sbd)) {if (debugEnabled) {logger.debug("Identified candidate component class: " + resource);}candidates.add(sbd);}else {if (debugEnabled) {logger.debug("Ignored because not a concrete top-level class: " + resource);}}}else {if (traceEnabled) {logger.trace("Ignored because not matching any filter: " + resource);}}}catch (Throwable ex) {throw new BeanDefinitionStoreException("Failed to read candidate component class: " + resource, ex);}}else {if (traceEnabled) {logger.trace("Ignored because not readable: " + resource);}}}}catch (IOException ex) {throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);}return candidates;}

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