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conditional注解是Springboot starter的基石，自动装配的时候会根据条件确定是否需要注入这个类。

含义：基于条件的注解。

作用：根据是否满足某个特定条件来决定是否创建某个特定的Bean。

意义：Springboot实现自动配置的关键基础能力。

@ConditionalOnBean:仅仅在当前上下文中存在某个对象时，才会实例化一个Bean

@ConditionalOnClass:某个class位于类路径上才会实例化一个Bean @ConditionalOnExpression:当表达式为true的时候，才会实例化一个Bean @ConditionalOnMissingBean:仅仅在当前上下文中不存在某个对象时，才会实例化一个Bean @ConditionalOnMissingClass:某个class不位于类路径上才会实例化一个Bean @ConditionalOnNotWebApplication:不是web应用

 

配置文件有这个配置

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

判断有这个配置才注入这个类 

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 {}

 

测试文件

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})//不用类似new ClassPathXmlApplicationContext()的方式，从已有的spring上下文取得已实例化的bean。通过ApplicationContextAware接口进行实现。public class SpringBootLearningApplicationTests implements ApplicationContextAware {    private ApplicationContext applicationContext;    @Override    public void setApplicationContext(ApplicationContext context) throws BeansException {        applicationContext = context;    }    @Test    public void testA() {        System.out.println("testA>>>"+applicationContext.getBean(ConditionalTest.class));    }}

关于ApplicationContextAware使用理解参考

可以看出这个ConditionalTest被注入了：

如果你去掉这个配置测试用例就会报错找不到这个bean：

自定义conditional注解实现

 

自定义注解引入condition接口实现类

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

实现一个自定义注解

实现Condition接口重写matches方法，符合条件返回true

public class MyCondition implements Condition {    @Override    public 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服务器地址spring.redis.master.host=r-xxx1.redis.rds.aliyuncs.com# Redis服务器连接端口spring.redis.master.port=6379# Redis服务器连接密码（默认为空）spring.redis.master.password=# Redis服务器地址spring.redis.follow.host=r-xxx2.redis.rds.aliyuncs.com# Redis服务器连接端口spring.redis.follow.port=6379# Redis服务器连接密码（默认为空）spring.redis.follow.password=

修改测试用例

@RunWith(SpringRunner.class)@SpringBootTest(classes = {SpringBootLearningApplication.class})//不用类似new ClassPathXmlApplicationContext()的方式，从已有的spring上下文取得已实例化的bean。通过ApplicationContextAware接口进行实现。public class SpringBootLearningApplicationTests implements ApplicationContextAware {    private ApplicationContext applicationContext;    @Override    public void setApplicationContext(ApplicationContext context) throws BeansException {        applicationContext = context;    }    @Test    public void testA() {        System.out.println("testA>>>"+applicationContext.getBean(ConditionalTest.class));        System.out.println("testA>>>"+applicationContext.getBean(ConditionalTest2.class));    }}

输出

 

重要的调试断点ClassPathScanningCandidateComponentProvider.scanCandidateComponents()

private Set
   
     scanCandidateComponents(String basePackage) {		Set
    
      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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