前言：
我们在使用SpringBoot开发相关项目时，经常会使用到@ConfigurationProperties注解，这个注解配合application.properties（或yml文件）使用，可以快速的为我们的Bean的成员变量进行赋值，常规用法如下：
// 创建bean
@ConfigurationProperties(prefix="person")
@Data
public class Person {
private int id;
private String name;
private String addr;
}
// 在application.properties文件中添加
person.name=jack
person.id=11
person.addr=china这样就可以为Person实例自动注入属性
那么SpringBoot是如何为bean实例注入属性值的呢？
1.关于自动注入属性值的功能猜想
SpringBoot会为每一个在配置文件中有匹配项的bean注入属性值，根据之前的 BeanPostProcessor功能可知，我们的Spring容器中应该有一个Bean实现BeanPostProcessor接口，然后在返回每一个bean之前，获取配置文件内容，找到对应属性，然后进行赋值操作
SpringBoot中确实有这么一个Bean，ConfigurationPropertiesBindingPostProcessor，实现了相关赋值功能，那么这个Bean是如何被注入到容器中的呢，下面亲随笔者一层层分析查找
2.SpringBoot项目启动之@SpringBootApplication
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {重要的注解就是@EnableAutoConfiguration，下面看下这个注解
@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {主要功能就是把EnableAutoConfigurationImportSelector注入到容器中，下面看下这个类
public class EnableAutoConfigurationImportSelector
extends AutoConfigurationImportSelector {
@Override
protected boolean isEnabled(AnnotationMetadata metadata) {
if (getClass().equals(EnableAutoConfigurationImportSelector.class)) {
return getEnvironment().getProperty(
EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
true);
}
return true;
}
}
// AutoConfigurationImportSelector
public class AutoConfigurationImportSelector
implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered {}AutoConfigurationImportSelector这个类实现了DeferredImportSelector接口，可知这个类应该在其selectImport方法中，应该获取了一个bean的列表，然后被这个bean列表的所有bean会被注入到容器中，下面重点分析下这个selectImport方法
对于@Import ImportSelector等接口注入bean到容器的方式不太明白的同学可以参考下这篇文章 Spring注入Bean的几种方式 @Import,ImportSelector,ImportBeanDefinitionRegistrar的详解_恐龙弟旺仔的博客-CSDN博客_@import @importselector
3.AutoConfigurationImportSelector.selectImport(AnnotationMetadata annotationMetadata)
该方法源码如下：
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
try {
// 1.获取META-INF/spring-autoconfigure-metadata.properties 路径下所有bean URL
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
AnnotationAttributes attributes = getAttributes(annotationMetadata);
// 2.重要操作在这里
List configurations = getCandidateConfigurations(annotationMetadata,
attributes);
configurations = removeDuplicates(configurations);
configurations = sort(configurations, autoConfigurationMetadata);
Set exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = filter(configurations, autoConfigurationMetadata);
fireAutoConfigurationImportEvents(configurations, exclusions);
// 3.经过中间的一系列操作返回configurations
return configurations.toArray(new String[configurations.size()]);
}
catch (IOException ex) {
throw new IllegalStateException(ex);
}
}
// getCandidateConfigurations(annotationMetadata,attributes)
protected List getCandidateConfigurations(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
// 主要就是为了获取META-INF/spring.factories 文件下EnableAutoConfiguration对应的value值，并返回该值
List configurations = SpringFactoriesLoader.loadFactoryNames(
getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
Assert.notEmpty(configurations,
"No auto configuration classes found in META-INF/spring.factories. If you "
+ "are using a custom packaging, make sure that file is correct.");
return configurations;
}通过以上分析可知：AutoConfigurationImportSelector类的主要功能就是为了加载META-INF/spring.factories文件下EnableAutoConfiguration对应的value值，这些value值对应一个个bean，Spring会将这些bean加载到容器中
我们看下是哪些bean（在spring-boot-autoconfigure-xxx.jar的META-INF/spring.factories中）
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
...
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
...
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
...
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration以上展示了部分类，可知，这些类都会被注入到Spring中，下面我们重点关注ConfigurationPropertiesAutoConfiguration类
4.ConfigurationPropertiesAutoConfiguration
1）ConfigurationPropertiesAutoConfiguration源码如下：
@Configuration
@EnableConfigurationProperties
public class ConfigurationPropertiesAutoConfiguration {
}
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import({EnableConfigurationPropertiesImportSelector.class})
public @interface EnableConfigurationProperties {
Class[] value() default {};
}该类没有其他方法，由其注解@EnableConfigurationProperties可知，整个类主要是为了注入EnableConfigurationPropertiesImportSelector这个类，
2）EnableConfigurationPropertiesImportSelector源码如下：
class EnableConfigurationPropertiesImportSelector implements ImportSelector {
public String[] selectImports(AnnotationMetadata metadata) {
MultiValueMap attributes = metadata.getAllAnnotationAttributes(EnableConfigurationProperties.class.getName(),
false);
Object[] type = attributes == null ? null : (Object[]) ((Object[]) attributes.getFirst("value"));
return type != null && type.length != 0
? new String[]{ConfigurationPropertiesBeanRegistrar.class.getName(),
ConfigurationPropertiesBindingPostProcessorRegistrar.class.getName()}
: new String[]{ConfigurationPropertiesBindingPostProcessorRegistrar.class.getName()};
}
}可知其会返回ConfigurationPropertiesBindingPostProcessorRegistrar类到容器中
3）ConfigurationPropertiesBindingPostProcessorRegistrar源码如下
public class ConfigurationPropertiesBindingPostProcessorRegistrar implements ImportBeanDefinitionRegistrar {
public static final String BINDER_BEAN_NAME = ConfigurationPropertiesBindingPostProcessor.class.getName();
private static final String METADATA_BEAN_NAME;
// 主要的注入方法
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
if (!registry.containsBeanDefinition(BINDER_BEAN_NAME)) {
BeanDefinitionBuilder meta = BeanDefinitionBuilder
.genericBeanDefinition(ConfigurationBeanFactoryMetaData.class);
BeanDefinitionBuilder bean = BeanDefinitionBuilder
.genericBeanDefinition(ConfigurationPropertiesBindingPostProcessor.class);
bean.addPropertyReference("beanMetaDataStore", METADATA_BEAN_NAME);
// 真正的注入功能就是这句话，将ConfigurationPropertiesBindingPostProcessor类注册进来
registry.registerBeanDefinition(BINDER_BEAN_NAME, bean.getBeanDefinition());
registry.registerBeanDefinition(METADATA_BEAN_NAME, meta.getBeanDefinition());
}
}
static {
METADATA_BEAN_NAME = BINDER_BEAN_NAME + ".store";
}
}总结：通过以上的一层层分析，我们终于到了ConfigurationPropertiesBindingPostProcessor类，也成功将该类注入到容器中，下面我们就分析下ConfigurationPropertiesBindingPostProcessor是如何将参数注入到bean中
具体分析过程可参考：spring boot 源码解析13-@ConfigurationProperties是如何生效的_一个努力的码农的博客-CSDN博客_configurationproperties源码 ，笔者不再分析
