Spring Boot自动装配原理源码分析

1.环境准备

使用IDEA Spring Initializr快速创建一个Spring Boot项目

添加一个Controller类

@RestController

public class HelloController {

    @RequestMapping("hello")

    public String hello() {

        return "hello";

    }

}

主配置类如下

@SpringBootApplication

public class SpringbootQuickstartApplication {

    public static void main(String[] args) {

        SpringApplication.run(SpringbootQuickstartApplication.class, args);

    }

}

2.注解分析

Spring Boot规定，项目的主配置类必须放在最外层包，也就是说，所有的类都必须放在主配置类的同级包或者子包里，这么做的用意是什么？我们点开@SpringBootApplication注解慢慢分析（下面代码中省略元注解）...

@SpringBootConfiguration

@EnableAutoConfiguration

@ComponentScan(excludeFilters = {

	@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),

	@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class)

})

public @interface SpringBootApplication {

@SpringBootApplication内部标注了三个注解：

@SpringBootConfiguration

进入源码中可以看见，@SpringBootConfiguration其实就是Spring中的@Configuration，用于标注配置类
```
@Configuration

public @interface SpringBootConfiguration {
```
@ComponentScan

这个注解也是Spring中的，它用来将指定包下需要装配的组件注册到容器中

@EnableAutoConfiguration

接下来才是今天的重头戏，Spring Boot自动配置的主角！

@AutoConfigurationPackage

@Import(AutoConfigurationImportSelector.class)

public @interface EnableAutoConfiguration {

3.自动装配的主角

进入@EnableAutoConfiguration源码你会发现这个注解中标注了两个注解@AutoConfigurationPackage和@Import

(1)、@AutoConfigurationPackage

点进该注解

@Import(AutoConfigurationPackages.Registrar.class)

public @interface AutoConfigurationPackage {

}

在点进Register，这是一个静态内部类

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

  @Override

  public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {

      register(registry, new PackageImport(metadata).getPackageName());

   }

  @Override

  public Set<Object> determineImports(AnnotationMetadata metadata) {

     return Collections.singleton(new PackageImport(metadata));

  }

}

我们在第一个方法处打一个断点debug一下，发现new PackageImport(metadata).getPackageName()的结果其实就是一个包名，这时我们很容易的可以想到，这个包就是Spring Boot主配置类所在的包

再看一眼metadata，果然，就是主配置类

因此，这个注解的作用就是将主配置类所在的包作为自动配置包进行管理

(2)、@Import(AutoConfigurationImportSelector.class)

@Import的作用就是导入一个类到IOC容器，我们先来看一下导入的这个类：自动配置导入选择器

源码里有一个方法selectImports，选择导入

@Override

public String[] selectImports(AnnotationMetadata annotationMetadata) {

   if (!isEnabled(annotationMetadata)) {

      return NO_IMPORTS;

   }

   AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader

         .loadMetadata(this.beanClassLoader);

   AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,

         annotationMetadata);

   return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

}

在点进getAutoConfigurationEntry，获取自动配置类

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,

      AnnotationMetadata annotationMetadata) {

   if (!isEnabled(annotationMetadata)) {

      return EMPTY_ENTRY;

   }

   AnnotationAttributes attributes = getAttributes(annotationMetadata);

   List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);

   configurations = removeDuplicates(configurations);

   Set<String> exclusions = getExclusions(annotationMetadata, attributes);

   checkExcludedClasses(configurations, exclusions);

   configurations.removeAll(exclusions);

   configurations = filter(configurations, autoConfigurationMetadata);

   fireAutoConfigurationImportEvents(configurations, exclusions);

   return new AutoConfigurationEntry(configurations, exclusions);

}

在getCandidateConfigurations方法处打一个断点，发现configurations的结果是所有的xxxAtuoConfiguration类，一共124个，请记住这个数字。

那么这些自动配置类是如何获取的呢，从哪里获取的呢？

我们继续点进getCandidateConfigurations，获取候选的配置

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {

	List<String> configurations =

        SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),

         getBeanClassLoader());

   Assert.notEmpty(configurations, "... ...");

   return configurations;

}

继续点，loadFactoryNames，加载工厂名，方法所在类中有一个常量FACTORIES_RESOURCE_LOCATION，看代码可以清晰的看到，这方法加载classpath下的所有jar包的META-INF/spring.factories文件，结果用一个HashMap存储

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {

		String factoryTypeName = factoryType.getName();

		return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());

	}

	private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {

		MultiValueMap<String, String> result = cache.get(classLoader);

		if (result != null) {

			return result;

		}

		try {

			Enumeration<URL> urls = (classLoader != null ?

					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :

					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

			result = new LinkedMultiValueMap<>();

			while (urls.hasMoreElements()) {

				URL url = urls.nextElement();

				UrlResource resource = new UrlResource(url);

				Properties properties = PropertiesLoaderUtils.loadProperties(resource);

				for (Map.Entry<?, ?> entry : properties.entrySet()) {

					String factoryTypeName = ((String) entry.getKey()).trim();

					for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {

						result.add(factoryTypeName, factoryImplementationName.trim());

					}

				}

			}

			cache.put(classLoader, result);

			return result;

		}

		catch (IOException ex) {

			throw new IllegalArgumentException("Unable to load factories from location [" +

					FACTORIES_RESOURCE_LOCATION + "]", ex);

		}

	}

打开spring-boot-autoconfigure-2.2.4.RELEASE.jar/META-INF/spring.factories，文件部分类容如下，你可以点进去看看第22~145行，确实是124个全类名

# 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.CloudServiceConnectorsAutoConfiguration,\

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.cassandra.CassandraDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration

... ...

4.类的加载时机

加载了这么多类我们又不一定全都用得到，设计师肯定会想办法让类在我们需要的时候才生效，我们随便点进一个类，可以看到一片飘红，因为我们并没有引入RabbitMQ相关依赖，再看一个注解ConditionalOnClass，意思就是存在某个指定的类才生效，类似的注解还有很多，都是@ConditionaOn xxx，在一定条件下类才会生效。

由于引入了web模块，WebMvcAutoConfiguration正常显示