spring boot 2.15 自動裝配源碼分析

昨天面試，面試官問了一個springboot 自動配置是怎麼實現的？結果回答的很模糊，今天專門整理一下

在搭建springboot項目時，我們可以簡化很多配置。那springboot是如何做到的呢？
首先先看下啓動類，啓動類只有一個@SpringBootApplication註解，

@SpringBootApplication
public class Application {
	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}
}

我們去看看@SpringBootApplication是怎麼實現的

@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 {

在這裏重點是@EnableAutoConfiguration，再去看看@EnableAutoConfiguration裏面是怎麼實現的

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

	Class<?>[] exclude() default {};

	String[] excludeName() default {};

}

EnableAutoConfiguration使用@Import註解將AutoConfigurationImportSelector導入並聲明爲一個Bean，跟蹤AutoConfigurationImportSelector的源碼，其中有這麼一個方法getCandidateConfigurations

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

這個方法利用SpringFactoriesLoader.loadFactoryNames去加載META-INF/spring.factories。
在這裏去看看loadFactoryNames如何實現

public final class SpringFactoriesLoader {

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

	private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);

	private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();

	private SpringFactoriesLoader() {
	}

	public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
		String factoryClassName = factoryClass.getName();
		return loadSpringFactories(classLoader).getOrDefault(factoryClassName, 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 factoryClassName = ((String) entry.getKey()).trim();
					for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
						result.add(factoryClassName, factoryName.trim());
					}
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}
}

META-INF/spring.factories文件中提供了Spring Boot的默認自動配置,可以隨便點開一個配置類：org.springframework.boot.autoconfigure.aop.AopAutoConfiguration

@Configuration
@ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class,AnnotatedElement.class })
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true",matchIfMissing = true)
public class AopAutoConfiguration {
		........
	}

這裏使用了上述註解實現自動裝配