问题

为什么我们在使用springboot的时候，只需要在maven中导入starter就能够使用呢？这里来分析一下

@SpringBootApplication注解

这个注解一般用在springboot程序的主启动类上，这个注解除去元注解，由下面几个注解构成

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

@SpringBootConfiguration是由下面两个注解构成

@Configuration
@Indexed

@EnableAutoConfiguration是由下面内容构成

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)

@AutoConfigurationPackage内容如下

@Import(AutoConfigurationPackages.Registrar.class)

所以@SpringBootApplication等价于下面内容

// @SpringBootConfiguration
@Configuration
@Indexed

// @EnableAutoConfiguration
@Import(AutoConfigurationPackages.Registrar.class)
@Import(AutoConfigurationImportSelector.class)

@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

可以发现实际就是导入了两个类，一个AutoConfigurationPackages.Registrar，一个AutoConfigurationImportSelector。

---

AutoConfigurationPackages.Registrar类

这个类的信息如下

	/**
	 * {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
	 * configuration.
	 */
	static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

		@Override
		public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
			register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
		}

		@Override
		public Set<Object> determineImports(AnnotationMetadata metadata) {
			return Collections.singleton(new PackageImports(metadata));
		}

	}

作用就是获取要扫描的包路径

---

AutoConfigurationImportSelector类

这个类的内容如下

里面实现了多个接口以Aware结尾的接口，其实就是完成某种资源的设置，Ordered就是用于指定bean加载顺序的，最重要的是DeferredImportSelector，这个接口继承了ImportSelector，ImportSelector就可以完成bean的注入工作

这个接口里面的group类里面就包含了要进行加载的bean信息

回到实现类，也就是AutoConfigurationImportSelector，去到process方法

@Override
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
   Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
         () -> String.format("Only %s implementations are supported, got %s",
               AutoConfigurationImportSelector.class.getSimpleName(),
               deferredImportSelector.getClass().getName()));
   AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
      .getAutoConfigurationEntry(annotationMetadata);
   this.autoConfigurationEntries.add(autoConfigurationEntry);
   for (String importClassName : autoConfigurationEntry.getConfigurations()) {
      this.entries.putIfAbsent(importClassName, annotationMetadata);
   }
}

将这个方法分解一下

   // 获取自动配置的类
   AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
      .getAutoConfigurationEntry(annotationMetadata);

   this.autoConfigurationEntries.add(autoConfigurationEntry);
   // 遍历自动配置的bean
   for (String importClassName : autoConfigurationEntry.getConfigurations()) {
   	  // 如果存在这个类就进行加入
      this.entries.putIfAbsent(importClassName, annotationMetadata);
   }

里面关键的就是获取所有的自动配置类，关键方法就是getAutoConfigurationEntry这个方法，进入这个方法

	/**
	 * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
	 * of the importing {@link Configuration @Configuration} class.
	 * @param annotationMetadata the annotation metadata of the configuration class
	 * @return the auto-configurations that should be imported
	 */
	protected AutoConfigurationEntry getAutoConfigurationEntry(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 = getConfigurationClassFilter().filter(configurations);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

里面的关键语句就是

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

功能就是完成获取所有要进行自动加载的bean，进入到方法

	/**
	 * Return the auto-configuration class names that should be considered. By default
	 * this method will load candidates using {@link ImportCandidates} with
	 * {@link #getSpringFactoriesLoaderFactoryClass()}. For backward compatible reasons it
	 * will also consider {@link SpringFactoriesLoader} with
	 * {@link #getSpringFactoriesLoaderFactoryClass()}.
	 * @param metadata the source metadata
	 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
	 * attributes}
	 * @return a list of candidate configurations
	 */
	protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
		List<String> configurations = new ArrayList<>(
				SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()));
		ImportCandidates.load(AutoConfiguration.class, getBeanClassLoader()).forEach(configurations::add);
		Assert.notEmpty(configurations,
				"No auto configuration classes found in META-INF/spring.factories nor in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. If you "
						+ "are using a custom packaging, make sure that file is correct.");
		return configurations;
	}

这个方法里面的

List<String> configurations = new ArrayList<>(
				SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()));

这个方法用于读取类路径下的所有 META-INF/spring.factories文件

下面的这行代码

ImportCandidates.load(AutoConfiguration.class, getBeanClassLoader()).forEach(configurations::add);

将其拆解为

ImportCandidates ic = ImportCandidates.load(AutoConfiguration.class, getBeanClassLoader());

ic.forEach(configurations::add);

可以发现就是先去获取所有自动加载bean，然后将其加入到一个集合中，进入到load方法

	/**
	 * Loads the names of import candidates from the classpath.
	 *
	 * The names of the import candidates are stored in files named
	 * {@code META-INF/spring/full-qualified-annotation-name.imports} on the classpath.
	 * Every line contains the full qualified name of the candidate class. Comments are
	 * supported using the # character.
	 * @param annotation annotation to load
	 * @param classLoader class loader to use for loading
	 * @return list of names of annotated classes
	 */
	public static ImportCandidates load(Class<?> annotation, ClassLoader classLoader) {
		Assert.notNull(annotation, "'annotation' must not be null");
		ClassLoader classLoaderToUse = decideClassloader(classLoader);
		String location = String.format(LOCATION, annotation.getName());
		Enumeration<URL> urls = findUrlsInClasspath(classLoaderToUse, location);
		List<String> importCandidates = new ArrayList<>();
		while (urls.hasMoreElements()) {
			URL url = urls.nextElement();
			importCandidates.addAll(readCandidateConfigurations(url));
		}
		return new ImportCandidates(importCandidates);
	}

里面最重要的语句就是

importCandidates.addAll(readCandidateConfigurations(url));

其实就是读取配置文件的信息，然后加入到集合中。读取的位置就是

META-INF/spring/%s.imports

实际上就是去读取

里面一共有144个默认加载项，然后再经过一些过滤操作，就会返回所有的配置类信息

	/**
	 * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
	 * of the importing {@link Configuration @Configuration} class.
	 * @param annotationMetadata the annotation metadata of the configuration class
	 * @return the auto-configurations that should be imported
	 */
	protected AutoConfigurationEntry getAutoConfigurationEntry(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 = getConfigurationClassFilter().filter(configurations);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

后面springboot就会去处理这些这些配置类。

---

springboot如何加载其他的starter

其实上面已经说了，springboot会去读取类路径下的所有META-INF/spring.factories文件，所以只需要在这个文件中写上自动配置的类即可，以mybatis-plus为例子，如下

---

总结

springboot在启动的时候会去读取org\springframework\boot\spring-boot-autoconfigure\2.7.10\spring-boot-autoconfigure-2.7.10.jar!\META-INF\spring\org.springframework.boot.autoconfigure.AutoConfiguration.imports这个文件中的所有配置。

springboot通过读取META-INF\spring.factories下的配置文件来支持其他starter的注入