Spring Boot自动装配原理

两个注解

  这个是面试热门问题了，如果只回答了spring.factories是不会留下好印象的，只有详细讲解这个过程，才会给面试官留下一个好印象。
  首先是@SpringBootApplication注解包含了@EnableAutoConfiguration注解，如源码（局部）：

@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
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  再看@EnableAutoConfiguration注解，源码：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration 
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  其余的信息可以不看，只需要看这两个就可以了。

AutoConfigurationImportSelector

  其实读源码，就是一步一步往下跟就行了，这个类的核心在于这个方法：

@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}
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  下一步就是调用getAutoConfigurationEntry方法，这个方法长这样：

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);
	}
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  这里面的核心代码就是getCandidateConfigurations，展开就是：

	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;
	}
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  而SpringFactoriesLoader，类似于JavaSE的ServiceLoader，从META-INFO/spring.factories里读取配置，然后加载配置类。为什么是，是在这里配置的：

public final class SpringFactoriesLoader {

	/**
	 * The location to look for factories.
	 * 
Can be present in multiple JAR files.
	 */
	public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
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面试技巧

  这么多东西也很难记住，只要说出@SpringBootApplication到@EnableAutoConfiguration，再到AutoConfigurationImportSelector，在这个类里会调用SpringFactoriesLoader，回答了这些就够了。