SpringBoot 自动配置原理

2018-02-01

JDK: 1.8
SpringBoot: 1.5.10.RELEASE

通过 @EnableAutoConfiguration 类上的注解@Import({EnableAutoConfigurationImportSelector.class})

的 EnableAutoConfigurationImportSelector 的源码：

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        try {
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
            List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
            configurations = this.removeDuplicates(configurations);
            configurations = this.sort(configurations, autoConfigurationMetadata);
            Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
            this.checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = this.filter(configurations, autoConfigurationMetadata);
            this.fireAutoConfigurationImportEvents(configurations, exclusions);
            return (String[])configurations.toArray(new String[configurations.size()]);
        } catch (IOException var6) {
            throw new IllegalStateException(var6);
        }
    }
}

其中：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
}

该方法中有个重要的方法：

SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());

而 getSpringFactoriesLoaderFactoryClass() 实质就是获取 EnableAutoConfiguration 的 class 信息：

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protected Class<?> getSpringFactoriesLoaderFactoryClass() {
	return EnableAutoConfiguration.class;
}

而从 loadFactoryNames 的源码：

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();

    try {
        Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
        ArrayList result = new ArrayList();

        while(urls.hasMoreElements()) {
            URL url = (URL)urls.nextElement();
            Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
            String factoryClassNames = properties.getProperty(factoryClassName);
            result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
        }

        return result;
    } catch (IOException var8) {
        throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
    }
}

中可以发现，其就是加载 META-INF/spring.factories 路径下的资源配置。该文件位于 spring-boot-autoconfigure 的 META-INF/spring.factories 中，定义了 SpringBoot 中所有预定义的自动配置类的类路径