springboot的@EnableAutoConfiguration起作用的原理

通常我们启动一个springboot项目会在启动方法中增加@SpringBootApplicatoin注解，该注解中包含了@EnableAutoConfiguration

@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 {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    String[] excludeName() default {};

    /**
     * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
     * for a type-safe alternative to String-based package names.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default {};

    /**
     * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
     * scan for annotated components. The package of each class specified will be scanned.
     * <p>
     * Consider creating a special no-op marker class or interface in each package that
     * serves no purpose other than being referenced by this attribute.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    Class<?>[] scanBasePackageClasses() default {};

}

那么@EnableAutoConfiguration是如何自动起作用的呢。我们先来看看@EnableAutoConfiguration的定义

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

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    String[] excludeName() default {};

}

我们看看EnableAutoConfiguration的注释定义:

Enable auto-configuration of the Spring Application Context, attempting to guess and
* configure beans that you are likely to need. Auto-configuration classes are usually
* applied based on your classpath and what beans you have defined.

大概的意思是:容器尝试猜测你想配置的bean，然后自动加载。

很欣喜的发现了@EnableAutoConfiguration里面引用了@Import(AutoConfigurationImportSelector.class)，直觉看起来像这个AutoConfigurationImportSelector里面引入了我们那些自动配置(数据源、拦截器)的类。

我们来看看AutoConfigurationImportSelector的定义:

public class AutoConfigurationImportSelector
implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered

原来AutoConfigurationImportSelector类实现了org.springframework.context.annotation.DeferredImportSelector

那么实现了该类的public String[] selectImports(AnnotationMetadata annotationMetadata)后，spring在解析bean的时候，就会自动调用该方法来加载拓展类，也就是selectImports方法返回的名称列表来加载类。

很容易起想到selectImports里面就返回了AutoEnableConfiguration的那些数据源、拦截器配置的类。！

接下来看看selectImports的具体实现:

    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        //从org.springframework.boot.autoconfigure.AutoConfigurationMetadataLoader的loadMetadata加载要自动配置的资源数据

        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
        AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
                autoConfigurationMetadata, annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }

看下AutoConfigurationMetadataLoader的源码看，可以看到默认加载的自动配置的路径为:/META-INF/spring-autoconfigure-metadata.properteis。

final class AutoConfigurationMetadataLoader {
    //默认加载自动配置的文件位置
    protected static final String PATH = "META-INF/"
            + "spring-autoconfigure-metadata.properties";

    private AutoConfigurationMetadataLoader() {
    }

    public static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
        return loadMetadata(classLoader, PATH);
    }

    static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
        try {
            Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
                    : ClassLoader.getSystemResources(path);
            Properties properties = new Properties();
            while (urls.hasMoreElements()) {
                properties.putAll(PropertiesLoaderUtils
                        .loadProperties(new UrlResource(urls.nextElement())));
            }
            return loadMetadata(properties);
        }
        catch (IOException ex) {
            throw new IllegalArgumentException(
                    "Unable to load @ConditionalOnClass location [" + path + "]", ex);
        }
    }

    static AutoConfigurationMetadata loadMetadata(Properties properties) {
        return new PropertiesAutoConfigurationMetadata(properties);
    }

    /**
     * {@link AutoConfigurationMetadata} implementation backed by a properties file.
     */
    private static class PropertiesAutoConfigurationMetadata
            implements AutoConfigurationMetadata {

        private final Properties properties;

        PropertiesAutoConfigurationMetadata(Properties properties) {
            this.properties = properties;
        }

        @Override
        public boolean wasProcessed(String className) {
            return this.properties.containsKey(className);
        }

        @Override
        public Integer getInteger(String className, String key) {
            return getInteger(className, key, null);
        }

        @Override
        public Integer getInteger(String className, String key, Integer defaultValue) {
            String value = get(className, key);
            return (value != null) ? Integer.valueOf(value) : defaultValue;
        }

        @Override
        public Set<String> getSet(String className, String key) {
            return getSet(className, key, null);
        }

        @Override
        public Set<String> getSet(String className, String key,
                Set<String> defaultValue) {
            String value = get(className, key);
            return (value != null) ? StringUtils.commaDelimitedListToSet(value)
                    : defaultValue;
        }

        @Override
        public String get(String className, String key) {
            return get(className, key, null);
        }

        @Override
        public String get(String className, String key, String defaultValue) {
            String value = this.properties.getProperty(className + "." + key);
            return (value != null) ? value : defaultValue;
        }

    }

}

可以看到配置的默认加载的配载类有如下,这些类都被@Confiration注解，并且都自动创建了一些@Bean来使用组件的特性，通过这些自动创建的bean来代替我们系统中手动声明bean,达到加快开发效率的效果。

 [org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration, org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration, org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration, org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration, org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration, org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration, org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration, org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration, org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration, org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration, org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration, org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration, org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration, org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration, org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration, org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration]

至此，springboot自动加载配置的原理也分析完毕。