一、简介

Spring致力于让Java开发更简单，SpringBoot致力于让使用Spring进行Java开发更简单，SpringCloud致力于基于SpringBoot构建微服务生态圈，让微服务开发更简单。随着这几年spring官网的更新可有看出spring发展的roadmap

网上有一个spring发展的时间线，也可以看下

随着近几年微服务的火爆，SpringBoot及SpringCloud被使用的越来越多，了解其内部原理显然越来越重要。

二、SpringBoot简介

Spring Boot将很多魔法带入了Spring应用程序的开发之中，其中最重要的是以下四个核心。
  自动配置：针对很多Spring应用程序常见的应用功能，Spring Boot能自动提供相关配置。
  起步依赖：告诉Spring Boot需要什么功能，它就能引入需要的库。
  命令行界面：这是Spring Boot的可选特性，借此你只需写代码就能完成完整的应用程序，
无需传统项目构建。
  Actuator：让你能够深入运行中的Spring Boot应用程序，一探究竟。

其中自动配置和起步依赖是目前和程序猿密切相关的。后面就重点分析下自动配置和起步依赖。而自动配置又是本篇的重点。

三、正题（基于springboot2.1.1分析）

1、Spring Boot的运行

以Spring Boot集成Dubbo为例，用idea基于Spring Initialzr很方便就能搭建Spring Boot项目结构，看下项目启动引导类

package com.jtt.hhl;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DubboServerApplication {

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

}

该类主要有两个作用：配置和启动引导。首先，这是主要的Spring配置类。虽然Spring Boot的自动配置免除了很多Spring配置，但你还需要进行少量配置来启用自动配置。@SpringBootApplication注解开启了Spring的组件扫描和Spring Boot的自动配置功能。自动配置就出现了。看下改注解

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.FilterType;
import org.springframework.core.annotation.AliasFor;

/**
 * Indicates a {@link Configuration configuration} class that declares one or more
 * {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration
 * auto-configuration} and {@link ComponentScan component scanning}. This is a convenience
 * annotation that is equivalent to declaring {@code @Configuration},
 * {@code @EnableAutoConfiguration} and {@code @ComponentScan}.
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @since 1.2.0
 */
@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 {};

}

 实际上@SpringBootApplication 将三个有用的注解组合在了一起，看其注释也很明确
  Spring的 @Configuration ：标明该类使用Spring基于Java的配置。Java配置相当于spring的xml配置。
  Spring的 @ComponentScan ：启用组件扫描，这样你写的Web控制器类和其他组件才能被自动发现并注册为Spring应用程序上下文里的Bean。
  Spring Boot 的 @EnableAutoConfiguration ： 这 个 不 起 眼 的 小 注 解 也 可 以 称 为@Abracadabra就是这一行配置开启了Spring Boot自动配置的魔力，让你不用再写成篇的配置了。

关于前两个注解可以参考前两遍文章【Spring实战】—-Spring配置文件的解析和【Spring实战】Spring注解配置工作原理源码解析，本文的分析重点是注解@EnableAutoConfiguration，接下来先看引导类的启动引导功能。

要部署Spring Boot应用程序有几种方式，其中包含传统的WAR文件部署。但这里的 main() 方法让你可以在命令行里把该应
用程序当作一个可执行JAR文件来运行。来看下SpringApplication.run(DubboServerApplication.class, args);

/**
	 * Static helper that can be used to run a {@link SpringApplication} from the
	 * specified sources using default settings and user supplied arguments.
	 * @param primarySources the primary sources to load
	 * @param args the application arguments (usually passed from a Java main method)
	 * @return the running {@link ApplicationContext}
	 */
	public static ConfigurableApplicationContext run(Class<?>[] primarySources,
			String[] args) {
		return new SpringApplication(primarySources).run(args);
	}

最终会调用构造函数，primarySources就是DubboServerApplication.class，

/**
	 * Create a new {@link SpringApplication} instance. The application context will load
	 * beans from the specified primary sources (see {@link SpringApplication class-level}
	 * documentation for details. The instance can be customized before calling
	 * {@link #run(String...)}.
	 * @param resourceLoader the resource loader to use
	 * @param primarySources the primary bean sources
	 * @see #run(Class, String[])
	 * @see #setSources(Set)
	 */
	@SuppressWarnings({ "unchecked", "rawtypes" })
	public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
		this.resourceLoader = resourceLoader;
		Assert.notNull(primarySources, "PrimarySources must not be null");
		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
		this.webApplicationType = WebApplicationType.deduceFromClasspath();
		setInitializers((Collection) getSpringFactoriesInstances(
				ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

然后调用run方法

/**
	 * Run the Spring application, creating and refreshing a new
	 * {@link ApplicationContext}.
	 * @param args the application arguments (usually passed from a Java main method)
	 * @return a running {@link ApplicationContext}
	 */
	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(
					args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners,
					applicationArguments);
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			exceptionReporters = getSpringFactoriesInstances(
					SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			prepareContext(context, environment, listeners, applicationArguments,
					printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass)
						.logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}

		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

run方法最重要的作用就是创建spring上下文环境，这里根据不用的应用环境创建不同的上下文

/**
	 * The class name of application context that will be used by default for non-web
	 * environments.
	 */
	public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
			+ "annotation.AnnotationConfigApplicationContext";

	/**
	 * The class name of application context that will be used by default for web
	 * environments.
	 */
	public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot."
			+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";

	/**
	 * The class name of application context that will be used by default for reactive web
	 * environments.
	 */
	public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."
			+ "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";


/**
	 * Strategy method used to create the {@link ApplicationContext}. By default this
	 * method will respect any explicitly set application context or application context
	 * class before falling back to a suitable default.
	 * @return the application context (not yet refreshed)
	 * @see #setApplicationContextClass(Class)
	 */
	protected ConfigurableApplicationContext createApplicationContext() {
		Class<?> contextClass = this.applicationContextClass;
		if (contextClass == null) {
			try {
				switch (this.webApplicationType) {
				case SERVLET:
					contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
					break;
				case REACTIVE:
					contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
					break;
				default:
					contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
				}
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Unable create a default ApplicationContext, "
								+ "please specify an ApplicationContextClass",
						ex);
			}
		}
		return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
	}

可以看出三个上下文都是基于注解的，默认的是AnnotationConfigApplicationContext，本例中classpath中没有web、servlet的配置（改判断就是在SpringApplication创建时初始化的），上下文环境就是AnnotationConfigApplicationContext，注意其无参构造函数

/**
     * Create a new AnnotationConfigApplicationContext that needs to be populated
     * through {@link #register} calls and then manually {@linkplain #refresh refreshed}.
     */
    public AnnotationConfigApplicationContext() {
        this.reader = new AnnotatedBeanDefinitionReader(this);   
        this.scanner = new ClassPathBeanDefinitionScanner(this);
    }

在this.reader操作中，重大作用的注解处理器已经添加完毕org.springframework.context.annotation.AnnotationConfigUtils#registerAnnotationConfigProcessors(org.springframework.beans.factory.support.BeanDefinitionRegistry, java.lang.Object)

然后准备上下文环境prepareContext(context, environment, listeners, applicationArguments, printedBanner);主要的工作就是加载引导类bean，本例中就是DubboServerApplication.class，将其注册到beanDefinitionMap中。后面有ConfigurationClassPostProcessor处理器对其处理，解析配置类，相当于使用xml配置时xml的解析（只不过时机不同）。

再看刷新上下文环境refreshContext(context);

@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

最终调用的AbstractApplicationContext中的refresh方法，是不是很熟悉了。简单说下：

1）、prepareRefresh()刷新前的预处理，属性设置及合法性检验等；

2）、obtainFreshBeanFactory();获取BeanFactory 为DefaultListableBeanFactory(在上下文初始化的时候已经创建);设置id（本例中是spring.application.name的值dubbo-server）；

3）、prepareBeanFactory(beanFactory);BeanFactory的预准备工作（BeanFactory进行一些设置）；

4）、postProcessBeanFactory(beanFactory);BeanFactory准备工作完成后进行的后置处理工作（提供给子类用的）；

5）、invokeBeanFactoryPostProcessors(beanFactory);激活各种BeanFactory处理器，执行BeanFactoryPostProcessor的方法。@Configration注解的处理器ConfigurationClassPostProcessor就是在这里调用的。

6）、registerBeanPostProcessors(beanFactory);注册BeanPostProcessor（Bean的后置处理器）【 intercept bean creation】，这里只是注册，调用的时机是getBean()的时候。

7)、initMessageSource();初始化MessageSource组件（做国际化功能；消息绑定，消息解析）；

8)、initApplicationEventMulticaster();初始化事件派发器；

9)、onRefresh();留给子容器（子类）

10)、registerListeners();给容器中将所有项目里面的ApplicationListener注册进来；

11)、finishBeanFactoryInitialization(beanFactory);初始化所有剩下的非懒加载的单实例bean（因为有的再之前使用的时候已经初始化过了，因此这里叫初始化剩下的非懒加载的单实例bean）；

12)、finishRefresh();完成BeanFactory的初始化创建工作；IOC容器就创建完成；

至此，spring上下文，也就是IOC容器创建完成了，从最初的XmlBeanFactory、ClassPathXmlApplicationContext再到web应用中的ContextLoaderListener，再到SpringBoot的SpringApplication及@SpringBootApplication注解，其实原理都是一样的：创建容器DefaultListableBeanFactory，注册相关的Bean（在spring中除了容器就是bean，后置处理器就是被当成bean处理的。只不过注解的基础bean注册是基于@SpringBootApplication的@Configuration及@ComponentScan注解，在refresh->invokeBeanFactoryPostProcessors(beanFactory)->ConfigurationClassPostProcessor处理器中ConfigurationClassParser进行解析的，其中由于该启动类还有@ComponentScan注解，因此还会进行组件扫描ComponentScanAnnotationParser，扫描规则为com.jtt.hhl基础包下的，不包含过滤规则@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class，主要是排除自动配置类及自定义配置规则；其他的config类，如本例中的ConfigTest是在扫描的时候处理的，如果是配置类还会再进行一次ConfigurationClassParser解析) 。而基于xml配置文件的是在refresh->obtainFreshBeanFactory()->loadBeanDefinitions解析xml配置文件），创建bean以及后置处理器处理（注意BeanFactoryPostProcessor和BeanPostProcessor处理时机，注解的处理都是在后置处理器中进行的），事件驱动模型。看下ConfigurationClassParser.java中的主要方法：对@PropertySource、@ComponentScan、@Import、@ImportResource、@Bean注解的处理，这也说明了@Configuration注解类中可以使用的注解有哪些。

/**
	 * Apply processing and build a complete {@link ConfigurationClass} by reading the
	 * annotations, members and methods from the source class. This method can be called
	 * multiple times as relevant sources are discovered.
	 * @param configClass the configuration class being build
	 * @param sourceClass a source class
	 * @return the superclass, or {@code null} if none found or previously processed
	 */
	@Nullable
	protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
			throws IOException {

		if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
			// Recursively process any member (nested) classes first
			processMemberClasses(configClass, sourceClass);
		}

		// Process any @PropertySource annotations
		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			else {
				logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
						"]. Reason: Environment must implement ConfigurableEnvironment");
			}
		}

		// Process any @ComponentScan annotations
		Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
		if (!componentScans.isEmpty() &&
				!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
			for (AnnotationAttributes componentScan : componentScans) {
				// The config class is annotated with @ComponentScan -> perform the scan immediately
				Set<BeanDefinitionHolder> scannedBeanDefinitions =
						this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
				// Check the set of scanned definitions for any further config classes and parse recursively if needed
				for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
					BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
					if (bdCand == null) {
						bdCand = holder.getBeanDefinition();
					}
					if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
						parse(bdCand.getBeanClassName(), holder.getBeanName());
					}
				}
			}
		}

		// Process any @Import annotations
		processImports(configClass, sourceClass, getImports(sourceClass), true);

		// Process any @ImportResource annotations
		AnnotationAttributes importResource =
				AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
		if (importResource != null) {
			String[] resources = importResource.getStringArray("locations");
			Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
			for (String resource : resources) {
				String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
				configClass.addImportedResource(resolvedResource, readerClass);
			}
		}

		// Process individual @Bean methods
		Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
		for (MethodMetadata methodMetadata : beanMethods) {
			configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
		}

		// Process default methods on interfaces
		processInterfaces(configClass, sourceClass);

		// Process superclass, if any
		if (sourceClass.getMetadata().hasSuperClass()) {
			String superclass = sourceClass.getMetadata().getSuperClassName();
			if (superclass != null && !superclass.startsWith("java") &&
					!this.knownSuperclasses.containsKey(superclass)) {
				this.knownSuperclasses.put(superclass, configClass);
				// Superclass found, return its annotation metadata and recurse
				return sourceClass.getSuperClass();
			}
		}

		// No superclass -> processing is complete
		return null;
	}

2、自动配置

上面说了注解@SpringBootApplication中的@Configuration及@ComponentScan注解，下面看一下和自动配置相关的注解@EnableAutoConfiguration，改注解的最终要的作用是导入了@Import(AutoConfigurationImportSelector.class)，这个也就是自动配置开启的地方，@Import也是向IOC容器中导入组件的一种方式，引申一下。

给容器中注册组件；
* 1）、包扫描+组件标注注解（@Controller/@Service/@Repository/@Component）[自己写的类]
* 2）、@Bean[导入的第三方包里面的组件]
* 3）、@Import[快速给容器中导入一个组件]
*     1）、@Import(要导入到容器中的组件)；容器中就会自动注册这个组件，id默认是全类名
*     2）、ImportSelector:返回需要导入的组件的全类名数组；
*     3）、ImportBeanDefinitionRegistrar:手动注册bean到容器中

@ComponentScan扫描时针对@Import注解处理，看下AutoConfigurationImportSelector的处理过程：

// Process any @Import annotations
processImports(configClass, sourceClass, getImports(sourceClass), true);

先看一下有意思的getImports(sourceClass)方法，主要是获取@Import注解信息

/**
	 * Returns {@code @Import} class, considering all meta-annotations.
	 */
	private Set<SourceClass> getImports(SourceClass sourceClass) throws IOException {
		Set<SourceClass> imports = new LinkedHashSet<>();
		Set<SourceClass> visited = new LinkedHashSet<>();
		collectImports(sourceClass, imports, visited);
		return imports;
	}

/**
	 * Recursively collect all declared {@code @Import} values. Unlike most
	 * meta-annotations it is valid to have several {@code @Import}s declared with
	 * different values; the usual process of returning values from the first
	 * meta-annotation on a class is not sufficient.
	 * <p>For example, it is common for a {@code @Configuration} class to declare direct
	 * {@code @Import}s in addition to meta-imports originating from an {@code @Enable}
	 * annotation.
	 * @param sourceClass the class to search
	 * @param imports the imports collected so far
	 * @param visited used to track visited classes to prevent infinite recursion
	 * @throws IOException if there is any problem reading metadata from the named class
	 */
	private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> visited)
			throws IOException {

		if (visited.add(sourceClass)) {
			for (SourceClass annotation : sourceClass.getAnnotations()) {
				String annName = annotation.getMetadata().getClassName();
				if (!annName.startsWith("java") && !annName.equals(Import.class.getName())) {
					collectImports(annotation, imports, visited);
				}
			}
			imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
		}
	}

本例中，这里的sourceClass就是com.jtt.hhl.DubboServerApplication，最终通过将java注解（@Target、@Retention、@Documented、@Inherited）排除，一层层递归到EnableAutoConfiguration注解的@Import(AutoConfigurationImportSelector.class)注解及@AutoConfigurationPackage中的@Import(AutoConfigurationPackages.Registrar.class)一步步看处理：

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
			Collection<SourceClass> importCandidates, boolean checkForCircularImports) {

		if (importCandidates.isEmpty()) {
			return;
		}

		if (checkForCircularImports && isChainedImportOnStack(configClass)) {
			this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
		}
		else {
			this.importStack.push(configClass);
			try {
				for (SourceClass candidate : importCandidates) {
					if (candidate.isAssignable(ImportSelector.class)) {
						// Candidate class is an ImportSelector -> delegate to it to determine imports
						Class<?> candidateClass = candidate.loadClass();
						ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
						ParserStrategyUtils.invokeAwareMethods(
								selector, this.environment, this.resourceLoader, this.registry);
						if (selector instanceof DeferredImportSelector) {
							this.deferredImportSelectorHandler.handle(
									configClass, (DeferredImportSelector) selector);
						}
						else {
							String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
							Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
							processImports(configClass, currentSourceClass, importSourceClasses, false);
						}
					}
					else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
						// Candidate class is an ImportBeanDefinitionRegistrar ->
						// delegate to it to register additional bean definitions
						Class<?> candidateClass = candidate.loadClass();
						ImportBeanDefinitionRegistrar registrar =
								BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
						ParserStrategyUtils.invokeAwareMethods(
								registrar, this.environment, this.resourceLoader, this.registry);
						configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
					}
					else {
						// Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
						// process it as an @Configuration class
						this.importStack.registerImport(
								currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
						processConfigurationClass(candidate.asConfigClass(configClass));
					}
				}
			}
			catch (BeanDefinitionStoreException ex) {
				throw ex;
			}
			catch (Throwable ex) {
				throw new BeanDefinitionStoreException(
						"Failed to process import candidates for configuration class [" +
						configClass.getMetadata().getClassName() + "]", ex);
			}
			finally {
				this.importStack.pop();
			}
		}
	}

第一个为（class org.springframework.boot.autoconfigure.AutoConfigurationPackages$Registrar）ImportBeanDefinitionRegistrar，只是将其加入到importBeanDefinitionRegistrars中。

看第二个org.springframework.boot.autoconfigure.AutoConfigurationImportSelector为ImportSelector，又是DeferredImportSelector类型的，因此走进this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);也只将其加入到deferredImportSelectors（延迟的导入选择器）中备用。什么时候用呢，继续往下看，解析完后会调用this.deferredImportSelectorHandler.process();

public void process() {
			List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
			this.deferredImportSelectors = null;
			try {
				if (deferredImports != null) {
					DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
					deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
					deferredImports.forEach(handler::register);
					handler.processGroupImports();
				}
			}
			finally {
				this.deferredImportSelectors = new ArrayList<>();
			}
		}

这里取出deferredImportSelectors中的AutoConfigurationImportSelector，开始处理，这里用到了java8的新特性:：及lambda表达式，重点关注下handler.processGroupImports();

public void processGroupImports() {
			for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
				grouping.getImports().forEach(entry -> {
					ConfigurationClass configurationClass = this.configurationClasses.get(
							entry.getMetadata());
					try {
						processImports(configurationClass, asSourceClass(configurationClass),
								asSourceClasses(entry.getImportClassName()), false);
					}
					catch (BeanDefinitionStoreException ex) {
						throw ex;
					}
					catch (Throwable ex) {
						throw new BeanDefinitionStoreException(
								"Failed to process import candidates for configuration class [" +
										configurationClass.getMetadata().getClassName() + "]", ex);
					}
				});
			}
		}

其中的lambda表达式，重点看，先看getImports()

/**
		 * Return the imports defined by the group.
		 * @return each import with its associated configuration class
		 */
		public Iterable<Group.Entry> getImports() {
			for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
				this.group.process(deferredImport.getConfigurationClass().getMetadata(),
						deferredImport.getImportSelector());
			}
			return this.group.selectImports();
		}
	}

这里又进行了process处理，这里的this.group就是在::register中赋值的

public void register(DeferredImportSelectorHolder deferredImport) {
			Class<? extends Group> group = deferredImport.getImportSelector()
					.getImportGroup();
			DeferredImportSelectorGrouping grouping = this.groupings.computeIfAbsent(
					(group != null ? group : deferredImport),
					key -> new DeferredImportSelectorGrouping(createGroup(group)));
			grouping.add(deferredImport);
			this.configurationClasses.put(deferredImport.getConfigurationClass().getMetadata(),
					deferredImport.getConfigurationClass());
		}

就是AutoConfigurationImportSelector的内部类AutoConfigurationGroup，因此这里的process就是调用内部类的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(getAutoConfigurationMetadata(),
							annotationMetadata);
			this.autoConfigurationEntries.add(autoConfigurationEntry);
			for (String importClassName : autoConfigurationEntry.getConfigurations()) {
				this.entries.putIfAbsent(importClassName, annotationMetadata);
			}
		}

会调用getAutoConfigurationEntry，获取自动配置条目

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

关键的部分：获取所有的自动配置信息List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);

/**
	 * Return the auto-configuration class names that should be considered. By default
	 * this method will load candidates using {@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 = 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;
	}

从哪里获取，classpath中的META-INF/spring.factories文件中，进入SpringFactoriesLoader类的下述方法

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


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);
		}
	}

实际上该方法在前面调用过（最一开始SpringApplication创建的时候调用setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));其中的getSpringFactoriesInstances方法就调用到了）因此cache中有值，就直接返回了key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的vule，注意这里的map是多值map（MultiValueMap）。看下spring.factories

很多地方都有,本例中一共 size = 119个，因此可以定制，看下dubbo中spring.factoreis内容

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.alibaba.boot.dubbo.autoconfigure.DubboAutoConfiguration


org.springframework.context.ApplicationListener=\
com.alibaba.boot.dubbo.context.event.OverrideDubboConfigApplicationListener,\
com.alibaba.boot.dubbo.context.event.WelcomeLogoApplicationListener,\
com.alibaba.boot.dubbo.context.event.AwaitingNonWebApplicationListener

因此这里获取的自动配置信息包含com.alibaba.boot.dubbo.autoconfigure.DubboAutoConfiguration，返回去看getAutoConfigurationEntry方法，会对配置信息进行过滤，去掉重复的removeDuplicates(configurations)，去掉扫描排除的，重点看下filter方法

private List<String> filter(List<String> configurations,
			AutoConfigurationMetadata autoConfigurationMetadata) {
		long startTime = System.nanoTime();
		String[] candidates = StringUtils.toStringArray(configurations);
		boolean[] skip = new boolean[candidates.length];
		boolean skipped = false;
		for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
			invokeAwareMethods(filter);
			boolean[] match = filter.match(candidates, autoConfigurationMetadata);
			for (int i = 0; i < match.length; i++) {
				if (!match[i]) {
					skip[i] = true;
					candidates[i] = null;
					skipped = true;
				}
			}
		}
		if (!skipped) {
			return configurations;
		}
		List<String> result = new ArrayList<>(candidates.length);
		for (int i = 0; i < candidates.length; i++) {
			if (!skip[i]) {
				result.add(candidates[i]);
			}
		}
		if (logger.isTraceEnabled()) {
			int numberFiltered = configurations.size() - result.size();
			logger.trace("Filtered " + numberFiltered + " auto configuration class in "
					+ TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)
					+ " ms");
		}
		return new ArrayList<>(result);
	}

首先会根据spring.factories（spring-boot-autoconfiguration-2.1.1RELRASE.jar）中的filter进行过滤，另外过滤是还会用到另一个文件中的内容spring-autoconfigure-metadata.properties

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

这里的过滤主要是根据spring-autoconfigure-metadata.properties文件中的内容进行过滤。简单看下OnClassCondition过滤的过程

@Override
	protected final ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses,
			AutoConfigurationMetadata autoConfigurationMetadata) {
		// Split the work and perform half in a background thread. Using a single
		// additional thread seems to offer the best performance. More threads make
		// things worse
		int split = autoConfigurationClasses.length / 2;
		OutcomesResolver firstHalfResolver = createOutcomesResolver(
				autoConfigurationClasses, 0, split, autoConfigurationMetadata);
		OutcomesResolver secondHalfResolver = new StandardOutcomesResolver(
				autoConfigurationClasses, split, autoConfigurationClasses.length,
				autoConfigurationMetadata, getBeanClassLoader());
		ConditionOutcome[] secondHalf = secondHalfResolver.resolveOutcomes();
		ConditionOutcome[] firstHalf = firstHalfResolver.resolveOutcomes();
		ConditionOutcome[] outcomes = new ConditionOutcome[autoConfigurationClasses.length];
		System.arraycopy(firstHalf, 0, outcomes, 0, firstHalf.length);
		System.arraycopy(secondHalf, 0, outcomes, split, secondHalf.length);
		return outcomes;
	}

这里将配置类一分为二用了多线程提高效率，看下怎么过滤的，就是判断classpath中有没有该类

private ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses,
				int start, int end, AutoConfigurationMetadata autoConfigurationMetadata) {
			ConditionOutcome[] outcomes = new ConditionOutcome[end - start];
			for (int i = start; i < end; i++) {
				String autoConfigurationClass = autoConfigurationClasses[i];
				if (autoConfigurationClass != null) {
                    //从properties中获取对应的类信息
					String candidates = autoConfigurationMetadata
							.get(autoConfigurationClass, "ConditionalOnClass");
					if (candidates != null) {
						outcomes[i - start] = getOutcome(candidates);  
                        //去classpath中看看有没有，没有则match为false，并显示对应的信息
					}
				}
			}
			return outcomes;
		}

private ConditionOutcome getOutcome(String className, ClassLoader classLoader) {
			if (ClassNameFilter.MISSING.matches(className, classLoader)) {
				return ConditionOutcome.noMatch(ConditionMessage
						.forCondition(ConditionalOnClass.class)
						.didNotFind("required class").items(Style.QUOTE, className));
			}
			return null;
		}


MISSING {

			@Override
			public boolean matches(String className, ClassLoader classLoader) {
				return !isPresent(className, classLoader);
			}

		};

public static boolean isPresent(String className, ClassLoader classLoader) {
			if (classLoader == null) {
				classLoader = ClassUtils.getDefaultClassLoader();
			}
			try {
				forName(className, classLoader);
				return true;
			}
			catch (Throwable ex) {
				return false;
			}
		}

		private static Class<?> forName(String className, ClassLoader classLoader)
				throws ClassNotFoundException {
			if (classLoader != null) {
				return classLoader.loadClass(className);
			}
			return Class.forName(className);
		}

其实最终就是基于Class.forName(className);进行判断的，这也是condition使用的底层支持。

值得注意的是本例中的DubboAutoConfiguration并没在其中，所以对应的outcomes为null，match返回的是ture，经过OnClassCondition、OnWebApplicationCondition及OnBeanCondition的过滤后最终配置类还剩10个配置类（包含DubboAutoConfiguration）条目。

至此，condition出现了，上面的importfilter分别对应注解@ConditionalOnBean、@ConditionalOnClass、@ConditionalOnWebApplication含义分别是仅仅在当前上下文中存在某个对象时，才会启用；某个class位于类路径上，才会启用，该注解的参数对应的类必须存在，否则不解析该注解修饰的配置类；在web应用下才会启用该配置类，看下dubbo的自动配置类

@Configuration
@ConditionalOnProperty(prefix = DUBBO_PREFIX, name = "enabled", matchIfMissing = true, havingValue = "true")
@ConditionalOnClass(AbstractConfig.class)
public class DubboAutoConfiguration {

还用到了@ConditionalOnProperty注解，配置属性dubbo.enabled的值存在或者缺失都会启用该配置，另外还必须存在AbstractConfig类。

回过头去继续看lambda表达式：

grouping.getImports().forEach(entry -> {
					ConfigurationClass configurationClass = this.configurationClasses.get(
							entry.getMetadata());
					try {
						processImports(configurationClass, asSourceClass(configurationClass),
								asSourceClasses(entry.getImportClassName()), false);
					}
					catch (BeanDefinitionStoreException ex) {
						throw ex;
					}
					catch (Throwable ex) {
						throw new BeanDefinitionStoreException(
								"Failed to process import candidates for configuration class [" +
										configurationClass.getMetadata().getClassName() + "]", ex);
					}
				});

针对上述过滤后剩余的10个配置类条目进行处理，继续调用

processImports(configurationClass, asSourceClass(configurationClass), asSourceClasses(entry.getImportClassName()), false);

这次上述两个条件都不满足，走到else当成配置类继续解析

else {
    // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
    // process it as an @Configuration class
    this.importStack.registerImport(
        currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
    processConfigurationClass(candidate.asConfigClass(configClass));
}

而其中的@EnableDubboConfig注解导入了@Import(DubboConfigConfigurationSelector.class)

/**
     * Single Dubbo Config Configuration
     *
     * @see EnableDubboConfig
     * @see DubboConfigConfiguration.Single
     */
    @EnableDubboConfig
    protected static class SingleDubboConfigConfiguration {
    }

因此会继续处理DubboConfigConfigurationSelector，这里采用的是递归处理。所有都处理完后得到ConfigClasses，继续处理配置类中的Bean，也就是用@Bean注解的bean信息，将其注册到IOC容器中。

// Read the model and create bean definitions based on its content
			if (this.reader == null) {
				this.reader = new ConfigurationClassBeanDefinitionReader(
						registry, this.sourceExtractor, this.resourceLoader, this.environment,
						this.importBeanNameGenerator, parser.getImportRegistry());
			}
			this.reader.loadBeanDefinitions(configClasses);
			alreadyParsed.addAll(configClasses);

/**
	 * Determine if an item should be skipped based on {@code @Conditional} annotations.
	 * @param metadata the meta data
	 * @param phase the phase of the call
	 * @return if the item should be skipped
	 */
	public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
		if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
			return false;
		}

		if (phase == null) {
			if (metadata instanceof AnnotationMetadata &&
					ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
				return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
			}
			return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
		}

		List<Condition> conditions = new ArrayList<>();
		for (String[] conditionClasses : getConditionClasses(metadata)) {
			for (String conditionClass : conditionClasses) {
				Condition condition = getCondition(conditionClass, this.context.getClassLoader());
				conditions.add(condition);
			}
		}

		AnnotationAwareOrderComparator.sort(conditions);

		for (Condition condition : conditions) {
			ConfigurationPhase requiredPhase = null;
			if (condition instanceof ConfigurationCondition) {
				requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
			}
			if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
				return true;
			}
		}

		return false;
	}

至此配置类就解析完毕了，总结下：

1、首先对springboot配置类（com.jtt.hhl.DubboServerApplication）利用配置后置处理器ConfigurationClassPostProcessor进行解析，解析过程中利用其上的ComponentScan注解对其他组件进行扫描注册，包含（自定义配置类com.jtt.hhl.config.ConfigTest，由于@Component是@Configuration的元注解）；利用其上的@EnableAutoConfiguration注解进行自动配置发现，主要是利用@Import导入AutoConfigurationImportSelector注解（解析时根据@Import注解导入的类型不同处理不同），读取META-INF/spring.factories中的配置类；还会对配置类上的@PropertySource注解、@ImportResource注解以及配置类内的@Bean注解进行处理。

2、自动配置的启用：根据spring.factories中的filter和spring-autoconfigure-metadata.properties中对应filter信息进行过滤，实际上就是自动配置的过程，如果classpath中有相应的配置就启用，继续进行配置类的解析操作，循环往复，直到所有启用的配置类都解析完毕，这个过程和使用xml配置时解析的过程类似。

3、@Conditional注解的处理，在配置类解析时会首先对配置类进行ConditionEvaluator处理（其实每个配置类都会进行处理，而且在loadBeanDefinitions(configClasses)配置类中的bean definition是也会进行@Conditional的处理（如果有的话）），判断条件是否满足，如果满足再继续进行配置类的解析（上面说com.alibaba.boot.dubbo.autoconfigure.DubboAutoConfiguration.OnClassCondition没在文件中，虽然当时没有被过滤，这里仍然会判断其上的注解条件），并将其中满足条件的bean注册到IOC容器中。因此在配置类com.alibaba.boot.dubbo.autoconfigure.DubboAutoConfiguration上的@Conditional注解在配置类解析时会进行处理。

至此，自动配置的全过程也就完毕了，自动配置的过程也就是SpringBoot启动类（也是配置类）的配置解析过程，配置类解析过程中会进行自动配置，自动配置就是读取META-INF/spring.factories中的配置类，然后利用@Conditional相关的注解，在classpath中判断对应的class是否存在（@ConditionalOnClass）、在propeties文件中判断对应的属性是否存在（@ConditionalOnProperty）、在IOC容器判断对应的bean是否存在（@ConditionalOnBean）而是否启用对应的配置，也是利用条件判断当自动配置不满足条件时可以实现自定义配置。

最后，没有实例化非懒加载单实例bean会在finishBeanFactoryInitialization(beanFactory);中进行实例化。

四、代码地址

https://github.com/honghailiang/springboot-dubbo

五、微信公众号