前言：本系列文章非本人原创，转自：http://tengj.top/2017/04/24/springboot0/

正文

我们开发任何一个Spring Boot项目，都会用到如下的启动类

 @SpringBootApplication

 public class Application {

     public static void main(String[] args) {

         SpringApplication.run(Application.class, args);

     }

 }

从上面代码可以看出，Annotation定义（@SpringBootApplication）和类定义（SpringApplication.run）最为耀眼，所以要揭开SpringBoot的神秘面纱，我们要从这两位开始就可以了。

SpringBootApplication背后的秘密

 package org.springframework.boot.autoconfigure;

 import java.lang.annotation.Annotation;

 import java.lang.annotation.Documented;

 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.context.annotation.ComponentScan;

 import org.springframework.core.annotation.AliasFor;

 @Target({java.lang.annotation.ElementType.TYPE})

 @Retention(RetentionPolicy.RUNTIME)

 @Documented

 @Inherited

 @SpringBootConfiguration

 @EnableAutoConfiguration

 @ComponentScan(excludeFilters={@org.springframework.context.annotation.ComponentScan.Filter(type=org.springframework.context.annotation.FilterType.CUSTOM, classes={org.springframework.boot.context.TypeExcludeFilter.class}), @org.springframework.context.annotation.ComponentScan.Filter(type=org.springframework.context.annotation.FilterType.CUSTOM, classes={AutoConfigurationExcludeFilter.class})})

 public @interface SpringBootApplication

 {

   @AliasFor(annotation=EnableAutoConfiguration.class)

   Class<?>[] exclude() default {};

   @AliasFor(annotation=EnableAutoConfiguration.class)

   String[] excludeName() default {};

   @AliasFor(annotation=ComponentScan.class, attribute="basePackages")

   String[] scanBasePackages() default {};

   @AliasFor(annotation=ComponentScan.class, attribute="basePackageClasses")

   Class<?>[] scanBasePackageClasses() default {};

 }

 /* Location:           C:\Users\Administrator\.m2\repository\org\springframework\boot\spring-boot-autoconfigure\2.1.8.RELEASE\spring-boot-autoconfigure-2.1.8.RELEASE.jar

  * Qualified Name:     org.springframework.boot.autoconfigure.SpringBootApplication

  * Java Class Version: 8 (52.0)

  * JD-Core Version:    0.7.0.1

  */

虽然定义使用了多个Annotation进行了原信息标注，但实际上重要的只有三个Annotation：

@Configuration（@SpringBootConfiguration点开查看发现里面还是应用了@Configuration）
@EnableAutoConfiguration
@ComponentScan

所以，如果我们使用如下的SpringBoot启动类，整个SpringBoot应用依然可以与之前的启动类功能对等：

 @Configuration

 @EnableAutoConfiguration

 @ComponentScan

 public class Application {

     public static void main(String[] args) {

         SpringApplication.run(Application.class, args);

     }

 }

每次写这3个比较累，所以写一个@SpringBootApplication方便点。接下来分别介绍这3个Annotation。

@Configuration

这里的@Configuration对我们来说不陌生，它就是JavaConfig形式的Spring Ioc容器的配置类使用的那个@Configuration，SpringBoot社区推荐使用基于JavaConfig的配置形式，所以，这里的启动类标注了@Configuration之后，本身其实也是一个IoC容器的配置类。
举几个简单例子回顾下，XML跟config配置方式的区别：

　　1、表达形式层面

　　基于XML配置的方式是这样：

 <?xml version="1.0" encoding="UTF-8"?>

 <beans xmlns="http://www.springframework.org/schema/beans"

        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd"

        default-lazy-init="true">

     <!--bean定义-->

 </beans>

　　基于JavaConfig的配置方式是这样：

 @Configuration

 public class MockConfiguration{

     //bean定义

 }

　　任何一个标注了@Configuration的Java类定义都是一个JavaConfig配置类。

　　2、注册bean定义层面

　　基于XML的配置形式是这样：

 <bean id="mockService" class="..MockServiceImpl">

     ...

 </bean>

　　基于JavaConfig的配置形式是这样的：

 @Configuration

 public class MockConfiguration{

     @Bean

     public MockService mockService(){

         return new MockServiceImpl();

     }

 }

　　任何一个标注了@Bean的方法，其返回值将作为一个bean定义注册到Spring的IoC容器，方法名将默认成该bean定义的id。

　　3、表达依赖注入关系层面

　　为了表达bean与bean之间的依赖关系，在XML形式中一般是这样：

 <bean id="mockService" class="..MockServiceImpl">

     <propery name ="dependencyService" ref="dependencyService" />

 </bean>

 <bean id="dependencyService" class="DependencyServiceImpl></bean>

　　基于JavaConfig的配置形式是这样的：

 @Configuration

 public class MockConfiguration{

     @Bean

     public MockService mockService(){

         return new MockServiceImpl(dependencyService());

     }

     @Bean

     public DependencyService dependencyService(){

         return new DependencyServiceImpl();

     }

 }

　　如果一个bean的定义依赖其他bean,则直接调用对应的JavaConfig类中依赖bean的创建方法就可以了。

@ComponentScan

@ComponentScan这个注解在Spring中很重要，它对应XML配置中的元素，@ComponentScan的功能其实就是自动扫描并加载符合条件的组件（比如@Component和@Repository等）或者bean定义，最终将这些bean定义加载到IoC容器中。

我们可以通过basePackages等属性来细粒度的定制@ComponentScan自动扫描的范围，如果不指定，则默认Spring框架实现会从声明@ComponentScan所在类的package进行扫描。

注：所以SpringBoot的启动类最好是放在root package下，因为默认不指定basePackages。

@EnableAutoConfiguration

个人感觉@EnableAutoConfiguration这个Annotation最为重要，所以放在最后来解读，大家是否还记得Spring框架提供的各种名字为@Enable开头的Annotation定义？比如@EnableScheduling、@EnableCaching、@EnableMBeanExport等，@EnableAutoConfiguration的理念和做事方式其实一脉相承，简单概括一下就是，借助@Import的支持，收集和注册特定场景相关的bean定义。

@EnableScheduling是通过@Import将Spring调度框架相关的bean定义都加载到IoC容器。
@EnableMBeanExport是通过@Import将JMX相关的bean定义加载到IoC容器。

而@EnableAutoConfiguration也是借助@Import的帮助，将所有符合自动配置条件的bean定义加载到IoC容器，仅此而已！

@EnableAutoConfiguration作为一个复合Annotation,其自身定义关键信息如下：

 /*

  * Copyright 2012-2019 the original author or authors.

  *

  * Licensed under the Apache License, Version 2.0 (the "License");

  * you may not use this file except in compliance with the License.

  * You may obtain a copy of the License at

  *

  *      https://www.apache.org/licenses/LICENSE-2.0

  *

  * Unless required by applicable law or agreed to in writing, software

  * distributed under the License is distributed on an "AS IS" BASIS,

  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

  * See the License for the specific language governing permissions and

  * limitations under the License.

  */

 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.autoconfigure.condition.ConditionalOnBean;

 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;

 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;

 import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;

 import org.springframework.boot.web.servlet.server.ServletWebServerFactory;

 import org.springframework.context.annotation.Conditional;

 import org.springframework.context.annotation.Configuration;

 import org.springframework.context.annotation.Import;

 import org.springframework.core.io.support.SpringFactoriesLoader;

 /**

  * 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. For example, if you

  * have {@code tomcat-embedded.jar} on your classpath you are likely to want a

  * {@link TomcatServletWebServerFactory} (unless you have defined your own

  * {@link ServletWebServerFactory} bean).

  * <p>

  * When using {@link SpringBootApplication}, the auto-configuration of the context is

  * automatically enabled and adding this annotation has therefore no additional effect.

  * <p>

  * Auto-configuration tries to be as intelligent as possible and will back-away as you

  * define more of your own configuration. You can always manually {@link #exclude()} any

  * configuration that you never want to apply (use {@link #excludeName()} if you don't

  * have access to them). You can also exclude them via the

  * {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied

  * after user-defined beans have been registered.

  * <p>

  * The package of the class that is annotated with {@code @EnableAutoConfiguration},

  * usually via {@code @SpringBootApplication}, has specific significance and is often used

  * as a 'default'. For example, it will be used when scanning for {@code @Entity} classes.

  * It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're

  * not using {@code @SpringBootApplication}) in a root package so that all sub-packages

  * and classes can be searched.

  * <p>

  * Auto-configuration classes are regular Spring {@link Configuration} beans. They are

  * located using the {@link SpringFactoriesLoader} mechanism (keyed against this class).

  * Generally auto-configuration beans are {@link Conditional @Conditional} beans (most

  * often using {@link ConditionalOnClass @ConditionalOnClass} and

  * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).

  *

  * @author Phillip Webb

  * @author Stephane Nicoll

  * @since 1.0.0

  * @see ConditionalOnBean

  * @see ConditionalOnMissingBean

  * @see ConditionalOnClass

  * @see AutoConfigureAfter

  * @see SpringBootApplication

  */

 @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 {};

 }

其中，最关键的要属@Import(EnableAutoConfigurationImportSelector.class)，借助EnableAutoConfigurationImportSelector，@EnableAutoConfiguration可以帮助SpringBoot应用将所有符合条件的@Configuration配置都加载到当前SpringBoot创建并使用的IoC容器。

借助于Spring框架原有的一个工具类：SpringFactoriesLoader的支持，@EnableAutoConfiguration可以智能的自动配置功效才得以大功告成！

自动配置幕后英雄：SpringFactoriesLoader详解
SpringFactoriesLoader属于Spring框架私有的一种扩展方案，其主要功能就是从指定的配置文件META-INF/spring.factories加载配置。

 /*

  * Copyright 2002-2018 the original author or authors.

  *

  * Licensed under the Apache License, Version 2.0 (the "License");

  * you may not use this file except in compliance with the License.

  * You may obtain a copy of the License at

  *

  *      https://www.apache.org/licenses/LICENSE-2.0

  *

  * Unless required by applicable law or agreed to in writing, software

  * distributed under the License is distributed on an "AS IS" BASIS,

  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

  * See the License for the specific language governing permissions and

  * limitations under the License.

  */

 package org.springframework.core.io.support;

 import java.io.IOException;

 import java.net.URL;

 import java.util.ArrayList;

 import java.util.Collections;

 import java.util.Enumeration;

 import java.util.List;

 import java.util.Map;

 import java.util.Properties;

 import org.apache.commons.logging.Log;

 import org.apache.commons.logging.LogFactory;

 import org.springframework.core.annotation.AnnotationAwareOrderComparator;

 import org.springframework.core.io.UrlResource;

 import org.springframework.lang.Nullable;

 import org.springframework.util.Assert;

 import org.springframework.util.ClassUtils;

 import org.springframework.util.ConcurrentReferenceHashMap;

 import org.springframework.util.LinkedMultiValueMap;

 import org.springframework.util.MultiValueMap;

 import org.springframework.util.ReflectionUtils;

 import org.springframework.util.StringUtils;

 /**

  * General purpose factory loading mechanism for internal use within the framework.

  *

  * <p>{@code SpringFactoriesLoader} {@linkplain #loadFactories loads} and instantiates

  * factories of a given type from {@value #FACTORIES_RESOURCE_LOCATION} files which

  * may be present in multiple JAR files in the classpath. The {@code spring.factories}

  * file must be in {@link Properties} format, where the key is the fully qualified

  * name of the interface or abstract class, and the value is a comma-separated list of

  * implementation class names. For example:

  *

  * <pre class="code">example.MyService=example.MyServiceImpl1,example.MyServiceImpl2</pre>

  *

  * where {@code example.MyService} is the name of the interface, and {@code MyServiceImpl1}

  * and {@code MyServiceImpl2} are two implementations.

  *

  * @author Arjen Poutsma

  * @author Juergen Hoeller

  * @author Sam Brannen

  * @since 3.2

  */

 public final class SpringFactoriesLoader {

     /**

      * 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 final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);

     private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();

     private SpringFactoriesLoader() {

     }

     /**

      * Load and instantiate the factory implementations of the given type from

      * {@value #FACTORIES_RESOURCE_LOCATION}, using the given class loader.

      * <p>The returned factories are sorted through {@link AnnotationAwareOrderComparator}.

      * <p>If a custom instantiation strategy is required, use {@link #loadFactoryNames}

      * to obtain all registered factory names.

      * @param factoryClass the interface or abstract class representing the factory

      * @param classLoader the ClassLoader to use for loading (can be {@code null} to use the default)

      * @throws IllegalArgumentException if any factory implementation class cannot

      * be loaded or if an error occurs while instantiating any factory

      * @see #loadFactoryNames

      */

     public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {

         Assert.notNull(factoryClass, "'factoryClass' must not be null");

         ClassLoader classLoaderToUse = classLoader;

         if (classLoaderToUse == null) {

             classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();

         }

         List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);

         if (logger.isTraceEnabled()) {

             logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames);

         }

         List<T> result = new ArrayList<>(factoryNames.size());

         for (String factoryName : factoryNames) {

             result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));

         }

         AnnotationAwareOrderComparator.sort(result);

         return result;

     }

     /**

      * Load the fully qualified class names of factory implementations of the

      * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given

      * class loader.

      * @param factoryClass the interface or abstract class representing the factory

      * @param classLoader the ClassLoader to use for loading resources; can be

      * {@code null} to use the default

      * @throws IllegalArgumentException if an error occurs while loading factory names

      * @see #loadFactories

      */

     public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {

         String factoryClassName = factoryClass.getName();

         return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());

     }

     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);

         }

     }

     @SuppressWarnings("unchecked")

     private static <T> T instantiateFactory(String instanceClassName, Class<T> factoryClass, ClassLoader classLoader) {

         try {

             Class<?> instanceClass = ClassUtils.forName(instanceClassName, classLoader);

             if (!factoryClass.isAssignableFrom(instanceClass)) {

                 throw new IllegalArgumentException(

                         "Class [" + instanceClassName + "] is not assignable to [" + factoryClass.getName() + "]");

             }

             return (T) ReflectionUtils.accessibleConstructor(instanceClass).newInstance();

         }

         catch (Throwable ex) {

             throw new IllegalArgumentException("Unable to instantiate factory class: " + factoryClass.getName(), ex);

         }

     }

 }

配合@EnableAutoConfiguration使用的话，它更多是提供一种配置查找的功能支持，即根据@EnableAutoConfiguration的完整类名org.springframework.boot.autoconfigure.EnableAutoConfiguration作为查找的Key,获取对应的一组@Configuration类

上图就是从SpringBoot的spring-boot-autoconfigure-2.1.8.RELEASE.jar依赖包中的META-INF/spring.factories配置文件中摘录的一段内容，可以很好地说明问题。

所以，@EnableAutoConfiguration自动配置的魔法骑士就变成了：从classpath中搜寻所有的META-INF/spring.factories配置文件，并将其中org.springframework.boot.autoconfigure.EnableutoConfiguration对应的配置项通过反射（Java Refletion）实例化为对应的标注了@Configuration的JavaConfig形式的IoC容器配置类，然后汇总为一个并加载到IoC容器。

深入探索SpringApplication执行流程

SpringApplication的run方法的实现是我们本次旅程的主要线路，该方法的主要流程大体可以归纳如下：

1）如果我们使用的是SpringApplication的静态run方法，那么，这个方法里面首先要创建一个SpringApplication对象实例，然后调用这个创建好的SpringApplication的实例方法。在SpringApplication实例初始化的时候，它会提前做几件事情：

根据classpath里面是否存在某个特征类（org.springframework.web.context.ConfigurableWebApplicationContext）来决定是否应该创建一个为Web应用使用的ApplicationContext类型。
使用SpringFactoriesLoader在应用的classpath中查找并加载所有可用的ApplicationContextInitializer。
使用SpringFactoriesLoader在应用的classpath中查找并加载所有可用的ApplicationListener。
推断并设置main方法的定义类。

2） SpringApplication实例初始化完成并且完成设置后，就开始执行run方法的逻辑了，方法执行伊始，首先遍历执行所有通过SpringFactoriesLoader可以查找到并加载的SpringApplicationRunListener。调用它们的started()方法，告诉这些SpringApplicationRunListener，“嘿，SpringBoot应用要开始执行咯！”。

3）创建并配置当前Spring Boot应用将要使用的Environment（包括配置要使用的PropertySource以及Profile）。

4）遍历调用所有SpringApplicationRunListener的environmentPrepared()的方法，告诉他们：“当前SpringBoot应用使用的Environment准备好了咯！”。

5）如果SpringApplication的showBanner属性被设置为true，则打印banner。

6）根据用户是否明确设置了applicationContextClass类型以及初始化阶段的推断结果，决定该为当前SpringBoot应用创建什么类型的ApplicationContext并创建完成，然后根据条件决定是否添加ShutdownHook，决定是否使用自定义的BeanNameGenerator，决定是否使用自定义的ResourceLoader，当然，最重要的，将之前准备好的Environment设置给创建好的ApplicationContext使用。

7） ApplicationContext创建好之后，SpringApplication会再次借助SpringFactoriesLoader，查找并加载classpath中所有可用的ApplicationContextInitializer，然后遍历调用这些ApplicationContextInitializer的initialize（applicationContext）方法来对已经创建好的ApplicationContext进行进一步的处理。

8）遍历调用所有SpringApplicationRunListener的contextPrepared()方法。

9）最核心的一步，将之前通过@EnableAutoConfiguration获取的所有配置以及其他形式的IoC容器配置加载到已经准备完毕的ApplicationContext。

10）遍历调用所有SpringApplicationRunListener的contextLoaded()方法。

11）调用ApplicationContext的refresh()方法，完成IoC容器可用的最后一道工序。

12）查找当前ApplicationContext中是否注册有CommandLineRunner，如果有，则遍历执行它们。

13）正常情况下，遍历执行SpringApplicationRunListener的finished()方法。（如果整个过程出现异常，则依然调用所有SpringApplicationRunListener的finished()方法，只不过这种情况下会将异常信息一并传入处理）
去除事件通知点后，整个流程如下：

总结

到此，SpringBoot的核心组件完成了基本的解析，综合来看，大部分都是Spring框架背后的一些概念和实践方式，SpringBoot只是在这些概念和实践上对特定的场景事先进行了固化和升华，而也恰恰是这些固化让我们开发基于Sping框架的应用更加方便高效。

参考

参考了《SpringBoot揭秘快速构建为服务体系》这本书的第三章，感兴趣的可以查阅。

巴特西

【转载】Spring boot学习记录（三）-启动原理解析

正文

SpringBootApplication背后的秘密

@Configuration

@ComponentScan

@EnableAutoConfiguration

深入探索SpringApplication执行流程

总结

参考

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