springboot源码解析
1、初始化SpringApplication
启动的第一步，先调用SpringApplication的静态函数run方法，run方法会调用SpringApplication的有参构造方法来初始化SpringApplication。
而SpringApplication的初始化方法主要做以下几件事情：
1、推断当前 WEB 应用类型，一共有三种：
NONE：无内嵌的web容器启动，这种模式springboot需要运行于外部的web容器中
SERVLET：使用内嵌的web容器启动
REACTIVE：使用spring5的新特性，响应式启动
2、从META-INF/spring.factories读取对应的ApplicationContextInitializer的类装配到集合中并去重
3、从META-INF/spring.factories读取对应ApplicationListener的类装配到集合中并去重*
SpringApplication的构造函数实例化了 初始化上下文的各种接口ApplicationContextInitializer以及监听器ApplicationListener，要注意的是这里的实例化，并不像平时的Spring Components一样通过注解和扫包完成，而是通过一种不依赖Spring上下文的加载方法，这样才能在Spring完成启动前做各种配置。Spring的解决方法是以接口的全限定名作为key，实现类的全限定名作为value记录在项目的META-INF/spring.factories文件中，然后通过SpringFactoriesLoader工具类提供静态方法进行类加载并缓存下来，spring.factories是Spring Boot的核心配置文件，目的是帮助Spring Boot完成自动化装配。
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
.........................................
//推断当前 WEB 应用类型
this.webApplicationType = WebApplicationType.deduceFromClasspath();
//从"META-INF/spring.factories"读取对应的ApplicationContextInitializer类的装配到集合中并去重
this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
//从"META-INF/spring.factories"读取对应ApplicationListener类的装配到集合中并去重
this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
//获取Main方法所在类，并赋值给mainApplicationClass
this.mainApplicationClass = this.deduceMainApplicationClass();
}
spring.factories的部分内容：
# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer
2、执行SpringApplication.run启动容器
启动的步骤主要是以下几个步骤：
1、stopWatch.start()：记录容器启动耗费的时间
2、this.getRunListeners(args)：从"META-INF/spring.factories"获取key为SpringApplicationRunListener的运行时监听器，其实就是EventPublishingRunListener监听器
3、listeners.starting() 向运行时监听器注册监听事件
4、ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments)根据监听器准备spring容器的环境变量
5、context = this.createApplicationContext()根据WEB 应用类型创建相应的容器，一共有三种：
NONE：AnnotationConfigApplicationContext
SERVLET：AnnotationConfigServletWebServerApplicationContext
REACTIVE：AnnotationConfigReactiveWebServerApplicationContext
6、this.prepareContext(context, environment, listeners, applicationArguments, printedBanner)这里主要是给容器设置前面准备好的环境变量，并且执行之前在spring.factories中读取的ApplicationContextInitializer类的initialize方法
7、this.refreshContext(context)刷新容器，这里比较重点后面单独分析
8、this.afterRefresh(context, applicationArguments)这里是空的方法，用于子类继承做一些容器启动后的拓展
9、监听器发布ApplicationStartedEvent事件和ApplicationReadyEvent事件
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
//记录容器启动耗费的时间
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
this.configureHeadlessProperty();
//从"META-INF/spring.factories"获取key为SpringApplicationRunListener的监听器，其实就是EventPublishingRunListener监听器
SpringApplicationRunListeners listeners = this.getRunListeners(args);
//遍历并集合中的监听器，并注册需要监听事件
listeners.starting();
Collection exceptionReporters;
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
//准备spring容器的环境变量
ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
this.configureIgnoreBeanInfo(environment);
Banner printedBanner = this.printBanner(environment);
//根据WEB 应用类型创建相应的容器
context = this.createApplicationContext();
exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
//这里主要是给容器设置前面准备好的环境变量，并且执行之前在spring.factories中读取的ApplicationContextInitializer类的initialize方法
this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//刷新容器
this.refreshContext(context);
//这里是空的方法，用于子类继承做一些容器启动后的拓展
this.afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
(new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
}
//监听器发布ApplicationStartedEvent事件
listeners.started(context);
try {
listeners.running(context);//发布ApplicationReadyEvent事件
}
......................................
//返回容器上下文
return context;
}
private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment, SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
context.setEnvironment(environment);
this.postProcessApplicationContext(context);
this.applyInitializers(context);
listeners.contextPrepared(context);
......................................
}
3、refresh()刷新容器
1、prepareRefresh()容器的预处理：记录启动时间，设置容器的状态，预留钩子方法给子类做属性设置，保存早期监听事件
2、obtainFreshBeanFactory()获取DefaultListableBeanFactory的作为工厂对象
3、prepareBeanFactory(beanFactory)设置工厂对象的属性，包括：类加载器、表达式解析器、添加后置处理器ApplicationContextAwareProcessor,系统环境属性Environment等
4、postProcessBeanFactory(beanFactory)钩子方法，预留给子类实现，在工厂对象准备刷新容器前做一些处理
5、invokeBeanFactoryPostProcessors(beanFactory)获取BeanDefinitionRegistryPostProcessor，遍历并根据优先级(继承order接口)执行其后置方法postProcessBeanDefinitionRegistry()方法。然后在获取所有的BeanFactoryPostProcessor的，再遍历并根据优先级(继承order接口)执行后置方法postProcessBeanFactory(),由此可见后置处理器BeanDefinitionRegistryPostProcessor是在BeanFactoryPostProcessor之前执行
6、registerBeanPostProcessors(beanFactory)获取所有的BeanPostProcessor，例如：InstantiationAwareBeanPostProcessor等，根据优先级(继承order接口)将所有的BeanPostProcessor注册到工厂对象中
7、initMessageSource()进行国际化
8、initApplicationEventMulticaster() 初始化的事件派发器
9、onRefresh()钩子方法，预留给子类进行拓展处理，其中springboot的web容器AnnotationConfigServletWebServerApplicationContext会在这里启动Tomcat
10、registerListeners()注册事件监听器，监听器需要实现 ApplicationListener 接口
11、finishBeanFactoryInitialization(beanFactory)初始化所有的单实例bean,主要方法在工厂对象的preInstantiateSingletons()方法，先获取RootBeanBefinition，在判断是否是factoryBean,是则调用factoryBean的方法创建bean,不是则调用getBean()方法进行bean的创建，getBean()方法是spring创建bean的核心类，我之前在spring源码分析那里分析过，这里不再赘述，主要流程就先从缓存中获取bean,获取不到就从头开始创建bean，并经过bean的实例化、属性赋值、初始化的生命周期，中间调用Aware类的方法和上面注册的BeanPostProcessor的前后置方法，完成IOC创建
12、finishRefresh()容器调用LifecycleProcessor生命周期处理器的onRefresh()和onClose()方法，然后调用publishEvent((ApplicationEvent)(new ContextRefreshedEvent(this)))发布容器刷新完成事件
public void refresh() throws BeansException, IllegalStateException {
synchronized(this.startupShutdownMonitor) {
StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
this.prepareRefresh();
ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
this.prepareBeanFactory(beanFactory);
try {
this.postProcessBeanFactory(beanFactory);
StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
this.invokeBeanFactoryPostProcessors(beanFactory);
this.registerBeanPostProcessors(beanFactory);
beanPostProcess.end();
this.initMessageSource();
this.initApplicationEventMulticaster();
this.onRefresh();
this.registerListeners();
this.finishBeanFactoryInitialization(beanFactory);
this.finishRefresh();
} catch (BeansException var10) {
if (this.logger.isWarnEnabled()) {
this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var10);
}
this.destroyBeans();
this.cancelRefresh(var10);
throw var10;
} finally {
this.resetCommonCaches();
contextRefresh.end();
}
}
}
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
.......................................
beanFactory.preInstantiateSingletons();
}
public void preInstantiateSingletons() throws BeansException {
List<String> beanNames = new ArrayList(this.beanDefinitionNames);
Iterator var2 = beanNames.iterator();
while(true) {
String beanName;
Object bean;
do {
while(true) {
RootBeanDefinition bd;
do {
do {
do {
if (!var2.hasNext()) {
var2 = beanNames.iterator();
while(var2.hasNext()) {
beanName = (String)var2.next();
Object singletonInstance = this.getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton)singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(() -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, this.getAccessControlContext());
} else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
return;
}
beanName = (String)var2.next();
bd = this.getMergedLocalBeanDefinition(beanName);
} while(bd.isAbstract());
} while(!bd.isSingleton());
} while(bd.isLazyInit());
if (this.isFactoryBean(beanName)) {
bean = this.getBean("&" + beanName);
break;
}
this.getBean(beanName);
}
} while(!(bean instanceof FactoryBean));
FactoryBean<?> factory = (FactoryBean)bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
SmartFactoryBean var10000 = (SmartFactoryBean)factory;
((SmartFactoryBean)factory).getClass();
isEagerInit = (Boolean)AccessController.doPrivileged(var10000::isEagerInit, this.getAccessControlContext());
} else {
isEagerInit = factory instanceof SmartFactoryBean && ((SmartFactoryBean)factory).isEagerInit();
}
if (isEagerInit) {
this.getBean(beanName);
}
}
}
4、springboot的自动化装配
@SpringBootApplication上的注解@EnableAutoConfiguration，通过@Import({AutoConfigurationImportSelector.class})注入AutoConfigurationImportSelector，AutoConfigurationImportSelector通过调用getAutoConfigurationEntry方法，方法中会调用getCandidateConfigurations方法，然后通过SpringFactoriesLoader.loadFactoryNames方法，从META-INF/spring.factories读取EnableAutoConfiguration的类为key,value为需要自动装配的xxxConfiguration配置类的数组，通过SpringFactoriesLoader机制创建对应的bean，注入到容器中，完成了自动装配。
这本来需要在spring的xml配置文件中去配置bean的操作就免去了 ，也就是说我们可以事先在xxxConfiguration中做好各种默认配置，然后通过加入到spring.factories中让spring自动读取并装配，实现了starter默认配置达到了约定大于配置的思想，这就是也就是springboot的自动装配
简单的流程如下：@EnableAutoConfiguration->@Import({AutoConfigurationImportSelector.class})->SpringFactoriesLoader.loadFactoryNames->META-INF/spring.factories->org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxxConfiguration，xxxConfiguration....
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
Class<?>[] exclude() default {};
String[] excludeName() default {};
}
AutoConfigurationImportSelector类及对应的SpringFactoriesLoader.loadFactoryNames及SpringFactoriesLoader.loadSpringFactories方法
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
if (!this.isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
} else {
AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
configurations = this.removeDuplicates(configurations);
Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
this.checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = this.getConfigurationClassFilter().filter(configurations);
this.fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
}
}
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;
}
}
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
String factoryTypeName = factoryType.getName();
return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
if (result != null) {
return result;
} else {
try {
Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
LinkedMultiValueMap result = new LinkedMultiValueMap();
while(urls.hasMoreElements()) {
URL url = (URL)urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
Iterator var6 = properties.entrySet().iterator();
while(var6.hasNext()) {
Entry<?, ?> entry = (Entry)var6.next();
String factoryTypeName = ((String)entry.getKey()).trim();
String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
int var10 = var9.length;
for(int var11 = 0; var11 < var10; ++var11) {
String factoryImplementationName = var9[var11];
result.add(factoryTypeName, factoryImplementationName.trim());
}
}
}
cache.put(classLoader, result);
return result;
} catch (IOException var13) {
throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
}
}
}
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
