Spring Boot作为目前最流行的Java开发框架，秉承“约定优于配置”原则，大大简化了Spring MVC繁琐的XML文件配置，基本实现零配置启动项目。
本文基于
Spring Boot 2.1.0.RELEASE版本了解Spring Boot如何启动
首先让我们看一下最简单的Spring Boot启动代码
@SpringBootApplication
public class DemoApplication {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
}
每一个使用过Spring Boot的同学对于上面的代码应该都非常熟悉了，通过这段代码即可启动Spring Boot应用。那么SpringApplication.run(DemoApplication.class, args)内部到底做了什么事情呢？
在查看具体代码之前，我们先了解一下SpringApplication内部大概的执行流程，如下图
从上图中可以看出run()是整个应用的入口，接着初始化SpringApplicationRunListener，Environment等实例，然后创建应用上下文对象，“准备”并“刷新”上下文，到这里Spring容器已基本启动完成，最后发送事件通知各个组件作出相应动作。
源码分析
在了解完大概的流程之后，下面开始深入源码分析Spring Boot具体的启动过程，首先进入入口方法run
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();
// ...
StopWatch主要是用来统计每项任务执行时长，例如Spring Boot启动占用总时长。
Started DemoApplication in 4.241 seconds (JVM running for 5.987)
getRunListeners()完成了SpringApplicationRunListener实例化工作，如何完成的呢？进入方法内部查看
private SpringApplicationRunListeners getRunListeners(String[] args) {
Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(
SpringApplicationRunListener.class, types, this, args));
}
SpringApplicationRunListeners和SpringApplicationRunListener不是同一个类，它们名称非常相似
查看SpringApplicationRunListeners源码
SpringApplicationRunListeners(Log log,
Collection<? extends SpringApplicationRunListener> listeners) {
this.log = log;
this.listeners = new ArrayList<>(listeners);
}
public void starting() {
for (SpringApplicationRunListener listener : this.listeners {
listener.starting();
}
}
public void environmentPrepared() {
// ....
}
public void contextPrepared() {
// ....
}
public void contextLoaded() {
// ....
}
public void started() {
// ....
}
public void running() {
// ....
}
它是
SpringApplicationRunListener的一个集合
观察SpringApplicationRunListeners所有方法，可以看出，它实际是一个用来发送SpringApplicationRunListener相关事件的工具类
接着继续观察getSpringFactoriesInstances源码，看它是如何实例化对象的（此方法后续多处使用）
private <T> Collection<T> getSpringFactoriesInstance(Class<T> type,
Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// 加载对象名称
Set<String> names = new LinkedHashSet<>(
SpringFactoriesLoader.loadFactoryNames(type,classLoader));
List<T> instances = createSpringFactoriesInstances(type parameterTypes,
classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
这里通过SpringFactoriesLoader.loadFactoryNames获取type对应的FactoryNames，不明白有什么用处？进入方法内部查看
public static List<String> loadFactoryNames(Class<?>factoryClass, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
return loadSpringFactories(classLoader).getOrDefaul(factoryClassName, Collections.emptyList());
}
继续进入loadSpringFactories方法内部
public static final String FACTORIES_RESOURCE_LOCATION ="META-INF/spring.factories";
private static Map<String, List<String>> loadSpringFactorie(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.ge(classLoader);
if (result != null) {
return result;
}
try {
// 获取 META-INF/spring.factories 对应的资源
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResource(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResource(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
// 读取文件内容
Properties properties =PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySe()) {
String factoryClassName = ((String)entry.getKey()).trim();
for (String factoryName :StringUtils.commaDelimitedListToStringArray(String) entry.getValue())) {
// 获取 factoryClassName 对应的多个valu（多个value用逗号分隔）
result.add(factoryClassName,factoryName.trim());
}
}
}
// 缓存已经读取到的内容
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to loadfactories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
看到这里可能会疑惑META-INF/spring.factories文件在哪里？文件里面有什么内容？
其实这个文件存放在Spring Boot和Spring Boot autoconfigure的jar包内部（有兴趣的同学可以自行下载jar包并解压查看），Spring Boot中的文件内容如下：
# 完整内容请查看原文件
# Run Listeners
org.springframework.boot.SpringApplicationRunListener=
org.springframework.boot.context.event.EventPublishingRunListener
# Application Listeners
org.springframework.context.ApplicationListener=
org.springframework.boot.ClearCachesApplicationListener,
org.springframework.boot.builder.ParentContextCloserApplicationListener,
org.springframework.boot.context.FileEncodingApplicationListener,
org.springframework.boot.context.config.AnsiOutputApplicationListener,
org.springframework.boot.context.config.ConfigFileApplicationListener,
org.springframework.boot.context.config.DelegatingApplicationListener,
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,
org.springframework.boot.context.logging.LoggingApplicationListener,
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener
可以看到SpringApplicationRunListener对应的值是EventPublishingRunListener
回到SpringFactoriesLoader.loadFactoryNames方法内部，可以发现方法获取的值实际上是factoryClass在META-INF/spring.factories中对应的实现类的集合
明白这个方法之后，再回到getSpringFactoriesInstances方法
private <T> Collection<T> getSpringFactoriesInstance(Class<T> type,
Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// 获取 SpringApplicationRunListener 对应的实现类的名称集合
Set<String> names = new LinkedHashSet<>(
SpringFactoriesLoader.loadFactoryNames(type,classLoader));
// 通过反射实例化对象
List<T> instances = createSpringFactoriesInstances(type parameterTypes,
classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
到此为止getRunListeners完成了SpringApplicationRunListener对应实现类的实例化，并回调其starting方法
SpringApplicationRunListeners listeners getRunListeners(args);
listeners.starting();
从上面分析得知，实际上调用的是EventPublishingRunListener的starting方法，那么方法内部做了什么呢？
public void starting() {
this.initialMulticaster.multicastEvent(
new ApplicationStartingEvent(this.application,this.args));
}
发送了一个ApplicationStartingEvent事件
继续查找ApplicationStartingEvent事件的消费者，从spring.factories中可以找到所有预定义的事件消费者
# Application Listeners
org.springframework.context.ApplicationListener=
org.springframework.boot.ClearCachesApplicationListener,
org.springframework.boot.builder.ParentContextCloserApplicationListener,
org.springframework.boot.context.FileEncodingApplicationListener,
org.springframework.boot.context.config.AnsiOutputApplicationListener,
org.springframework.boot.context.config.ConfigFileApplicationListener,
org.springframework.boot.context.config.DelegatingApplicationListener,
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,
org.springframework.boot.context.logging.LoggingApplicationListener,
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener
# Application Listeners
org.springframework.context.ApplicationListener=
org.springframework.boot.autoconfigure.BackgroundPreinitializer
接下来要做的就是从这些消费者中找出ApplicationStartingEvent事件的消费者（查找过程省略），找到以下两个消费者
-
LoggingApplicationListener
初始化日志系统 -
LiquibaseServiceLocatorApplicationListener
（参数liquibase.servicelocator.ServiceLocator）如果存在，则使用springboot相关的版本进行替代
了解完ApplicationStartingEvent事件之后，回到run方法继续往下探究prepareEnvironment
private ConfigurableEnvironment prepareEnvironment(
SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// 创建Environment对象
ConfigurableEnvironment environment =getOrCreateEnvironment();
configureEnvironment(environment,applicationArguments.getSourceArgs());
// 发布ApplicationEnvironmentPreparedEvent事件
listeners.environmentPrepared(environment);
bindToSpringApplication(environment);
if (!this.isCustomEnvironment) {
environment = new EnvironmentConverte(getClassLoader())
.convertEnvironmentIfNecessary(environment,deduceEnvironmentClass());
}
ConfigurationPropertySources.attach(environment);
return environment;
}
这里又发布了一个ApplicationEnvironmentPreparedEvent事件，继续查找事件监听对象
- FileEncodingApplicationListener
检查系统文件编码格式是否符合环境变量中配置的文件编码格式（如果存在相关设置 - spring.mandatory-file-encoding），如果编码不符合，则抛出异常阻止Spring启动 - AnsiOutputApplicationListener
是否开启AnsiOutput - DelegatingApplicationListener
代理context.listener.classes配置的监听者 - ClasspathLoggingApplicationListener
日志输出classpath - LoggingApplicationListener
配置日志系统，logging.config, logging.level...等 - ConfigFileApplicationListener
这是一个比较重要的监听对象，具体的方法实现如下
private void onApplicationEnvironmentPreparedEvent(
ApplicationEnvironmentPreparedEvent event) {
List<EnvironmentPostProcessor> postProcessors =loadPostProcessors();
postProcessors.add(this);
AnnotationAwareOrderComparator.sort(postProcessors);
for (EnvironmentPostProcessor postProcessor :postProcessors) {
postProcessor.postProcessEnvironmen(event.getEnvironment(),
event.getSpringApplication());
}
}
List<EnvironmentPostProcessor> loadPostProcessors() {
return SpringFactoriesLoader.loadFactorie(EnvironmentPostProcessor.class,
getClass().getClassLoader());
}
通过spring.factories，可以看到这里加载以下EnvironmentPostProcessor对象
- CloudFoundryVcapEnvironmentPostProcessor
- SpringApplicationJsonEnvironmentPostProcessor
- SystemEnvironmentPropertySourceEnvironmentPostProcessor
- ConfigFileApplicationListener
很多同学可能会疑问ConfigFileApplicationListener并不存在spring.factories文件中，这里为什么会有它呢？
实际上ConfigFileApplicationListener在onApplicationEnvironmentPreparedEvent方法中，将自身添加到EnvironmentPostProcessor对象列表中。
我们主要关注ConfigFileApplicationListener的postProcessEnvironment方法
public void postProcessEnvironment(ConfigurableEnvironmentenvironment,
SpringApplication application) {
addPropertySources(environment,application.getResourceLoader());
}
protected void addPropertySources(ConfigurableEnvironmentenvironment,
ResourceLoader resourceLoader) {
RandomValuePropertySource.addToEnvironment(environment);
// 读取applicaiton.yml, application.properties等配置文件
new Loader(environment, resourceLoader).load();
}
ConfigFileApplicationListener监听到ApplicationEnvironmentPreparedEvent事件之后开始读取本地配置文件
关于Spring如何读取本地配置文件，请前往Spring Boot源码分析-配置文件加载原理
创建ApplicationContext对象
protected ConfigurableApplicationContextcreateApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
// 根据webApplicationType创建对应上下文对象
switch (this.webApplicationType) {
case SERVLET:
contextClass = Class.forNam(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
break;
case REACTIVE:
contextClass = Class.forNam(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
break;
default:
contextClass = Class.forNam(DEFAULT_CONTEXT_CLASS);
}
}
catch (ClassNotFoundException ex) {
throw new IllegalStateException(
"Unable create a defaultApplicationContext, "
+ "please specify anApplicationContextClass",
ex);
}
}
return (ConfigurableApplicationContext)BeanUtils.instantiateClass(contextClass);
}
这里是根据webApplicationType决定创建什么类型的ApplicationContext对象，那么webApplicationType是何时赋值的呢？
public SpringApplication(ResourceLoader resourceLoader,Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must notbe null");
this.primarySources = new LinkedHashSet<>(Arrays.asLis(primarySources));
// 初始化webApplicationType
this.webApplicationType =WebApplicationType.deduceFromClasspath();
setInitializers((Collection) getSpringFactoriesInstance(
ApplicationContextInitializer.class));
setListeners((Collection) getSpringFactoriesInstance(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass(;
}
从上面可以看出是通过WebApplicationType.deduceFromClasspath方法初始化的webApplicationType，继续跟踪代码
private static final String WEBFLUX_INDICATOR_CLASS = "org."
+ "springframework.web.reactive.DispatcherHandler";
private static final String WEBMVC_INDICATOR_CLASS = "org.springframework."
+ "web.servlet.DispatcherServlet";
private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer";
private static final String[] SERVLET_INDICATOR_CLASSES = { "javax.servlet.Servlet",
"org.springframework.web.context.ConfigurableWebApplicationContext" };
static WebApplicationType deduceFromClasspath() {
if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null)
&& !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS null)
&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS null)) {
return WebApplicationType.REACTIVE;
}
for (String className : SERVLET_INDICATOR_CLASSES) {
if (!ClassUtils.isPresent(className, null)) {
return WebApplicationType.NONE;
}
}
return WebApplicationType.SERVLET;
}
从上面代码中可以看出Spring是通过当前classpath下是否存在相应的类，从而决定webApplicationType类型
初始化ApplicationContext对象
private void prepareContext(ConfigurableApplicationContextcontext,
ConfigurableEnvironment environment,SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments, BannerprintedBanner) {
// 初始化context
context.setEnvironment(environment);
postProcessApplicationContext(context);
applyInitializers(context);
// 发送ApplicationContextInitializedEvent消息
listeners.contextPrepared(context);
if (this.logStartupInfo) {
logStartupInfo(context.getParent() == null);
logStartupProfileInfo(context);
}
// Add boot specific singleton beans
ConfigurableListableBeanFactory beanFactory =context.getBeanFactory();
beanFactory.registerSingleto("springApplicationArguments", applicationArguments);
if (printedBanner != null) {
beanFactory.registerSingleton("springBootBanner",printedBanner);
}
if (beanFactory instanceof DefaultListableBeanFactory) {
((DefaultListableBeanFactory) beanFactory)
.setAllowBeanDefinitionOverridin(this.allowBeanDefinitionOverriding);
}
Set<Object> sources = getAllSources();
Assert.notEmpty(sources, "Sources must not be empty");
// 注册DemoApplication
load(context, sources.toArray(new Object[0]));
listeners.contextLoaded(context);
}
这里注册了DemoApplication到Spring容器中，为后续bean扫描做准备
接下来继续深入refreshContext方法，可以发现实际上是执行了AbstractApplicationContext.refresh方法
public void refresh() throws BeansException,IllegalStateException {
synchronized (this.startupShutdownMonitor) {
prepareRefresh();
ConfigurableListableBeanFactory beanFactory =obtainFreshBeanFactory();
prepareBeanFactory(beanFactory);
try {
postProcessBeanFactory(beanFactory);
// 完成bean的加载
invokeBeanFactoryPostProcessors(beanFactory);
registerBeanPostProcessors(beanFactory);
initMessageSource();
initApplicationEventMulticaster();
onRefresh();
registerListeners();
finishBeanFactoryInitialization(beanFactory);
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered duringcontext initialization - " +
"cancelling refresh attempt: " + ex;
}
destroyBeans();
cancelRefresh(ex);
throw ex;
}
finally {
resetCommonCaches();
}
}
}
refresh方法内部做了很多事情。比如：完成BeanFactory设置，BeanFactoryPostProcessor、BeanPostProcessor接口回调，Bean加载，国际化配置等。
到此为止Spring基本完成了容器的初始化工作，最后在调用callRunners方法，执行ApplicationRunner、CommandLineRunner接口。
private void callRunners(ApplicationContext context,ApplicationArguments args) {
List<Object> runners = new ArrayList<>();
runners.addAll(context.getBeansOfTyp(ApplicationRunner.class).values());
runners.addAll(context.getBeansOfTyp(CommandLineRunner.class).values());
AnnotationAwareOrderComparator.sort(runners);
for (Object runner : new LinkedHashSet<>(runners)) {
if (runner instanceof ApplicationRunner) {
callRunner((ApplicationRunner) runner, args);
}
if (runner instanceof CommandLineRunner) {
callRunner((CommandLineRunner) runner, args);
}
}
}
整个启动过程的核心方法是refresh，此方法内部承载大部分容器启动所需的工作。由于篇幅原因，后续再进行refresh内部源码分析，了解Spring Boot加载Bean的整个过程。