真的好奇害死猫！之前写过几个SpringBoot应用，但是一直没搞明白应用到底是怎么启动的，心里一直有点膈应。好吧，趁有空去看了下源码，写下这篇博客作为学习记录吧！
个人拙见，若哪里有理解不对的地方，请各路大神指正，小弟不胜感激！
一.应用启动类
@SpringBootApplication
public class DemoApplication {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
}开发SpirngBoot应用时，入口类就这简单的几行。但是却完成了N多服务的初始化、加载和发布。那么这几行代码究竟干了什么呢，SpringBoot应用到底是怎么启动的。
二.@SpringBootApplication注解
2.1.SpringBootApplication注解
@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 {@SpringBootApplication=@SpringBootConfiguration+@EnableAutoConfiguration+@ComponentScan
2.2.@SpringBootConfiguration
/**
* Indicates that a class Spring Boot application
* {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
* standard {@code @Configuration} annotation so that configuration can be found
* automatically (for example in tests).
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}SpringBootConfiguration注解和Spring的@Configuration注解作用一样。标注当前类是配置类，并会将当前类内声明的一个或多个以@Bean注解标记的方法的实例纳入到spring容器中。比如容器加载时，会生成Hello的Bean加载到IOC容器中。
@SpringBootConfiguration
public class ExampleConfig {
@Bean
public void Hello(){
System.out.println("hello");
}
}2.3.@EnableAutoConfiguration
@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}这个注解是SpringBoot能进行自动配置的关键。@Import注解用于导入配置类，我们看下导入类EnableAutoConfigurationImportSelector。容器刷新时，会调用AutoConfigurationImportSelector类的selectImports方法，扫描META-INF/spring.factories文件自动配置类（key为EnableAutoConfiguration），然后Spring容器处理配置类。（对Spring的一些加载过程不清晰，我是相当的迷啊）
2.4.@ComponentScan
@ComponentScan(excludeFilters = {
@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
/**
* Configures component scanning directives for use with @{@link Configuration} classes.
* Provides support parallel with Spring XML's {@code <context:component-scan>} element.
*
* <p>Either {@link #basePackageClasses} or {@link #basePackages} (or its alias
* {@link #value}) may be specified to define specific packages to scan. If specific
* packages are not defined, scanning will occur from the package of the
* class that declares this annotation.
*/
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan @ComponentScan扫描指定的包路径，若未指定包路径，则以声明这个注解的类作为基本包路径。比如@SpringBootApplication就没有指定包路径，则DemoApplication的包路径将作为扫描的基本包路径，因此强烈建议将主类放在顶层目录下。
excludeFilters属性指定哪些类型不符合组件扫描的条件，会在扫描的时候过滤掉。
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class)比如上面这段代码。@Filter声明了过滤器类型类为自定义类型（需要实现TypeFilter接口），过滤器为AutoConfigurationExcludeFilter。当match方法为true，返回扫描类对象，否则过滤掉。但是要注意@ComponentScan的key为excludeFilters，因此这些类型将在包扫描的时候过滤掉，也就是说，ComponentScan在扫描时，发现当前扫描类满足macth的条件(match返回true)，是不会将该类加载到容器的。
//metadataReader 表示读取到的当前正在扫描的类的信息
//metadataReaderFactory 表示可以获得到其他任何类的信息
@Override
public boolean match(MetadataReader metadataReader,
MetadataReaderFactory metadataReaderFactory) throws IOException {
return isConfiguration(metadataReader) && isAutoConfiguration(metadataReader);
}
//该类是带有Configuration注解的配置类
private boolean isConfiguration(MetadataReader metadataReader) {
return metadataReader.getAnnotationMetadata()
.isAnnotated(Configuration.class.getName());
}
//该类是否为spring.factory配置的自动配置类
private boolean isAutoConfiguration(MetadataReader metadataReader) {
return getAutoConfigurations()
.contains(metadataReader.getClassMetadata().getClassName());
}三.run(DemoApplication.class, args)解析
3.1.进入SpringApplication
public static ConfigurableApplicationContext run(Class<?>[] primarySources,
String[] args) {
return new SpringApplication(primarySources).run(args);
}我们根据DemoApplication跟进代码，发现其调用的SpringApplication类的run方法。这个方法就干了2件事：一是创建SpringApplication对象，二是启动SpringApplication。
3.2.SpringApplication构造器分析
1.构造器
public SpringApplication(Class<?>... primarySources) {
this(null, primarySources);
}
/**
* Create a new {@link SpringApplication} instance. The application context will load
* beans from the specified primary sources
*/
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
//根据应用是否存在某些类推断应用类型，分为响应式web应用，servlet类型web应用和非web应用，在后面用于确定实例化applicationContext的类型
this.webApplicationType = WebApplicationType.deduceFromClasspath();
//设置初始化器，读取spring.factories文件key ApplicationContextInitializer对应的value并实例化
//ApplicationContextInitializer接口用于在Spring上下文被刷新之前进行初始化的操作
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
//设置监听器，读取spring.factories文件key ApplicationListener对应的value并实例化
// interface ApplicationListener<E extends ApplicationEvent> extends EventListener
//ApplicationListener继承EventListener，实现了观察者模式。对于Spring框架的观察者模式实现，它限定感兴趣的事件类型需要是ApplicationEvent类型事件
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
//没啥特别作用，仅用于获取入口类class对象
this.mainApplicationClass = deduceMainApplicationClass();
}在构造器里主要干了2件事，一个设置初始化器，二是设置监听器。
2.设置初始化器
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
return getSpringFactoriesInstances(type, new Class<?>[] {});
}
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// Use names and ensure unique to protect against duplicates
Set<String> names = new LinkedHashSet<>(
//从类路径的META-INF处读取相应配置文件spring.factories，然后进行遍历，读取配置文件中Key(type)对应的value
SpringFactoriesLoader.loadFactoryNames(type, classLoader));
//将names的对象实例化
List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}根据入参type类型ApplicationContextInitializer.class从类路径的META-INF处读取相应配置文件spring.factories并实例化对应Initializer。上面这2个函数后面会反复用到。
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer3.设置监听器
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));和设置初始化器一个套路，通过getSpringFactoriesInstances函数实例化监听器。
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.LiquibaseServiceLocatorApplicationListener3.3.run(String... args)解析
1.run函数
/**
* Run the Spring application, creating and refreshing a new ApplicationContext
*/
public ConfigurableApplicationContext run(String... args) {
//计时器
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
//设置java.awt.headless系统属性为true，Headless模式是系统的一种配置模式。
// 在该模式下，系统缺少了显示设备、键盘或鼠标。但是服务器生成的数据需要提供给显示设备等使用。
// 因此使用headless模式，一般是在程序开始激活headless模式，告诉程序，现在你要工作在Headless mode下，依靠系统的计算能力模拟出这些特性来
configureHeadlessProperty();
//获取监听器集合对象
SpringApplicationRunListeners listeners = getRunListeners(args);
//发出开始执行的事件。
listeners.starting();
try {
//根据main函数传入的参数，创建DefaultApplicationArguments对象
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
//根据扫描到的监听器对象和函数传入参数，进行环境准备。
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
configureIgnoreBeanInfo(environment);
Banner printedBanner = printBanner(environment);
context = createApplicationContext();
//和上面套路一样，读取spring.factories文件key SpringBootExceptionReporter对应的value
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
//和上面的一样，context准备完成之后，将触发SpringApplicationRunListener的contextPrepared执行
refreshContext(context);
//其实啥也没干。但是老版本的callRunners好像是在这里执行的。
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
//发布ApplicationStartedEvent事件，发出结束执行的事件
listeners.started(context);
//在某些情况下，我们希望在容器bean加载完成后执行一些操作，会实现ApplicationRunner或者CommandLineRunner接口
//后置操作，就是在容器完成刷新后，依次调用注册的Runners，还可以通过@Order注解设置各runner的执行顺序。
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;
}2.获取run listeners
SpringApplicationRunListeners listeners = getRunListeners(args);和构造器设置初始化器一个套路，根据传入type SpringApplicationRunListener去扫描spring.factories文件，读取type对应的value并实例化。然后利用实例化对象创建SpringApplicationRunListeners对象。
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListenerEventPublishingRunListener的作用是发布SpringApplicationEvent事件。
EventPublishingRunListener更像是被监听对象，这个命名让我有点迷。
public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {
......
@Override
public void starting() {
this.initialMulticaster.multicastEvent(
new ApplicationStartingEvent(this.application, this.args));
}
@Override
public void environmentPrepared(ConfigurableEnvironment environment) {
this.initialMulticaster.multicastEvent(new ApplicationEnvironmentPreparedEvent(
this.application, this.args, environment));
}
........
}3.发出开始执行的事件
listeners.starting();继续跟进starting函数，
public void starting() {
this.initialMulticaster.multicastEvent(
new ApplicationStartingEvent(this.application, this.args));
}
//获取ApplicationStartingEvent类型的事件后，发布事件
@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
invokeListener(listener, event);
}
}
}
//继续跟进invokeListener方法,最后调用ApplicationListener监听者的onApplicationEvent处理事件
private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
try {
listener.onApplicationEvent(event);
}
catch (ClassCastException ex) {
.....
}
}
这个后面也会反复遇到，比如listeners.running(context)。
这里是典型的观察者模式。
//观察者：监听<E extends ApplicationEvent>类型事件
ApplicationListener<E extends ApplicationEvent> extends EventListener
//事件类型：
Event extends SpringApplicationEvent extends ApplicationEvent extends EventObject
//被观察者：发布事件
EventPublishingRunListener implements SpringApplicationRunListenerSpringApplication根据当前事件Event类型，比如ApplicationStartingEvent，查找到监听ApplicationStartingEvent的观察者EventPublishingRunListener，调用观察者的onApplicationEvent处理事件。
4.环境准备
//根据main函数传入的参数，创建DefaultApplicationArguments对象
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
//根据扫描到的listeners对象和函数传入参数，进行环境准备。
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);ApplicationArguments提供运行application的参数，后面会作为一个Bean注入到容器。这里重点说下prepareEnvironment方法做了些什么。
private ConfigurableEnvironment prepareEnvironment(
SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// Create and configure the environment
ConfigurableEnvironment environment = getOrCreateEnvironment();
configureEnvironment(environment, applicationArguments.getSourceArgs());
//和listeners.starting一样的流程
listeners.environmentPrepared(environment);
//上述完成了环境的创建和配置，传入的参数和资源加载到environment
//绑定环境到SpringApplication
bindToSpringApplication(environment);
if (!this.isCustomEnvironment) {
environment = new EnvironmentConverter(getClassLoader())
.convertEnvironmentIfNecessary(environment, deduceEnvironmentClass());
}
ConfigurationPropertySources.attach(environment);
return environment;
}这段代码核心有3个。
- configureEnvironment，用于基本运行环境的配置。
- 发布事件ApplicationEnvironmentPreparedEvent。和发布ApplicationStartingEvent事件的流程一样。
- 绑定环境到SpringApplication
5.创建ApplicationContext
context = createApplicationContext();传说中的IOC容器终于来了。
在实例化context之前，首先需要确定context的类型，这个是根据应用类型确定的。应用类型webApplicationType在构造器已经推断出来了。
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch (this.webApplicationType) {
case SERVLET:
//应用为servlet类型的web应用
contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
break;
case REACTIVE:
//应用为响应式web应用
contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
break;
default:
//应用为非web类型的应用
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);
}获取context类型后，进行实例化，这里根据class类型获取无参构造器进行实例化。
public static <T> T instantiateClass(Class<T> clazz) throws BeanInstantiationException {
Assert.notNull(clazz, "Class must not be null");
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
//clazz.getDeclaredConstructor()获取无参的构造器，然后进行实例化
return instantiateClass(clazz.getDeclaredConstructor());
}
catch (NoSuchMethodException ex) {
.......
}比如web类型为servlet类型，就会实例化org.springframework.boot.web.servlet.context.
AnnotationConfigServletWebServerApplicationContext类型的context。
6.context前置处理阶段
private void prepareContext(ConfigurableApplicationContext context,
ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments, Banner printedBanner) {
//关联环境
context.setEnvironment(environment);
//ApplicationContext预处理，主要配置Bean生成器以及资源加载器
postProcessApplicationContext(context);
//调用初始化器，执行initialize方法，前面set的初始化器终于用上了
applyInitializers(context);
//发布contextPrepared事件，和发布starting事件一样，不多说
listeners.contextPrepared(context);
if (this.logStartupInfo) {
logStartupInfo(context.getParent() == null);
logStartupProfileInfo(context);
}
// Add boot specific singleton beans
ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
//bean, springApplicationArguments,用于获取启动application所需的参数
beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
//加载打印Banner的Bean
if (printedBanner != null) {
beanFactory.registerSingleton("springBootBanner", printedBanner);
}
if (beanFactory instanceof DefaultListableBeanFactory) {
((DefaultListableBeanFactory) beanFactory)
.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
}
// Load the sources，根据primarySources加载resource。primarySources:一般为主类的class对象
Set<Object> sources = getAllSources();
Assert.notEmpty(sources, "Sources must not be empty");
//构造BeanDefinitionLoader并完成定义的Bean的加载
load(context, sources.toArray(new Object[0]));
//发布ApplicationPreparedEvent事件，表示application已准备完成
listeners.contextLoaded(context);
}7.刷新容器
private void refreshContext(ConfigurableApplicationContext context) {
refresh(context);
// 注册一个关闭容器时的钩子函数,在jvm关闭时调用
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}调用父类AbstractApplicationContext刷新容器的操作，具体的还没看。
@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();
}
......
}8.后置操作，调用Runners
后置操作，就是在容器完成刷新后，依次调用注册的Runners，还可以通过@Order注解设置各runner的执行顺序。
Runner可以通过实现ApplicationRunner或者CommandLineRunner接口。
private void callRunners(ApplicationContext context, ApplicationArguments args) {
List<Object> runners = new ArrayList<>();
runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
runners.addAll(context.getBeansOfType(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);
}
}
}根据源码可知，runners收集从容器获取的ApplicationRunner和CommandLineRunner类型的Bean，然后依次执行。
9.发布ApplicationReadyEvent事件
listeners.running(context);应用启动完成，可以对外提供服务了，在这里发布ApplicationReadyEvent事件。流程还是和starting时一样。