本文基于SpringBoot 2.1.12.RELEASE
一個(gè)常見的SpringBoot啟動(dòng)類為
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}
其中 @SpringBootApplication 為@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan三者的聚合
其中@SpringBootConfiguration與@Configuration無異， ComponentScan也不用多說，@EnableAutoConfiguation則實(shí)際上用了@Import({AutoConfigurationImportSelector.class})，用于將指定目錄下的配置類進(jìn)行加載，其原理類似SPI，也無須多講。
1. SpringApplication#run()方法
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
return run(new Class<?>[] { primarySource }, args);
}
// 上一級(jí)調(diào)用的接口
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
return new SpringApplication(primarySources).run(args);
}
可以看到，run()方法最后返回的是 new SpringApplication(primarySources).run(args)
2. SpringApplication 的構(gòu)造函數(shù)
接下來，進(jìn)入此處的SpringApplication的構(gòu)造函數(shù)，到最底層調(diào)用的是
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
// 1.將配置類放入
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
// 2.判斷webApplication的類型，枚舉有 NONE SERVLET REACTIVE
this.webApplicationType = WebApplicationType.deduceFromClasspath();
// 3.加載ApplicationContextInitializer配置并實(shí)例化，關(guān)聯(lián)到SpringApplication#initializer
//底層是SpringFactoriesLoader.loadFactoryNames(type, classLoader)
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
// 4.與上一句類似 加載ApplicationListener配置并實(shí)例化，關(guān)聯(lián)到SpringApplication#listener
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
// 5.推斷應(yīng)用引導(dǎo)類
this.mainApplicationClass = deduceMainApplicationClass();
}
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));中具體的調(diào)用路徑為
// 加載制定目錄下對應(yīng)類型的類，并初始化后返回實(shí)例
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// 作用是獲取META-INF/spring.factories下的所有ApplicationContextInitializer類
Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
// 初始化對應(yīng)的類
List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
// 排序
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
// 將所有的ApplicationContextInitializer 關(guān)聯(lián)到SpringApplication#initializers中
public void setInitializers(Collection<? extends ApplicationContextInitializer<?>> initializers) {
this.initializers = new ArrayList<>();
this.initializers.addAll(initializers);
}
this.mainApplicationClass = deduceMainApplicationClass();的具體函數(shù)為
// 推斷引導(dǎo)類
private Class<?> deduceMainApplicationClass() {
try {
// 獲取當(dāng)前線程執(zhí)行棧
StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
for (StackTraceElement stackTraceElement : stackTrace) {
// 哪個(gè)類包含main方法
if ("main".equals(stackTraceElement.getMethodName())) {
return Class.forName(stackTraceElement.getClassName());
}
}
}
catch (ClassNotFoundException ex) {
// Swallow and continue
}
return null;
}
3. SpringApplication(primarySources).run方法—— 真正啟動(dòng)
// 創(chuàng)建并refresh一個(gè)ApplicationContext
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
//設(shè)置系統(tǒng)屬性『java.awt.headless』，為true則啟用headless模式支持
configureHeadlessProperty();
//通過*SpringFactoriesLoader*檢索*META-INF/spring.factories*，
//找到聲明的所有SpringApplicationRunListener的實(shí)現(xiàn)類并將其實(shí)例化，
//之后逐個(gè)調(diào)用其started()方法，廣播SpringBoot要開始執(zhí)行了
SpringApplicationRunListeners listeners = getRunListeners(args);
//發(fā)布應(yīng)用開始啟動(dòng)事件
listeners.starting();
try {
//初始化參數(shù)
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
//創(chuàng)建并配置當(dāng)前SpringBoot應(yīng)用將要使用的Environment（包括配置要使用的PropertySource以及Profile）,
//并遍歷調(diào)用所有的SpringApplicationRunListener的environmentPrepared()方法，廣播Environment準(zhǔn)備完畢。
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
configureIgnoreBeanInfo(environment);
//打印banner
Banner printedBanner = printBanner(environment);
//創(chuàng)建應(yīng)用上下文
context = createApplicationContext();
//通過*SpringFactoriesLoader*檢索*META-INF/spring.factories*，獲取并實(shí)例化異常分析器
exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
//為ApplicationContext加載environment，之后逐個(gè)執(zhí)行ApplicationContextInitializer的initialize()方法來進(jìn)一步封裝ApplicationContext，
//并調(diào)用所有的SpringApplicationRunListener的contextPrepared()方法，【EventPublishingRunListener只提供了一個(gè)空的contextPrepared()方法】，
//之后初始化IoC容器，并調(diào)用SpringApplicationRunListener的contextLoaded()方法，廣播ApplicationContext的IoC加載完成，
//這里就包括通過**@EnableAutoConfiguration**導(dǎo)入的各種自動(dòng)配置類。
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//刷新上下文
refreshContext(context);
//再一次刷新上下文,其實(shí)是空方法，可能是為了后續(xù)擴(kuò)展。
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
}
//發(fā)布應(yīng)用已經(jīng)啟動(dòng)的事件
listeners.started(context);
//遍歷所有注冊的ApplicationRunner和CommandLineRunner，并執(zhí)行其run()方法。
//我們可以實(shí)現(xiàn)自己的ApplicationRunner或者CommandLineRunner，來對SpringBoot的啟動(dòng)過程進(jìn)行擴(kuò)展。
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
//應(yīng)用已經(jīng)啟動(dòng)完成的監(jiān)聽事件
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}
}
其中，SpringApplicationRunListeners listeners = getRunListeners(args);的getRunListeners中調(diào)用了getSpringFactorieInstances方法，其作用就是加載制定目錄下的配置類
private SpringApplicationRunListeners getRunListeners(String[] args) {
Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
// 獲取配置的SpringApplicationRunListener
return new SpringApplicationRunListeners(logger,
getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}
3.1 創(chuàng)建上下文
context = createApplicationContext()創(chuàng)建上下文的語句，其源碼為
/**
默認(rèn)上下文
*/
public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot."
+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";
/**
reactive上下文
*/
public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."
+ "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";
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);
}
即在默認(rèn)web環(huán)境下，上下文的實(shí)現(xiàn)為AnnotationConfigServletWebServerApplicationContext
3.2 上下文運(yùn)行前準(zhǔn)備
prepareContext(context, environment, listeners, applicationArguments, printedBanner);的源碼為
private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
// 設(shè)置環(huán)境 必須在refresh之前 原因?yàn)槿羰褂?AbstractApplicationContext#createEnvironment，則由BeanFactoryPostProcessor實(shí)現(xiàn)environment屬性的裝載，但是無法保證在PostProcessor中最早執(zhí)行
context.setEnvironment(environment);
// 上下文后置處理 處理resourceLoader,classLoader以及conversionService
postProcessApplicationContext(context);
// 迭代執(zhí)行所有的ApplicationContextInitializer的實(shí)現(xiàn)類
applyInitializers(context);
// 發(fā)布 contextPrepared的事件
listeners.contextPrepared(context);
if (this.logStartupInfo) {
logStartupInfo(context.getParent() == null);
logStartupProfileInfo(context);
}
// Add boot specific singleton beans
ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
if (printedBanner != null) {
beanFactory.registerSingleton("springBootBanner", printedBanner);
}
if (beanFactory instanceof DefaultListableBeanFactory) {
//允許Override
((DefaultListableBeanFactory) beanFactory)
.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
}
// 讀取所有的源信息 參照第一步的 primarySources
Set<Object> sources = getAllSources();
Assert.notEmpty(sources, "Sources must not be empty");
// 將所有的上下文配置源(sources)讀取并放入上下文中
load(context, sources.toArray(new Object[0]));
// 執(zhí)行SpringApplicationRunListener#contextLoaded回調(diào)
// 發(fā)布ApplicationLoadedEvent
listeners.contextLoaded(context);
}
可以看到，在此期間內(nèi)，listeners發(fā)送了兩次事件，一次是contextPrepared，一次是contextLoaded
3.3 refreshContext
內(nèi)部為
private void refreshContext(ConfigurableApplicationContext context) {
refresh(context);
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}
其實(shí)就是調(diào)用的((AbstractApplicationContext) applicationContext).refresh()
至此，我們基本上把SpringBoot的啟動(dòng)的大致過程摸清楚了。
在查詢資料的過程中，發(fā)現(xiàn)一篇文章寫的不錯(cuò)，是側(cè)重于啟動(dòng)過程中Tomcat的處理，有興趣可以看看
