前言:springboot相信基本上所有的人都使用過,但是對於一些初學者可能只是知道如何使用,但是對於它實現的原理不太熟悉,今天跟大家一起去分析下它的啓動源碼。其實也是比較簡單,相信通過這篇文章能對一些初學者有一些幫助,在學習這篇文章之前最好有spring的基礎知識。
一、引入問題
今天我們是以web應用爲例來分析springboot的啓動原理,首先我們看如下的代碼
@SpringBootApplication
public class HellobootApplication {
public static void main(String[] args) {
SpringApplication.run(HellobootApplication.class, args);
}
}
我們知道當我們run這個main方法時,web應用程序就能啓動。那麼首先請大家思考如下問題
1、springboot如何和spring容器關聯上
2、我們沒有看到有tomcat容器,爲什麼能夠支撐起web應用
3、當我們引用第三方的starter時,爲什麼會自動實例化一些類,我們並沒有掃描到第三方的包,甚至我們對第三方的包的路徑都不知道
帶着上面三個問題,我們一起來看下springboot的啓動的原理吧
二、源碼分析(1)
首先分析上面第一個問題(springboot如何和spring容器關聯上)
SpringApplication#run()
public static ConfigurableApplicationContext run(Class<?> primarySource,
String... args) {
//看這裏
return run(new Class<?>[] { primarySource }, args);
}
run()
public static ConfigurableApplicationContext run(Class<?>[] primarySources,
String[] args) {
//看這個run方法
return new SpringApplication(primarySources).run(args);
}
繼續記者往下看
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();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
configureIgnoreBeanInfo(environment);
Banner printedBanner = printBanner(environment);
//從這個方法名字看,大概就能猜測出來意思了,這裏就是創建spring的context
context = createApplicationContext();
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
//這個方法名字,看起來是不是很熟悉
refreshContext(context);
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
listeners.started(context);
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;
}
這個方法,我們重點看兩個地方
(1)createApplicationContext()
(2)refreshContext(context)
如果對spring源碼有了解的話,我們大概能猜到這兩個方法的意思,第一個時創建spring上下文,第二個就是刷新上下文,這兩個地方其實就實現了springboot–>spring容器
比如我們單元測試的時候,經常編寫如下代碼進行啓動spring容器
ClassPathXmlApplicationContext app = new ClassPathXmlApplicationContext("xxx.xml")
1、我們先看createApplicationContext()方法
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch (this.webApplicationType) {
case SERVLET:
//重點看加載的這個class的路徑
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);
}
}
//後面就是通過反射創建ApplicationContext上下文了,後面不再繼續跟了
return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}
這裏我們重點看一下加載的是哪個ApplicationContext的上下文,因爲ApplicationContext有很多子類,我們看下常量(DEFAULT_SERVLET_WEB_CONTEXT_CLASS)對應的class是哪個
/**
* The class name of application context that will be used by default for web
* environments.
*/
public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot."
+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";
所以這裏創建的ApplicationContext的上下文是AnnotationConfigServletWebServerApplicationContext,這裏很重要,大家先記一下(不同的ApplicationContext有些方法實現的細節是不一樣的)
2、接着看上面的refreshContext(context)
private void refreshContext(ConfigurableApplicationContext context) {
//繼續看該方法
refresh(context);
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}
refresh()
protected void refresh(ApplicationContext applicationContext) {
Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
//這裏就調用了spring的context包中的容器刷新了
((AbstractApplicationContext) applicationContext).refresh();
}
分析到這裏,相信大家都知道springboot如何和spring容器相關聯的了
三、源碼分析(2)
上面介紹了springboot如何與spring容器進行關聯的,接着我們看web應用時,我們沒有將應用放入tomcat中時,爲何能實現web應用
上面有一個地方,我讓大家重點記住創建ApplicationContext時,到底是創建了哪個ApplicationContext的子類,其實是創建了AnnotationConfigServletWebServerApplicationContext那麼我們來看下ApplicationContext.refresh()方法
AbstractApplicationContext.refresh()
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.
//咱們重點看這裏,上面的英文註釋寫的也很詳細,可以在子類中實例化其他bean
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
這個方法我相信對spring源碼有了解的人都非常熟悉這個方法,這個方法就是spring代碼的入口,非常重要。這裏其實是實現了模板方法設計模式,這裏是定義了基本的骨架,有些方法可以在不同的子類有不同的實現,我們上面看到創建的springboot的web應用程序默認創建的是AnnotationConfigServletWebServerApplicationContext,我們來看下它的onRefresh()方法(在他的父類ServletWebServerApplicationContext中)
ServletWebServerApplicationContext.onRefresh()
@Override
protected void onRefresh() {
super.onRefresh();
try {
//重點看這裏
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
咱們重點看createWebServer(),這裏從方法名就能看出創建web服務
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
//創建web服務在這裏,這裏是工廠方法
this.webServer = factory.getWebServer(getSelfInitializer());
}
else if (servletContext != null) {
try {
getSelfInitializer().onStartup(servletContext);
}
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context",
ex);
}
}
initPropertySources();
}
上面是工廠方法,有Jetty和tomcat,我們這裏點tomcat進去看看
TomcatServletWebServerFactory.getWebServer()
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? this.baseDirectory
: createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath());
Connector connector = new Connector(this.protocol);
tomcat.getService().addConnector(connector);
customizeConnector(connector);
tomcat.setConnector(connector);
tomcat.getHost().setAutoDeploy(false);
configureEngine(tomcat.getEngine());
for (Connector additionalConnector : this.additionalTomcatConnectors) {
tomcat.getService().addConnector(additionalConnector);
}
prepareContext(tomcat.getHost(), initializers);
return getTomcatWebServer(tomcat);
}
這裏相信大家就很清晰了,這裏自己創建了一個Tomcat,最後會調用tomcat.start()這裏我就不繼續往下跟了,感興趣的可以繼續往下看看。另外如果感興趣的同學可以看看這裏的Tomcat的設置和我們下載的tomcat中的server.xml文件進行對比一下,看看有什麼相似的地方
這篇文章就先分析到這裏吧,還留了一個最後一個問題(如何自動加載第三方的starter),我們到下一篇文章再詳細的分析