SpringApplication.run(xxx.class, args)背后的東東——實現(xiàn)原理
現(xiàn)在springboot已經(jīng)成為web應(yīng)用開發(fā)的事實標準，所以為了能更好的應(yīng)用springboot特性，有必要深入研究下背后的實現(xiàn)原理。
在分析之前，先拋出幾個問題，后續(xù)章節(jié)也是圍繞解決這幾個問題展開：
1、springboot是如何依賴幾個注解就零配置啟動spring容器?
2、springboot開發(fā)出來的jar，如何實現(xiàn)內(nèi)嵌tomcat？
3、springboot開發(fā)經(jīng)常用到的springmvc中的dispathservlet是如何關(guān)聯(lián)到內(nèi)嵌tomcat?
首先咱們直接從main方法上的 @SpringBootApplication 組合注解開始，里面實現(xiàn)零配置的注解就是 @EnableAutoConfiguration ，它利用 @Import 一個 AutoConfigurationImportSelector，該類主要利用類SPI的機制，獲取META-INFO下面需要自動裝配的 @Configuration，至于SPI后續(xù)將詳細介紹。
說到這里，估計大家會有一個疑問，springboot如何一開始就知道這個重要 @Import 注解呢，肯定有個注解解析的地方，下面需要說的就是spring容器的啟動（跟著main方法走）：
context = this.createApplicationContext()
這里是創(chuàng)建重要的ApplicationContext，其實就是BeanFactory，不過springboot會自動推斷創(chuàng)建針對servlet應(yīng)用的 AnnotationConfigServletWebServerApplicationContext。
org.springframework.context.support.AbstractApplicationContext#refresh
熟悉spring源碼的同學(xué)就會知道，這里就是刷新容器的地方，
org.springframework.context.support.AbstractApplicationContext#invokeBeanFactoryPostProcessors
org.springframework.context.annotation.ConfigurationClassParser#processImports
這個方法里面就有對 @Import 注解做解析
解析邏輯比較復(fù)雜，有興趣可以自己看下，說白了就是開頭講的找到需要自動裝配的 @Configuration 對應(yīng)的類，這里列出來三個重要的如下：
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {
@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {
其實這三個都是通過 @AutoConfigureAfter 依賴關(guān)系加載的，加載這些的目的就是為了getBean時可以拿到具體的實例對象。
咱們再回到刷新spring器中的方法中
org.springframework.context.support.AbstractApplicationContext#onRefresh
該方法對應(yīng)servlet的實現(xiàn)是
org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext#onRefresh
里面就是創(chuàng)建WebServer，可以理解為就是咱們的tomcat，不過它是由工廠方法創(chuàng)建的
protected ServletWebServerFactory getWebServerFactory() {
String[] beanNames = this.getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
核心代碼來了，這里就會觸發(fā)getBean，去拿到 ServletWebServerFactory的實例，也就會觸發(fā)Bean的生命周期調(diào)用，
@Configuration(proxyBeanMethods = false)
class ServletWebServerFactoryConfiguration {
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedTomcat {
@Bean
TomcatServletWebServerFactory tomcatServletWebServerFactory(
ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
ObjectProvider<TomcatContextCustomizer> contextCustomizers,
ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
factory.getTomcatConnectorCustomizers()
.addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatContextCustomizers()
.addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatProtocolHandlerCustomizers()
.addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
}
其實就是到這里來獲取 TomcatServletWebServerFactory 實例，這個也就是前面自動配置 @Import掃描出來的 ServletWebServerFactoryAutoConfiguration 導(dǎo)入的beandefinition
這個工廠實現(xiàn)了 ErrorPageRegistry 接口
org.springframework.boot.web.server.ErrorPageRegistry
而ServletWebServerFactory在getBean的生命周期中會觸發(fā)BeanPostProcessor實現(xiàn)類的調(diào)用
public class ErrorPageRegistrarBeanPostProcessor implements BeanPostProcessor, BeanFactoryAware {
private ListableBeanFactory beanFactory;
private List<ErrorPageRegistrar> registrars;
@Override
public void setBeanFactory(BeanFactory beanFactory) {
Assert.isInstanceOf(ListableBeanFactory.class, beanFactory,
"ErrorPageRegistrarBeanPostProcessor can only be used with a ListableBeanFactory");
this.beanFactory = (ListableBeanFactory) beanFactory;
}
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if (bean instanceof ErrorPageRegistry) {
postProcessBeforeInitialization((ErrorPageRegistry) bean);
}
return bean;
}
也就是實例化 TomcatServletWebServerFactory 之前，會去先實例化 ErrorPageRegistrar
private Collection<ErrorPageRegistrar> getRegistrars() {
if (this.registrars == null) {
// Look up does not include the parent context
this.registrars = new ArrayList<>(
this.beanFactory.getBeansOfType(ErrorPageRegistrar.class, false, false).values());
this.registrars.sort(AnnotationAwareOrderComparator.INSTANCE);
this.registrars = Collections.unmodifiableList(this.registrars);
}
return this.registrars;
}
跟著代碼，實例化 ErrorPageRegistrar 又會先去實例化 DispatcherServletPath，它只有一個實現(xiàn)類 DispatcherServletRegistrationBean
protected ErrorPageCustomizer(ServerProperties properties, DispatcherServletPath dispatcherServletPath) {
this.properties = properties;
this.dispatcherServletPath = dispatcherServletPath;
}
public DispatcherServletRegistrationBean(DispatcherServlet servlet, String path) {
super(servlet);
Assert.notNull(path, "Path must not be null");
this.path = path;
super.addUrlMappings(getServletUrlMapping());
}
這里就看到熟悉的 DispatcherServlet了，有人可能會問，DispatcherServlet哪里來的？其實又要回到前面掃描處理的三個自動裝配類中的 DispatcherServletAutoConfiguration
至此，我們終于通過spring容器的啟動，將DispatcherServlet實例化出來了，同理負責創(chuàng)建tomcat容器的TomcatServletWebServerFactory也實例化出來了。
下面就是負責創(chuàng)建內(nèi)嵌tomcat了，以及建立tomcat與DispatcherServlet實例的關(guān)聯(lián)
org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory#getWebServer
tomcat容器啟動后，因為它實現(xiàn)了servlet規(guī)范，內(nèi)部會回調(diào)實現(xiàn)了所有實現(xiàn)了ServletContainerInitializer類的onStartup方法
javax.servlet.ServletContainerInitializer#onStartup
具體代碼需要關(guān)聯(lián)tomcat源碼，這里只是簡單記錄下，后續(xù)再深入分析tomcat實現(xiàn)原理
org.apache.catalina.core.StandardContext#startInternal
while(var27.hasNext()) {
Entry entry = (Entry)var27.next();
try {
((ServletContainerInitializer)entry.getKey()).onStartup((Set)entry.getValue(), this.getServletContext());
} catch (ServletException var22) {
log.error(sm.getString("standardContext.sciFail"), var22);
ok = false;
break;
}
}
而TomcatStarter正好實現(xiàn)了ServletContainerInitializer接口，所以tomcat啟動會觸發(fā)這個方法的調(diào)用
org.springframework.boot.web.embedded.tomcat.TomcatStarter#onStartup
方法內(nèi)部調(diào)用了spring中實現(xiàn)了ServletContextInitializer接口的類的onStartup方法，恰好又是DispatcherServletRegistrationBean，而它的父類RegistrationBean就將DispatcherServlet綁定到tomcat容器的ServletContext上了
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletRegistrationBean
org.springframework.boot.web.servlet.RegistrationBean#onStartup
最后，tomcat容器啟動了，spring容器也啟動了，訪問tomcat的請求也可以通過關(guān)聯(lián)的DispatcherServlet進行分發(fā)處理了，咱們前面的三個問題也應(yīng)該有答案了。
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