种类
spring-boot支持使用jetty、netty、tomcat、undertow作为嵌入式web容器
获取web容器实例
public void refresh() throws BeansException, IllegalStateException {
synchronized(this.startupShutdownMonitor) {
this.prepareRefresh();
ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
this.prepareBeanFactory(beanFactory);
try {
this.postProcessBeanFactory(beanFactory);
this.invokeBeanFactoryPostProcessors(beanFactory);
this.registerBeanPostProcessors(beanFactory);
this.initMessageSource();
this.initApplicationEventMulticaster();
this.onRefresh();
this.registerListeners();
this.finishBeanFactoryInitialization(beanFactory);
this.finishRefresh();
} catch (BeansException var9) {
if (this.logger.isWarnEnabled()) {
this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
}
this.destroyBeans();
this.cancelRefresh(var9);
throw var9;
} finally {
this.resetCommonCaches();
}
}
}
protected void onRefresh() throws BeansException {
}
protected void finishRefresh() {
this.clearResourceCaches();
this.initLifecycleProcessor();
this.getLifecycleProcessor().onRefresh();
this.publishEvent((ApplicationEvent)(new ContextRefreshedEvent(this)));
LiveBeansView.registerApplicationContext(this);
}
spring-framework初始化的步骤如上，而spring-boot的嵌入式web容器实例化在onRefresh方法里进行。
spring-boot支持两种容器方案，一种为阻塞式的servlet，另一种为非阻塞的reactive，spring-boot在启动时会根据classpath下文件推断当前使用的是何种容器方案，从而使用ReactiveWebServerApplicationContext或ServletWebServerApplicationContext类实例，它们都继承了spring-framework的GenericWebApplicationContext类，并重写了onRefresh方法。
servlet方案容器的实例化
ServletWebServerApplicationContext.java
@Override
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
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();
}
protected ServletWebServerFactory getWebServerFactory() {
// Use bean names so that we don't consider the hierarchy
String[] beanNames = getBeanFactory()
.getBeanNamesForType(ServletWebServerFactory.class);
if (beanNames.length == 0) {
throw new ApplicationContextException(
"Unable to start ServletWebServerApplicationContext due to missing "
+ "ServletWebServerFactory bean.");
}
if (beanNames.length > 1) {
throw new ApplicationContextException(
"Unable to start ServletWebServerApplicationContext due to multiple "
+ "ServletWebServerFactory beans : "
+ StringUtils.arrayToCommaDelimitedString(beanNames));
}
return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
}
容器类型存在成员变量webServer中，在实例化容器前，会先判断webServer是否已经被实例化，webServer是一个接口，所有的嵌入式容器启动类都实现了这个接口。
getWebServerFactory方法会从beanFactory中获取实现了ServletWebServerFactory接口的类实例，通过其getWebServer方法可以实例化具体的web容器实例并放入实现了WebServer的类中返回。
上一段提到需要从beanFactory中获取实现了ServletWebServerFactory接口的类实例，这个类实例来自于spring-boot提供的auto-configuration功能，专用的嵌入式容器jar包会自动配置ServletWebServerFactory的实现。
servlet方案容器的运行
@Override
protected void finishRefresh() {
super.finishRefresh();
WebServer webServer = startWebServer();
if (webServer != null) {
publishEvent(new ServletWebServerInitializedEvent(webServer, this));
}
}
private WebServer startWebServer() {
WebServer webServer = this.webServer;
if (webServer != null) {
webServer.start();
}
return webServer;
}
上文说到容器的实例化在onRefresh方法进行，而容器的启动则是在finishRefresh方法进行，同样的，ServletWebServerApplicationContext类重写了GenericWebApplicationContext类的finishRefresh方法。
该方法直接调用了webServer的start方法启动容器。就这样，项目容器的WebServer接口实现类便会被启动，具体的启动方法因容器而异，具体启动的实现可参考NettyWebServer、TomcatWebServer、JettyWebServer、UndertowWebServer、UndertowServletWebServer，spring-boot提供了这几种嵌入式容器启动的实现。
reactive方案容器的实例化
@Override
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start reactive web server",
ex);
}
}
private void createWebServer() {
ServerManager serverManager = this.serverManager;
if (serverManager == null) {
this.serverManager = ServerManager.get(getWebServerFactory());
}
initPropertySources();
}
protected ReactiveWebServerFactory getWebServerFactory() {
// Use bean names so that we don't consider the hierarchy
String[] beanNames = getBeanFactory()
.getBeanNamesForType(ReactiveWebServerFactory.class);
if (beanNames.length == 0) {
throw new ApplicationContextException(
"Unable to start ReactiveWebApplicationContext due to missing "
+ "ReactiveWebServerFactory bean.");
}
if (beanNames.length > 1) {
throw new ApplicationContextException(
"Unable to start ReactiveWebApplicationContext due to multiple "
+ "ReactiveWebServerFactory beans : "
+ StringUtils.arrayToCommaDelimitedString(beanNames));
}
return getBeanFactory().getBean(beanNames[0], ReactiveWebServerFactory.class);
}
与servlet方案的容器实例化类似，reactive方案的容器实例化仅仅将获取容器实例的接口从ServletWebServerFactory变成了ReactiveWebServerFactory，嵌入式容器jar包会提供实现了ServletWebServerFactory接口的bean。
reactive方案容器的运行
@Override
protected void finishRefresh() {
super.finishRefresh();
WebServer webServer = startReactiveWebServer();
if (webServer != null) {
publishEvent(new ReactiveWebServerInitializedEvent(webServer, this));
}
}
private WebServer startReactiveWebServer() {
ServerManager serverManager = this.serverManager;
ServerManager.start(serverManager, this::getHttpHandler);
return ServerManager.getWebServer(serverManager);
}
public static void start(ServerManager manager,
Supplier<HttpHandler> handlerSupplier) {
if (manager != null && manager.server != null) {
manager.handler = handlerSupplier.get();
manager.server.start();
}
}
与servlet方案容器的实现一致，直接调用start方法运行即可。
