在refresh → onRefresh的时候
createWebServer:177, ServletWebServerApplicationContext (org.springframework.boot.web.servlet.context)
onRefresh:158, ServletWebServerApplicationContext (org.springframework.boot.web.servlet.context)
refresh:545, AbstractApplicationContext (org.springframework.context.support)
refresh:143, ServletWebServerApplicationContext (org.springframework.boot.web.servlet.context)
refresh:758, SpringApplication (org.springframework.boot)
refresh:750, SpringApplication (org.springframework.boot)
refreshContext:397, SpringApplication (org.springframework.boot)
run:315, SpringApplication (org.springframework.boot)
run:1237, SpringApplication (org.springframework.boot)
run:1226, SpringApplication (org.springframework.boot)
main:10, SpringWebDemoApplication (com.lunac.learning.java.demo)
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
getBeanFactory().registerSingleton("webServerGracefulShutdown",
new WebServerGracefulShutdownLifecycle(this.webServer));
getBeanFactory().registerSingleton("webServerStartStop",
new WebServerStartStopLifecycle(this, this.webServer));
}
...
通过ServletWebServerFactory来创建具体的webServer
具体容器的选择，取决于ServletWebServerFactory 的实现类。
/**
* Returns the {@link ServletWebServerFactory} that should be used to create the
* embedded {@link WebServer}. By default this method searches for a suitable bean in
* the context itself.
* @return a {@link ServletWebServerFactory} (never {@code null})
*/
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);
}
如上，会从beanFactory中找到ServletWebServerFactory 的实现类，如果没有，或者大于1个，都会报错。（tomcat 、nettty、undertower）只能选择一个。这个选择通过控制ServletWebServerFactory 的实现类来达成的。
首先，有一个统一配置类，配置好了Tomcat、Jetty、UnderTower各自factory的bean：TomcatServletWebServerFactory，JettyServletWebServerFactory，EmbeddedUndertow
但他们是按需加载的（Condition），比如TomcatServletWebServerFactory 只有在classpath中出现Tomcat类的时候才会被加载，Jetty、UnderTower也是类似。
package org.springframework.boot.autoconfigure.web.servlet;
import java.util.stream.Collectors;
import javax.servlet.Servlet;
import io.undertow.Undertow;
import org.apache.catalina.startup.Tomcat;
import org.apache.coyote.UpgradeProtocol;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.util.Loader;
import org.eclipse.jetty.webapp.WebAppContext;
import org.xnio.SslClientAuthMode;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.SearchStrategy;
import org.springframework.boot.web.embedded.jetty.JettyServerCustomizer;
import org.springframework.boot.web.embedded.jetty.JettyServletWebServerFactory;
import org.springframework.boot.web.embedded.tomcat.TomcatConnectorCustomizer;
import org.springframework.boot.web.embedded.tomcat.TomcatContextCustomizer;
import org.springframework.boot.web.embedded.tomcat.TomcatProtocolHandlerCustomizer;
import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
import org.springframework.boot.web.embedded.undertow.UndertowBuilderCustomizer;
import org.springframework.boot.web.embedded.undertow.UndertowDeploymentInfoCustomizer;
import org.springframework.boot.web.embedded.undertow.UndertowServletWebServerFactory;
import org.springframework.boot.web.servlet.server.ServletWebServerFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
/**
* Configuration classes for servlet web servers
* <p>
* Those should be {@code @Import} in a regular auto-configuration class to guarantee
* their order of execution.
*
* @author Phillip Webb
* @author Dave Syer
* @author Ivan Sopov
* @author Brian Clozel
* @author Stephane Nicoll
* @author Raheela Asalm
* @author Sergey Serdyuk
*/
@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;
}
}
/**
* Nested configuration if Jetty is being used.
*/
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedJetty {
@Bean
JettyServletWebServerFactory JettyServletWebServerFactory(
ObjectProvider<JettyServerCustomizer> serverCustomizers) {
JettyServletWebServerFactory factory = new JettyServletWebServerFactory();
factory.getServerCustomizers().addAll(serverCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
}
/**
* Nested configuration if Undertow is being used.
*/
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedUndertow {
@Bean
UndertowServletWebServerFactory undertowServletWebServerFactory(
ObjectProvider<UndertowDeploymentInfoCustomizer> deploymentInfoCustomizers,
ObjectProvider<UndertowBuilderCustomizer> builderCustomizers) {
UndertowServletWebServerFactory factory = new UndertowServletWebServerFactory();
factory.getDeploymentInfoCustomizers()
.addAll(deploymentInfoCustomizers.orderedStream().collect(Collectors.toList()));
factory.getBuilderCustomizers().addAll(builderCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
}
而什么情况下classpath下会出现Tomcat类内，就是pom引入了Tomcat依赖的情况（spring-boot-starter-tomcat），这其实是默认情况。
如果要用Jetty，就要引入对应的依赖，并去掉tomcat依赖（否则就会出现大于一个的错误）
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-jetty</artifactId>
</dependency>