SpringBoot嵌入式Servlet容器与定制化组件超详细讲解

嵌入式Servlet容器

1、原理分析

2、Servlet容器切换

3、定制Servlet容器配置

定制化组件

嵌入式Servlet容器

在Spring Boot中，默认支持的web容器有 Tomcat, Jetty, 和 Undertow

1、原理分析

那么这些web容器是怎么注入的呢？我们一起来分析一下

当SpringBoot应用启动发现当前是Web应用，它会创建一个web版的ioc容器ServletWebServerApplicationContext

这个类下面有一个createWebServer()方法，当执行关键代码ServletWebServerFactory factory = this.getWebServerFactory();时，它会在系统启动的时候寻找 ServletWebServerFactory（Servlet 的web服务器工厂—> 用于生产Servlet 的web服务器）

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = this.getServletContext();
    if (webServer == null && servletContext == null) {
        StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
        // 获取ServletWebFactory
        ServletWebServerFactory factory = this.getWebServerFactory();
        createWebServer.tag("factory", factory.getClass().toString());
        // 这里会去调用系统中获取到的web容器工厂类的getWebServer()方法
        this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});
        createWebServer.end();
        this.getBeanFactory().registerSingleton("webServerGracefulShutdown", new WebServerGracefulShutdownLifecycle(this.webServer));
        this.getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer));
    } else if (servletContext != null) {
        try {
            this.getSelfInitializer().onStartup(servletContext);
        } catch (ServletException var5) {
            throw new ApplicationContextException("Cannot initialize servlet context", var5);
        }
    }
    this.initPropertySources();
}

获取ServletWebFactory

protected ServletWebServerFactory getWebServerFactory() {
    String[] beanNames = this.getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
    if (beanNames.length == 0) {
        throw new MissingWebServerFactoryBeanException(this.getClass(), ServletWebServerFactory.class, WebApplicationType.SERVLET);
    } else if (beanNames.length > 1) {
        throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
    } else {
        return (ServletWebServerFactory)this.getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
    }
}

SpringBoot底层默认有很多的WebServer工厂：TomcatServletWebServerFactory,，JettyServletWebServerFactory和 UndertowServletWebServerFactory

那么究竟返回哪一个工厂呢？

我们需要分析一下底层的自动配置类，ServletWebServerFactoryAutoConfiguration

@AutoConfiguration
@AutoConfigureOrder(-2147483648)
@ConditionalOnClass({ServletRequest.class})
@ConditionalOnWebApplication(
    type = Type.SERVLET
)
@EnableConfigurationProperties({ServerProperties.class})
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, EmbeddedTomcat.class, EmbeddedJetty.class, EmbeddedUndertow.class})
public class ServletWebServerFactoryAutoConfiguration {
    public ServletWebServerFactoryAutoConfiguration() {
    }
    ...

它引入了一个配置类ServletWebServerFactoryConfiguration，这个类里面会根据动态判断系统中到底导入了那个Web服务器的包，然后去创建对应的web容器工厂，spring-boot-starter-web这个依赖默认导入tomcat，所以我们系统会创建TomcatServletWebServerFactory，由这个工厂创建tomcat容器并启动

一旦我们获取到web Server的工厂类，createWebServer()方法就会去调用this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});

根据断点一直深入，我们可以发现，Tomcat, Jetty, 和 Undertow的工厂类最后都会去调用getWebServer()方法，设置了链接参数，例如TomcatServletWebServerFactory的getWebServer()方法

在方法的最后，它会执行return this.getTomcatWebServer(tomcat);，跟着断点深入，我们发现它会去调用对应web容器类的构造方法，如TomcatWebServer的构造方法，启动tomcat容器

public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
    this.monitor = new Object();
    this.serviceConnectors = new HashMap();
    Assert.notNull(tomcat, "Tomcat Server must not be null");
    this.tomcat = tomcat;
    this.autoStart = autoStart;
    this.gracefulShutdown = shutdown == Shutdown.GRACEFUL ? new GracefulShutdown(tomcat) : null;
    // 初始化方法initialize---会调用this.tomcat.start();启动容器
    this.initialize();
}

2、Servlet容器切换

Spring Boot默认使用的是tomcat容器，那如果我们想要使用Undertow应该如何切换呢

只需要修改pom文件即可，排除web启动器中tomcat相关的依赖

然后导入Undertow相关启动器

<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-undertow</artifactId>
</dependency>

3、定制Servlet容器配置

如果想要自己定义一个Servlet容器，可以通过哪些途径呢？

通过分析ServletWebServerFactoryAutoConfiguration绑定了ServerProperties配置类可知，我们想要修改容器的配置，只需要修改配置文件中对应的server.xxx配置项即可

创建一个配置类，通过@Configuration+@Bean的方式，向容器中注入一个ConfigurableServletWebServerFactory类的实现类，ConfigurableServletWebServerFactory是ServletWebServerFactory类的子类，提供了很多方法供我们使用

代码样例如下

package com.decade.config;
import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class MyConfig {
    @Bean
    public ConfigurableServletWebServerFactory defineWebServletFactory() {
        final TomcatServletWebServerFactory tomcatServletWebServerFactory = new TomcatServletWebServerFactory();
        tomcatServletWebServerFactory.setPort(8081);
        return tomcatServletWebServerFactory;
    }
}

自定义一个ServletWebServerFactoryCustomizer类，它的下面有一个customize()方法，能把配置文件的值和ServletWebServerFactory 进行绑定