4_7.springboot2.x嵌入式servlet容器自动配置原理

概述

Spring Boot对所支持的Servlet Web服务器实现做了建模抽象:

Servlet容器类型	WebServer模型接口	WebServer工厂实现类
Tomcat	TomcatWebServer	TomcatServletWebServerFactory
Jetty	JettyWebServer	JettyServletWebServerFactory
Undertow	UndertowWebServer	UndertowServletWebServerFactory

基于此模型概念,在一个Servlet Web应用中，Spring Boot会使用上表中所说的WebServer工厂组件生成相应的WebServer实例。而这里的WebServer工厂组件又是从哪里来的呢 ？ 这就是自动配置类ServletWebServerFactoryAutoConfiguration的任务了。

自动配置类ServletWebServerFactoryAutoConfiguration首先通过注解声明自己的生效条件:类 ServletRequest 存在于 classpath 上时才生效,也就是要求javax.servlet-api包必须被引用;当前应用必须是Spring MVC应用才生效；在以上条件被满足时，ServletWebServerFactoryAutoConfiguration引入了如下三个配置类 :

EmbeddedTomcat

EmbeddedJetty

EmbeddedUndertow

这三个配置类是ServletWebServerFactoryConfiguration的嵌套配置类，它们会分别检测classpath上存在的类，从而判断当前应用使用的是Tomcat/Jetty/Undertow中的哪一个Servlet Web服务器,从而决定定义相应的工厂组件bean : TomcatServletWebServerFactory/JettyServletWebServerFactory/UndertowServletWebServerFactory。

详解：

ServletWebServerFactoryAutoConfiguration

/**
* EnableAutoConfiguration Auto-configuration for servlet web servers.
*
*/
@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
// 仅在类 ServletRequest 存在于 classpath 上时才生效
@ConditionalOnClass(ServletRequest.class)
// 仅在当前应用是 Servlet Web 应用时才生效
@ConditionalOnWebApplication(type = Type.SERVLET)
// 确保前缀为 server 的配置参数加载到 bean ServerProperties
@EnableConfigurationProperties(ServerProperties.class)
// 1. 导入 ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar 以注册
// BeanPostProcessor : WebServerFactoryCustomizerBeanPostProcessor 和
// ErrorPageRegistrarBeanPostProcessor
// 2. 导入 EmbeddedTomcat/EmbeddedJetty/EmbeddedUndertow 这三个属于
// ServletWebServerFactoryConfiguration 的嵌套配置类，这三个配置类会分别检测
// classpath上存在的类，从而判断当前应用使用的是 Tomcat/Jetty/Undertow,
// 从而决定定义哪一个 Servlet Web服务器的工厂 bean :
// TomcatServletWebServerFactory/JettyServletWebServerFactory/UndertowServletWebServerFactory
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {

    // 定义 bean ServletWebServerFactoryCustomizer，这里与properties中serverProperties配置联系，解释了为什么配置文件也可修改servlet容器配置
    @Bean
    public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(
            ServerProperties serverProperties) {
        return new ServletWebServerFactoryCustomizer(serverProperties);
    }

    // 针对当前Servlet容器是Tomcat时定义该 bean，用于定制化 TomcatServletWebServerFactory
    @Bean
    // 仅在类 org.apache.catalina.startup.Tomcat 存在于 classpath 上时才生效
    @ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
    public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
            ServerProperties serverProperties) {
        return new TomcatServletWebServerFactoryCustomizer(serverProperties);
    }

    /**
     * Registers a WebServerFactoryCustomizerBeanPostProcessor. Registered via
     * ImportBeanDefinitionRegistrar for early registration.
     * 这是一个 ImportBeanDefinitionRegistrar， 它会向容器注入两个 BeanPostProcessor :
     * 1. WebServerFactoryCustomizerBeanPostProcessor
     * 该 BeanPostProcessor 会搜集容器中所有的 WebServerFactoryCustomizer，对当前应用所采用的
     * WebServerFactory 被初始化前进行定制
     * 2. ErrorPageRegistrarBeanPostProcessor
     * 该 BeanPostProcessor 会搜集容器中所有的 ErrorPageRegistrar，添加到当前应用所采用的
     * ErrorPageRegistry 中,实际上，这里的 ErrorPageRegistry 会是 ConfigurableWebServerFactory,
     * 具体实现上来讲，是一个 ConfigurableTomcatWebServerFactory,ConfigurableJettyWebServerFactory
     * 或者 ConfigurableUndertowWebServerFactory,分别对应 Tomcat, Jetty, Undertow 这三种
     * Servlet Web 容器的工厂类
     */
    public static class BeanPostProcessorsRegistrar
            implements ImportBeanDefinitionRegistrar, BeanFactoryAware {

        private ConfigurableListableBeanFactory beanFactory;

        @Override
        public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
            if (beanFactory instanceof ConfigurableListableBeanFactory) {
                this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;
            }
        }

        @Override
        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
                                            BeanDefinitionRegistry registry) {
            if (this.beanFactory == null) {
                return;
            }
            registerSyntheticBeanIfMissing(registry,
                    "webServerFactoryCustomizerBeanPostProcessor",
                    WebServerFactoryCustomizerBeanPostProcessor.class);
            registerSyntheticBeanIfMissing(registry,
                    "errorPageRegistrarBeanPostProcessor",
                    ErrorPageRegistrarBeanPostProcessor.class);
        }

        private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,
                                                    String name, Class<?> beanClass) {
            if (ObjectUtils.isEmpty(
                    this.beanFactory.getBeanNamesForType(beanClass, true, false))) {
                RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);
                beanDefinition.setSynthetic(true);
                registry.registerBeanDefinition(name, beanDefinition);
            }
        }

    }

ServletWebServerFactoryConfiguration

ServletWebServerFactoryConfiguration是一个针对ServletWebServerFactory进行配置的配置类。它通过检测应用classpath存在的类，从而判断当前应用要使用哪个Servlet容器:Tomcat,Jetty还是Undertow。检测出来之后，定义相应的Servlet Web服务器工厂组件bean

Servlet容器类型	WebServerFactory实现类
Tomcat	TomcatServletWebServerFactory
Jetty	JettyServletWebServerFactory
Undertow	UndertowServletWebServerFactory

注意，以上三个实现类都继承自抽象基类AbstractServletWebServerFactory,实现了接口WebServerFactory,ErrorPageRegistry。该特征会被WebServerFactoryCustomizerBeanPostProcessor和ErrorPageRegistrarBeanPostProcessor使用，用于对WebServerFactory进行定制化，以及设置相应的错误页面。

@Configuration
class ServletWebServerFactoryConfiguration {

@Configuration
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedTomcat {

@Bean
public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
return new TomcatServletWebServerFactory();
}

}

/**
* Nested configuration if Jetty is being used.
*/
@Configuration
@ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedJetty {

@Bean
public JettyServletWebServerFactory JettyServletWebServerFactory() {
return new JettyServletWebServerFactory();
}

}

/**
* Nested configuration if Undertow is being used.
*/
@Configuration
@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedUndertow {

@Bean
public UndertowServletWebServerFactory undertowServletWebServerFactory() {
return new UndertowServletWebServerFactory();
}

}

}

查看代码：

这里以TomcatServletWebServerFactory:

注意这里此方法返回值：getTomcatWebServer(tomcat);查看此方法：传入两个参数，只要得到端口号大于0就默认启动

同时查看：其中TomcatWebServer类构造方法：

这时候tomcat启动开始。查看initialize();方法

private void initialize() throws WebServerException {
		logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));
		synchronized (this.monitor) {
			try {
				addInstanceIdToEngineName();

				Context context = findContext();
				context.addLifecycleListener((event) -> {
					if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {
						// Remove service connectors so that protocol binding doesn't
						// happen when the service is started.
						removeServiceConnectors();
					}
				});

				// Start the server to trigger initialization listeners
				this.tomcat.start();

				// We can re-throw failure exception directly in the main thread
				rethrowDeferredStartupExceptions();

				try {
					ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
				}
				catch (NamingException ex) {
					// Naming is not enabled. Continue
				}

				// Unlike Jetty, all Tomcat threads are daemon threads. We create a
				// blocking non-daemon to stop immediate shutdown
				startDaemonAwaitThread();
			}
			catch (Exception ex) {
				stopSilently();
				destroySilently();
				throw new WebServerException("Unable to start embedded Tomcat", ex);
			}
		}
	}

可以看到有启动： this.tomcat.start();启动！

思考

如何对嵌入式容器的配置修改是怎么生效的

两种方法:1、修改properties

2、配置servlet容器定制器

@Bean
public WebServerFactoryCustomizer<TomcatServletWebServerFactory> myWebServerFactoryCustomizer() {
    return new WebServerFactoryCustomizer<TomcatServletWebServerFactory>() {
        public void customize(TomcatServletWebServerFactory factory) {
            //设置相关配置
            factory.setPort(8080);
        }
    };
}

思考：怎么修改的原理？

springboot版本对照：

1.5.x:-->

//导入后置处理器的注册器，作用给容器导入组件

@Import(BeanPostProcessorsRegistrar.class

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication
//导入后置处理器的注册器
@Import(BeanPostProcessorsRegistrar.class)
public class EmbeddedServletContainerAutoConfiguration {.....}

2.x:将其作为内部类导入组件

@Configuration
@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 {
    .....
    public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {
    ...

        @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) {
       if (this.beanFactory == null) {
      return;
           }
        //导入servlet定制器的后置处理器
       registerSyntheticBeanIfMissing(registry, "webServerFactoryCustomizerBeanPostProcessor",
       WebServerFactoryCustomizerBeanPostProcessor.class);
       registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor",
         ErrorPageRegistrarBeanPostProcessor.class);
            }
     }

}

观察方法：BeanPostProcessorsRegistrar，导入servlet定制器的后置处理器

作用：后置处理器：bean初始化前后（创建完对象，还没赋值赋值）执行初始化工作

WebServerFactoryCustomizerBeanPostProcessor

@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
   if (bean instanceof WebServerFactory) {
      postProcessBeforeInitialization((WebServerFactory) bean);
   }
   return bean;
}

初始化之前：判断如果当前初始化组件是WebServerFactory 的一个组件，就调用postProcessBeforeInitialization此方法，

private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {
LambdaSafe.callbacks(WebServerFactoryCustomizer.class, getCustomizers(), webServerFactory)
.withLogger(WebServerFactoryCustomizerBeanPostProcessor.class)
.invoke((customizer) -> customizer.customize(webServerFactory));
}

此方法：获取所有的定制器，调用每一个定制器的customize方法，来给servlet容器进行属性赋值

getCustomizers()方法以上调用：

private Collection<WebServerFactoryCustomizer<?>> getCustomizers() {
if (this.customizers == null) {
// Look up does not include the parent context
this.customizers = new ArrayList<>(getWebServerFactoryCustomizerBeans());
this.customizers.sort(AnnotationAwareOrderComparator.INSTANCE);
this.customizers = Collections.unmodifiableList(this.customizers);
}
return this.customizers;
}

this.customizers = new ArrayList<>(getWebServerFactoryCustomizerBeans());此方法点进去：
@SuppressWarnings({ "unchecked", "rawtypes" })
private Collection<WebServerFactoryCustomizer<?>> getWebServerFactoryCustomizerBeans() {
return (Collection) this.beanFactory.getBeansOfType(WebServerFactoryCustomizer.class, false, false).values();
}

作用：从容器中获取所有该类型的组件：WebServerFactoryCustomizer这里该组件我们可以定制。

思考2？在properties中是怎样定制servlet容器的？？

@Configuration
@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 {......}

2.x:中直接在servelt容器工厂的自动配置类：@EnableConfigurationProperties(ServerProperties.class)注解，向容器中添加ServerProperties组件，在SpringBoot中xxxProperties类都是和配置文件映射的类，用户可以通过配置文件配置类中属性。例如：用户可以通过server.port配置容器端口号。

1.5.x:serverProperties中实现方法。

嵌入式servlet容器自动配置原理的步骤

1）、SpringBoot根据导入的依赖情况，容器中导入的ServletWebServerFactoryConfiguration配置类，给容器添加相应的TomcatServletWebServerFactory，通过TomcatServletWebServerFactory的getWebServer（）启动servelt容器

2）、容器中某个组件要创建对象就会惊动后置处理器；@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,

只要是嵌入式的Servlet容器工厂，后置处理器就工作，用与定制嵌入式容器的配置修改，

3）、后置处理器，执行postProcessBeforeInitialization（）方法，调用定制器的定制方法；