SpringBoot Source Learning Series Embedded Servlet Container

SpringBoot Source Learning Series Embedded Servlet containers principle start

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1, a brief preface blog

SpringBoot automatic configuration is the essence of SpringBoot for SpringBoot specific implementation is not very clear to the reader, I can read the source code to learn column , which has for some blog SpringBoot source of learning, content is relatively simple, more suitable for entry-learning

For SpringBoot project is no need to configure Tomcat, jetty, etc. Servlet container, either directly start the application class, SpringBoot Why can do so simple? The reason is the use of embedded Servlet container, the default is to use Tomcat, and what specific reason? Why start the application you can start the embedded Tomcat Servlet container or other? ok, this blog had SpringBoot Embedded Servlet starts principles of brief

Preparing the environment:

• SmartGit
• IntelliJ IDEA
• Maven
• SpringBoot2.2.1

This blog SpringBoot first create a project, based on the latest version 2.2.1, first carry out the necessary application code to read the source code before practice, practice before learning applications, help to understand the source code

In SpringBoot official website to find relevant description Embedded Servlet container:

Under the hood, Spring Boot uses a different type of ApplicationContext for embedded servlet container support. The ServletWebServerApplicationContext is a special type of WebApplicationContext that bootstraps itself by searching for a single ServletWebServerFactory bean. Usually a TomcatServletWebServerFactory, JettyServletWebServerFactory, or UndertowServletWebServerFactory has been auto-configured.

This is important information, a simple translation of what, in which he mentions ServletWebServerApplicationContext This is a special class of ApplicationContext, that is, start time, used to scan ServletWebServerFactory type of class, ServletWebServerFactory class is an interface class, is a specific category TomcatServletWebServerFactory, JettyServletWebServerFactory, or UndertowServletWebServerFactory etc.

2, custom servlet container

Since then define how to configure the Embedded Servlet container? Figure corresponding descriptions found in the official documentation:

Method 1: Modify the application configuration

As can be seen from the official document support is arranged as follows, so to modify the servlet container configuration, you can directly modify the application configuration file:

• 网络设置: 监听端口(server.port)、服务器地址(server.address)等等
• Session设置: 会话是否持久 (server.servlet.session.persistent),会话超时(server.servlet.session.timeout), 会话数据的位置 (server.servlet.session.store-dir), 会话对应的cookie配置 (server.servlet.session.cookie.*) 等等
• 错误管理: 错误页面位置 (server.error.path)等等
• SSL设置：具体参考Configure SSL
• HTTP compression：具体参考Enable HTTP Response Compression

方法2：自定义WebServerFactoryCustomizer定制器类

从文档里还找到了通过新建自定义的WebServerFactoryCustomizer类来实现属性配置修改，WebServerFactoryCustomizer也就是一种定制器类：

import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
import org.springframework.stereotype.Component;

/**
 * <pre>
 *  自定义的WebServerFactory定制器类
 * </pre>
 * @author nicky
 * <pre>
 * 修改记录
 *    修改后版本:     修改人：  修改日期: 2019年12月01日  修改内容:
 * </pre>
 */
@Component
public class WebServerFactoryCustomizationBean implements WebServerFactoryCustomizer<ConfigurableServletWebServerFactory> {

    @Override
    public void customize(ConfigurableServletWebServerFactory server) {
        server.setPort(8081);
    }

}

ok，官方文档里提供了如上的代码实例，当然也可以通过@bean注解进行设置，代码实例如：

@Configuration
public class MyServerConfig {

    /**
     * 自定义的WebServerFactory定制器类
     * @return
     */
    @Bean
    public WebServerFactoryCustomizer<ConfigurableServletWebServerFactory> webServerFactoryCustomizer(){
        return new WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>() {
            @Override
            public void customize(ConfigurableServletWebServerFactory factory) {
                factory.setPort(8082);
            }
        };
    }
}

3、变换servlet容器

SpringBoot2.2.1版本支持的内嵌servlet容器有tomcat、jetty(适用于长连接)、undertow(高并发性能不错，但是默认不支持jsp)，不过项目默认使用的是Tomcat的

我们可以找新建的一个SpringBoot项目，要求是集成了spring-boot-starter-web的工程，在pom文件右键->Diagrams->Show Dependencies，可以看到对应的jar关系图：

ok，从图可以看出，SpringBoot默认使用是Servlet容器是Tomcat，然后如果要切换其它嵌入式Servlet容器，要怎么实现？我们可以在图示选择spring-boot-starter-tomcat右键exclusion，然后引入其它的servlet容器，pom配置如图：

<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>

4、servlet容器启动原理

ok，有了前面应用方面的学习之后，就可以简单跟一下Springboot是怎么对servlet容器进行自动配置的？内嵌的默认Tomcat容器是怎么样启动的？

从之前博客的学习，可以知道Springboot的自动配置都是通过一些AutoConfiguration类进行自动配置的，所以同理本博客也找一些对应的类，ServletWebServerFactoryAutoConfiguration 就是嵌入式servlet容器的自动配置类，简单跟一下其源码

@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)//使ServerProperties配置类起效
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })//@Import是Spring框架的注解，作用是将对应组件加载到容器，这里关键的是BeanPostProcessorsRegistrar，一个后置处理类
public class ServletWebServerFactoryAutoConfiguration {

    @Bean
    public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties) {
        return new ServletWebServerFactoryCustomizer(serverProperties);
    }

//Tomcat的定制器类，起作用的条件是有Tomcat对应jar有引入项目的情况，默认是引入的，所以会执行Tomcat的servletWeb工厂定制类
    @Bean
    @ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
    public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
            ServerProperties serverProperties) {
        return new TomcatServletWebServerFactoryCustomizer(serverProperties);
    }

    ....
    
    //注册重要的后置处理器类WebServerFactoryCustomizerBeanPostProcessor，在ioc容器启动的时候会调用后置处理器
    public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {

        private ConfigurableListableBeanFactory beanFactory;
        
        //设置ConfigurableListableBeanFactory
        @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);
            }
        }

    }

}

从自动配置类里，我们并不能很明确地理解自动配置是怎么运行的，只看重关键的一些信息点，比如注册了Tomcat的ServletWebServer工厂的定制器类，方法是tomcatServletWebServerFactoryCustomizer，还有一个后置处理类BeanPostProcessorsRegistrar，后置处理是Spring源码里是很关键的，所以这里可以继续点一下TomcatServletWebServerFactoryCustomizer，Tomcat的webServer工厂定制器类

也是一个WebServerFactoryCustomizer类型的类，从前面的应用学习，这个类是进行servlet容器的一些定制

这个是关键的方法，主要是拿ServerProperties配置类里的信息进行特定属性定制

所以，这里就可以知道Tomcat的配置是通过定制器类TomcatServletWebServerFactoryCustomizer进行定制的，其工厂类是TomcatServletWebServerFactory

TomcatServletWebServerFactory工厂类进行Tomcat对象的创建，必要参数的自动配置

ok，简单跟了一下源码之后，我们知道了TomcatServletWebServerFactoryCustomizer是Tomcat的定制器类，Tomcat对象的创建是通过TomcatServletWebServerFactory类的，然后，有个疑问，这个定制器类是什么时候创建的？为什么一启动Application类，嵌入式的Tomcat也启动了？打成jar格式的Springboot项目，只要运行jar命令，不需要启动任何servlet容器，项目也是正常运行的？然后这个BeanPostProcessorsRegistrar后置处理类有什么作用？ok，带着这些疑问，我们还是用调试一下源码

如图，打断点调试，看看Tomcat定制器是怎么创建的？

定制器类被调用了，其对应的工厂类也会起作用，打个断点，看看工厂类是怎么调用的？

ok，启动Application类，在idea里调试，如图，可以跟着调用顺序，一点点跟源码，如图所示，调用了Springboot的run方法

run方法里的刷新上下文方法，refreshContext其实也就是创建ioc容器，初始化ioc容器，并创建容器的每一个组件

这里注意到了，调用到了ServletWebServerApplicationContext类的refresh方法，ServletWebServerApplicationContext类前面也介绍到了，这个类是一种特殊的ApplicationContext类，也就是一些ioc的上下文类，作用于WebServer类型的类

创建webServer类，先创建ioc容器，调用基类的onRefresh方法，然后再调用createWebServer方法

ioc的servletContext组件没被创建的情况，调用ServletWebServerFactory类获取WebServer类，有servletContext的情况，直接从ioc容器获取

扫描ioc容器里是否有对应的ServletWebServerFactory类，TomcatServletWebServerFactory是其中一种，通过调试，是有扫描到的，所以从ioc容器里将这个bean对应的信息封装到ServletWebServerFactory对象

接着是ioc容器创建bean的过程，这个一个比较复杂的过程，因为是单例的，所以是调用singleObjects进行存储

bean被创建之后，调用了后置处理器，这个其实就是Spring的源码里的bean的创建过程，后置处理器是很关键的，在bean被创建，还没进行属性赋值时候，就调用了后置处理器

关键点，这里是检测是否有WebServerFactory工厂类，前面的调试发现已经有Tomcat的WebServer工厂类，所以是会调用后置处理器的

调用了WebServerFactoryCustomizerBeanPostProcessor这个后置处理类，然后拿到一个WebServerFactoryCustomizer定制器类，也就是TomcatWebServerFactoryCustomizer，通过后置处理器调用定制方法customize

然后WebServerFactoryCustomizerBeanPostProcessor这个后置处理器是什么注册的？往前翻Springboot的自动配置类，在这里找到了WebServerFactoryCustomizerBeanPostProcessor的注册

ok，继续调试源码，BeanWrapperImpl创建bean实例

ok，Tomcat定制器类被调用了，是通过后置处理器调用的

然后就是之前跟过的定制方法customize执行：

Tomcat的WebServer工厂类创建Tomcat对象实例，进行属性配置，引擎设置等等

端口有设置就创建TomcatwebServer对象

TomcatWebServer启动Tomcat，如图代码所示：

ok，跟了源码，您是否有一个疑问？Tomcat的工厂类TomcatServletWebServerFactory是什么时候创建的？好的，返回前面配置类看看，如图，这里用import引入了EmbeddedTomcat类

ok，EmbeddedTomcat是一个内部的配置类，条件是有引入Tomcat对应的jar，就会自动创建工厂类，很显然，Springboot默认是有引入的

ok, after learning the source code is relatively simple, the idea is very clear, Springboot of ServletWebServerFactoryAutoConfiguration class is automatically configured Embedded Servlet container, the main role of this class is to create TomcatServletWebServerFactory factory class to create custom class TomcatServletWebServerFactoryCustomizer, create FilterRegistrationBean class, at the same time a crucial step is to register post-processors webServerFactoryCustomizerBeanPostProcessor, then Springboot Application class started, it will run method, run through a series of call creates ioc container by onRefresh method ServletWebServerApplicationContext, and then through createWebServer method, the method will go createWebServer ioc container scans a corresponding ServletWebServerFactory factory class (TomcatServletWebServerFactory is one), scanned, triggered webServerFactoryCustomizerBeanPostProcessor postprocessor class, the processor retrieves TomcatServletWebServerFactoryCustomizer custom class and calls the custom customize method, which when workers Class work, call getWebServer methods Tomcat configuration attributes and engine settings, etc., and then create TomcatWebServer start Tomcat container, ok, this blog is simply about the boot process with embedded Tomcat container, you can see a strong Springboot, or based on Spring creation bean instance framework, such as the post-processor mentioned in this blog, bean and engineering processes, are achieved through the Spring framework