springboot 自动注入servlet原理

前置内容： servlet的多种注册方式

SpringBoot启用 ServletContainerInitializer

ServletContainerInitializer-SPI

SPI部分内容参照 servlet的多种注册方式

开启SPI

默认情况下，springboot 使用 embedded-tomcat,没有启用ServletContainerInitializer，这就需要我们做改造。

pom.xml

 <dependency>
            <groupId>org.springframework.bootgroupId>
            <artifactId>spring-boot-starter-webartifactId>
            <exclusions>
                <exclusion>
                    <groupId>org.springframework.bootgroupId>
                    <artifactId>spring-boot-starter-tomcatartifactId>
                exclusion>
            exclusions>
        dependency>

        
        <dependency>
            <groupId>javax.servletgroupId>
            <artifactId>javax.servlet-apiartifactId>
            <version>3.1.0version>
            <scope>providedscope>
        dependency>
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App.java 继承 SpringBootServletInitializer

@SpringBootApplication
public class App extends SpringBootServletInitializer {

 /*   public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
*/

/*    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(App.class);
    }*/
}
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springboot SPI

增加Filter配置类

@Configuration
public class SpringFilterConfiguration {

    @Bean
    public FilterRegistrationBean<SpringFilter1> SpringFilter1() {
        FilterRegistrationBean<SpringFilter1> registrationBean = new FilterRegistrationBean<>();
        registrationBean.setFilter(new SpringFilter1());
        registrationBean.setName("springfilter11");
        registrationBean.addUrlPatterns("/*");
        registrationBean.setOrder(11);
        return registrationBean;
    }

    @Bean
    public FilterRegistrationBean<SpringFilter2> SpringFilter2() {
        FilterRegistrationBean<SpringFilter2> registrationBean = new FilterRegistrationBean<>();
        registrationBean.setFilter(new SpringFilter2());
        registrationBean.setName("springfilter22");
        registrationBean.addUrlPatterns("/*");
        registrationBean.setOrder(22);
        return registrationBean;
    }
}
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源码分析

1. SpringServletContainerInitializer

@HandlesTypes(WebApplicationInitializer.class) //lookup 实现WebApplicationInitializer的类
public class SpringServletContainerInitializer implements ServletContainerInitializer {
	@Override
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {
		List<WebApplicationInitializer> initializers = new ArrayList<>(webAppInitializerClasses.size());

		for (Class<?> waiClass : webAppInitializerClasses) {
			initializers.add((WebApplicationInitializer) ReflectionUtils.accessibleConstructor(waiClass).newInstance());
		}

		AnnotationAwareOrderComparator.sort(initializers);
		for (WebApplicationInitializer initializer : initializers) { //  2. 此时值为App.class
			initializer.onStartup(servletContext); 
		}
	}
}
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2.App

类图

App为我们的主类，它的泪如如下。

3.SpringBootServletInitializer.onStartup(servletContext)

程序的最终入口就在此了。

public abstract class SpringBootServletInitializer implements WebApplicationInitializer {
	
	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		WebApplicationContext rootApplicationContext = createRootApplicationContext(servletContext); // 3.1
		if (rootApplicationContext != null) {
			servletContext.addListener(new SpringBootContextLoaderListener(rootApplicationContext, servletContext)); //3.2
		}
	}

	//3.1
	protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
		SpringApplicationBuilder builder = createSpringApplicationBuilder(); // new SpringApplicationBuilder()
		builder.main(getClass()); // getClass() == App.class
		
		//parent部分略....

		builder.initializers(new ServletContextApplicationContextInitializer(servletContext));  //3.1.1
		builder.contextFactory((webApplicationType) -> new AnnotationConfigServletWebServerApplicationContext()); //3.1.2 factory 直接返回new AnnotationConfigServletWebServerApplicationContext(); extends ServletWebServerApplicationContext
		builder = configure(builder); //do nothing 
		builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext)); //3.1.3
		SpringApplication application = builder.build(); // 返回 SpringApplication
		application.addPrimarySources(Collections.singleton(getClass())); //3.1.4
		
		application.setRegisterShutdownHook(false);
		return run(application);
	}

	protected WebApplicationContext run(SpringApplication application) {
		return (WebApplicationContext) application.run(); //执行 SpringApplication.run()
	}

}
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4. SpringApplication.run()

SpringApplication.run()的逻辑不再赘述， 最终会调用AbstractApplicationContext.onRefresh(),而此时的ApplicationContext的具体实现类为AnnotationConfigServletWebServerApplicationContext

public class SpringApplication {
	private List<ApplicationContextInitializer<?>> initializers;		 //3.1.1
	private List<ApplicationListener<?>> listeners;				//3.1.3
	private ApplicationContextFactory applicationContextFactory = ApplicationContextFactory.DEFAULT; //3.1.2
	private Set<Class<?>> primarySources;  //3.1.4

	public ConfigurableApplicationContext run(String... args) {
		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
		ConfigurableApplicationContext context = null;
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting(bootstrapContext, this.mainApplicationClass);

		ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
		ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
		configureIgnoreBeanInfo(environment);
		Banner printedBanner = printBanner(environment);
		context = createApplicationContext(); // applicationContextFactory.create()//3.1.2 return AnnotationConfigServletWebServerApplicationContext();
		context.setApplicationStartup(this.applicationStartup);
		prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
		refreshContext(context); // ConfigurableApplicationContext.refresh();   ---> AbstractApplicationContext.onRefresh();
		afterRefresh(context, applicationArguments);
		Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
		if (this.logStartupInfo) {
			new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);
		}
		listeners.started(context, timeTakenToStartup);
		callRunners(context, applicationArguments);
		
		return context;

	}

}
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5.AnnotationConfigServletWebServerApplicationContext.onRefresh()

类图

ServletWebServerApplicationContext.onRefresh()

public class ServletWebServerApplicationContext extends GenericWebApplicationContext
		implements ConfigurableWebServerApplicationContext {
	@Override
	protected void onRefresh() {
		super.onRefresh();
		createWebServer(); // factory.getWebServer(getSelfInitializer());
	}


	private void selfInitialize(ServletContext servletContext) throws ServletException {
		prepareWebApplicationContext(servletContext);
		registerApplicationScope(servletContext);
		WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
		for (ServletContextInitializer beans : getServletContextInitializerBeans()) {  beanFactory.getBeanNamesForType(ServletContextInitializer.class);
			beans.onStartup(servletContext); //6
		}
	}
}
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6.ServletContextInitializer.onStartup

类图

AbstractFilterRegistrationBean.addRegistration()

//6. FilterRegistrationBean -> AbstractFilterRegistrationBean -> DynamicRegistrationBean  -> RegistrationBean
public abstract class AbstractFilterRegistrationBean<T extends Filter> extends DynamicRegistrationBean<Dynamic> {

	@Override
	protected Dynamic addRegistration(String description, ServletContext servletContext) {
		Filter filter = getFilter();
		return servletContext.addFilter(getOrDeduceName(filter), filter);
	}

}
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