springboot整合springmvc源碼分析（2）--承上啓下

根據上章留下的猜想，我們開始對其進行源碼分析，一探究竟 

<modelVersion>4.0.0</modelVersion>
    <packaging>pom</packaging>
    <parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.2.5.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.example</groupId>
	<artifactId>springmvc-demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>springmvc-demo</name>
	<description>Demo project for Spring Boot</description>

	<properties>
		<java.version>1.8</java.version>
	</properties>

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
			<version>2.1.4.RELEASE</version>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
			<exclusions>
				<exclusion>
					<groupId>org.junit.vintage</groupId>
					<artifactId>junit-vintage-engine</artifactId>
				</exclusion>
			</exclusions>
		</dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

</project>

我們maven依賴好spring-boot-starter-web後我們進到spring-web依賴jar中看看：

這裏確確實實有我們上章所提到的spi加載該接口的類

org.springframework.web.SpringServletContainerInitializer

我們進到這個類看下：

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

	@Override
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {

		List<WebApplicationInitializer> initializers = new LinkedList<>();

		if (webAppInitializerClasses != null) {
			for (Class<?> waiClass : webAppInitializerClasses) {
				// Be defensive: Some servlet containers provide us with invalid classes,
				// no matter what @HandlesTypes says...
				if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
						WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
					try {
						initializers.add((WebApplicationInitializer)
								ReflectionUtils.accessibleConstructor(waiClass).newInstance());
					}
					catch (Throwable ex) {
						throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
					}
				}
			}
		}

		if (initializers.isEmpty()) {
			servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
			return;
		}

		servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
		AnnotationAwareOrderComparator.sort(initializers);
		for (WebApplicationInitializer initializer : initializers) {
			initializer.onStartup(servletContext);
		}
	}

}

這裏你會發現這不就是我們上一章所做的事嗎？跟上一章提到的知識點一模一樣

這裏AbstractDispatcherServletInitializer實現了WebApplicationInitializer接口

以下是該實現類的部分核心內容：

@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		super.onStartup(servletContext);
		registerDispatcherServlet(servletContext);
	}

	
	protected void registerDispatcherServlet(ServletContext servletContext) {
		String servletName = getServletName();
		Assert.hasLength(servletName, "getServletName() must not return null or empty");

		WebApplicationContext servletAppContext = createServletApplicationContext();
		Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null");

		FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
		Assert.notNull(dispatcherServlet, "createDispatcherServlet(WebApplicationContext) must not return null");
		dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());

		ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
		if (registration == null) {
			throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
					"Check if there is another servlet registered under the same name.");
		}

		registration.setLoadOnStartup(1);
		registration.addMapping(getServletMappings());
		registration.setAsyncSupported(isAsyncSupported());

		Filter[] filters = getServletFilters();
		if (!ObjectUtils.isEmpty(filters)) {
			for (Filter filter : filters) {
				registerServletFilter(servletContext, filter);
			}
		}

		customizeRegistration(registration);
	}

在registerDispatcherServlet方法中你會發現springmvc的東西以及我們上章的做法添加一個servlet：

FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);

protected FrameworkServlet createDispatcherServlet(WebApplicationContext servletAppContext) {
		return new DispatcherServlet(servletAppContext);
	}

從這裏我們可以看見，確確實實是創建了一個springmvc的核心控制器DispatcherServlet，然後將其放入tomcat的servlet容器中

至此我們是不是可以愉快的斷定springboot整合springmvc的方式和我們上章（springboot整合springmvc源碼分析（1）--前言）的猜想一致呢？

答案：非也 

why?

不急，要不我們在SpringServletContainerInitializer這個類的onStartup中打上個斷定，然後啓動我們的程序

@SpringBootApplication
public class DemoApplication {
	public static void main(String[] args) {
		SpringApplication.run(DemoApplication.class, args);
	}
}

當你啓動程序之後，你就會發現，爲什麼不進斷點？why?是不是springboot創建tomcat不是我們上章提到的tomcat.addWebapp("/","d:\\tomcat\\")，所以就不會通過spi自動加載我們SpringServletContainerInitializer呢？或許是；

下章我們就這個問題再深入討論