[Java知识库]Tomcat启动时，加载Spring容器的过程是什么？

Tomcat启动时，加载Spring容器的过程是什么？

1、Tomcat(Servlet3.0规范的web容器)启动时，会查找ServletContainerInitializer接口实现类；
2、 Spring框架提供了一个SpringServletContainerInitializer类，实现了ServletContainerInitializer接口。

在这个类中的onStartup（）方法，需要一个Set<Class<?>> webAppInitializerClasses类型的参数，也就是WebApplicationInitializer接口的实现类。

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
	@Override
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {
		//WebApplicationInitializer接口的实现类集合
		List<WebApplicationInitializer> initializers = new LinkedList<>();

		if (webAppInitializerClasses != null) {
			//遍历参数WebApplicationInitializer接口的实现类集合
			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 {
						//将当前对象加入到新创建的List集合
						initializers.add((WebApplicationInitializer)
								ReflectionUtils.accessibleConstructor(waiClass).newInstance());
					}
					catch (Throwable ex) {
						//抛出容器初始化异常
						throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
					}
				}
			}
		}
		
		//如果新创建的集合为空
		if (initializers.isEmpty()) {
			//输出日志：在类路径上未检测到spring webapplicationinitializer类型
			servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
			return;
		}

		servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
		//排序
		AnnotationAwareOrderComparator.sort(initializers);
		//遍历List集合
		for (WebApplicationInitializer initializer : initializers) {
			//调用当前对象（我们写的配置类）的onStartup()，也就是我们写的配置类的onStartup()，进行初始化配置
			initializer.onStartup(servletContext);
		}
	}

}

3、WebApplicationInitializer接口，又有抽象类实现它

public interface WebApplicationInitializer {
	void onStartup(ServletContext servletContext) throws ServletException;

}

4、AbstractContextLoaderInitializer 实现了WebApplicationInitializer 接口

AbstractDispatcherServletInitializer类继承了AbstractContextLoaderInitializer

AbstractAnnotationConfigDispatcherServletInitializer继承了AbstractDispatcherServletInitializer

为了创建容器，我们就必须写WebApplicationInitializer接口真正的实现类DispatcherServletConfig；

//AbstractContextLoaderInitializer 
public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer {

	protected final Log logger = LogFactory.getLog(getClass());

	//重写接口的方法onStartup()
	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		registerContextLoaderListener(servletContext);
	}

	//注册监听器
	protected void registerContextLoaderListener(ServletContext servletContext) {
		WebApplicationContext rootAppContext = createRootApplicationContext();
		if (rootAppContext != null) {
			ContextLoaderListener listener = new ContextLoaderListener(rootAppContext);
			listener.setContextInitializers(getRootApplicationContextInitializers());
			servletContext.addListener(listener);
		}
		else {
			logger.debug("No ContextLoaderListener registered, as " +
					"createRootApplicationContext() did not return an application context");
		}
	}
	
	@Nullable
	protected abstract WebApplicationContext createRootApplicationContext();

	@Nullable
	protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() {
		return null;
	}

}

5、AbstractDispatcherServletInitializer

public abstract class AbstractDispatcherServletInitializer extends AbstractContextLoaderInitializer {
	public static final String DEFAULT_SERVLET_NAME = "dispatcher";
	
	//重写onStartup()
	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		//调用父类onStartup()方法
		super.onStartup(servletContext);
		//注册DispatcherServlet（SpringMVC核心控制器）
		registerDispatcherServlet(servletContext);
	}
	
	//注册DispatcherServlet（SpringMVC核心控制器）
	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());
		
		//向容器中添加DispatcherServlet
		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.");
		}
		
		//设置加载级别
		registration.setLoadOnStartup(1);
		//加载Mapping映射
		registration.addMapping(getServletMappings());
		//设置异步请求
		registration.setAsyncSupported(isAsyncSupported());
		
		//获取过滤器
		Filter[] filters = getServletFilters();
		if (!ObjectUtils.isEmpty(filters)) {
			for (Filter filter : filters) {
			//注册过滤器
				registerServletFilter(servletContext, filter);
			}
		}

		customizeRegistration(registration);
	}

6、AbstractAnnotationConfigDispatcherServletInitializer

public abstract class AbstractAnnotationConfigDispatcherServletInitializer
		extends AbstractDispatcherServletInitializer {

	@Override
	@Nullable
	 //加载Spring容器：Root WebApplicationContext(Spring)
	protected WebApplicationContext createRootApplicationContext() {
		//获取子类的对象数组
		Class<?>[] configClasses = getRootConfigClasses();
		if (!ObjectUtils.isEmpty(configClasses)) {		//如果通过ObjectUtils查到的configClasses不为空
			//创建AnnotationConfigWebApplicationContext容器
			AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
			//将数组注册至容器
			context.register(configClasses);
			//返回容器
			return context;
		}
		else {
			return null;
		}
	}

	 // 加载Spring容器：Serlvet WebApplicationContext(Spring MVC)
	@Override
	protected WebApplicationContext createServletApplicationContext() {
		//创建容器
		AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
		//获取子类的对象数组
		Class<?>[] configClasses = getServletConfigClasses();
		if (!ObjectUtils.isEmpty(configClasses)) { //如果通过ObjectUtils查到的configClasses不为空
			context.register(configClasses);//将数组注册至容器
		}
		return context;	//返回容器
	}

	
	@Nullable
	protected abstract Class<?>[] getRootConfigClasses();

	@Nullable
	protected abstract Class<?>[] getServletConfigClasses();

}

7、在我们自己写的DispatcherServletConfig就必须重写三个方法
和onStartup（）方法，加载监听器和过滤器

	// 加载Spring容器：Root WebApplicationContext(Spring)
	//AppRootConfig该类对Serlvet Root WebApplicationContext容器配置
	@Override
	protected Class<?>[] getRootConfigClasses() {
		return new Class[] {AppRootConfig.class};
	}

	// 加载Spring容器：Serlvet WebApplicationContext(Spring MVC)
	//SpringMvcConfig该类对Serlvet WebApplicationContext容器配置
	@Override
	protected Class<?>[] getServletConfigClasses() {
		return new Class[] {SpringMvcConfig.class};
	}

	// 设置DispatcherServlet映射路径为处理所有请求
	@Override
	protected String[] getServletMappings() {
		return new String[] {"/"};
	}