Spring5 源码阅读笔记（5）基于注解的方法替换 web.xml

替换配置servlet

servlet 容器启动的时候，会先加载指定路径下的文件里的类。

也就是加载所有实现了 javax.servlet.ServletContainerInitializer 的类。调用它的 onStartup 方法。

@HandlesTypes(LoadServlet.class)
public class MyServletContainerInitializer implements ServletContainerInitializer {
	//tomcat启动会调用
    @Override				//set里面是所有LoadServlet的实例
    public void onStartup(Set<Class<?>> set, ServletContext servletContext) throws ServletException {
        Iterator var4;
        if (set != null) {
            var4 = set.iterator();
            while (var4.hasNext()) {
                Class<?> clazz = (Class<?>) var4.next();
                if (!clazz.isInterface() && !Modifier.isAbstract(clazz.getModifiers()) && LoadServlet.class.isAssignableFrom(clazz)) {
                    try {
                        ((LoadServlet) clazz.newInstance()).loadOnstarp(servletContext);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }
}

自定义接口 LoadServlet

public interface LoadServlet {
    void loadOnstarp(ServletContext servletContext);
}

自己实现一个 LoadServlet：

public class LoadServletImpl implements LoadServlet {
    @Override
    public void loadOnstarp(ServletContext servletContext) {
        //在这里加入自己写的servlet
        ServletRegistration.Dynamic initServlet = servletContext.addServlet("initServlet", "com.xiangxue.jack.servlet.InitServlet");
        //<load-on-startup>值大于等于0可以随着tomcat启动而加载
        initServlet.setLoadOnStartup(1);
        //拦截url路径
        initServlet.addMapping("/init");

    }
}

通过这一步可以实现配置 servlet 的功能

替换配置ContextLoaderListener和DispatcherServlet

LoadServlet 的作用和 spring 里的 WebApplicationInitializer 类似

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

@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);
		}
	}
}

跟 onStartup：
类 WebApplicationInitializer

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

ContextLoaderListener

跟 onStartup：
类 AbstractContextLoaderInitializer

@Override
public void onStartup(ServletContext servletContext) throws ServletException {
	registerContextLoaderListener(servletContext);
}

跟 registerContextLoaderListener：

protected void registerContextLoaderListener(ServletContext servletContext) {

	//创建spring上下文，注册了SpringContainer
	WebApplicationContext rootAppContext = createRootApplicationContext();
	if (rootAppContext != null) {
		//创建监听器
		/*
			形如这种配置
		* <listener>
			  <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
			<!--<listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>-->
		  </listener>
		*
		* */
		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");
	}
}

跟 createRootApplicationContext：

@Override
@Nullable
protected WebApplicationContext createRootApplicationContext() {
	Class<?>[] configClasses = getRootConfigClasses();
	if (!ObjectUtils.isEmpty(configClasses)) {
		AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
		context.register(configClasses);
		return context;
	}
	else {
		return null;
	}
}

跟 getRootConfigClasses：
自己定义一个子类

public class WebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    //父容器
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[]{SpringContainer.class};
    }

    //SpringMVC配置子容器
    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[]{MvcContainer.class};
    }

    //获取DispatcherServlet的映射信息
    @Override
    protected String[] getServletMappings() {
        return new String[]{"/"};
    }

    @Override
    protected Filter[] getServletFilters() {
        return super.getServletFilters();
    }
}

//不扫描有@Controller注解的类
@ComponentScan(value = "com.xiangxue.jack",excludeFilters = {
        @ComponentScan.Filter(type = FilterType.ANNOTATION,classes = {Controller.class})
})
public class SpringContainer {
}

Spring 上下文的 refresh() 在哪里

类 ContextLoaderListener 里有这么一个方法，会由 tomcat 去调：

@Override
public void contextInitialized(ServletContextEvent event) {
	initWebApplicationContext(event.getServletContext());
}

跟 initWebApplicationContext：

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
	if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
		throw new IllegalStateException(
				"Cannot initialize context because there is already a root application context present - " +
				"check whether you have multiple ContextLoader* definitions in your web.xml!");
	}

	servletContext.log("Initializing Spring root WebApplicationContext");
	Log logger = LogFactory.getLog(ContextLoader.class);
	if (logger.isInfoEnabled()) {
		logger.info("Root WebApplicationContext: initialization started");
	}
	long startTime = System.currentTimeMillis();

	try {
		// Store context in local instance variable, to guarantee that
		// it is available on ServletContext shutdown.
		if (this.context == null) {
			this.context = createWebApplicationContext(servletContext);
		}
		if (this.context instanceof ConfigurableWebApplicationContext) {
			ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
			if (!cwac.isActive()) {
				// The context has not yet been refreshed -> provide services such as
				// setting the parent context, setting the application context id, etc
				if (cwac.getParent() == null) {
					// The context instance was injected without an explicit parent ->
					// determine parent for root web application context, if any.
					ApplicationContext parent = loadParentContext(servletContext);
					cwac.setParent(parent);
				}
				configureAndRefreshWebApplicationContext(cwac, servletContext);
			}
		}
		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
		if (ccl == ContextLoader.class.getClassLoader()) {
			currentContext = this.context;
		}
		else if (ccl != null) {
			currentContextPerThread.put(ccl, this.context);
		}

		if (logger.isInfoEnabled()) {
			long elapsedTime = System.currentTimeMillis() - startTime;
			logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");
		}

		return this.context;
	}
	catch (RuntimeException | Error ex) {
		logger.error("Context initialization failed", ex);
		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
		throw ex;
	}
}

跟 configureAndRefreshWebApplicationContext：

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
	if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
		// The application context id is still set to its original default value
		// -> assign a more useful id based on available information
		String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
		if (idParam != null) {
			wac.setId(idParam);
		}
		else {
			// Generate default id...
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
					ObjectUtils.getDisplayString(sc.getContextPath()));
		}
	}

	wac.setServletContext(sc);
	String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
	if (configLocationParam != null) {
		wac.setConfigLocation(configLocationParam);
	}

	// The wac environment's #initPropertySources will be called in any case when the context
	// is refreshed; do it eagerly here to ensure servlet property sources are in place for
	// use in any post-processing or initialization that occurs below prior to #refresh
	ConfigurableEnvironment env = wac.getEnvironment();
	if (env instanceof ConfigurableWebEnvironment) {
		((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
	}

	customizeContext(sc, wac);
	//这里
	wac.refresh();
}

DispatcherServlet

类 AbstractDispatcherServletInitializer

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

	//创建springmvc的上下文，注册了MvcContainer类
	WebApplicationContext servletAppContext = createServletApplicationContext();
	Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null");

	//创建DispatcherServlet
	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.");
	}

	/*
	* 如果该元素的值为负数或者没有设置，则容器会当Servlet被请求时再加载。
		如果值为正整数或者0时，表示容器在应用启动时就加载并初始化这个servlet，
		值越小，servlet的优先级越高，就越先被加载
	* */
	registration.setLoadOnStartup(1);
							//骨架
	registration.addMapping(getServletMappings());
	registration.setAsyncSupported(isAsyncSupported());
						//骨架
	Filter[] filters = getServletFilters();
	if (!ObjectUtils.isEmpty(filters)) {
		for (Filter filter : filters) {
			registerServletFilter(servletContext, filter);
		}
	}

	customizeRegistration(registration);
}

跟 createServletApplicationContext：

@Override
protected WebApplicationContext createServletApplicationContext() {
	AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
								//骨架
	Class<?>[] configClasses = getServletConfigClasses();
	if (!ObjectUtils.isEmpty(configClasses)) {
		context.register(configClasses);
	}
	return context;
}

跟 getServletConfigClasses：
类 AbstractAnnotationConfigDispatcherServletInitializer

又是前面出现过的自己定义的类

public class WebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    //父容器
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[]{SpringContainer.class};
    }

    //SpringMVC配置子容器
    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[]{MvcContainer.class};
    }

    //获取DispatcherServlet的映射信息
    @Override
    protected String[] getServletMappings() {
        return new String[]{"/"};
    }

    @Override
    protected Filter[] getServletFilters() {
        return super.getServletFilters();
    }
}

这次跟 MvcContainer：

@ComponentScan(value = "com.xiangxue.jack",includeFilters = {
        @ComponentScan.Filter(type = FilterType.ANNOTATION,classes = {Controller.class})
},useDefaultFilters = false)
public class MvcContainer {
}

SpringMVC 上下文的 refresh() 在哪里

容器启动的时候，tomcat 会调 DispatcherServlet 的 init 方法，这个方法在它的父类 HttpServletBean 里：

@Override
public final void init() throws ServletException {

	// Set bean properties from init parameters.
	PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
	if (!pvs.isEmpty()) {
		try {
			BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
			ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
			bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
			initBeanWrapper(bw);
			bw.setPropertyValues(pvs, true);
		}
		catch (BeansException ex) {
			if (logger.isErrorEnabled()) {
				logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
			}
			throw ex;
		}
	}

	// Let subclasses do whatever initialization they like.
	initServletBean();
}

跟 initServletBean：
类 FrameworkServlet

@Override
protected final void initServletBean() throws ServletException {
	getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
	if (logger.isInfoEnabled()) {
		logger.info("Initializing Servlet '" + getServletName() + "'");
	}
	long startTime = System.currentTimeMillis();

	try {
		this.webApplicationContext = initWebApplicationContext();
		initFrameworkServlet();
	}
	catch (ServletException | RuntimeException ex) {
		logger.error("Context initialization failed", ex);
		throw ex;
	}

	if (logger.isDebugEnabled()) {
		String value = this.enableLoggingRequestDetails ?
				"shown which may lead to unsafe logging of potentially sensitive data" :
				"masked to prevent unsafe logging of potentially sensitive data";
		logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
				"': request parameters and headers will be " + value);
	}

	if (logger.isInfoEnabled()) {
		logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
	}
}

跟 initWebApplicationContext：

protected WebApplicationContext initWebApplicationContext() {
	//这里会从servletContext中获取到父容器，就是通过监听器加载的容器
	WebApplicationContext rootContext =
			WebApplicationContextUtils.getWebApplicationContext(getServletContext());
	WebApplicationContext wac = null;

	if (this.webApplicationContext != null) {
		// A context instance was injected at construction time -> use it
		wac = this.webApplicationContext;
		if (wac instanceof ConfigurableWebApplicationContext) {
			ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
			if (!cwac.isActive()) {
				// The context has not yet been refreshed -> provide services such as
				// setting the parent context, setting the application context id, etc
				if (cwac.getParent() == null) {
					// The context instance was injected without an explicit parent -> set
					// the root application context (if any; may be null) as the parent
					cwac.setParent(rootContext);
				}
				//容器加载
				configureAndRefreshWebApplicationContext(cwac);
			}
		}
	}
	if (wac == null) {
		// No context instance was injected at construction time -> see if one
		// has been registered in the servlet context. If one exists, it is assumed
		// that the parent context (if any) has already been set and that the
		// user has performed any initialization such as setting the context id
		wac = findWebApplicationContext();
	}
	if (wac == null) {
		// No context instance is defined for this servlet -> create a local one
		wac = createWebApplicationContext(rootContext);
	}

	if (!this.refreshEventReceived) {
		// Either the context is not a ConfigurableApplicationContext with refresh
		// support or the context injected at construction time had already been
		// refreshed -> trigger initial onRefresh manually here.
		synchronized (this.onRefreshMonitor) {
			onRefresh(wac);
		}
	}

	if (this.publishContext) {
		// Publish the context as a servlet context attribute.
		String attrName = getServletContextAttributeName();
		getServletContext().setAttribute(attrName, wac);
	}

	return wac;
}

跟 configureAndRefreshWebApplicationContext：

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
	if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
		// The application context id is still set to its original default value
		// -> assign a more useful id based on available information
		if (this.contextId != null) {
			wac.setId(this.contextId);
		}
		else {
			// Generate default id...
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
					ObjectUtils.getDisplayString(getServletContext().getContextPath()) + '/' + getServletName());
		}
	}

	wac.setServletContext(getServletContext());
	wac.setServletConfig(getServletConfig());
	wac.setNamespace(getNamespace());
	wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

	// The wac environment's #initPropertySources will be called in any case when the context
	// is refreshed; do it eagerly here to ensure servlet property sources are in place for
	// use in any post-processing or initialization that occurs below prior to #refresh
	ConfigurableEnvironment env = wac.getEnvironment();
	if (env instanceof ConfigurableWebEnvironment) {
		((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
	}

	postProcessWebApplicationContext(wac);
	applyInitializers(wac);
	//这里
	wac.refresh();
}

Spring 上下文是 SpringMVC上下文的父上下文。