Spring MVC源码——Root WebApplicationContext

Spring MVC源码——Root WebApplicationContext

打算开始读一些框架的源码,先拿 Spring MVC 练练手,欢迎点击这里访问我的源码注释.

目录

• Spring MVC源码——Root WebApplicationContext
  • 上下文层次结构
  • Root WebApplicationContext 初始化和销毁
    • ContextLoaderListener
    • ContextLoader
      • 初始化上下文
      • 销毁上下文
    • Servlet 3.0+ 中初始化
  • 参考资料

---

Spring MVC 的文档一开始就给出了这样的两段示例:

public class MyWebApplicationInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletCxt) {

        // Load Spring web application configuration
        AnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();
        ac.register(AppConfig.class);
        ac.refresh();

        // Create and register the DispatcherServlet
        DispatcherServlet servlet = new DispatcherServlet(ac);
        ServletRegistration.Dynamic registration = servletCxt.addServlet("app", servlet);
        registration.setLoadOnStartup(1);
        registration.addMapping("/app/*");
    }
}

web.xml :

<web-app>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/app-context.xml</param-value>
    </context-param>

    <servlet>
        <servlet-name>app</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value></param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>app</servlet-name>
        <url-pattern>/app/*</url-pattern>
    </servlet-mapping>

</web-app>

我们按照 web.xml 中的实例来看一下 Spring MVC 初始化过程.

上下文层次结构

Spring MVC 的上下文有如下这样的层级:

图中的 Servlet WebApplicationContext 是与 DispatcherServlet 绑定的上下文, 其中还有 controllers、ViewResolver、HandlerMapping 等组件.

Root WebApplicationContext 不是必须的上下文, 在需要时,可以用来在多个 DispatcherServlet 间共享一些 bean.

Root WebApplicationContext 初始化和销毁

ContextLoaderListener

web.xml 中配置的 ContextLoaderListener 用于启动和终止 Spring 的 root WebApplicationContext.

ContextLoaderListener 本身的代码十分简单:

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

	public ContextLoaderListener() {
	}

	public ContextLoaderListener(WebApplicationContext context) {
		super(context);
	}

	/**
	 * Initialize the root web application context.
	 * Servlet 上下文初始化,调用父类的方法初始化 WebApplicationContext
	 */
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}

	/**
	 * Close the root web application context.
	 * Servlet 上下文被销毁,调用父类的方法销毁 WebApplicationContext
	 */
	@Override
	public void contextDestroyed(ServletContextEvent event) {
		closeWebApplicationContext(event.getServletContext());
		// 销毁 ServletContext 上实现了 DisposableBean 的属性并移除他们
		ContextCleanupListener.cleanupAttributes(event.getServletContext());
	}
}

ContextLoader

ContextLoaderListener 直接调用了父类 ContextLoader 的方法来初始化和销毁上下文.

ContextLoaderListener 在创建上下文时,会尝试读取 contextClass context-param 来指定上下文的类型,被指定的类需要实现 ConfigurableWebApplicationContext 接口. 如果没有获取到,默认会使用 WebApplicationContext.

初始化上下文时,会尝试读取 contextConfigLocation context-param, 作为 xml 文件的路径.

初始化上下文

initWebApplicationContext() 方法如下:

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
	// 检查有没有绑定上下文
	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");
	}
	// 初始化开始时间
	long startTime = System.currentTimeMillis();

	try {
		// Store context in local instance variable, to guarantee that
		// it is available on ServletContext shutdown.
		// 保存上下文到本地实例变量中，保证上细纹能在 ServletContext 关闭时访问到
		if (this.context == null) {
			// 创建上下文
			this.context = createWebApplicationContext(servletContext);
		}
		if (this.context instanceof ConfigurableWebApplicationContext) {
			// 如果上下文实现了 ConfigurableWebApplicationContext
			ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
			if (!cwac.isActive()) {
				// 上下文还没有刷新，设置 父上下文（如果能找到），并且刷新
				// 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);
				}
				// 设置和刷新上下文
				configureAndRefreshWebApplicationContext(cwac, servletContext);
			}
		}
		// 将上下文绑定到 servletContext 的属性上
		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

		// 获取当前线程的上下文类加载器
		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
		if (ccl == ContextLoader.class.getClassLoader()) {
			currentContext = this.context;
		}
		else if (ccl != null) {
			// 如果有线程上下文类加载器，而且不是 ContextLoader 本身的类加载器，放入到 currentContextPerThread 中。这是一个 static 的域
			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) {
	    // 发生异常, 把异常绑定到上下文对应的属性上,之后不会再进行初始化
		logger.error("Context initialization failed", ex);
		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
		throw ex;
	}
}

initWebApplicationContext() 方法调用了 createWebApplicationContext() 方法来创建上下文;调用了 configureAndRefreshWebApplicationContext() 来对实现了 ConfigurableWebApplicationContext 接口的上下文做初始化.

---

createWebApplicationContext() 会调用 determineContextClass() 来获取上下文类型的 Class 对象.

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
	// 获取上下文类型
	Class<?> contextClass = determineContextClass(sc);
	// 检查是否实现了 ConfigurableWebApplicationContext
	if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
		throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
				"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
	}
	// 实例化
	return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}

protected Class<?> determineContextClass(ServletContext servletContext) {
	// 获取 serveltContext 的 'contextClass' 初始化参数。
	String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
	if (contextClassName != null) {
		// 指定过上下文类型，加载类
		try {
			return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
		}
		catch (ClassNotFoundException ex) {
			throw new ApplicationContextException(
					"Failed to load custom context class [" + contextClassName + "]", ex);
		}
	}
	else {
		// 去默认策略里获取默认的上下文类型名称
		contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
		try {
			// 加载类
			return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
		}
		catch (ClassNotFoundException ex) {
			throw new ApplicationContextException(
					"Failed to load default context class [" + contextClassName + "]", ex);
		}
	}
}

---

configureAndRefreshWebApplicationContext() 方法

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
	// 用可以获取到的信息，获取一个更有意义的上下文
	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
		// 获取 ServletContext 的 'contextId' 初始化参数。
		String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
		if (idParam != null) {
			wac.setId(idParam);
		}
		else {
			// Generate default id...
			// 生成默认 id
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
					ObjectUtils.getDisplayString(sc.getContextPath()));
		}
	}
	// 设置 servletContext 属性
	wac.setServletContext(sc);
	// 设置配置文件路径
	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
	// 初始化属性源， 确保 servlet 属性源到位并能够在任何 refresh 之前的后期处理和初始化中使用
	ConfigurableEnvironment env = wac.getEnvironment();
	if (env instanceof ConfigurableWebEnvironment) {
		((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
	}

	// 在设置了配置文件之后上下文刷新之前，自定义上下文
	customizeContext(sc, wac);
	wac.refresh();
}

protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
	// 根据 ServletContext 的 'contextInitializerClasses' 和 'globalInitializerClasses' 初始化参数 加载 ApplicationContextInitializer 的 class
	List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
			determineContextInitializerClasses(sc);

	for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
		// 获取范型参数类型
		Class<?> initializerContextClass =
				GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
		// 检查 Initializer 是否适用于当前上下文对象
		if (initializerContextClass != null && !initializerContextClass.isInstance(wac)) {
			throw new ApplicationContextException(String.format(
					"Could not apply context initializer [%s] since its generic parameter [%s] " +
					"is not assignable from the type of application context used by this " +
					"context loader: [%s]", initializerClass.getName(), initializerContextClass.getName(),
					wac.getClass().getName()));
		}
		// 创建 Initializer 实例，并添加到 contextInitializers
		this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
	}
	// 根据 org.springframework.core.Ordered 和 org.springframework.core.annotation.Order 排序，如果没有实现或注解，会被排到最后
	AnnotationAwareOrderComparator.sort(this.contextInitializers);
	// 执行每个 initializer 的 initialize() 方法
	for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
		initializer.initialize(wac);
	}
}

销毁上下文

closeWebApplicationContext() 方法如下:

public void closeWebApplicationContext(ServletContext servletContext) {
	servletContext.log("Closing Spring root WebApplicationContext");
	try {
		// 如果 context 是 ConfigurableWebApplicationContext 调用 close() 方法
		if (this.context instanceof ConfigurableWebApplicationContext) {
			((ConfigurableWebApplicationContext) this.context).close();
		}
	}
	finally {
		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
		if (ccl == ContextLoader.class.getClassLoader()) {
			currentContext = null;
		}
		else if (ccl != null) {
			currentContextPerThread.remove(ccl);
		}
		// 移除 servletContext 中的 context 属性
		servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
	}
}

Servlet 3.0+ 中初始化

Servlet 3.0+ 中可以通过 ServletContext 的 addlistener() 方法来添加监听器.因此可以先把 Spring 容器先创建好,再传给 ContextLoaderListener 的构造器.这里就不自己写例子了,选了单元测试中的 ContextLoaderTests.testContextLoaderListenerWithDefaultContext() 方法:

public void testContextLoaderListenerWithDefaultContext() {
	MockServletContext sc = new MockServletContext("");
	sc.addInitParameter(ContextLoader.CONFIG_LOCATION_PARAM,
			"/org/springframework/web/context/WEB-INF/applicationContext.xml " +
			"/org/springframework/web/context/WEB-INF/context-addition.xml");
	ServletContextListener listener = new ContextLoaderListener();
	ServletContextEvent event = new ServletContextEvent(sc);
	listener.contextInitialized(event);
	String contextAttr = WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE;
	WebApplicationContext context = (WebApplicationContext) sc.getAttribute(contextAttr);
	assertTrue("Correct WebApplicationContext exposed in ServletContext", context instanceof XmlWebApplicationContext);
	assertTrue(WebApplicationContextUtils.getRequiredWebApplicationContext(sc) instanceof XmlWebApplicationContext);
	LifecycleBean lb = (LifecycleBean) context.getBean("lifecycle");
	assertTrue("Has father", context.containsBean("father"));
	assertTrue("Has rod", context.containsBean("rod"));
	assertTrue("Has kerry", context.containsBean("kerry"));
	assertTrue("Not destroyed", !lb.isDestroyed());
	assertFalse(context.containsBean("beans1.bean1"));
	assertFalse(context.containsBean("beans1.bean2"));
	listener.contextDestroyed(event);
	assertTrue("Destroyed", lb.isDestroyed());
	assertNull(sc.getAttribute(contextAttr));
	assertNull(WebApplicationContextUtils.getWebApplicationContext(sc));
}

参考资料

芋道源码