Spring MVC源码——Root WebApplicationContext

Posted 2021-11-25 fjh1994

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
    • Servlet 3.0+ 中初始化
  • 参考资料

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.

---

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