Spring mvc上下文层次

简单记录下对spring mvc上下文层次的理解。

至少两个容器？

刚接触spring mvc时，我看着教程，写下了下面这样的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/root-context.xml</param-value>
    </context-param>

    <servlet>
        <servlet-name>dispatcherServlet</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>/WEB-INF/servlet-context.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

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

</web-app>

然后有了这么一个印象：Spring mvc会创建两个容器，他们是父子关系，父容器里放service和dao，子容器里放controller。

今天我试图找到创建两个容器的原因，然而我并没有找到它，反而发现两个容器不是必要的（其实官网文档里就有写）。

DispatcherServlet初始化上下文的过程

先来看一下DispatcherServlet如何初始化它的spring上下文，实际上是它父类FrameworkServlet的initWebApplicationContext方法：

/**
 * Initialize and publish the WebApplicationContext for this servlet.
 * <p>Delegates to {@link #createWebApplicationContext} for actual creation
 * of the context. Can be overridden in subclasses.
 * @return the WebApplicationContext instance
 * @see #FrameworkServlet(WebApplicationContext)
 * @see #setContextClass
 * @see #setContextConfigLocation
 */
protected WebApplicationContext initWebApplicationContext() {
        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);
                    }
                    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.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

        return wac;
    }

这个方法在调用createWebApplicationContext()来创建一个 WebApplicationContext之前，会先检查this.webApplicationContext是否被注入了一个ConfigurableWebApplicationContext，然后尝试去ServletContext中找一个容器，最后才是去创建一个rootContext的子容器。

一个容器

从初始化过程中可以看出，DispatcherServlet并不一定会创建自己的容器。

如果使用前面的web.xml配置，确实是会创建两个容器（由ContextLoaderListener创建的根容器和DispatchServlet的子容器），两个容器都会绑定到ServletContext。

如果使用纯代码的方式来配置，像官网给出的例子这样：

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/*");
    }
}

使用Spring Boot时，默认情况下也只会创建一个容器。当然使用web.xml配置也可以实现一个应用只创建一个容器，这里就先不放例子了。

结论

懒得写了。。就写这么多吧。
实际上官网的文档有这么一段：

For many applications having a single WebApplicationContext is simple and sufficient. It is also possible to have a context hierarchy where one root WebApplicationContext is shared across multiple DispatcherServlet (or other Servlet) instances, each with its own child WebApplicationContext configuration.

一个应用可以只有一个容器，多个容器是可选的。

多个DispatcherServlet的例子

// TODO 以后再加