SpringMvc在SpringBoot环境和Web环境中上下文的关系

ゞ浴缸里的玫瑰 2022-06-06 10:17 241阅读 0赞

之前有人在我的项目中提出issue，咨询在SpringBoot中的bean是不是由谁来管理的问题（SpringMvc还是Spring）来管理（[https://github.com/cmlbeliever/SpringBootLearning/issues/2][https_github.com_cmlbeliever_SpringBootLearning_issues_2]）

其实一开始我也是挺懵逼的，之前没有怎么了解过这些细节，既然提出问题了，当然要找出个所以然。

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在Web环境中，是分为SpringMvc管理的子容器，和Spring管理的父容器。父子容器的关系就像类的继承一样，子容器可以获取父容器的bean，反之则是不可以的。

如何验证是两个容器呢？  
只要在Controller与Service中实现ApplicationContextAware接口，就可已得知对应的管理容器：  
Controller中：  
![这里写图片描述][SouthEast]  
Service中：  
![这里写图片描述][SouthEast 1]

那么在SpringBoot中则是同一个容器管理的（AnnotationConfigEmbeddedWebApplicationContext）：  
Controller：  
![这里写图片描述][SouthEast 2]  
Service：  
![这里写图片描述][SouthEast 3]

可以看出在boot环境中使用的是相同的容器管理的，因为AnnotationConfigEmbeddedWebApplicationContext是在包org.springframework.boot.context.embedded中的，那么就可以理解为boot容器管理bean。(当然也可以理解为Spring管理，因为AnnotationConfigEmbeddedWebApplicationContext也只是拓展WebApplicationContext)。

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由于SpringMvc的入口都是DispatcherServlet，那是什么原因造成在Web环境中有父子容器，在boot环境中只有一个容器呢？这个当然需要剖析源码咯：

直接debug就可以了，在FrameworkServlet.initWebApplicationContext方法中初始化web上下文环境。

protected WebApplicationContext initWebApplicationContext() {
            WebApplicationContext rootContext =
                    WebApplicationContextUtils.getWebApplicationContext(getServletContext());
            WebApplicationContext wac = null;
    //因为调用时Spring容器不是webApplicationContext类型的，所以在webApplicationContext对象时为空
            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();
            }
            //这里是重点，从这里新建SpringMvc容器
            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;
        }

当web容器文不存在时，会去创建一个新的web容器：

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
            Class<?> contextClass = getContextClass();
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Servlet with name '" + getServletName() +
                        "' will try to create custom WebApplicationContext context of class '" +
                        contextClass.getName() + "'" + ", using parent context [" + parent + "]");
            }
            if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
                throw new ApplicationContextException(
                        "Fatal initialization error in servlet with name '" + getServletName() +
                        "': custom WebApplicationContext class [" + contextClass.getName() +
                        "] is not of type ConfigurableWebApplicationContext");
            }
            ConfigurableWebApplicationContext wac =
                    (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    
            wac.setEnvironment(getEnvironment());
            //设置父容器，这里的parent即为spring上下文
            wac.setParent(parent);
            wac.setConfigLocation(getContextConfigLocation());
    
            configureAndRefreshWebApplicationContext(wac);
    
            return wac;
        }

当web容器初始化成功后，发出ContextRefreshedEvent事件，  
DispatcherServlet接收到事件后调用initStrategies方法进行web相关的mapping和adapter设置

protected void initStrategies(ApplicationContext context) {
            initMultipartResolver(context);
            initLocaleResolver(context);
            initThemeResolver(context);
            initHandlerMappings(context);
            initHandlerAdapters(context);
            initHandlerExceptionResolvers(context);
            initRequestToViewNameTranslator(context);
            initViewResolvers(context);
            initFlashMapManager(context);
        }

在SpringBoot环境中，其他流程都是一致的，但是用的容器是AnnotationConfigEmbeddedWebApplicationContext而且还是WebApplicationContext子类，在FrameworkServlet.setApplicationContext时，就对webApplicationContext 进行了赋值

public void setApplicationContext(ApplicationContext applicationContext) {
            if (this.webApplicationContext == null && applicationContext instanceof WebApplicationContext) {
                this.webApplicationContext = (WebApplicationContext) applicationContext;
                this.webApplicationContextInjected = true;
            }
        }

所以在initWebApplicationContext方法初始化的时候用的就是这个全局的上下文，没有新建一个SpringMvc专用的Web上下文  
![这里写图片描述][SouthEast 4]

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总结：  
在Web环境中是由Spring和SpringMvc两个容器组成的，在SpringBoot环境中只有一个容器AnnotationConfigEmbeddedWebApplicationContext。也就是可以说是由SpringBoot容器管理的。

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SpringBootLearning是对springboot学习与研究项目，是根据实际项目的形式对进行配置与处理，欢迎star与fork。  
\[oschina 地址\]  
[http://git.oschina.net/cmlbeliever/SpringBootLearning][http_git.oschina.net_cmlbeliever_SpringBootLearning]  
\[github 地址\]  
[https://github.com/cmlbeliever/SpringBootLearning][https_github.com_cmlbeliever_SpringBootLearning]

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[https_github.com_cmlbeliever_SpringBootLearning_issues_2]: https://github.com/cmlbeliever/SpringBootLearning/issues/2
[SouthEast]: /images/20220606/0f4638d8ad1646d5ab582be8dcd62cf6.png
[SouthEast 1]: /images/20220606/1ebcdcc529804d9dae9227ce3eabf7cf.png
[SouthEast 2]: /images/20220606/a18765bb39ef413c8b76de8d699911d8.png
[SouthEast 3]: /images/20220606/da2df9f23b514afba49fb07bae4b33ca.png
[SouthEast 4]: /images/20220606/c8b597e2cf8c44329070e15019092232.png
[http_git.oschina.net_cmlbeliever_SpringBootLearning]: http://git.oschina.net/cmlbeliever/SpringBootLearning
[https_github.com_cmlbeliever_SpringBootLearning]: https://github.com/cmlbeliever/SpringBootLearning