Spring MVC 九:Context层级(基于配置)

/ 146 / 2023-09-24 18:41:06

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Context层级的问题,前面文章应该已经说清楚了。

只不过,前面文章是以注解方式举例说明的,通过配置方式怎么体现Context层级呢?有必要也说一下,毕竟现在很多项目都是基于xml配置实现的。

web.xml

基于配置的Spring MVC的入口就是web.xml文件,毕竟web.xml是基于web的应用的祖先入口......

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         id="WebApp_ID" version="3.1">
  <!--1、启动Spring的容器 -->
  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:applicationContext.xml</param-value>
  </context-param>

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

一定要搞清楚的是,context-param配置是用来指定的Spring容器的配置文件所在路径的。必须是和ContextLoaderListener一起配合起作用的。ContextLoaderListener读取context-param的配置来完成Spring IoC容器的初始化的。

Spring IoC容器和Spring MVC的Servlet Web ApplicationContext容器不是必须要分开的,也可以配置为同一个容器。

比如以上配置不指定(不配置Spring IoC容器),也就是去掉contextConfigLocation以及ContextLoaderListener,让Spring MVC的容器担负起Spring IoC容器的职责,也是可以的。

web.xml下面继续配置spring MVC的xml文件所在路径,DispathcerServlet初始化的时候会读取。


  <!--2、springmvc的前端控制器,拦截所有请求 -->
  <servlet>
    <servlet-name>dispatcherServlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>classpath:springmvc.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>

Spring-mvc.xml

名字是可以在web.xml文件中指定的,不指定的话默认就是dispatcherServlet名-dispatcher.xml(需要读一下SpringMVC源码确认)。

如果你想要Spring IoC容器和Spring MvC的web applicationContext容器分开的话,就在Spring-mvc.xml文件中指定包扫描路径仅扫描controller,否则,全扫描即可。反之,Spring Ioc容器存在的话,Ioc容器的配置文件的扫描路径也要排除掉Controller的扫描:

    <context:component-scan base-package="org.example.service">
        <context:exclude-filter type="annotation"
                                expression="org.springframework.stereotype.Controller" />
    </context:component-scan>

Servlet和根容器的关系

下图一目了然的说明了两者之间的关系:
image.png

Servlet容器存放Controller、VIewResolver、HanderMapping等DispatcherServlet的相关对象,根容器可以存放其他Service、Repositories等对象。

一个DispatcherServlet可以对应的有一个Servlet WebApplicationContext容器,一个Web应用可以有多个DispatcherServlet(这种应用其实比较少见),所以一个应用可以有多个Servlet WebApplicationContext容器。但是一般情况下,即使有多个Servlet容器,一个应用也希望只有一个根容器,以便在不同的Servlet容器之间共享根容器的对象。

根容器初始化过程

xml配置文件的情况下,根容器要依靠ContextLoaderListener来初始化。ContextLoaderListener是Spring MVC实现的ServletContextListener接口的实现类,ServletContextListener是Java Servlet的接口。Servlet标准约定,在Servlet容器初始化的过程中,会回调ServletContextListener接口的contextInitialized方法。

所以如果我们在web.xml文件中配置了ContextLoaderListener,那么,Tomcat在Servlet容器初始化的过程中就会回调ContextLoaderListener的contextInitialized方法:

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

该方法会调用initWebApplicationContext方法,这个方法我们在前面文章中其实已经分析过了,我们再来看一下:

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);
                }
            }
      ...省略n行代码

最终会调用到configureAndRefreshWebApplicationContext方法:

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);
        //就是在这儿读取web.xml的contextConfigLocation的
        String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
        if (configLocationParam != null) {
            wac.setConfigLocation(configLocationParam);
        }
      ...

configureAndRefreshWebApplicationContext方法会读取web.xml中contextConfigLocation配置,Spring IoC容器的初始化配置文件applicationContext.xml文件名、以及所在位置就是在这儿被读入的。

读入配置文件信息之后,调用refresh方法:

        // 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);
        wac.refresh();
    }

refresh方法是Spring framework的方法,具体就不在这儿深入研究了,我们只是简单跟踪一下配置文件的加载过程:

    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
            ...

在refresh方法的obtainFreshBeanFactory()方法中:

    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        return getBeanFactory();
    }

继续跟踪refreshBeanFactory();方法:

@Override
    protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);
            this.beanFactory = beanFactory;
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

继续跟踪,loadBeanDefinitions(beanFactory);方法,会调用到XmlWebApplicationContext类中:

@Override
    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        // Configure the bean definition reader with this context's
        // resource loading environment.
        beanDefinitionReader.setEnvironment(getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        // Allow a subclass to provide custom initialization of the reader,
        // then proceed with actually loading the bean definitions.
        initBeanDefinitionReader(beanDefinitionReader);
        loadBeanDefinitions(beanDefinitionReader);
    }

最后一个方法调用,loadBeanDefinitions方法:

    protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            for (String configLocation : configLocations) {
                reader.loadBeanDefinitions(configLocation);
            }
        }
    }

终于发现了根据configLocation通过loadBeanDefinitions方法读取配置文件、调用beandefinition的代码逻辑。

可以发现代码是可以支持多个配置文件的!

从代码的角度,我们也完成了xml配置方式下,SpringMVC与spring framework集成,完成spring Ioc容器的初始化过程!

与前面两篇文:Spring MVC 四:Context层级/Spring MVC 五 - Spring MVC的配置和DispatcherServlet初始化过程:Context层级结合,我们就完成了基于注解的、以及基于xml配置文件两种方式下的Spring MVC框架下,Spring IoC容器的初始化代码的分析!

上一篇 Spring MVC 八 - 内置过滤器


Spring MVC 九:Context层级(基于配置)
作者
许可协议
CC BY 4.0
发布于
2023-09-24
修改于
2024-07-14
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