【springmvc】1. DispatcherServlet的注入

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发布于 2020-03-24 17:11:21

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发布于 2020-03-24 17:11:21

文章被收录于专栏：后端技术

参考Spring Boot与Spring MVC集成启动过程源码分析

1. refresh

1.1 Springboot启动流程中调用refresh

从SpringApplication.run一路运行到refresh方法：

protected void refresh(ApplicationContext applicationContext) {
    Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
    ((AbstractApplicationContext) applicationContext).refresh();
}

在启动web服务时，此处的applicationContext类型是AnnotationConfigServletWebServerApplicationContext，原因省略。

1.2 refresh流程中调用createWebServer

此处AbstractApplicationContext执行refresh方法，进而调用onRefresh：

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            ...
            try {
                ...
                // Initialize other special beans in specific context subclasses.
                onRefresh();
                ...
            }
            ...
        }
    }

AbstractApplicationContext.refresh->ServletWebServerApplicationContext.onRefresh：

// ServletWebServerApplicationContext
@Override
    protected void onRefresh() {
        super.onRefresh();
        try {
            createWebServer();
        }
        catch (Throwable ex) {
            ...
        }
    }

private void createWebServer() {
        WebServer webServer = this.webServer;
        ServletContext servletContext = getServletContext();
        if (webServer == null && servletContext == null) {
            ServletWebServerFactory factory = getWebServerFactory();
            this.webServer = factory.getWebServer(getSelfInitializer());
        }
        else if (servletContext != null) {
            ...
        }
        initPropertySources();
    }

调试到此处会进入第一个if语句：

getSelfInitializer()实际上创建了下图的ServletWebServerApplicationContext::lambda，实际上调用ServletWebServerApplicationContext::selfInitialize，它的作用很重要。
factory.getWebServer做了如下事情：
1. 将TomcatEmbeddedContext注入Tomcat的host
2. 将包括ServletWebServerApplicationContext::lambda的其它初始器注入TomcatStarter
3. 将TomcatStarter注入Context(TomcatEmbeddedContext)

1.3 附录：关于Servlet

为防止读不懂上一小节，特此附上关于对接Servlet3.0的规范。(主要介绍ServletContainerInitializer，关于filter只是拓展阅读) servlet和filter的区别

Filter可认为是Servlet的一种“变种”，它主要用于对用户请求进行预处理，也可以对HttpServletResponse进行后处理，是个典型的处理链。
它与Servlet的区别在于：它不能直接向用户生成响应。完整的流程是：Filter对用户请求进行预处理，接着将请求交给Servlet进行处理并生成响应，最后Filter再对服务器响应进行后处理。

servlet filter生命周期

servlet filter都是在容器启动时初始化，调用init；都是在容器关闭时随着销毁，调用destroy

Servlet3.0后，定义servlet和filter有三种方式：

@WebServlet ，@WebFilter
调用ServletContext#addServlet等方法

在javax.servlet.ServletContextListener#contextInitialized(ServletContextEvent sce)回调函数中调用
在javax.servlet.ServletContainerInitializer#onStartup(Set<Class<?>> c, ServletContext ctx)回调函数中调用

2. 回调selfInitialize注入DispatcherServlet

2.1 获取ServletContextInitializer的Bean

根据上一章，selfInitialize会在Tomcat容器启动时间接被回调：

private void selfInitialize(ServletContext servletContext) throws ServletException {
        prepareWebApplicationContext(servletContext);
        registerApplicationScope(servletContext);
        WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
        for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
            beans.onStartup(servletContext);
        }
    }

    /**
     * Returns {@link ServletContextInitializer}s that should be used with the embedded
     * web server. By default this method will first attempt to find
     * {@link ServletContextInitializer}, {@link Servlet}, {@link Filter} and certain
     * {@link EventListener} beans.
     * @return the servlet initializer beans
     */
    protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
        return new ServletContextInitializerBeans(getBeanFactory());
    }

代码中调用getServletContextInitializerBeans()，获取类型为ServletContextInitializer的Bean，并回调onStartup方法。并此处我们不展开分析方法，有需要的可以看源码验证一下。
被回调的Bean就包括DispatcherServletRegistrationBean。其注入的地方，和它的效果，我们下文讲解

2.2 DispatcherServletRegistrationBean

2.2.1 注入

spring-boot-autoconfigure/META-INF/spring.factories中有一段配置：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\

看下其实现

..
@Configuration(proxyBeanMethods = false)
..
public class DispatcherServletAutoConfiguration {

    /*
     * The bean name for a DispatcherServlet that will be mapped to the root URL "/"
     */
    public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";

    /*
     * The bean name for a ServletRegistrationBean for the DispatcherServlet "/"
     */
    public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";

    @Configuration(proxyBeanMethods = false)
    @Conditional(DefaultDispatcherServletCondition.class)
    @ConditionalOnClass(ServletRegistration.class)
    @EnableConfigurationProperties({ HttpProperties.class, WebMvcProperties.class })
    protected static class DispatcherServletConfiguration {

        @Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public DispatcherServlet dispatcherServlet(HttpProperties httpProperties, WebMvcProperties webMvcProperties) {
            DispatcherServlet dispatcherServlet = new DispatcherServlet();
            ..
            return dispatcherServlet;
        }
        ..

    }

    @Configuration(proxyBeanMethods = false)
    ..
    protected static class DispatcherServletRegistrationConfiguration {

        @Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
        @ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet,
                WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfig) {
            DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet,
                    webMvcProperties.getServlet().getPath());
            registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
            registration.setLoadOnStartup(webMvcProperties.getServlet().getLoadOnStartup());
            multipartConfig.ifAvailable(registration::setMultipartConfig);
            return registration;
        }

    }

利用Bean机制:

创建DispatcherServlet，并作为Bean注入
接受DispatcherServlet为参数，创建DispatcherServletRegistrationBean，Bean再作为Bean注入。

2.2.2 回调

DispatcherServletRegistrationBean继承ServletContextInitializer接口，因此也会被回调。其回调实现在RegistrationBean中：

@Override
public final void onStartup(ServletContext servletContext) throws ServletException {
  String description = getDescription();
  f (!isEnabled()) {
    logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
    return;
  }
    register(description, servletContext);
}

register方法是一个模板方法，调用子类DynamicRegistrationBean的实现：

@Override
protected final void register(String description, ServletContext servletContext) {
    D registration = addRegistration(description, servletContext);
    if (registration == null) {
        logger.info(StringUtils.capitalize(description) + " was not registered " + "(possibly already registered?)");
        return;
    }
    configure(registration);
}

addRegistration方法又是一个模板方法，调用子类ServletRegistrationBean的实现，

@Override
  protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
    String name = getServletName();
    return servletContext.addServlet(name, this.servlet);
}

当前处于tomcat启动环境中，调用servletContext.addServlet(javax.servlet.ServletContext)注入了this.servlet，也就是在上文注入的DispatcherServlet。至此，DispatcherServlet已完成注入

总结

spring利用Tomcat的Servlet中，ServletContainerInitializer会被回调的机制，实现了TomcatStarter，来回调spring自己实现的ServletContextInitializer回调函数。
ServletWebServerApplicationContext::selfInitialize在回调时会获取类型为ServletContextInitializer的Bean并调用其回调函数onStartup，其中包括DispatcherServletRegistrationBean
DispatcherServletRegistrationBean利用springboot的自动配置机制事先被注入为Bean，在回调执行时将DispatcherServlet注入Context中，从而注入到了Tomcat中。

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