springboot mvc自動配置（二）註冊DispatcherServlet到ServletContext

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http://www.javashuo.com/article/p-mbkpimnl-cg.html

 

正文

上一篇文章中，咱們看到了DispatcherServlet和DispatcherServletRegistrationBean這兩個Bean的自動配置。DispatcherServlet咱們很熟悉，DispatcherServletRegistrationBean負責將DispatcherServlet註冊到ServletContext當中。

 

DispatcherServletRegistrationBean的類圖

既然該類的職責是負責註冊DispatcherServlet，那麼咱們得知道何時觸發註冊操做。爲此，咱們先看看DispatcherServletRegistrationBean這個類的類圖

 

註冊DispatcherServlet流程

ServletContextInitializer

咱們看到，最上面是一個ServletContextInitializer接口。咱們能夠知道，實現該接口意味着是用來初始化ServletContext的。咱們看看該接口

public interface ServletContextInitializer {
    void onStartup(ServletContext servletContext) throws ServletException;
}

 

RegistrationBean

看看RegistrationBean是怎麼實現onStartup方法的

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

調用了內部register方法，跟進它

protected abstract void register(String description, ServletContext servletContext);

這是一個抽象方法

 

DynamicRegistrationBean

再看DynamicRegistrationBean是怎麼實現register方法的

@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);
}

跟進addRegistration方法

protected abstract D addRegistration(String description, ServletContext servletContext);

同樣是一個抽象方法

 

ServletRegistrationBean

再看ServletRegistrationBean是怎麼實現addRegistration方法的

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

咱們看到，這裏直接將DispatcherServlet給add到了servletContext當中。

 

總結

總的來講，其實就是觸發了初始化ServletContext時候的回調接口onStartup方法，然後直接將DispatcherServlet做爲一個Servlet給add到ServletContext當中。