知其所以然之web.xml

今天有人问我web.xml的配置，早些年看过的配置就用了一次，后来又都用了Java Config，把web.xml都忘了个干净，略是尴尬，因此再开个专题《知其所以然》来细究一些java杂七杂八的细节问题。
今天先来谈谈web.xml的前身今世。
首先明确一个概念，web.xml不是必须的用xml形式存在的，一样可以使用Java Config完成.
当时被人唬住了。web.xml 不是在项目中必须出现的，可以使用JavaConfig 替代。！！在朝花夕拾中之Spring WebMvc 中提到继承了 AbstractAnnotationConfigDispatcherServletInitializer 的WebMVCInitializer的类，就是web.xml的java config 形式。
web.xml 主要作用是通知 Servlet Containers (Tomcat) 从哪些类加载 & Url映射 & filter是哪些 & 等等。在servlet 3.0之后，@WebServlet, @WebListener 这些annotations 可以做这类的标记工作了，Spring 也早就在框架里集成了这些功能

下面放出博主实现的web.xml的Java Config 版本。

WebMVCInitializer

public class WebMVCInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
    @Override
    protected Class[] getRootConfigClasses() {
        return new Class[]{
            Application.class, MyBatisConfig.class
        };
    }

    @Override
    protected Class[] getServletConfigClasses() {
        return new Class[]{};
    }

    @Override
    protected String[] getServletMappings() {
        return new String[]{
                "/"
        };
    }
    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        super.onStartup(servletContext);
        servletContext.addListener(new SessionListener());
    }
}

父类AbstractAnnotationConfigDispatcherServletInitializer

public abstract class AbstractAnnotationConfigDispatcherServletInitializer
        extends AbstractDispatcherServletInitializer {

    /**
     * {@inheritDoc}
     * 
This implementation creates an {@link AnnotationConfigWebApplicationContext},
     * providing it the annotated classes returned by {@link #getRootConfigClasses()}.
     * Returns {@code null} if {@link #getRootConfigClasses()} returns {@code null}.
     */
    @Override
    protected WebApplicationContext createRootApplicationContext() {
        Class[] configClasses = getRootConfigClasses();
        if (!ObjectUtils.isEmpty(configClasses)) {
            AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext();
            rootAppContext.register(configClasses);
            return rootAppContext;
        }
        else {
            return null;
        }
    }

    /**
     * {@inheritDoc}
     * 
This implementation creates an {@link AnnotationConfigWebApplicationContext},
     * providing it the annotated classes returned by {@link #getServletConfigClasses()}.
     */
    @Override
    protected WebApplicationContext createServletApplicationContext() {
        AnnotationConfigWebApplicationContext servletAppContext = new AnnotationConfigWebApplicationContext();
        Class[] configClasses = getServletConfigClasses();
        if (!ObjectUtils.isEmpty(configClasses)) {
            servletAppContext.register(configClasses);
        }
        return servletAppContext;
    }

    /**
     * Specify {@link org.springframework.context.annotation.Configuration @Configuration}
     * and/or {@link org.springframework.stereotype.Component @Component} classes to be
     * provided to the {@linkplain #createRootApplicationContext() root application context}.
     * @return the configuration classes for the root application context, or {@code null}
     * if creation and registration of a root context is not desired
     */
    protected abstract Class[] getRootConfigClasses();

    /**
     * Specify {@link org.springframework.context.annotation.Configuration @Configuration}
     * and/or {@link org.springframework.stereotype.Component @Component} classes to be
     * provided to the {@linkplain #createServletApplicationContext() dispatcher servlet
     * application context}.
     * @return the configuration classes for the dispatcher servlet application context or
     * {@code null} if all configuration is specified through root config classes.
     */
    protected abstract Class[] getServletConfigClasses();

}

博主一直都觉得注释写的真好，加上函数名，基本上作什么用，改怎么用都解释的很清楚了。

最后插一句，xml配置不一定一直好于Java Config。