基于代码的Servlet容器配置

大部分情况下，使用Spring是采用基于配置的方式，通常情况下是通过web.xml文件对容器的行为进行配置. 从servlet3.0开始，spring提供了基于代码的配置，以下是个例子:


public class MyWebApplicationInitializer implements WebApplicationInitializer {
    @Override
    public void onStartup(ServletContext container) {
        XmlWebApplicationContext appContext = new XmlWebApplicationContext();
        appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
        ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet(appContext));
        registration.setLoadOnStartup(1);
        registration.addMapping("/");
    }
}

WebApplicationInitializer的实现会自动被检测到并且被用于初始化容器的配置. 除此之外, spring还提供了一种AbstractDispatcherServletInitializer抽象类来提供更简洁的方式对容器进行初始化, 这也是spring推荐的使用方式.

使用AbstractDispatcherServletInitializer进行初始化还可以分为使用代码和使用XML配置文件的方式：

注意：这里说的是初始化DispatcherServlet的配置，而上面说的是利用代码方式初始化Servlet容器，两者不要混淆.

//使用代码方式配置
public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return null;
    }
    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class[] { MyWebConfig.class };
    }
    @Override
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }
}
//使用XML方式配置
public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {
    @Override
    protected WebApplicationContext createRootApplicationContext() {
        return null;
    }
    @Override
    protected WebApplicationContext createServletApplicationContext() {
        XmlWebApplicationContext cxt = new XmlWebApplicationContext();
        cxt.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
        return cxt;
    }
    @Override
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }
}

另外，AbstractDispatcherServletInitializer抽象类还提供了关于filter，isAsyncSupport等配置的支持，具体可以参阅其文档，这里就不做过多阐述.

查阅AbstractDispatcherServletInitializer的源码可以发现，它是典型的模板模式的实现，将对WebApplicationInitializer的实现进行抽象，但是整个方法的实现过程没有太多变化.

public void onStartup(ServletContext servletContext) throws ServletException {
    super.onStartup(servletContext);
    registerDispatcherServlet(servletContext);
}
protected void registerDispatcherServlet(ServletContext servletContext) {
    String servletName = getServletName();
    Assert.hasLength(servletName, "getServletName() may not return empty or null");
    WebApplicationContext servletAppContext = createServletApplicationContext();
    Assert.notNull(servletAppContext,
            "createServletApplicationContext() did not return an application " +
            "context for servlet [" + servletName + "]");
    DispatcherServlet dispatcherServlet = new DispatcherServlet(servletAppContext);
    ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
    Assert.notNull(registration,
            "Failed to register servlet with name '" + servletName + "'." +
            "Check if there is another servlet registered under the same name.");
    registration.setLoadOnStartup(1);
    registration.addMapping(getServletMappings());
    registration.setAsyncSupported(isAsyncSupported());
    Filter[] filters = getServletFilters();
    if (!ObjectUtils.isEmpty(filters)) {
        for (Filter filter : filters) {
            registerServletFilter(servletContext, filter);
        }
    }
    customizeRegistration(registration);
}

参考文献

官方文档：基于代码配置Servlet容器