4.5.4. 注册组件中的 DispatcherServlet

本节将介绍如何将一个应用程序组件中的 servlet 和 filter 配置传递到使用该组件的应用程序。为了避免web.xml文件中的代码重复，需要在组件中使用特殊的 ServletRegistrationManager bean 注册 servlet 和 filter。

关于 Servlet 注册的最常见情况在示例HTTP servlet 注册中介绍。我们考虑一个更复杂的例子：一个应用程序组件带有一个用于处理 Web 请求的自定义 DispatcherServlet 的实现。

这个 servlet 从 demo-dispatcher-spring.xml 文件加载配置，如果要该 servlet 正常工作，应该在源码根目录（例如 web/src）先创建一个同名的空文件。

public class WebDispatcherServlet extends DispatcherServlet {
    private volatile boolean initialized = false;
    @Override
    public String getContextConfigLocation() {
        String configFile = "demo-dispatcher-spring.xml";
        File baseDir = new File(AppContext.getProperty("cuba.confDir"));
        String[] tokenArray = new StrTokenizer(configFile).getTokenArray();
        StringBuilder locations = new StringBuilder();
        for (String token : tokenArray) {
            String location;
            if (ResourceUtils.isUrl(token)) {
                location = token;
            } else {
                if (token.startsWith("/"))
                    token = token.substring(1);
                File file = new File(baseDir, token);
                if (file.exists()) {
                    location = file.toURI().toString();
                } else {
                    location = "classpath:" + token;
                }
            }
            locations.append(location).append(" ");
        }
        return locations.toString();
    }
    @Override
    protected WebApplicationContext initWebApplicationContext() {
        WebApplicationContext wac = findWebApplicationContext();
        if (wac == null) {
            ApplicationContext parent = AppContext.getApplicationContext();
            wac = createWebApplicationContext(parent);
        }
        onRefresh(wac);
        String attrName = getServletContextAttributeName();
        getServletContext().setAttribute(attrName, wac);
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                    "' as ServletContext attribute with name [" + attrName + "]");
        }
        return wac;
    }
    @Override
    public void init(ServletConfig config) throws ServletException {
        if (!initialized) {
            super.init(config);
            initialized = true;
        }
    }
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        _service(response);
    }
    @Override
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        _service(res);
    }
    private void _service(ServletResponse res) throws IOException {
        String testMessage = AppContext.getApplicationContext().getBean(Messages.class).getMainMessage("testMessage");
        res.getWriter()
                .write("WebDispatcherServlet test message: " + testMessage);
    }
}

要注册 DispatcherServlet，必须手动对此类进行加载、实例化、初始化，否则不同的类加载器可能会在 SingleWAR/SingleUberJAR 部署的情况下引发问题。而且，自定义 DispatcherServlet 应该需要进行双重初始化 - 第一次手动初始化，第二次由 servlet 容器初始化。

下面是一个初始化 WebDispatcherServlet 的组件示例：

@Component
public class WebInitializer {
    private static final String WEB_DISPATCHER_CLASS = "com.demo.comp.web.WebDispatcherServlet";
    private static final String WEB_DISPATCHER_NAME = "web_dispatcher_servlet";
    private final Logger log = LoggerFactory.getLogger(WebInitializer.class);
    @Inject
    private ServletRegistrationManager servletRegistrationManager;
    @EventListener
    public void initialize(ServletContextInitializedEvent e) {
        Servlet webDispatcherServlet = servletRegistrationManager.createServlet(e.getApplicationContext(), WEB_DISPATCHER_CLASS);
        ServletContext servletContext = e.getSource();
        try {
            webDispatcherServlet.init(new AbstractWebAppContextLoader.CubaServletConfig(WEB_DISPATCHER_NAME, servletContext));
        } catch (ServletException ex) {
            throw new RuntimeException("Failed to init WebDispatcherServlet");
        }
        servletContext.addServlet(WEB_DISPATCHER_NAME, webDispatcherServlet)
                .addMapping("/webd/*");
    }
}

注入的 ServletRegistrationManager bean 的 createServlet() 方法从 ServletContextInitializedEvent 获取应用程序上下文，并获取 WebDispatcherServlet 类的完全限定名。要初始化 servlet，需要传递从 ServletContextInitializedEvent 获得的 ServletContext 实例和 servlet 名称。addMapping() 方法用于定义通过 URL：/webd/ 访问 servlet 的 HTTP 映射。