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Servlet – Upload、Download、Async、动态注册

Upload-上传

随着3.0版本的发布,文件上传终于成为Servlet规范的一项内置特性,不再依赖于像 Commons FileUpload 之类组件,因此在服务端进行文件上传编程变得不费吹灰之力.

要上传文件, 必须利用 multipart/form-data 设置HTML表单的 enctype 属性,且 method 必须为 POST :

<form action="simple_file_upload_servlet.do" method="POST" enctype="multipart/form-data">     <table align="center" border="1" width="50%">         <tr>             <td>Author:</td>             <td><input type="text" name="author"></td>         </tr>         <tr>             <td>Select file to Upload:</td>             <td><input type="file" name="file"></td>         </tr>         <tr>             <td><input type="submit" value="上传"></td>         </tr>     </table> </form>

服务端Servlet主要围绕着 @MultipartConfig 注解和 Part 接口:

处理上传文件的Servlet必须用 @MultipartConfig 注解标注:

@MultipartConfig属性	描述
`fileSizeThreshold`	The size threshold after which the file will be written to disk
`location`	The directory location where files will be stored
`maxFileSize`	The maximum size allowed for uploaded files.
`maxRequestSize`	The maximum size allowed for multipart/form-data requests

在一个由多部件组成的请求中, 每一个表单域(包括非文件域), 都会被封装成一个 Part , HttpServletRequest 中提供如下两个方法获取封装好的 Part :

HttpServletRequest	描述
`Part getPart(String name)`	Gets the Part with the given name.
`Collection<Part> getParts()`	Gets all the Part components of this request, provided that it is of type multipart/form-data.

Part 中提供了如下常用方法来获取/操作上传的文件/数据:

Part	描述
`InputStream getInputStream()`	Gets the content of this part as an InputStream
`void write(String fileName)`	A convenience method to write this uploaded item to disk.
`String getSubmittedFileName()`	Gets the file name specified by the client(需要有Tomcat 8.x 及以上版本支持)
`long getSize()`	Returns the size of this fille.
`void delete()`	Deletes the underlying storage for a file item, including deleting any associated temporary disk file.
`String getName()`	Gets the name of this part
`String getContentType()`	Gets the content type of this part.
`Collection<String> getHeaderNames()`	Gets the header names of this Part.
`String getHeader(String name)`	Returns the value of the specified mime header as a String.

文件流解析

通过抓包获取到客户端上传文件的数据格式:

------WebKitFormBoundaryXJ6TxfJ9PX5hJHGh Content-Disposition: form-data; name="author"  feiqing ------WebKitFormBoundaryXJ6TxfJ9PX5hJHGh Content-Disposition: form-data; name="file"; filename="memcached.txt" Content-Type: text/plain  ------WebKitFormBoundaryXJ6TxfJ9PX5hJHGh--

可以看到:: A. 如果HTML表单输入项为文本( <input type="text"/> ),将只包含一个请求头 Content-Disposition .

在Servlet中处理上传文件时, 需要:

<code>- 通过查看是否存在`Content-Type`标头, 检验一个Part是封装的普通表单域,还是文件域. - 若有`Content-Type`存在, 但文件名为空, 则表示没有选择要上传的文件. - 如果有文件存在, 则可以调用`write()`方法来写入磁盘, 调用同时传递一个绝对路径, 或是相对于`@MultipartConfig`注解的`location`属性的相对路径. </code>

SimpleFileUploadServlet

/**  * @author jifang.  * @since 2016/5/8 16:27.  */ @MultipartConfig @WebServlet(name = "SimpleFileUploadServlet", urlPatterns = "/simple_file_upload_servlet.do") public class SimpleFileUploadServlet extends HttpServlet {      protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         response.setContentType("text/html;charset=UTF-8");         PrintWriter writer = response.getWriter();         Part file = request.getPart("file");         if (!isFileValid(file)) {             writer.print("<h1>请确认上传文件是否正确!");         } else {             String fileName = file.getSubmittedFileName();             String saveDir = getServletContext().getRealPath("/WEB-INF/files/");             mkdirs(saveDir);             file.write(saveDir + fileName);              writer.print("<h3>Uploaded file name: " + fileName);             writer.print("<h3>Size: " + file.getSize());             writer.print("<h3>Author: " + request.getParameter("author"));         }     }      private void mkdirs(String saveDir) {         File dir = new File(saveDir);         if (!dir.exists()) {             dir.mkdirs();         }     }      private boolean isFileValid(Part file) {         // 上传的并非文件         if (file.getContentType() == null) {             return false;         }         // 没有选择任何文件         else if (Strings.isNullOrEmpty(file.getSubmittedFileName())) {             return false;         }         return true;     } }

善用 WEB-INF
存放在 /WEB-INF/ 目录下的资源无法在浏览器地址栏直接访问, 利用这一特点可将某些受保护资源存放在 WEB-INF 目录下, 禁止用户直接访问(如用户上传的可执行文件,如JSP等),以防被恶意执行, 造成服务器信息泄露等危险.

getServletContext().getRealPath("/WEB-INF/")

文件名乱码

当文件名包含中文时,可能会出现乱码,其解决方案与 POST 相同:

```
request.setCharacterEncoding("UTF-8");
```
避免文件同名如果上传同名文件,会造成文件覆盖.因此可以为每份文件生成一个唯一ID,然后连接原始文件名:

private String generateUUID() {     return UUID.randomUUID().toString().replace("-", "_"); }

目录打散

如果一个目录下存放的文件过多, 会导致文件检索速度下降,因此需要将文件打散存放到不同目录中, 在此我们采用Hash打散法(根据文件名生成Hash值, 取Hash值的前两个字符作为二级目录名), 将文件分布到一个二级目录中:

private String generateTwoLevelDir(String destFileName) {     String hash = Integer.toHexString(destFileName.hashCode());     return String.format("%s/%s", hash.charAt(0), hash.charAt(1)); }

采用Hash打散的好处是:在根目录下最多生成16个目录,而每个子目录下最多再生成16个子子目录,即一共256个目录,且分布较为均匀.

示例-简易存储图片服务器

需求: 提供上传图片功能, 为其生成外链, 并提供下载功能(见下)

客户端

<html> <head>     <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">     <title>IFS</title> </head> <body> <form action="ifs_upload.action" method="POST" enctype="multipart/form-data">     <table align="center" border="1" width="50%">         <tr>             <td>Select A Image to Upload:</td>             <td><input type="file" name="image"></td>         </tr>         <tr>             <td> </td>             <td><input type="submit" value="上传"></td>         </tr>     </table> </form> </body> </html>

服务端

@MultipartConfig @WebServlet(name = "ImageFileUploadServlet", urlPatterns = "/ifs_upload.action") public class ImageFileUploadServlet extends HttpServlet {      private Set<String> imageSuffix = new HashSet<>();      private static final String SAVE_ROOT_DIR = "/images";      {         imageSuffix.add(".jpg");         imageSuffix.add(".png");         imageSuffix.add(".jpeg");     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         request.setCharacterEncoding("UTF-8");         response.setContentType("text/html;charset=UTF-8");         PrintWriter writer = response.getWriter();         Part image = request.getPart("image");         String fileName = getFileName(image);         if (isFileValid(image, fileName) && isImageValid(fileName)) {             String destFileName = generateDestFileName(fileName);             String twoLevelDir = generateTwoLevelDir(destFileName);              // 保存文件             String saveDir = String.format("%s/%s/", getServletContext().getRealPath(SAVE_ROOT_DIR), twoLevelDir);             makeDirs(saveDir);             image.write(saveDir + destFileName);              // 生成外链             String ip = request.getLocalAddr();             int port = request.getLocalPort();             String path = request.getContextPath();             String urlPrefix = String.format("http://%s:%s%s", ip, port, path);             String urlSuffix = String.format("%s/%s/%s", SAVE_ROOT_DIR, twoLevelDir, destFileName);             String url = urlPrefix + urlSuffix;             String result = String.format("<a href=%s>%s</a><hr/><a href=ifs_download.action?location=%s>下载</a>",                     url,                     url,                     saveDir + destFileName);             writer.print(result);         } else {             writer.print("Error : Image Type Error");         }     }      /**      * 校验文件表单域有效      *      * @param file      * @param fileName      * @return      */     private boolean isFileValid(Part file, String fileName) {         // 上传的并非文件         if (file.getContentType() == null) {             return false;         }         // 没有选择任何文件         else if (Strings.isNullOrEmpty(fileName)) {             return false;         }          return true;     }      /**      * 校验文件后缀有效      *      * @param fileName      * @return      */     private boolean isImageValid(String fileName) {         for (String suffix : imageSuffix) {             if (fileName.endsWith(suffix)) {                 return true;             }         }         return false;     }      /**      * 加速图片访问速度, 生成两级存放目录      *      * @param destFileName      * @return      */     private String generateTwoLevelDir(String destFileName) {         String hash = Integer.toHexString(destFileName.hashCode());         return String.format("%s/%s", hash.charAt(0), hash.charAt(1));     }      private String generateUUID() {         return UUID.randomUUID().toString().replace("-", "_");     }      private String generateDestFileName(String fileName) {         String destFileName = generateUUID();         int index = fileName.lastIndexOf(".");         if (index != -1) {             destFileName += fileName.substring(index);         }         return destFileName;     }      private String getFileName(Part part) {         String[] elements = part.getHeader("content-disposition").split(";");         for (String element : elements) {             if (element.trim().startsWith("filename")) {                 return element.substring(element.indexOf("=") + 1).trim().replace("/"", "");             }         }         return null;     }      private void makeDirs(String saveDir) {         File dir = new File(saveDir);         if (!dir.exists()) {             dir.mkdirs();         }     } }

由于 getSubmittedFileName() 方法需要有Tomcat 8.X以上版本的支持, 因此为了通用期间, 我们自己解析 content-disposition 请求头, 获取filename.

Download-下载

文件下载是向客户端响应二进制数据(而非字符),浏览器不会直接显示这些内容,而是会弹出一个下载框, 提示下载信息.

为了将资源发送给浏览器, 需要在Servlet中完成以下工作:

使用 Content-Type 响应头来规定响应体的MIME类型, 如 image/pjpeg 、 application/octet-stream ;
添加 Content-Disposition 响应头,赋值为 attachment;filename=xxx.yyy , 设置文件名;
使用 response.getOutputStream() 给浏览器发送二进制数据;

文件名中文乱码

当文件名包含中文时( attachment;filename=文件名.后缀名 ),在下载框中会出现乱码, 需要对文件名编码后在发送, 但不同的浏览器接收的编码方式不同:

<code> * FireFox: Base64编码   * 其他大部分Browser: URL编码 </code>

因此最好将其封装成一个通用方法:

private String filenameEncoding(String filename, HttpServletRequest request) throws IOException {     // 根据浏览器信息判断     if (request.getHeader("User-Agent").contains("Firefox")) {         filename = String.format("=?utf-8?B?%s?=", BaseEncoding.base64().encode(filename.getBytes("UTF-8")));     } else {         filename = URLEncoder.encode(filename, "utf-8");     }     return filename; }

示例-IFS下载功能

/**  * @author jifang.  * @since 2016/5/9 17:50.  */ @WebServlet(name = "ImageFileDownloadServlet", urlPatterns = "/ifs_download.action") public class ImageFileDownloadServlet extends HttpServlet {      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         response.setContentType("application/octet-stream");         String fileLocation = request.getParameter("location");         String fileName = fileLocation.substring(fileLocation.lastIndexOf("/") + 1);         response.setHeader("Content-Disposition", "attachment;filename=" + filenameEncoding(fileName, request));          ByteStreams.copy(new FileInputStream(fileLocation), response.getOutputStream());     }      @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         doGet(req, resp);     } }

Async-异步处理

Servlet / Filter 默认会一直占用请求处理线程, 直到它完成任务.如果任务耗时长久, 且并发用户请求量大, Servlet容器将会遇到超出线程数的风险.

Servlet 3.0 中新增了一项特性, 用来处理异步操作. 当 Servlet / Filter 应用程序中有一个/多个长时间运行的任务时, 你可以选择将任务分配给一个新的线程, 从而将当前请求处理线程返回到线程池中,释放线程资源,准备为下一个请求服务.

异步Servlet/Filter

异步支持

@WebServlet / @WebFilter 注解提供了新的 asyncSupport 属性:

@WebFilter(asyncSupported = true) @WebServlet(asyncSupported = true)

同样部署描述符中也添加了 <async-supportted/> 标签:

<servlet>     <servlet-name>HelloServlet</servlet-name>     <servlet-class>com.fq.web.servlet.HelloServlet</servlet-class>     <async-supported>true</async-supported> </servlet>

Servlet/Filter

支持异步处理的 Servlet / Filter 可以通过在 ServletRequest 中调用 startAsync() 方法来启动新线程:

ServletRequest	描述
`AsyncContext startAsync()`	Puts this request into asynchronous mode, and initializes its AsyncContext with the original (unwrapped) ServletRequest and ServletResponse objects.
`AsyncContext startAsync(ServletRequest servletRequest, ServletResponse servletResponse)`	Puts this request into asynchronous mode, and initializes its AsyncContext with the given request and response objects.

注意:

1. 只能将原始的 ServletRequest / ServletResponse 或其包装器(Wrapper/Decorator,详见 Servlet – Listener、Filter、Decorator )传递给第二个 startAsync() 方法.

2. 重复调用 startAsync() 方法会返回相同的 AsyncContext 实例, 如果在不支持异步处理的 Servlet / Filter 中调用, 会抛出 java.lang.IllegalStateException 异常.

3. AsyncContext 的 start() 方法不会造成方法阻塞.

这两个方法都返回 AsyncContext 实例, AsyncContext 中提供了如下常用方法:

AsyncContext	描述
`void start(Runnable run)`	Causes the container to dispatch a thread, possibly from a managed thread pool, to run the specified Runnable.
`void dispatch(String path)`	Dispatches the request and response objects of this AsyncContext to the given path.
`void dispatch(ServletContext context, String path)`	Dispatches the request and response objects of this AsyncContext to the given path scoped to the given context.
`void addListener(AsyncListener listener)`	Registers the given AsyncListener with the most recent asynchronous cycle that was started by a call to one of the ServletRequest.startAsync() methods.
`ServletRequest getRequest()`	Gets the request that was used to initialize this AsyncContext by calling ServletRequest.startAsync() or ServletRequest.startAsync(ServletRequest, ServletResponse).
`ServletResponse getResponse()`	Gets the response that was used to initialize this AsyncContext by calling ServletRequest.startAsync() or ServletRequest.startAsync(ServletRequest, ServletResponse).
`boolean hasOriginalRequestAndResponse()`	Checks if this AsyncContext was initialized with the original or application-wrapped request and response objects.
`void setTimeout(long timeout)`	Sets the timeout (in milliseconds) for this AsyncContext.

在异步 Servlet / Filter 中需要完成以下工作, 才能真正达到异步的目的:

调用 AsyncContext 的 start() 方法, 传递一个执行长时间任务的 Runnable ;
任务完成时, 在 Runnable 内调用 AsyncContext 的 complete() 方法或 dispatch() 方法

示例-改造文件上传

在前面的图片存储服务器中, 如果上传图片过大, 可能会耗时长久,为了提升服务器性能, 可将其改造为异步上传(其改造成本较小):

@Override protected void doPost(final HttpServletRequest request, final HttpServletResponse response) throws ServletException, IOException {     final AsyncContext asyncContext = request.startAsync();     asyncContext.start(new Runnable() {         @Override         public void run() {             try {                 request.setCharacterEncoding("UTF-8");                 response.setContentType("text/html;charset=UTF-8");                 PrintWriter writer = response.getWriter();                 Part image = request.getPart("image");                 final String fileName = getFileName(image);                 if (isFileValid(image, fileName) && isImageValid(fileName)) {                     String destFileName = generateDestFileName(fileName);                     String twoLevelDir = generateTwoLevelDir(destFileName);                      // 保存文件                     String saveDir = String.format("%s/%s/", getServletContext().getRealPath(SAVE_ROOT_DIR), twoLevelDir);                     makeDirs(saveDir);                     image.write(saveDir + destFileName);                     // 生成外链                     String ip = request.getLocalAddr();                     int port = request.getLocalPort();                     String path = request.getContextPath();                     String urlPrefix = String.format("http://%s:%s%s", ip, port, path);                     String urlSuffix = String.format("%s/%s/%s", SAVE_ROOT_DIR, twoLevelDir, destFileName);                     String url = urlPrefix + urlSuffix;                     String result = String.format("<a href=%s>%s</a><hr/><a href=ifs_download.action?location=%s>下载</a>",                             url,                             url,                             saveDir + destFileName);                     writer.print(result);                 } else {                     writer.print("Error : Image Type Error");                 }                 asyncContext.complete();             } catch (ServletException | IOException e) {                 LOGGER.error("error: ", e);             }         }     }); }

注意: Servlet异步支持只适用于长时间运行,且想让用户知道执行结果的任务. 如果只有长时间, 但用户不需要知道处理结果,那么只需提供一个 Runnable 提交给 Executor , 并立即返回即可.

AsyncListener

Servlet 3.0 还新增了一个 AsyncListener 接口, 以便通知用户在异步处理期间发生的事件, 该接口会在异步操作的启动 / 完成 / 失败 / 超时情况下调用其对应方法:

ImageUploadListener

/**  * @author jifang.  * @since 2016/5/10 17:33.  */ public class ImageUploadListener implements AsyncListener {      @Override     public void onComplete(AsyncEvent event) throws IOException {         System.out.println("onComplete...");     }      @Override     public void onTimeout(AsyncEvent event) throws IOException {         System.out.println("onTimeout...");     }      @Override     public void onError(AsyncEvent event) throws IOException {         System.out.println("onError...");     }      @Override     public void onStartAsync(AsyncEvent event) throws IOException {         System.out.println("onStartAsync...");     } }

与其他监听器不同, 他没有 @WebListener 标注 AsyncListener 的实现, 因此必须对有兴趣收到通知的每个 AsyncContext 都手动注册一个 AsyncListener :

asyncContext.addListener(new ImageUploadListener());

动态注册

动态注册是Servlet 3.0新特性,它不需要重新加载应用便可安装新的 Web对象 ( Servlet / Filter / Listener 等).

API支持

为了使动态注册成为可能, ServletContext 接口添加了如下方法用于 创建/添加 Web对象:

ServletContext	描述
Create
`<T extends Servlet> T createServlet(Class<T> clazz)`	Instantiates the given Servlet class.
`<T extends Filter> T createFilter(Class<T> clazz)`	Instantiates the given Filter class.
`<T extends EventListener> T createListener(Class<T> clazz)`	Instantiates the given EventListener class.
Add
`ServletRegistration.Dynamic addServlet(String servletName, Servlet servlet)`	Registers the given servlet instance with this ServletContext under the given servletName.
`FilterRegistration.Dynamic addFilter(String filterName, Filter filter)`	Registers the given filter instance with this ServletContext under the given filterName.
`<T extends EventListener> void addListener(T t)`	Adds the given listener to this ServletContext.
Create & And
`ServletRegistration.Dynamic addServlet(String servletName, Class<? extends Servlet> servletClass)`	Adds the servlet with the given name and class type to this servlet context.
`ServletRegistration.Dynamic addServlet(String servletName, String className)`	Adds the servlet with the given name and class name to this servlet context.
`FilterRegistration.Dynamic addFilter(String filterName, Class<? extends Filter> filterClass)`	Adds the filter with the given name and class type to this servlet context.
`FilterRegistration.Dynamic addFilter(String filterName, String className)`	Adds the filter with the given name and class name to this servlet context.
`void addListener(Class<? extends EventListener> listenerClass)`	Adds a listener of the given class type to this ServletContext.
`void addListener(String className)`	Adds the listener with the given class name to this ServletContext.

其中 addServlet() / addFilter() 方法的返回值是 ServletRegistration.Dynamic / FilterRegistration.Dynamic ,他们都是 Registration.Dynamic 的子接口,用于动态配置 Servlet / Filter 实例.

示例-DynamicServlet

动态注册DynamicServlet, 注意: 并未使用 web.xml 或 @WebServlet 静态注册 DynamicServlet 实例, 而是用 DynRegListener 在服务器启动时动态注册.

DynamicServlet

/**  * @author jifang.  * @since 2016/5/13 16:41.  */ public class DynamicServlet extends HttpServlet {      private String dynamicName;      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         response.getWriter().print("<h1>DynamicServlet, MyDynamicName: " + getDynamicName() + "</h1>");     }      public String getDynamicName() {         return dynamicName;     }      public void setDynamicName(String dynamicName) {         this.dynamicName = dynamicName;     } }

DynRegListener

@WebListener public class DynRegListener implements ServletContextListener {      @Override     public void contextInitialized(ServletContextEvent sce) {         ServletContext context = sce.getServletContext();          DynamicServlet servlet;         try {             servlet = context.createServlet(DynamicServlet.class);         } catch (ServletException e) {             servlet = null;         }          if (servlet != null) {             servlet.setDynamicName("Hello fQ Servlet");             ServletRegistration.Dynamic dynamic = context.addServlet("dynamic_servlet", servlet);             dynamic.addMapping("/dynamic_servlet.do");         }      }      @Override     public void contextDestroyed(ServletContextEvent sce) {     } }

容器初始化

在使用类似SpringMVC这样的MVC框架时,需要首先注册 DispatcherServlet 到 web.xml 以完成URL的转发映射:

<!-- 配置SpringMVC --> <servlet>     <servlet-name>mvc</servlet-name>     <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>     <init-param>         <param-name>contextConfigLocation</param-name>         <param-value>classpath:spring/mvc-servlet.xml</param-value>     </init-param>     <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping>     <servlet-name>mvc</servlet-name>     <url-pattern>*.do</url-pattern> </servlet-mapping>

在Servlet 3.0中,通过 Servlet容器初始化 ,可以自动完成Web对象的首次注册,因此可以省略这个步骤.

API支持

容器初始化的核心是 javax.servlet.ServletContainerInitializer 接口,他只包含一个方法:

ServletContainerInitializer	描述
`void onStartup(Set<Class<?>> c, ServletContext ctx)`	Notifies this ServletContainerInitializer of the startup of the application represented by the given ServletContext.

在执行任何 ServletContext 监听器之前, 由Servlet容器自动调用 onStartup() 方法.

注意: 任何实现了 ServletContainerInitializer 的类必须使用 @HandlesTypes 注解标注, 以声明该初始化程序可以处理这些类型的类.

实例-SpringMVC初始化

利用Servlet容器初始化, SpringMVC可实现容器的零配置注册.

SpringServletContainerInitializer

@HandlesTypes(WebApplicationInitializer.class) public class SpringServletContainerInitializer implements ServletContainerInitializer {      @Override     public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)             throws ServletException {          List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();          if (webAppInitializerClasses != null) {             for (Class<?> waiClass : webAppInitializerClasses) {                 // Be defensive: Some servlet containers provide us with invalid classes,                 // no matter what @HandlesTypes says...                 if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&                         WebApplicationInitializer.class.isAssignableFrom(waiClass)) {                     try {                         initializers.add((WebApplicationInitializer) waiClass.newInstance());                     }                     catch (Throwable ex) {                         throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);                     }                 }             }         }          if (initializers.isEmpty()) {             servletContext.log("No Spring WebApplicationInitializer types detected on classpath");             return;         }          AnnotationAwareOrderComparator.sort(initializers);         servletContext.log("Spring WebApplicationInitializers detected on classpath: " + initializers);          for (WebApplicationInitializer initializer : initializers) {             initializer.onStartup(servletContext);         }     }  }

SpringMVC为 ServletContainerInitializer 提供了实现类 SpringServletContainerInitializer 通过查看源代码可以知道,我们只需提供 WebApplicationInitializer 的实现类到classpath下, 即可完成对所需 Servlet / Filter / Listener 的注册.

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

详细可参考 springmvc基于java config的实现

javax.servlet.ServletContainerInitializer

org.springframework.web.SpringServletContainerInitializer

元数据文件<strong>javax.servlet.ServletContainerInitializer</strong>只有一行内容(即实现了<code style="font-style: inherit;">ServletContainerInitializer</code>类的全限定名),该文本文件必须放在jar包的<strong>META-INF/services</strong>目录下.

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