Servlet

標籤： Java與Web

Upload-上傳

隨着3.0版本的發佈,文件上傳終於成爲Servlet規範的一項內置特性,不再依賴於像Commons FileUpload之類組件,因此在服務端進行文件上傳編程變得不費吹灰之力.

客戶端

要上傳文件, 必須利用multipart/form-data設置HTML表單的enctype屬性,且method必須爲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="上傳"></td>
        </tr>
    </table>
</form>

服務端

服務端Servlet主要圍繞着@MultipartConfig註解和Part接口:

處理上傳文件的Servlet必須用@MultipartConfig註解標註:

@MultipartConfig屬性	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

在一個由多部件組成的請求中, 每一個表單域(包括非文件域), 都會被封裝成一個Part,HttpServletRequest中提供如下兩個方法獲取封裝好的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中提供瞭如下常用方法來獲取/操作上傳的文件/數據:

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

通過抓包獲取到客戶端上傳文件的數據格式:

------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表單輸入項爲文本(<input type="text"/>),將只包含一個請求頭Content-Disposition.; B. 如果HTML表單輸入項爲文件(<input type="file"/>), 則包含兩個頭:
Content-Disposition與Content-Type.

在Servlet中處理上傳文件時, 需要:

- 通過查看是否存在`Content-Type`標頭, 檢驗一個Part是封裝的普通表單域,還是文件域.
- 若有`Content-Type`存在, 但文件名爲空, 則表示沒有選擇要上傳的文件.
- 如果有文件存在, 則可以調用`write()`方法來寫入磁盤, 調用同時傳遞一個絕對路徑, 或是相對於`@MultipartConfig`註解的`location`屬性的相對路徑.

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>請確認上傳文件是否正確!");
        } 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) {
        // 上傳的並非文件
        if (file.getContentType() == null) {
            return false;
        }
        // 沒有選擇任何文件
        else if (Strings.isNullOrEmpty(file.getSubmittedFileName())) {
            return false;
        }
        return true;
    }
}

優化

善用WEB-INF
存放在/WEB-INF/目錄下的資源無法在瀏覽器地址欄直接訪問, 利用這一特點可將某些受保護資源存放在WEB-INF目錄下, 禁止用戶直接訪問(如用戶上傳的可執行文件,如JSP等),以防被惡意執行, 造成服務器信息泄露等危險.

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

文件名亂碼
當文件名包含中文時,可能會出現亂碼,其解決方案與POST相同:

request.setCharacterEncoding("UTF-8");

避免文件同名
如果上傳同名文件,會造成文件覆蓋.因此可以爲每份文件生成一個唯一ID,然後連接原始文件名:

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

目錄打散
如果一個目錄下存放的文件過多, 會導致文件檢索速度下降,因此需要將文件打散存放到不同目錄中, 在此我們採用Hash打散法(根據文件名生成Hash值, 取Hash值的前兩個字符作爲二級目錄名), 將文件分佈到一個二級目錄中:

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

採用Hash打散的好處是:在根目錄下最多生成16個目錄,而每個子目錄下最多再生成16個子子目錄,即一共256個目錄,且分佈較爲均勻.

示例-簡易存儲圖片服務器

需求: 提供上傳圖片功能, 爲其生成外鏈, 並提供下載功能(見下)

客戶端

<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>&nbsp;</td>
            <td><input type="submit" value="上傳"></td>
        </tr>
    </table>
</form>
</body>
</html>

服務端

@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);

            // 生成外鏈
            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>下載</a>",
                    url,
                    url,
                    saveDir + destFileName);
            writer.print(result);
        } else {
            writer.print("Error : Image Type Error");
        }
    }

    /**
     * 校驗文件表單域有效
     *
     * @param file
     * @param fileName
     * @return
     */
    private boolean isFileValid(Part file, String fileName) {
        // 上傳的並非文件
        if (file.getContentType() == null) {
            return false;
        }
        // 沒有選擇任何文件
        else if (Strings.isNullOrEmpty(fileName)) {
            return false;
        }

        return true;
    }

    /**
     * 校驗文件後綴有效
     *
     * @param fileName
     * @return
     */
    private boolean isImageValid(String fileName) {
        for (String suffix : imageSuffix) {
            if (fileName.endsWith(suffix)) {
                return true;
            }
        }
        return false;
    }

    /**
     * 加速圖片訪問速度, 生成兩級存放目錄
     *
     * @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();
        }
    }
}

由於getSubmittedFileName()方法需要有Tomcat 8.X以上版本的支持, 因此爲了通用期間, 我們自己解析content-disposition請求頭, 獲取filename.

Download-下載

文件下載是向客戶端響應二進制數據(而非字符),瀏覽器不會直接顯示這些內容,而是會彈出一個下載框, 提示下載信息.

爲了將資源發送給瀏覽器, 需要在Servlet中完成以下工作:

使用Content-Type響應頭來規定響應體的MIME類型, 如image/pjpeg、application/octet-stream;
添加Content-Disposition響應頭,賦值爲attachment;filename=xxx.yyy, 設置文件名;
使用response.getOutputStream()給瀏覽器發送二進制數據;

文件名中文亂碼

當文件名包含中文時(attachment;filename=文件名.後綴名),在下載框中會出現亂碼, 需要對文件名編碼後在發送, 但不同的瀏覽器接收的編碼方式不同:

    * FireFox: Base64編碼
    * 其他大部分Browser: URL編碼

因此最好將其封裝成一個通用方法:

private String filenameEncoding(String filename, HttpServletRequest request) throws IOException {
    // 根據瀏覽器信息判斷
    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下載功能

/**
 * @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-異步處理

Servlet/Filter默認會一直佔用請求處理線程, 直到它完成任務.如果任務耗時長久, 且併發用戶請求量大, Servlet容器將會遇到超出線程數的風險.

Servlet 3.0 中新增了一項特性, 用來處理異步操作. 當Servlet/Filter應用程序中有一個/多個長時間運行的任務時, 你可以選擇將任務分配給一個新的線程, 從而將當前請求處理線程返回到線程池中,釋放線程資源,準備爲下一個請求服務.

異步Servlet/Filter

異步支持
@WebServlet/@WebFilter註解提供了新的asyncSupport屬性:

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

同樣部署描述符中也添加了<async-supportted/>標籤:

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

Servlet/Filter
支持異步處理的Servlet/Filter可以通過在ServletRequest中調用startAsync()方法來啓動新線程:

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. 只能將原始的ServletRequest/ServletResponse或其包裝器(Wrapper/Decorator,詳見Servlet - Listener、Filter、Decorator)傳遞給第二個startAsync()方法.
2. 重複調用startAsync()方法會返回相同的AsyncContext實例, 如果在不支持異步處理的Servlet/Filter中調用, 會拋出java.lang.IllegalStateException異常.
3. AsyncContext的start()方法不會造成方法阻塞.

這兩個方法都返回AsyncContext實例, AsyncContext中提供瞭如下常用方法:

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.

在異步Servlet/Filter中需要完成以下工作, 才能真正達到異步的目的:

調用AsyncContext的start()方法, 傳遞一個執行長時間任務的Runnable;
任務完成時, 在Runnable內調用AsyncContext的complete()方法或dispatch()方法

示例-改造文件上傳

在前面的圖片存儲服務器中, 如果上傳圖片過大, 可能會耗時長久,爲了提升服務器性能, 可將其改造爲異步上傳(其改造成本較小):

@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);
                    // 生成外鏈
                    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>下載</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異步支持只適用於長時間運行,且想讓用戶知道執行結果的任務. 如果只有長時間, 但用戶不需要知道處理結果,那麼只需提供一個Runnable提交給Executor, 並立即返回即可.

AsyncListener

Servlet 3.0 還新增了一個AsyncListener接口, 以便通知用戶在異步處理期間發生的事件, 該接口會在異步操作的啓動/完成/失敗/超時情況下調用其對應方法:

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...");
    }
}

與其他監聽器不同, 他沒有@WebListener標註AsyncListener的實現, 因此必須對有興趣收到通知的每個AsyncContext都手動註冊一個AsyncListener:

asyncContext.addListener(new ImageUploadListener());

動態註冊

動態註冊是Servlet 3.0新特性,它不需要重新加載應用便可安裝新的Web對象(Servlet/Filter/Listener等).

API支持

爲了使動態註冊成爲可能, ServletContext接口添加了如下方法用於 創建/添加 Web對象:

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,他們都是Registration.Dynamic的子接口,用於動態配置Servlet/Filter實例.

示例-DynamicServlet

動態註冊DynamicServlet, 注意: 並未使用web.xml或@WebServlet靜態註冊DynamicServlet實例, 而是用DynRegListener在服務器啓動時動態註冊.

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

在使用類似SpringMVC這樣的MVC框架時,需要首先註冊DispatcherServlet到web.xml以完成URL的轉發映射:

<!-- 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,通過Servlet容器初始化,可以自動完成Web對象的首次註冊,因此可以省略這個步驟.

API支持

容器初始化的核心是javax.servlet.ServletContainerInitializer接口,他只包含一個方法:

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

在執行任何ServletContext監聽器之前, 由Servlet容器自動調用onStartup()方法.

注意: 任何實現了ServletContainerInitializer的類必須使用@HandlesTypes註解標註, 以聲明該初始化程序可以處理這些類型的類.

實例-SpringMVC初始化

利用Servlet容器初始化, SpringMVC可實現容器的零配置註冊.

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爲ServletContainerInitializer提供了實現類SpringServletContainerInitializer通過查看源代碼可以知道,我們只需提供WebApplicationInitializer的實現類到classpath下, 即可完成對所需Servlet/Filter/Listener的註冊.

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

詳細可參考springmvc基於java config的實現

javax.servlet.ServletContainerInitializer

org.springframework.web.SpringServletContainerInitializer

元數據文件javax.servlet.ServletContainerInitializer只有一行內容(即實現了ServletContainerInitializer類的全限定名),該文本文件必須放在jar包的META-INF/services目錄下.

Servlet - Upload、Download、Async、動態註冊

Servlet

Upload-上傳

客戶端

服務端

文件流解析

優化

示例-簡易存儲圖片服務器

Download-下載

文件名中文亂碼

示例-IFS下載功能

Async-異步處理

異步Servlet/Filter

示例-改造文件上傳

AsyncListener

動態註冊

API支持

示例-DynamicServlet

容器初始化

API支持

實例-SpringMVC初始化

Python 潮流週刊#50：我最喜歡的 Python 3.13 新特性！

Spring MVC 實踐 - Component

網絡與RPC

Redis與Java - 數據結構

Spring MVC 實踐 - Base

Servlet - Upload、Download、Async、動態註冊

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