Servlet - Upload、Download、Async

时间：2016-05-11 09:39:31 阅读：434 评论：0 收藏：0 [点我收藏+]

标签：

Upload、Download、Async

标签： Java与Web

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.

客户端

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

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.; B. 如果HTML表单输入项为文件(<input type="file"/>), 则包含两个头:
Content-Disposition与Content-Type.

在Servlet中处理上传文件时, 需要:

- 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.

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;
    }
}

优化

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>&nbsp;</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:

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

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());

Servlet - Upload、Download、Async

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原文地址：http://blog.csdn.net/zjf280441589/article/details/51371389

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