翻译：使用Servlet实现文件上传
原文地址：http://www.tutorialspoint.com/servlets/servlets-file-uploading.htm
一个Servlet可以通过HTML表单标签将文件上传到服务器。支待上传的有文本、图像及任何文件。
创建文件上传表单：
下面的html代码创建了一个上传表单。创建过程需要注意以下几点：
l form标签中的method属性必须设置为POST，即GET方法是不可以的。
l form标签中的enctype属性应该设置为multipart/form-data。
l from标签中的action属性应该与服务器后台的servlet映射路径相同。接下来的实例，我们将使用UploadServlet实现文件上传。
l 要上传一个文件，你应该使用一个<input type=”file”.../>标记。要多个文件上传，必须包含多个具有不同的名称属性值的<input type=”file”.../>标记。The browser associates a Browse button with each of them。
| <html> <head> <title>File Uploading Form</title> </head> <body> <h3>File Upload:</h3> Select a file to upload: <br /> <form action="UploadServlet" method="post" enctype="multipart/form-data"> <input type="file" name="file" size="50" /> <br /> <input type="submit" value="Upload File" /> </form> </body> </html> |
以上代码将得到以下效果。你可以在本地PC上选择一个文件。当你点击“Upload File”，表单将会随着你选择的文件一起被提交。
后台servlet：
以下UploadServlet servlet将接收上传的文件并将其保存入<Tomcat-installation-directory>/webapps/data文件夹。这个文件夹的名称可以通过外部配置文件web.xml中的context-param元素内容增加。代码如下：
<web-app> .... <context-param> <description>Location to store uploaded file</description> <param-name>file-upload</param-name> <param-value> c:"apache-tomcat-5.5.29"webapps"data" </param-value> </context-param> .... </web-app> |
以下是实现了多文件同时上传功能的UploadServlet。在此之前您必须确定以下几点：
l 以下实例依赖F ileUpload类，所以您须将最新版的commons-fileupload.x.x.jar放到您的classpath下。可以从这里下载：http://commons.apache.org/fileupload/。
l FileUpload类依赖于Commons IO包，所以您须将最新版commons-fileupload.x.x.jar放到您的classpath下。可以从这里下载：http://commons.apache.org/io/。
l 在测试以下例子的时候，您应该上传小于maxFileSize的文件，否则无法上传。
l 事先确定你已经建议好文件夹：c:"temp和c:"apache-tomcat-5.5.29"webapps"data。
// Import required java libraries import java.io.*; import java.util.*; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.commons.fileupload.FileItem; import org.apache.commons.fileupload.FileUploadException; import org.apache.commons.fileupload.disk.DiskFileItemFactory; import org.apache.commons.fileupload.servlet.ServletFileUpload; import org.apache.commons.io.output.*; public class UploadServlet extends HttpServlet { private boolean isMultipart; private String filePath; private int maxFileSize = 50 * 1024; private int maxMemSize = 4 * 1024; private File file ; public void init( ){ // Get the file location where it would be stored. filePath = getServletContext().getInitParameter("file-upload"); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, java.io.IOException { // Check that we have a file upload request isMultipart = ServletFileUpload.isMultipartContent(request); response.setContentType("text/html"); java.io.PrintWriter out = response.getWriter( ); if( !isMultipart ){ out.println("<html>"); out.println("<head>"); out.println("<title>Servlet upload</title>"); out.println("</head>"); out.println("<body>"); out.println("<p>No file uploaded</p>"); out.println("</body>"); out.println("</html>"); return; } DiskFileItemFactory factory = new DiskFileItemFactory(); // maximum size that will be stored in memory factory.setSizeThreshold(maxMemSize); // Location to save data that is larger than maxMemSize. factory.setRepository(new File("c:""temp")); // Create a new file upload handler ServletFileUpload upload = new ServletFileUpload(factory); // maximum file size to be uploaded. upload.setSizeMax( maxFileSize ); try{ // Parse the request to get file items. List fileItems = upload.parseRequest(request); // Process the uploaded file items Iterator i = fileItems.iterator(); out.println("<html>"); out.println("<head>"); out.println("<title>Servlet upload</title>"); out.println("</head>"); out.println("<body>"); while ( i.hasNext () ) { FileItem fi = (FileItem)i.next(); if ( !fi.isFormField () ) { // Get the uploaded file parameters String fieldName = fi.getFieldName(); String fileName = fi.getName(); String contentType = fi.getContentType(); boolean isInMemory = fi.isInMemory(); long sizeInBytes = fi.getSize(); // Write the file if( fileName.lastIndexOf("""") >= 0 ){ file = new File( filePath + fileName.substring( fileName.lastIndexOf(""""))) ; }else{ file = new File( filePath + fileName.substring(fileName.lastIndexOf("""")+1)) ; } fi.write( file ) ; out.println("Uploaded Filename: " + fileName + "<br>"); } } out.println("</body>"); out.println("</html>"); }catch(Exception ex) { System.out.println(ex); } } public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, java.io.IOException { throw new ServletException("GET method used with " + getClass( ).getName( )+": POST method required."); } } |
编译并运行Servlet
编译以上UploadServlet并在web.xml中创建必须的实体，如下：
<servlet> <servlet-name>UploadServlet</servlet-name> <servlet-class>UploadServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>UploadServlet</servlet-name> <url-pattern>/UploadServlet</url-pattern> </servlet-mapping> |
现在可以尝试使用你创建的HTML表单上传文件。当你访问http://localhost:8080/UploadFile.htm，浏览器里将会显示如下效果，您可以从本地上传你想要上传的任何文件。
如果您的servlet脚本运行成功，您的文件上传在c:"apache-tomcat-5.5.29"webapps"data"directory文件夹。