Servlets 简明教程

Servlets - File Uploading

Servlet 可以与 HTML 表单标记一起使用，允许用户将文件上载到服务器。上传的文件可以是文本文件、图像文件或任何文档。

Creating a File Upload Form

以下 HTM 代码创建一个上传程序表单。以下是需要注意的重要事项：

表单 method 属性应设置为 POST 方法，不能使用 GET 方法
表单 enctype 属性应设置为 multipart/form-data 。
表单 action 属性应设置为一个 servlet 文件，它将处理后端服务器上的文件上传。以下示例使用 UploadServlet servlet 上传文件。
要上传单个文件，应使用具有属性 type="file" 的单个 <input …/> 标记。为了允许上传多个文件，包含名称属性中具有不同值的多个输入标记。浏览器将一个浏览按钮与它们中的每一个关联。

<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>

这将显示以下结果，该结果允许从本地 PC 中选择一个文件，当用户单击“上传文件”时，表单将与所选文件一起提交：

File Upload:
Select a file to upload:




NOTE: This is just dummy form and would not work.

Writing Backend Servlet

以下是 UploadServlet servlet，它将负责接受上传的文件并将其存储在目录 <Tomcat-installation-directory>/webapps/data 中。此目录名称还可以使用外部配置（例如 web.xml 中的 context-param 元素）如下所示添加：

<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 UploadServlet 的源代码，它可以一次处理多个文件上传。在继续之前，请确保以下内容：

以下示例依赖于 FileUpload，因此请确保 classpath 中有最新版本的 commons-fileupload.x.x.jar 文件。你可以从 https://commons.apache.org/fileupload/ 下载它。
FileUpload 依赖于 Commons IO，因此请确保你的 classpath 中有最新版本的 commons-io-x.x.jar 文件。你可以从 https://commons.apache.org/io/ 下载它。
在测试以下示例时，应上传一个大小小于 maxFileSize 的文件，否则文件将不会上传。
确保你已经提前创建了目录 c:\temp 和 c:\apache-tomcat8.0.28\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.");
      }
   }
}

Compile and Running Servlet

编译上述 servlet UploadServlet，并在 web.xml 文件中创建必需的条目，如下所示。

<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>

现在尝试使用您在上面创建的 HTML 表单上传文件。当您尝试 [role="bare"] [role="bare"]http://localhost:8080/UploadFile.htm 时，它将显示以下结果，这将帮助您上传本地计算机上的任何文件。

File Upload:
Select a file to upload:

如果您的 servlet 脚本运行良好，您的文件应上传到 c:\apache-tomcat-8.0.28\webapps\data\ 目录中。