Servlet - 文件上传

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

创建文件上传表单

下面的 HTM 代码创建一个上传者表单。以下是需要注意的要点 -

form 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。
表单操作属性应设置为 servlet 文件，该文件将在后端服务器处理文件上传。以下示例是使用UploadServlet servlet 上传文件。
要上传单个文件，您应该使用属性 type="file" 的单个 <input .../> 标记。要允许上传多个文件，请包含多个具有不同 name 属性值的输入标签。浏览器将一个“浏览”按钮与每个按钮相关联。

 
<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，允许从本地 PC 选择文件，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.

编写后端Servlet

以下是 Servlet UploadServlet，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。还可以使用外部配置（例如web.xml 中的context-param元素）添加此目录名称，如下所示 -

 
<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，它可以同时处理多个文件上传。在继续之前，您必须确保以下内容 -

以下示例取决于 FileUpload，因此请确保类路径中具有最新版本的commons-fileupload.xxjar文件。您可以从https://commons.apache.org/fileupload/下载它。
FileUpload 依赖于 Commons IO，因此请确保类路径中具有最新版本的commons-io-xxjar文件。您可以从https://commons.apache.org/io/下载它。
在测试以下示例时，您应该上传一个小于maxFileSize 的文件，否则文件将不会上传。
确保您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.");
      }
   }
}

编译并运行Servlet

编译上面的 servlet UploadServlet 并在 web.xml 文件中创建所需的条目，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 http://localhost:8080/UploadFile.htm 时，它将显示以下结果，这将帮助您从本地计算机上传任何文件。

File Upload:

Select a file to upload:

如果您的 servlet 脚本工作正常，您的文件应该上传到 c:\apache-tomcat8.0.28\webapps\data\ 目录中。