Servlet - 檔案上傳

Servlet可以與HTML表單標籤一起使用，允許使用者將檔案上傳到伺服器。上傳的檔案可以是文字檔案、影像檔案或任何文件。

建立檔案上傳表單

以下HTML程式碼建立一個上傳表單。需要注意以下幾點：

表單的method屬性應設定為POST方法，不能使用GET方法。
表單的enctype屬性應設定為multipart/form-data。
表單的action屬性應設定為處理檔案上傳的Servlet檔案。以下示例使用UploadServlet Servlet來上傳檔案。
要上傳單個檔案，應使用單個<input .../>標籤，其屬性type="file"。要允許上傳多個檔案，請包含多個input標籤，併為name屬性使用不同的值。瀏覽器會為每個標籤關聯一個“瀏覽”按鈕。

 
<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>

這將顯示以下結果，允許使用者從本地PC選擇檔案，當用戶點選“上傳檔案”時，表單將與所選檔案一起提交：

 
File Upload: 
Select a file to upload: 
 
 

 
 

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

編寫後臺Servlet

以下是Servlet UploadServlet，它負責接收上傳的檔案並將其儲存在目錄<Tomcat安裝目錄>/webapps/data中。此目錄名稱也可以使用外部配置（例如web.xml中的context-param元素）新增，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的原始碼，它可以一次處理多個檔案上傳。在繼續之前，請確保：

以下示例依賴於FileUpload，所以請確保你的classpath中包含最新版本的commons-fileupload.x.x.jar檔案。你可以從 https://commons.apache.org/fileupload/ 下載。
FileUpload依賴於Commons IO，所以請確保你的classpath中包含最新版本的commons-io-x.x.jar檔案。你可以從 https://commons.apache.org/io/ 下載。
測試以下示例時，應上傳大小小於maxFileSize的檔案，否則檔案將無法上傳。
請確保提前建立了c:\temp和c:\apache-tomcat8.0.28\webapps\data目錄。

// 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.");
      }
   }
}

編譯和執行Servlet

編譯上面的Servlet UploadServlet，並在web.xml檔案中建立必要的條目，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>

現在嘗試使用上面建立的HTML表單上傳檔案。當你嘗試訪問https://:8080/UploadFile.htm時，它將顯示以下結果，這將幫助你從本地計算機上傳任何檔案。

File Upload:

Select a file to upload:

如果你的Servlet指令碼執行正常，你的檔案應該上傳到c:\apache-tomcat8.0.28\webapps\data\目錄。

列印頁面