Recipe 8.3 Uploading One File at a Time

Problem

You want to create a component that can receive a client file upload and store the file in a local directory.

Solution

Create a servlet that uses the com.oreilly.servlet.MultipartRequest class from Jason Hunter's cos.jar archive.

Discussion

The MultipartRequest class includes several overloaded constructors. The one used in Example 8-3 takes the javax.servlet.http.HttpServletRequest object, the path to the directory where you want to save uploaded files, and the size limit for the file as parameters. In Example 8-3, if the client uploads a file that exceeds 5 MB, then the UploadServlet throws a java.io.IOException . You can allow this exception to be managed by an error-page element in web.xml for IOExceptions , as Example 8-3 does, or use a try/catch block in the upload servlet to deal with errors.

See Chapter 9 for how to declare error pages for the web application.

With MultipartRequest , as soon as the code instantiates the object, the object is handling the file upload; in other words, you do not have to call a method to commence managing the upload.

The servlet in Example 8-3 initiates the file upload and then displays the name of the uploaded file(s).

Example 8-3. A servlet that uses the MultipartRequest class

 package com.jspservletcookbook;            import javax.servlet.*; import javax.servlet.http.*;  import com.oreilly.servlet.MultipartRequest;  import java.util.Enumeration; public class UploadServlet extends HttpServlet {  private String webTempPath;  public void init( )     webTempPath = getServletContext( ).getRealPath("/") + "data";         }     public void doPost(HttpServletRequest request,                          HttpServletResponse response)         throws ServletException, java.io.IOException {                 //file limit size of 5 MB  MultipartRequest mpr = new MultipartRequest(           request,webTempPath,5 * 1024 * 1024);         Enumeration enum = mpr.getFileNames( );  response.setContentType("text/html");         java.io.PrintWriter out = response.getWriter( );              out.println("<html>");         out.println("<head>");         out.println("<title>Servlet upload</title>");           out.println("</head>");         out.println("<body>");  for (int i = 1; enum.hasMoreElements( );i++)             out.println("The name of uploaded file " + i +               " is: " + mpr.getFilesystemName((String) enum.nextElement( ))                  + "<br><br>");  out.println("</body>");         out.println("</html>");                   }      public void doGet(HttpServletRequest request,                         HttpServletResponse response)         throws ServletException, java.io.IOException {  throw new ServletException("GET method used with " +             getClass( ).getName( )+": POST method required.");  }  }

The code generates the path to the save directory by calling javax.servlet.ServletContext.getRealPath("/") to get an absolute pathname to the root of the web application (as in h:\home\ ). Then the code adds the name of the directory where the file will be saved ( data ).

This directory name could also be added using an external configuration such as a context-param element in web.xml . See Recipe 8.6 for details.

The method MultipartRequest.getFilesystemName( StringName ) returns the filename from the client's filesystem. The file can be saved on the server end with its original filename, or you can use a different MultipartRequest constructor that takes as a parameter a FileRenamePolicy object. This constructor looks like:

 MultipartRequest(javax.servlet.http.HttpServletRequest request,                  java.lang.String saveDirectory, int maxPostSize,                  FileRenamePolicy policy)

There are a few versions of the MultipartRequest constructor with the FileRenamePolicy parameter, which is used to rename uploaded files (see Recipe 8.5). Example 8-3 also throws a ServletException if the UploadServlet is requested with a GET method, which is not allowed with file uploads.

Problem

Solution

Discussion

Example 8-3. A servlet that uses the MultipartRequest class

See Also