Servlet Upload File and Download File is a common task in java web application. Since I have written a lot about java servlet recently, I thought to provide a sample example of servlet file upload to server and then download from server to client.
Servlet Upload File
Our use case is to provide a simple HTML page where client can select a local file to be uploaded to server. On submission of request to upload the file, our servlet program will upload the file into a directory in the server and then provide the URL through which user can download the file. For security reason, user will not be provided direct URL for downloading the file, rather they will be given a link to download the file and our servlet will process the request and send the file to user. We will create a dynamic web project in Eclipse and the project structure will look like below image. 
Let’s look into all the components of our web application and understand the implementation.
HTML Page for Java Uploading File to Server
We can upload a file to server by sending a post request to servlet and submitting the form. We can’t use GET method for uploading file. Another point to note is that enctype of form should be multipart/form-data. To select a file from user file system, we need to use input element with type as file. So we can have a simple HTML page index.html for uploading file as:
<html>
<head></head>
<body>
<form action="UploadDownloadFileServlet" method="post" enctype="multipart/form-data">
Select File to Upload:<input type="file" name="fileName">
<br>
<input type="submit" value="Upload">
</form>
</body>
</html>
Server File Location for File Upload
We need to store file into some directory at server, we can have this directory hardcoded in program but for better flexibility, we will keep it configurable in deployment descriptor context params. Also we will add our upload file html page to the welcome file list. Our web.xml file will look like below:
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" xmlns="https://java.sun.com/xml/ns/javaee" xsi:schemaLocation="https://java.sun.com/xml/ns/javaee https://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0">
<display-name>ServletFileUploadDownloadExample</display-name>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
</welcome-file-list>
<context-param>
<param-name>tempfile.dir</param-name>
<param-value>tmpfiles</param-value>
</context-param>
</web-app>
ServletContextListener for File Upload Location
Since we need to read context parameter for file location and create a File object from it, we can write a ServletContextListener to do it when context is initialized. We can set absolute directory location and File object as context attribute to be used by other servlets. Our ServletContextListener implementation code is like below.
package com.journaldev.servlet;
import java.io.File;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class FileLocationContextListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent servletContextEvent) {
String rootPath = System.getProperty("catalina.home");
ServletContext ctx = servletContextEvent.getServletContext();
String relativePath = ctx.getInitParameter("tempfile.dir");
File file = new File(rootPath + File.separator + relativePath);
if(!file.exists()) file.mkdirs();
System.out.println("File Directory created to be used for storing files");
ctx.setAttribute("FILES_DIR_FILE", file);
ctx.setAttribute("FILES_DIR", rootPath + File.separator + relativePath);
}
public void contextDestroyed(ServletContextEvent servletContextEvent) {
//do cleanup if needed
}
}
File Upload Download Servlet
Update: Servlet Specs 3 added support to upload files on server in the API, so we won’t need to use any third party API. Please check out Servlet 3 Upload File. For File upload, we will use Apache Commons FileUpload utility, for our project we are using version 1.3, FileUpload depends on Apache Commons IO jar, so we need to place both in the lib directory of the project, as you can see that in above image for project structure. We will use DiskFileItemFactory factory that provides a method to parse the HttpServletRequest object and return list of FileItem. FileItem provides useful method to get the file name, field name in form, size and content type details of the file that needs to be uploaded. To write file to a directory, all we need to do it create a File object and pass it as argument to FileItem write() method. Since the whole purpose of the servlet is to upload file, we will override init() method to initialise the DiskFileItemFactory object instance of the servlet. We will use this object in the doPost() method implementation to upload file to server directory. Once the file gets uploaded successfully, we will send response to client with URL to download the file, since HTML links use GET method,we will append the parameter for file name in the URL and we can utilise the same servlet doGet() method to implement file download process. For implementing download file servlet, first we will open the InputStream for the file and use ServletContext.getMimeType() method to get the MIME type of the file and set it as response content type. We will also need to set the response content length as length of the file. To make sure that client understand that we are sending file in response, we need to set “Content-Disposition” header with value as "attachment; filename=“fileName”. Once we are done with setting response configuration, we can read file content from InputStream and write it to ServletOutputStream and the flush the output to client. Our final implementation of UploadDownloadFileServlet servlet looks like below.
package com.journaldev.servlet;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Iterator;
import java.util.List;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
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;
@WebServlet("/UploadDownloadFileServlet")
public class UploadDownloadFileServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private ServletFileUpload uploader = null;
@Override
public void init() throws ServletException{
DiskFileItemFactory fileFactory = new DiskFileItemFactory();
File filesDir = (File) getServletContext().getAttribute("FILES_DIR_FILE");
fileFactory.setRepository(filesDir);
this.uploader = new ServletFileUpload(fileFactory);
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String fileName = request.getParameter("fileName");
if(fileName == null || fileName.equals("")){
throw new ServletException("File Name can't be null or empty");
}
File file = new File(request.getServletContext().getAttribute("FILES_DIR")+File.separator+fileName);
if(!file.exists()){
throw new ServletException("File doesn't exists on server.");
}
System.out.println("File location on server::"+file.getAbsolutePath());
ServletContext ctx = getServletContext();
InputStream fis = new FileInputStream(file);
String mimeType = ctx.getMimeType(file.getAbsolutePath());
response.setContentType(mimeType != null? mimeType:"application/octet-stream");
response.setContentLength((int) file.length());
response.setHeader("Content-Disposition", "attachment; filename=\"" + fileName + "\"");
ServletOutputStream os = response.getOutputStream();
byte[] bufferData = new byte[1024];
int read=0;
while((read = fis.read(bufferData))!= -1){
os.write(bufferData, 0, read);
}
os.flush();
os.close();
fis.close();
System.out.println("File downloaded at client successfully");
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
if(!ServletFileUpload.isMultipartContent(request)){
throw new ServletException("Content type is not multipart/form-data");
}
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.write("<html><head></head><body>");
try {
List<FileItem> fileItemsList = uploader.parseRequest(request);
Iterator<FileItem> fileItemsIterator = fileItemsList.iterator();
while(fileItemsIterator.hasNext()){
FileItem fileItem = fileItemsIterator.next();
System.out.println("FieldName="+fileItem.getFieldName());
System.out.println("FileName="+fileItem.getName());
System.out.println("ContentType="+fileItem.getContentType());
System.out.println("Size in bytes="+fileItem.getSize());
File file = new File(request.getServletContext().getAttribute("FILES_DIR")+File.separator+fileItem.getName());
System.out.println("Absolute Path at server="+file.getAbsolutePath());
fileItem.write(file);
out.write("File "+fileItem.getName()+ " uploaded successfully.");
out.write("<br>");
out.write("<a href=\"UploadDownloadFileServlet?fileName="+fileItem.getName()+"\">Download "+fileItem.getName()+"</a>");
}
} catch (FileUploadException e) {
out.write("Exception in uploading file.");
} catch (Exception e) {
out.write("Exception in uploading file.");
}
out.write("</body></html>");
}
}
The sample execution of the project is shown in below images. 
Download Servlet File Upload Download Project
You can download Apache Commons IO jar and Apache Commons FileUpload jar from below URLs. https://commons.apache.org/proper/commons-fileupload/download_fileupload.cgi https://commons.apache.org/proper/commons-io/download_io.cgi
Download Servlet File Upload Download Example Project
Check out next article in the series about Servlet Exception Handling.
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About the author
Java and Python Developer for 20+ years, Open Source Enthusiast, Founder of https://www.askpython.com/, https://www.linuxfordevices.com/, and JournalDev.com (acquired by DigitalOcean). Passionate about writing technical articles and sharing knowledge with others. Love Java, Python, Unix and related technologies. Follow my X @PankajWebDev
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