Submit HTML form data using Java to retrieve a download from a jsp application

Question

I have an obscure issue. I am in the process of porting some Perl to Java and one of the methods in the Perl code posts to a jsp app and downloads a zip file. The working part o

Answer 1

First of all sorry for the example I have posted in the comment. I thought you were trying to perform a simple POST request by just submitting some values. I have missed that you were also uploading a file. So since you are also uploading a file (note that you don't just pass a filename here) this is a mutlipart request (you see that in your jsp: enctype="multipart/form-data") and it has a little more work to do that programmatically.

When you are submitting a multipart form the request looks like this:

-----------------------------253171262814565
Content-Disposition: form-data; name="myfile"; filename="test.bin"
Content-Type: application/octet-stream

Test data    
-----------------------------253171262814565
Content-Disposition: form-data; name="submit"

Submit
-----------------------------253171262814565--

That strange alphanumerics are called a boundary (are chosen randomly by the browser) and are used to distringuish between submitted fields (files or normal fields). In the above example the submitted fields were:

• a field named "myfile" that was an in the form. The user selected the file named test.bin. The file data was a single text line Test data
• a normal field value of "Submit" of a button

Below I have copied the code from an example at code.java.net that provides a class that can help you in creating such requests.

Please modify any hard-coded urls and files to match your case. I have not tried it out but it should give you the idea of how to create a request like the above one. (Note that there are two classes: One that constructs the request and one with a main() method to test it)

Side note: The change of line character \r\n does not depend on the system but it is a part of how multipart requests are constructed. So you should use it as is no matter if your system is Windows or Linux

I hope that helps

package net.codejava.networking;

import java.io.BufferedReader;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.List;

/**
 * This utility class provides an abstraction layer for sending multipart HTTP
 * POST requests to a web server.
 * @author www.codejava.net
 *
 */
public class MultipartUtility {
    private final String boundary;
    private static final String LINE_FEED = "\r\n";
    private HttpURLConnection httpConn;
    private String charset;
    private OutputStream outputStream;
    private PrintWriter writer;

    /**
     * This constructor initializes a new HTTP POST request with content type
     * is set to multipart/form-data
     * @param requestURL
     * @param charset
     * @throws IOException
     */
    public MultipartUtility(String requestURL, String charset)
            throws IOException {
        this.charset = charset;

        // creates a unique boundary based on time stamp
        boundary = "===" + System.currentTimeMillis() + "===";

        URL url = new URL(requestURL);
        httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setDoOutput(true); // indicates POST method
        httpConn.setDoInput(true);
        httpConn.setRequestProperty("Content-Type",
                "multipart/form-data; boundary=" + boundary);
        outputStream = httpConn.getOutputStream();
        writer = new PrintWriter(new OutputStreamWriter(outputStream, charset),
                true);
    }

    /**
     * Adds a form field to the request
     * @param name field name
     * @param value field value
     */
    public void addFormField(String name, String value) {
        writer.append("--" + boundary).append(LINE_FEED);
        writer.append("Content-Disposition: form-data; name=\"" + name + "\"")
                .append(LINE_FEED);
        writer.append("Content-Type: text/plain; charset=" + charset).append(
                LINE_FEED);
        writer.append(LINE_FEED);
        writer.append(value).append(LINE_FEED);
        writer.flush();
    }

    /**
     * Adds a upload file section to the request
     * @param fieldName name attribute in 
     * @param uploadFile a File to be uploaded
     * @throws IOException
     */
    public void addFilePart(String fieldName, File uploadFile)
            throws IOException {
        String fileName = uploadFile.getName();
        writer.append("--" + boundary).append(LINE_FEED);
        writer.append(
                "Content-Disposition: form-data; name=\"" + fieldName
                        + "\"; filename=\"" + fileName + "\"")
                .append(LINE_FEED);
        writer.append(
                "Content-Type: "
                        + URLConnection.guessContentTypeFromName(fileName))
                .append(LINE_FEED);
        writer.append("Content-Transfer-Encoding: binary").append(LINE_FEED);
        writer.append(LINE_FEED);
        writer.flush();

        FileInputStream inputStream = new FileInputStream(uploadFile);
        byte[] buffer = new byte[4096];
        int bytesRead = -1;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.flush();
        inputStream.close();

        writer.append(LINE_FEED);
        writer.flush();    
    }

    /**
     * Completes the request and receives response from the server.
     * @return a list of Strings as response in case the server returned
     * status OK, otherwise an exception is thrown.
     * @throws IOException
     */
    public List finish() throws IOException {
        List response = new ArrayList();

        writer.append(LINE_FEED).flush();
        writer.append("--" + boundary + "--").append(LINE_FEED);
        writer.close();

        // checks server's status code first
        int status = httpConn.getResponseCode();
        if (status == HttpURLConnection.HTTP_OK) {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    httpConn.getInputStream()));
            String line = null;
            while ((line = reader.readLine()) != null) {
                response.add(line);
            }
            reader.close();
            httpConn.disconnect();
        } else {
            throw new IOException("Server returned non-OK status: " + status);
        }

        return response;
    }
}

//---------------------------------------------------------

package net.codejava.networking;

import java.io.File;
import java.io.IOException;
import java.util.List;

/**
 * This program demonstrates a usage of the MultipartUtility class.
 * @author www.codejava.net
 *
 */
public class MultipartFileUploader {

    public static void main(String[] args) {
        String charset = "UTF-8";
        File uploadFile1 = new File("e:/Test/PIC1.JPG");
        File uploadFile2 = new File("e:/Test/PIC2.JPG");
        String requestURL = "http://localhost:8080/FileUploadSpringMVC/uploadFile.do";

        try {
            MultipartUtility multipart = new MultipartUtility(requestURL, charset);
            multipart.addFormField("description", "Cool pix");
            multipart.addFilePart("fileUpload", uploadFile1);
            multipart.addFilePart("fileUpload", uploadFile2);

            List response = multipart.finish();
            System.out.println("SERVER REPLIED:");
            for (String line : response) {
                System.out.println(line);
            }
        } catch (IOException ex) {
            System.out.println("ERROR: " + ex.getMessage());
            ex.printStackTrace();
        }
    }
}