How to convert InputStream to virtual File

Question

I have a method which expects the one of the input variable to be of java.io.File type but what I get is only InputStream. Also, I cannot change the signature of the method.

Answer 1

Something like this should work. Note that for simplicity, I've used a Java 7 feature (try block with closeable resource), and IOUtils from Apache commons-io. If you can't use those it'll be a little longer, but the same idea.

import org.apache.commons.io.IOUtils;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

public class StreamUtil {

    public static final String PREFIX = "stream2file";
    public static final String SUFFIX = ".tmp";

    public static File stream2file (InputStream in) throws IOException {
        final File tempFile = File.createTempFile(PREFIX, SUFFIX);
        tempFile.deleteOnExit();
        try (FileOutputStream out = new FileOutputStream(tempFile)) {
            IOUtils.copy(in, out);
        }
        return tempFile;
    }

}

Answer 2

You can't. The input stream is just a generic stream of data and there is no guarantee that it actually originates from a File. If someone created an InputStream from reading a web service or just converted a String into an InputStream, there would be no way to link this to a file. So the only thing you can do is actually write data from the stream to a temporary file (e.g. using the File.createTempFile method) and feed this file into your method.