Upload component (from Vaadin 7.1.13) that remembers the files it has uploaded and asks, in case a duplicate file is uploaded, if we wish to overwrite it.

RestrictingUpload

import com.vaadin.ui.Upload;
import org.apache.commons.io.IOUtils;
import org.vaadin.dialogs.ConfirmDialog;

import java.io.ByteArrayOutputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;

/**
 * Upload component that remembers the files it has uploaded and asks, in case a duplicate file is uploaded, if
 * we wish to overwrite it.
 *
 * Uses Apache Commons IO 2.4 (http://mvnrepository.com/artifact/commons-io/commons-io/2.4)
 * and the ConfirmDialog addon from Vaadin Directory (https://vaadin.com/directory#addon/confirmdialog)
 *
 * @author Ruiu Gabriel Mihai (gabriel.ruiu@mail.com)
 */
public class RestrictingUpload extends Upload implements Upload.SucceededListener, Upload.Receiver {

    private List<String> uploadedFilenames;
    private ByteArrayOutputStream latestUploadedOutputStream;

    public RestrictingUpload() {
        setCaption("Upload");
        setButtonCaption("Upload file");
        addSucceededListener(this);
        setReceiver(this);
        uploadedFilenames = new ArrayList<String>();
    }

    @Override
    public OutputStream receiveUpload(String filename, String mimeType) {
        latestUploadedOutputStream = new ByteArrayOutputStream();
        return latestUploadedOutputStream;
    }

    @Override
    public void uploadSucceeded(SucceededEvent event) {
        if (fileExistsInSystem(event.getFilename())) {
            confirmOverwrite(event.getFilename());
        } else {
            uploadedFilenames.add(event.getFilename());
        }
    }

    private void confirmOverwrite(final String filename) {
        ConfirmDialog confirmDialog = new ConfirmDialog();
        String message = String.format("The file %s already exists in the system. Overwrite?", filename);
        confirmDialog.show(getUI(), "Overwrite?", message, "Overwrite", "Cancel", new ConfirmDialog.Listener() {
            @Override
            public void onClose(ConfirmDialog dialog) {
                if (dialog.isConfirmed()) {
                    copyFileToSystem(filename);
                }
            }
        });
    }

    private void copyFileToSystem(String filename) {
        try {
            IOUtils.write(latestUploadedOutputStream.toByteArray(), new FileOutputStream(filename));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e2) {
            e2.printStackTrace();
        }
    }

    private boolean fileExistsInSystem(String filename) {
        return uploadedFilenames.contains(filename);
    }
}