Creating RESTful API's with Spark microframework using Java

REST-API-with-Spark-Microframework.java

import spark.Request;

import javax.servlet.MultipartConfigElement;
import javax.servlet.ServletException;
import javax.servlet.http.Part;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardCopyOption;
import java.util.stream.Collectors;

import static spark.Spark.*;

public class Server {
    private static String STORAGE = "storage";

    public static void main(String[] args) {
        File storageDir = new File(STORAGE);
        if (!storageDir.isDirectory()) storageDir.mkdir();

        post("/upload", (req, res) -> uploadFile(req));
        get("/download/:file", (req, res) -> downloadFile(req.params(":file")));
        get("/count", (req, res) -> countFiles());
        delete("/delete/:file", (req, res) -> deleteFile(req.params(":file")));

    }

    private static String uploadFile(Request request) {
        // TO allow for multipart file uploads
        request.attribute("org.eclipse.jetty.multipartConfig", new MultipartConfigElement(""));

        try {
            // "file" is the key of the form data with the file itself being the value
            Part filePart = request.raw().getPart("file");
            // The name of the file user uploaded
            String uploadedFileName = filePart.getSubmittedFileName();
            InputStream stream = filePart.getInputStream();

            // Write stream to file under storage folder
            Files.copy(stream, Paths.get(STORAGE).resolve(uploadedFileName), 
                       StandardCopyOption.REPLACE_EXISTING);
        } catch (IOException | ServletException e) {
            return "Exception occurred while uploading file" + e.getMessage();
        }

        return "File successfully uploaded";
    }

    private static String downloadFile(String fileName) {
        Path filePath = Paths.get(STORAGE).resolve(fileName);
        File file = filePath.toFile();
        if (file.exists()) {
            try {
                // Read from file and join all the lines into a string
                return Files.readAllLines(filePath).stream().collect(Collectors.joining());
            } catch (IOException e) {
                return "Exception occurred while reading file" + e.getMessage();
            }

        }
        return "File doesn't exist. Cannot download";
    }

    private static int countFiles() {
        // Count the number of files in the storage folder
        return new File(STORAGE).listFiles().length;
    }

    private static String deleteFile(String fileName) {
        File file = Paths.get(STORAGE).resolve(fileName).toFile();
        if (file.exists()) {
            file.delete();
            return "File deleted";
        } else {
            return "File " + fileName + " doesn't exist";
        }
    }

}