Question

I have an html-form with a field that will take a url pointing to a (possibly binary) file (e.g. an image). The url can point to a local resource that the server can’t reach, so it needs to be fetched by the client. (This fetching is another point, my question is about writing the servlet.)

I want to upload the result of this fetch to a Java servlet (through a http POST request) and put it into an array of bytes. Since this is a low-level representation, I’m not sure if I should use Apache’s FileUpload.

Another point that confused me, is that FileItems in FileUpload only provide an OutputStream where I was expecting an InputStream to read the file.

I’m stuck after studying FileUpload. I may be that my basic knowledge of web-app development falls short.

How should I build this functionality? Please note that I’m new to this corner of Java development (although my general (i.e. SE) Java skills are very good).

Answer 1

Just keep using FileUpload, don’t reinvent it yourself. This will be a waste of time.

I’m not sure why you think that FileItem only provides an OutputStream, there’s also an getInputStream() method. Just write it to a ByteArrayOutputStream.

InputStream input = fileItem.getInputStream();
ByteArrayOutputStream output = new ByteArrayOutputStream();
IOUtils.copy(input, output);
byte[] bytes = output.toByteArray();
// ...

You only need to keep in mind that this may explode your server’s memory usage if there are a lot of users uploading files at the same time. Each byte of a byte array eats one byte of JVM’s memory. So imagine you’ve 100 simultaneous users who upload each a 10MB file, then 1GB of server memory is wasted to this.