java - Losing content in InputStream after performing operation

Question

I am performing data check using an external scan tool. In the process I pass the InputStream into the scan tool. The tool scans the stream and respond back with a boolean value. Using this boolean value I will decide If I would want to save the stream to a file or not. During the process the tool will not reset the inputstream, and it will be left in a EOF-state. Which makes it unusable. And hence the file I am creating will have 0 Bytes.

There is no way I can alter the tool that is doing this. So I would need to find out a way not to affect the InputStream content. But at the same time perform scan and writing of the Input Stream.

Here is my code,

    FileOutputStream fos = null;
    ReadableByteChannel rbc = null;
    try {
        log.info(masterId + " : DOWNLOADING FILE FROM [ " + url + " ]");

        HttpURLConnection httpcon = (HttpURLConnection) urlConnection;
        //rbc = Channels.newChannel(httpcon.getInputStream());  -- this was my another approach
        //InputStream inputStream = Channels.newInputStream(rbc);
        if (!scanTool.scan(IOUtils.toByteArray(httpcon.getInputStream()))) {
            // exit with fail code
        }
        rbc = Channels.newChannel(httpcon.getInputStream());
        fos = new FileOutputStream(filePath);
        fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
        fos.close();
        rbc.close();
        return 1;
    } catch (FileNotFoundException ex) {
        //Handle exception
    }

In the above code snippet,

        if (!scanTool.scan(IOUtils.toByteArray(httpcon.getInputStream()))) {
            // exit with fail code
        }

This is my new insertion. I am calling the scanTool to perform an operation to check a condition, only if it passes then go ahead and write the file. Now while doing this I am losing the content in the inputStream.

What am i missing here. I even tried using BufferedInputSteam. Still not working out. also tried the another approach where I construct new readable byte channel and then construct new InputStream out of it. As shown in the commented code. But no fruit.

question from:https://stackoverflow.com/questions/65847446/losing-content-in-inputstream-after-performing-operation

Answer 1

Given that you have already converted the InputStream to byte[], you can just write out the same byte array after the scan:

byte[] bytes = IOUtils.toByteArray(httpcon.getInputStream());
if (!scanTool.scan(bytes)) {
        // exit with fail code
}
try(FileOutputStream fos = new FileOutputStream(filePath)) {
    fos.write(bytes);
}