Unzipping a file from InputStream and returning another InputStream in Java

Question

I am trying to write a function which will accept an InputStream with Zip file data and would return another InputStream with unzipped data.

The Zip file will only contain a single file and thus there is no requirement of creating directories, etc.

I tried looking at ZipInputStream and others but I am confused by so many different types of streams in Java.

Answer 1

Unless I'm missing something, you should absolutely try and get ZipInputStream to work and there's no reason it shouldn't (I've certainly used it on several occasions).

What you should do is try and get ZipInputStream to work and if you can't, post the code and we'll help you with whatever problems you're having.

Whatever you do though, don't try and reinvent its functionality.

Answer 2

GZipinputstream is for streams (or files) ziped as gzip (".gz" extension). It doesn't have any header information.

If you have a real zip file, you have to user ZipFile to open the file, ask for the list of files (one in your example) and ask for the decompressed input stream.

Your method would be something like:

// ITS PSEUDOCODE!!

private InputStream extractOnlyFile(String path) {
   ZipFile zf = new ZipFile(path);
   Enumeration e = zf.entries();
   ZipEntry entry = (ZipEntry) e.nextElement(); // your only file
   return zf.getInputStream(entry);
}

EDIT:

Ok, if you have an inputstream you can use (as cletus says) ZipInputStream. The code would be something like:

// ITS PSEUDOCODE!!

private InputStream extractOnlyFile(InputStream input) {
   ZipInputStream zipinput = new ZipInputStream(input);
   ZipEntry entry = zipinput.getNextEntry(); // reads the header info and position in the begining of the file
   return zipinput;
}

EDIT: working example

Important: if you have the file in your PC you can use ZipFile class to access it randomly

This is a test I've made now in my PC:

import java.io.FileInputStream;
import java.util.zip.ZipEntry;
import java.util.zip.ZipInputStream;


public class Main {
    public static void main(String[] args) throws Exception
    {
        FileInputStream fis = new FileInputStream("c:/inas400.zip");
        ZipInputStream zis = new ZipInputStream(fis);
        ZipEntry entry;
            // while there are entries I process them
        while ((entry = zis.getNextEntry()) != null)
        {
            System.out.println("entry: " + entry.getName() + ", " + entry.getSize());
                    // consume all the data from this entry
            while (zis.available() > 0)
                zis.read();
                    // I could close the entry, but getNextEntry does it automatically
                    // zis.closeEntry()
        }
    }
}

Answer 3

If you can change the input data I would suggested you to use GZIPInputStream.

GZipInputStream is different from ZipInputStream since you only have one data inside it. So the whole input stream represents the whole file. In ZipInputStream the whole stream contains also the structure of the file(s) inside it, which can be many.