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I want to download multiple zip files as one zip file for a request. I have zip file paths like C, https://test12.zip etc. So how can I download these files as a one zip file. I have been searching this for a while. All i got is examples for downloading multiple files(local) and zip them. This is what i tried for downloading one file. For multiple files it won't work.

URL url = new URL("https://test12.zip");
URLConnection connection = url.openConnection();
InputStream stream = connection.getInputStream();
BufferedOutputStream outs = new BufferedOutputStream(response.getOutputStream());

        int len;
        byte[] buf = new byte[1024];
        while ((len = stream.read(buf)) > 0) {
            outs.write(buf, 0, len);
        }
        outs.close();

Any help would be much appreciated.

sparrow
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  • can't you just download all files and extract them and then make a large zip out of them? – Japu_D_Cret Mar 31 '17 at 13:37
  • `https://test12.zip` isn't valid. Your question is unclear. Are you asking to download `a.zip` and `b.zip` as a single file like `ab.zip`? You can't do that. You can't download what doesn't exist. The server must have a pre-existing zip file containing all contents. You'll have to download them individually and combining them into a single archive. – Christopher Schneider Mar 31 '17 at 18:59

1 Answers1

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A ZIP file consists of two parts: First the compressed file entries (filename, attributes and data) and at the end of the file there is a central directory containing a list of all entries, again with filename and attributes.

Hence, you can not directly combine or concatenate zip files. In Java you can only decompress the downloaded zip files on-the-fly (without storing them in the file-system) and at the same time using the decompressed content to create a new combined ZIP file:

First create a ZipOutputStream for the zip file you want to create.

Then use the InputStream of each download and use it with a ZipInputStream. Iterates through all the entries in every ZipInputStream and for each entry create a new identical entry in the ZipOutputStream and copy the content from the ZipInputStream to the ZipOutputStream.

How to use ZipInputStream see for example: https://stackoverflow.com/a/36648504/150978

Note that this process requires to decompress and afterwards re-compress the file content. Depending on the archive size this can result in a high utilization of one CPU core.

JMax
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