Best way to detect if a stream is zipped in Java

Question

What is the best way to find out i java.io.InputStream contains zipped data?

Answer 1

Not very elegant, but reliable:

If the Stream can be read via ZipInputStream, it should be zipped.

Answer 2

The magic bytes for the ZIP format are 50 4B. You could test the stream (using mark and reset - you may need to buffer) but I wouldn't expect this to be a 100% reliable approach. There would be no way to distinguish it from a US-ASCII encoded text file that began with the letters PK.

The best way would be to provide metadata on the content format prior to opening the stream and then treat it appropriately.

Answer 3

You could check that the first four bytes of the stream are the local file header signature that starts the local file header that proceeds every file in a ZIP file, as shown in the spec here to be 50 4B 03 04.

A little test code shows this to work:

byte[] buffer = new byte[4];

try {
    ZipOutputStream zos = new ZipOutputStream(new FileOutputStream("so.zip"));
    ZipEntry ze = new ZipEntry("HelloWorld.txt");
    zos.putNextEntry(ze);
    zos.write("Hello world".getBytes());
    zos.close();

    FileInputStream is = new FileInputStream("so.zip");
    is.read(buffer);
    is.close();
}
catch(IOException e) {
    e.printStackTrace();
}

for (byte b : buffer) { 
    System.out.printf("%H ",b);
}

Gave me this output:

50 4B 3 4