stanfordnlp/CoreNLP · error · RuntimeIOException

Resource or file looks like a gzip file, but is not

Error message

Resource or file looks like a gzip file, but is not: ${textFileOrUrl}

What it means

If the resource name ends with .gz, IOUtils wraps the opened stream in a GZIPInputStream. If the stream is not actually gzip-compressed, wrapping throws and the method rethrows RuntimeIOException 'Resource or file looks like a gzip file, but is not: <name>'.

Solutions

1. Verify the file really is gzip: run `file x.gz` or `gunzip -t x.gz`; re-download/recreate if corrupt.
2. Rename the file to drop .gz if it's actually uncompressed.
3. Remove the .gz suffix from the name passed to IOUtils so it won't try to ungzip.
4. Check the download step's HTTP status; don't save error pages as .gz.

Example fix

// before
InputStream in = IOUtils.getInputStreamFromURLOrClasspathOrFileSystem("model.ser.gz"); // not gzip
// after
File f = new File("model.ser.gz");
try (java.util.zip.GZIPInputStream test = new java.util.zip.GZIPInputStream(new java.io.FileInputStream(f))) {
  // ok: really gzip
}
InputStream in = IOUtils.getInputStreamFromURLOrClasspathOrFileSystem(f.getPath());

try (InputStream test = new FileInputStream(f)) {
  byte[] magic = test.readNBytes(2);
  if (f.getName().endsWith(".gz") && !(magic[0] == 0x1f && magic[1] == (byte)0x8b)) {
    throw new IllegalStateException(f + " has .gz name but is not gzip");
  }
}

boolean isRealGzip(File f) throws IOException {
  try (InputStream in = new FileInputStream(f)) {
    int b0 = in.read(), b1 = in.read();
    return b0 == 0x1f && b1 == 0x8b;
  }
}

try {
  InputStream in = IOUtils.getInputStreamFromURLOrClasspathOrFileSystem(name);
} catch (RuntimeIOException e) {
  if (e.getMessage().contains("looks like a gzip file")) {
    log.severe("Corrupt/mislabeled .gz: " + e.getMessage());
  }
}

When it happens

Trigger: Opening a file whose name ends in .gz but whose content is plain text, an HTML error page (e.g. failed download), a truncated/corrupt gzip file, or already-decompressed data left with a .gz suffix.

Common situations: Download interrupted or proxy returned an HTML 404 page saved as models.ser.gz; a preprocessing step gunzipped in place without renaming; copy truncated by disk full.

Related errors

• Could not open jar file:
• Could not read from double initial LOP scales file
• Could not read from double initial weight file
• Could not read from float initial weight file
• argsToProperties could not read properties file: " + file

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/6ce120cb36759df1. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/io/IOUtils.java:512

        try {
          // Maybe this happens to be some other format of URL?
          URL u = new URL(textFileOrUrl);
          URLConnection uc = u.openConnection();
          in = uc.getInputStream();
        } catch (IOException e2) {
          // Don't make the original exception a cause, since it is usually bogus
          throw new IOException("Unable to open \"" +
                  textFileOrUrl + "\" as " + "class path, filename or URL"); // , e2);
        }
      }
    }

    // If it is a GZIP stream then ungzip it
    if (textFileOrUrl.endsWith(".gz")) {
      try {
        in = new GZIPInputStream(in);
      } catch (Exception e) {
        throw new RuntimeIOException("Resource or file looks like a gzip file, but is not: " + textFileOrUrl, e);
      }
    }

    // buffer this stream.  even gzip streams benefit from buffering,
    // such as for the shift reduce parser [cdm 2016: I think this is only because default buffer is small; see below]
    in = new BufferedInputStream(in);

    return in;
  }


  // todo [cdm 2015]: I think GZIPInputStream has its own buffer and so we don't need to buffer in that case.
  // todo: Though it's default size is 512 bytes so need to make 8K in constructor. Or else buffering outside gzip is faster
  // todo: final InputStream is = new GZIPInputStream( new FileInputStream( file ), 65536 );
  /**
   * Quietly opens a File. If the file ends with a ".gz" extension,
   * automatically opens a GZIPInputStream to wrap the constructed
   * FileInputStream.

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