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static final int BUFFER = 512;
static final int TOOBIG = 0x6400000; // max size of unzipped data, 100MB
static final int TOOMANY = 1024;     // max number of files
// ...

private String validateFilename(String filename, String intendedDir) {
  File f = new File(filename);
  String canonicalPath = f.getCanonicalPath(); 

  File iD = new File(intendedDir);
  String canonicalID = iD.getCanonicalPath();
  
  if (canonicalPath.startsWith(canonicalID)) {
    return canonicalPath;
  } else {
    throw new IllegalStateException("File is outside extraction target directory.");
  }
}

public final void unzip(String filename) throws java.io.IOException{
  FileInputStream fis = new FileInputStream(filename);
  ZipInputStream zis = new ZipInputStream(new BufferedInputStream(fis));
  ZipEntry entry;
  int entries = 0;
  try {
    while ((entry = zis.getNextEntry()) != null) {
      System.out.println("Extracting: " + entry);
      int count;
      byte data[] = new byte[BUFFER]; // Write the files to the disk, but ensure that the file is not insanely big
      int total = 0;
      String name = validateFilename(entry.getName(), ".");
      FileOutputStream fos = new FileOutputStream(name);
      BufferedOutputStream dest = new BufferedOutputStream(fos, BUFFER);
      while (total <= TOOBIG && (count = zis.read(data, 0, BUFFER)) != -1) {
        dest.write(data, 0, count);
        total += count;
      }
      dest.flush();
      dest.close();
      zis.closeEntry();
      entries++;
      if (entries > TOOMANY) {
        throw new IllegalStateException("Too many files to unzip.");
      }
      if (total > TOOBIG) {
        throw new IllegalStateException("File being unzipped is too big.");
      }
    }
  } finally {
    zis.close();
  }
}