I was not able to find a Java build-in solution to read the file comment of a ZIP.
Reading the comment of a ZipEntry can easily be done by invoking ZipEntry.getComment(), but you cannot read the global archive comment of the ZIP file with it.
Therefore, I have implemented the following solution. It reads n bytes from the end of the ZIP file and searches (backward) for the magic sequence that indicates the end of the ZIP contents.
The ZIP file comment can be found 22 bytes after the beginning of that magic sequence…
public static String extractZipComment (String filename) {
String retStr = null;
try {
File file = new File(filename);
int fileLen = (int)file.length();
FileInputStream in = new FileInputStream(file);
/* The whole ZIP comment (including the magic byte sequence)
* MUST fit in the buffer
* otherwise, the comment will not be recognized correctly
*
* You can safely increase the buffer size if you like
*/
byte[] buffer = new byte[Math.min(fileLen, 8192)];
int len;
in.skip(fileLen - buffer.length);
if ((len = in.read(buffer)) > 0) {
retStr = getZipCommentFromBuffer (buffer, len);
}
in.close();
} catch (Exception e) {
e.printStackTrace();
}
return retStr;
}
private static String getZipCommentFromBuffer (byte[] buffer, int len) {
byte[] magicDirEnd = {0x50, 0x4b, 0x05, 0x06};
int buffLen = Math.min(buffer.length, len);
// Check the buffer from the end
for (int i = buffLen-magicDirEnd.length-22; i >= 0; i--) {
boolean isMagicStart = true;
for (int k=0; k < magicDirEnd.length; k++) {
if (buffer[i+k] != magicDirEnd[k]) {
isMagicStart = false;
break;
}
}
if (isMagicStart) {
// Magic Start found!
int commentLen = buffer[i+20] + buffer[i+21]*256;
int realLen = buffLen - i - 22;
System.out.println ("ZIP comment found at buffer position " + (i+22) + " with len="+commentLen+", good!");
if (commentLen != realLen) {
System.out.println ("WARNING! ZIP comment size mismatch: directory says len is "+
commentLen+", but file ends after " + realLen + " bytes!");
}
String comment = new String (buffer, i+22, Math.min(commentLen, realLen));
return comment;
}
}
System.out.println ("ERROR! ZIP comment NOT found!");
return null;
}
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Tags: Java
