Main.java Source code

Introduction

Here is the source code for Main.java

Source

//package com.java2s;
//License from project: Apache License 

import java.io.IOException;

import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.FileChannel;

public class Main {
    private static final int ZIP_EOCD_REC_MIN_SIZE = 22;
    private static final int ZIP_EOCD_REC_SIG = 0x06054b50;
    private static final int UINT16_MAX_VALUE = 0xffff;
    private static final int ZIP_EOCD_COMMENT_LENGTH_FIELD_OFFSET = 20;

    public static long getCommentLength(final FileChannel fileChannel) throws IOException {
        // End of central directory record (EOCD)
        // Offset    Bytes     Description[23]
        // 0           4       End of central directory signature = 0x06054b50
        // 4           2       Number of this disk
        // 6           2       Disk where central directory starts
        // 8           2       Number of central directory records on this disk
        // 10          2       Total number of central directory records
        // 12          4       Size of central directory (bytes)
        // 16          4       Offset of start of central directory, relative to start of archive
        // 20          2       Comment length (n)
        // 22          n       Comment
        // For a zip with no archive comment, the
        // end-of-central-directory record will be 22 bytes long, so
        // we expect to find the EOCD marker 22 bytes from the end.

        final long archiveSize = fileChannel.size();
        if (archiveSize < ZIP_EOCD_REC_MIN_SIZE) {
            throw new IOException("APK too small for ZIP End of Central Directory (EOCD) record");
        }
        // ZIP End of Central Directory (EOCD) record is located at the very end of the ZIP archive.
        // The record can be identified by its 4-byte signature/magic which is located at the very
        // beginning of the record. A complication is that the record is variable-length because of
        // the comment field.
        // The algorithm for locating the ZIP EOCD record is as follows. We search backwards from
        // end of the buffer for the EOCD record signature. Whenever we find a signature, we check
        // the candidate record's comment length is such that the remainder of the record takes up
        // exactly the remaining bytes in the buffer. The search is bounded because the maximum
        // size of the comment field is 65535 bytes because the field is an unsigned 16-bit number.
        final long maxCommentLength = Math.min(archiveSize - ZIP_EOCD_REC_MIN_SIZE, UINT16_MAX_VALUE);
        final long eocdWithEmptyCommentStartPosition = archiveSize - ZIP_EOCD_REC_MIN_SIZE;
        for (int expectedCommentLength = 0; expectedCommentLength <= maxCommentLength; expectedCommentLength++) {
            final long eocdStartPos = eocdWithEmptyCommentStartPosition - expectedCommentLength;

            final ByteBuffer byteBuffer = ByteBuffer.allocate(4);
            fileChannel.position(eocdStartPos);
            fileChannel.read(byteBuffer);
            byteBuffer.order(ByteOrder.LITTLE_ENDIAN);

            if (byteBuffer.getInt(0) == ZIP_EOCD_REC_SIG) {
                final ByteBuffer commentLengthByteBuffer = ByteBuffer.allocate(2);
                fileChannel.position(eocdStartPos + ZIP_EOCD_COMMENT_LENGTH_FIELD_OFFSET);
                fileChannel.read(commentLengthByteBuffer);
                commentLengthByteBuffer.order(ByteOrder.LITTLE_ENDIAN);

                final int actualCommentLength = commentLengthByteBuffer.getShort(0);
                if (actualCommentLength == expectedCommentLength) {
                    return actualCommentLength;
                }
            }
        }
        throw new IOException("ZIP End of Central Directory (EOCD) record not found");
    }
}