2.1 ByteBuffer 正确使用姿势

static {
    final char[] DIGITS = "0123456789abcdef".toCharArray();
    for (int i = 0; i < 256; i++) {
        HEXDUMP_TABLE[i << 1] = DIGITS[i >>> 4 & 0x0F];
        HEXDUMP_TABLE[(i << 1) + 1] = DIGITS[i & 0x0F];
    }
    int i;
    // Generate the lookup table for hex dump paddings
    for (i = 0; i < HEXPADDING.length; i++) {
        int padding = HEXPADDING.length - i;
        StringBuilder buf = new StringBuilder(padding * 3);
        for (int j = 0; j < padding; j++) {
            buf.append("   ");
        }
        HEXPADDING[i] = buf.toString();
    }
    // Generate the lookup table for the start-offset header in each row (up to 64KiB).
    for (i = 0; i < HEXDUMP_ROWPREFIXES.length; i++) {
        StringBuilder buf = new StringBuilder(12);
        buf.append(NEWLINE);
        buf.append(Long.toHexString(i << 4 & 0xFFFFFFFFL | 0x100000000L));
        buf.setCharAt(buf.length() - 9, '|');
        buf.append('|');
        HEXDUMP_ROWPREFIXES[i] = buf.toString();
    }
    // Generate the lookup table for byte-to-hex-dump conversion
    for (i = 0; i < BYTE2HEX.length; i++) {
        BYTE2HEX[i] = ' ' + StringUtil.byteToHexStringPadded(i);
    }
    // Generate the lookup table for byte dump paddings
    for (i = 0; i < BYTEPADDING.length; i++) {
        int padding = BYTEPADDING.length - i;
        StringBuilder buf = new StringBuilder(padding);
        for (int j = 0; j < padding; j++) {
            buf.append(' ');
        }
        BYTEPADDING[i] = buf.toString();
    }
    // Generate the lookup table for byte-to-char conversion
    for (i = 0; i < BYTE2CHAR.length; i++) {
        if (i <= 0x1f || i >= 0x7f) {
            BYTE2CHAR[i] = '.';
        } else {
            BYTE2CHAR[i] = (char) i;
        }
    }
}
/**
 * 打印所有内容
 * @param buffer
 */
public static void debugAll(ByteBuffer buffer) {
    int oldlimit = buffer.limit();
    buffer.limit(buffer.capacity());
    StringBuilder origin = new StringBuilder(256);
    appendPrettyHexDump(origin, buffer, 0, buffer.capacity());
    System.out.println("+--------+-------------------- all ------------------------+----------------+");
    System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), oldlimit);
    System.out.println(origin);
    buffer.limit(oldlimit);
}
/**
 * 打印可读取内容
 * @param buffer
 */
public static void debugRead(ByteBuffer buffer) {
    StringBuilder builder = new StringBuilder(256);
    appendPrettyHexDump(builder, buffer, buffer.position(), buffer.limit() - buffer.position());
    System.out.println("+--------+-------------------- read -----------------------+----------------+");
    System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), buffer.limit());
    System.out.println(builder);
}
private static void appendPrettyHexDump(StringBuilder dump, ByteBuffer buf, int offset, int length) {
    if (isOutOfBounds(offset, length, buf.capacity())) {
        throw new IndexOutOfBoundsException(
                "expected: " + "0 <= offset(" + offset + ") <= offset + length(" + length
                        + ") <= " + "buf.capacity(" + buf.capacity() + ')');
    }
    if (length == 0) {
        return;
    }
    dump.append(
            "         +-------------------------------------------------+" +
                    NEWLINE + "         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |" +
                    NEWLINE + "+--------+-------------------------------------------------+----------------+");
    final int startIndex = offset;
    final int fullRows = length >>> 4;
    final int remainder = length & 0xF;
    // Dump the rows which have 16 bytes.
    for (int row = 0; row < fullRows; row++) {
        int rowStartIndex = (row << 4) + startIndex;
        // Per-row prefix.
        appendHexDumpRowPrefix(dump, row, rowStartIndex);
        // Hex dump
        int rowEndIndex = rowStartIndex + 16;
        for (int j = rowStartIndex; j < rowEndIndex; j++) {
            dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
        }
        dump.append(" |");
        // ASCII dump
        for (int j = rowStartIndex; j < rowEndIndex; j++) {
            dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
        }
        dump.append('|');
    }
    // Dump the last row which has less than 16 bytes.
    if (remainder != 0) {
        int rowStartIndex = (fullRows << 4) + startIndex;
        appendHexDumpRowPrefix(dump, fullRows, rowStartIndex);
        // Hex dump
        int rowEndIndex = rowStartIndex + remainder;
        for (int j = rowStartIndex; j < rowEndIndex; j++) {
            dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
        }
        dump.append(HEXPADDING[remainder]);
        dump.append(" |");
        // Ascii dump
        for (int j = rowStartIndex; j < rowEndIndex; j++) {
            dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
        }
        dump.append(BYTEPADDING[remainder]);
        dump.append('|');
    }
    dump.append(NEWLINE +
            "+--------+-------------------------------------------------+----------------+");
}
private static void appendHexDumpRowPrefix(StringBuilder dump, int row, int rowStartIndex) {
    if (row < HEXDUMP_ROWPREFIXES.length) {
        dump.append(HEXDUMP_ROWPREFIXES[row]);
    } else {
        dump.append(NEWLINE);
        dump.append(Long.toHexString(rowStartIndex & 0xFFFFFFFFL | 0x100000000L));
        dump.setCharAt(dump.length() - 9, '|');
        dump.append('|');
    }
}
public static short getUnsignedByte(ByteBuffer buffer, int index) {
    return (short) (buffer.get(index) & 0xFF);
}