这是因为MySQL对于BLOB类型的数据默认使用了UTF-8编码，而UTF-8编码是一种变长编码。因此，解决此问题的方法是在将Byte数组写入MySQL前，将其转换为字符集为latin1的字符串，这样可以保证长度不再发生变化。具体代码实现如下：

// byte数组转为latin1编码的字符串
public static String latin1ByteArrayToString(byte[] bytes) {
    Charset latin1Charset = Charset.forName("ISO-8859-1");
    CharBuffer buffer = latin1Charset.decode(ByteBuffer.wrap(bytes));
    return buffer.toString();
}

// latin1编码的字符串转为byte数组
public static byte[] latin1StringToByteArray(String str) {
    Charset latin1Charset = Charset.forName("ISO-8859-1");
    ByteBuffer buffer = latin1Charset.encode(CharBuffer.wrap(str));
    byte[] bytes = new byte[buffer.limit()];
    buffer.get(bytes);
    return bytes;
}

// 将byte数组写入MySQL
public static void writeByteArrayToMySQL(byte[] bytes) {
    try {
        String latin1Str = latin1ByteArrayToString(bytes);
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/mydb", "user", "password");
        PreparedStatement ps = conn.prepareStatement("INSERT INTO mytable (mycolumn) VALUES (?)");
        ps.setString(1, latin1Str);
        ps.executeUpdate();
    } catch (SQLException e) {
        e.printStackTrace();
    }
}

// 从MySQL读取byte数组
public static byte[] readByteArrayFromMySQL() {
    try {
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/mydb", "user", "password");
        PreparedStatement ps = conn.prepareStatement("SELECT mycolumn FROM mytable WHERE id = ?");
        ps.setInt(1, 1);
        ResultSet rs = ps.executeQuery();
        if (rs.next()) {
            String latin1Str = rs.getString("mycolumn");
            return latin1StringToByteArray(latin1Str);
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return null;
}