Split ~200mb log4j log file by day - bash

Question

I have a log file formatted as follows and I want to split it into multiple files by day (ie. log-2017-10-2, log-2017-10-3 etc). I've seen people do it with awk but I'm not sure how to handle stack traces because java.io.Exception is a new line. Is there any convenient way to achieve this?
2017-10-02 04:26:02,534 INFO XXXXXXXXXXXXXXXXX
2017-10-03 04:26:02,543 INFO XXXXXXXXXXXX
2017-10-04 04:26:02,544 INFO XXXXXXXXX
2017-10-04 04:26:02,546 INFO XXXXXXXXXXXXX
2017-10-04 04:26:02,549 INFO XXXXXXXXXXX
2017-10-04 04:53:02,787 WARN class.class.class: [FetcherXXXXXX], Error in fetch XXXXXXXXXXXXXXXXXXXXXX
java.io.IOException: Connection to X was disconnected before the response was read
at XXXXXXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXX
2017-10-05 04:26:02,549 INFO XXXXXXXXXXX
Final file contents will be:
log-2017-10-2:
2017-10-02 04:26:02,534 INFO XXXXXXXXXXXXXXXXX
log-2017-10-3:
2017-10-03 04:26:02,543 INFO XXXXXXXXXXXX
log-2017-10-4:
2017-10-04 04:26:02,544 INFO XXXXXXXXX
2017-10-04 04:26:02,546 INFO XXXXXXXXXXXXX
2017-10-04 04:26:02,549 INFO XXXXXXXXXXX
2017-10-04 04:53:02,787 WARN class.class.class: [FetcherXXXXXX], Error in fetch XXXXXXXXXXXXXXXXXXXXXX
java.io.IOException: Connection to X was disconnected before the response was read
at XXXXXXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXX
at XXXXXXXXXXXXXXXX
log-2017-10-5:
2017-10-05 04:26:02,549 INFO XXXXXXXXXXX

Answer 1

awk to the rescue!
$ awk --posix 'BEGIN{f="log-header"}
$1~/^[0-9]{4}-[0-9]{2}-[0-9]{2}$/{f="log-"$1} {print > f}' log
if there are too many dates (corresponding to too many open files) you may need to close files at one point. For few hundred it should work as is.
The initial log file (log-header) is set in case your log doesn't start with the checked regex.

Answer 2

awk solution:
awk '/^[0-9]{4}-[0-9]{2}-[0-9]{2} /{
if (fn && !a[$1]++) close(fn);
fn="log-"$1
}{ print > fn }' logfile
/^[0-9]{4}-[0-9]{2}-[0-9]{2} / - on encountering line starting with date string
if(fn && !a[$1]++) close(fn) - close the previous opened file descriptor for the previous "date"
fn="log-"$1 - constructing filename
Viewing results:
$ head log-*
==> log-2017-10-02 <==
2017-10-02 04:26:02,534 INFO XXXXXXXXXXXXXXXXX
==> log-2017-10-03 <==
2017-10-03 04:26:02,543 INFO XXXXXXXXXXXX
==> log-2017-10-04 <==
2017-10-04 04:26:02,544 INFO XXXXXXXXX
2017-10-04 04:26:02,546 INFO XXXXXXXXXXXXX
2017-10-04 04:26:02,549 INFO XXXXXXXXXXX
2017-10-04 04:53:02,787 WARN class.class.class: [FetcherXXXXXX], Error in fetch XXXXXXXXXXXXXXXXXXXXXX
java.io.IOException: Connection to X was disconnected before the response was read
&XXXXXXXXXXXXXXXXXXXX
&XXXXXXXXXXXXXXXXXXXX
&XXXXXXXXXXXXXXXXXXXXX
&XXXXXXXXXXXXXXXX
&XXXXXXXXXXXXXXXX
==> log-2017-10-05 <==
2017-10-05 04:26:02,549 INFO XXXXXXXXXXX