My DataBase can't accept conntecion

we have e test db server, we use jdbc to contect.
but after a whie, The test db server can not accept connect.

useps aux | grep postgrescommand :
postgres 16904 0.0 0.0 46036 3948 ? S 17:03 0:00 /usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data
postgres 16906 0.0 0.0 46144 2192 ? Ss 17:03 0:00 postgres: writer process
postgres 16907 0.0 0.0 46036 1088 ? Ss 17:03 0:00 postgres: wal writer process
postgres 16908 0.0 0.0 46840 2072 ? Ss 17:03 0:00 postgres: autovacuum launcher process
postgres 16909 0.0 0.0 14248 1012 ? Ss 17:03 0:00 postgres: stats collector process
postgres 16910 0.0 0.0 46972 2992 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40608) idle
postgres 16911 0.0 0.0 46708 2656 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40609) idle
postgres 16912 0.0 0.0 46708 2628 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40610) idle
postgres 16913 0.0 0.0 46708 2628 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40611) idle
postgres 16914 0.0 0.0 46708 2632 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40612) idle
postgres 16915 0.0 0.0 46708 2636 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40613) idle
postgres 16916 0.0 0.0 46708 2632 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40614) idle
postgres 16917 0.0 0.1 47088 4136 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40615) idle
postgres 16918 0.0 0.1 47060 4236 ? Ss 17:03 0:00 postgres: postgres edu 172.29.0.40(40616) idle
postgres 16919 0.0 0.1 47052 4136 ? Ss 17:03 0:02 postgres: postgres edu 172.29.0.40(40617) idle
postgres 21619 0.0 0.0 46708 2636 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38803) idle
postgres 21620 0.0 0.0 46708 2636 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38804) idle
postgres 21621 0.0 0.0 46708 2640 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38805) idle
postgres 21622 0.0 0.0 46708 2644 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38806) idle
postgres 21623 0.0 0.0 46708 2644 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38807) idle
postgres 21624 0.0 0.0 46708 2636 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38808) idle
postgres 21625 0.0 0.0 46708 2640 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38809) idle
postgres 21626 0.0 0.0 46708 2644 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38810) idle
postgres 21627 0.0 0.0 46708 2640 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38811) idle
postgres 21628 0.0 0.1 47104 4836 ? Ss 19:43 0:00 postgres: postgres edu 172.29.0.40(38812) INSERT
...

kill -9 pidcommand to kill all of them in the past? and the test db server is shut down.
how to analyze the reason of these? and how to solve?