nmap show port is closed even though I opened it - centos

Question

My VPS is running on CentOS 7.2 , I opened a port by firewall-cmd --zone=public --add-port=8006/tcp --permanent and have already type the firewall-cmd --reload command, but when I check the port by nmap, nmap -p 8006 ip-addressxxx, it still shows it is closed. Here is some information may help:
[root#localhost ~]# systemctl status firewalld
● firewalld.service - firewalld - dynamic firewall daemon
Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled)
Active: active (running) since Fri 2017-04-07 02:06:50 EDT; 3 days ago
Docs: man:firewalld(1)
Main PID: 663 (firewalld)
CGroup: /system.slice/firewalld.service
└─663 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid
Apr 07 02:06:50 localhost.localdomain systemd[1]: Starting firewalld - dynamic firewall daemon...
Apr 07 02:06:50 localhost.localdomain systemd[1]: Started firewalld - dynamic firewall daemon.
Apr 10 02:03:42 localhost.localdomain firewalld[663]: ERROR: ALREADY_ENABLED: 80:tcp
Apr 10 02:03:49 localhost.localdomain firewalld[663]: ERROR: ALREADY_ENABLED: 8006:tcp
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[root#localhost ~]# firewall-cmd --list-all
public (active)
target: default
icmp-block-inversion: no
interfaces: ens3
sources:
services: dhcpv6-client ssh
ports: 8009/tcp 80/tcp 8080/tcp 8006/tcp
protocols:
masquerade: no
forward-ports:
sourceports:
icmp-blocks:
rich rules:
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[root#localhost ~]# firewall-cmd --list-ports
8009/tcp 80/tcp 8080/tcp 8006/tcp
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[root#localhost ~]# netstat -plunt
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address
State PID/Program name
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 992/sshd
tcp6 0 0 :::8009 :::* LISTEN 1027/java
tcp6 0 0 :::3306 :::* LISTEN 1383/mysqld
tcp6 0 0 :::80 :::* LISTEN 1027/java
tcp6 0 0 :::22 :::* LISTEN 992/sshd
tcp6 0 0 127.0.0.1:8006 :::* LISTEN 1027/java

Answer 1

Revisited my answer
The process you have listening on port 8006 is only listening on the loopback interface, 127.0.0.1, it should be listening on 0.0.0.0. See the sshd process in your process list 0.0.0.0:22 it works fine.
Use something like netcat to test. This will open a port on 8006 on the 0.0.0.0 interface, which is open to the world because of your firewall rules
On your VPS Try:
nc -l 8006
and then scan with nmap again and you will see the port is open, provided your firewall rules are in place.
You want to see this in the process list
tcp6 0 0 0.0.0.0:8006 :::* LISTEN 1027/java
and not
tcp6 0 0 127.0.0.1:8006 :::* LISTEN 1027/java