我看着这个：Why does Jenkins say "This Jenkins instance appears to be offline"
我在CentOS Linux版本7.5.1804（核心）上安装了jenkins，它是Mac上的虚拟机。
我尝试修改以下网址：
/var/lib/jenkins/hudson.model.UpdateCenter.xml
三个网址一一对应（我在中国）：
http://updates.jenkins.io/update-center.json
https://updates.jenkins.io/update-center.json
https://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json
<?xml version='1.1' encoding='UTF-8'?>
<sites>
<site>
<id>default</id>
<url>http://updates.jenkins.io/update-center.json</url>
</site>
</sites>
然后我重新启动詹金斯：
⚡ root@centos-linux > service jenkins restart
Restarting jenkins (via systemctl): [ OK ]
但仍处于离线状态，我不知道为什么：
This Jenkins instance appears to be offline.
This Jenkins instance appears to be offline.
防火墙已关闭：
✘ ⚡ root@centos-linux > systemctl status firewalld
● firewalld.service
Loaded: masked (/dev/null; bad)
Active: inactive (dead)
Jan 22 16:44:01 localhost.localdomain systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Jan 22 16:44:01 localhost.localdomain systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Jan 22 16:44:01 localhost.localdomain systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Jan 22 16:51:09 centos-linux.shared systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Jan 22 16:52:38 centos-linux.shared systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Jan 22 16:52:50 centos-linux.shared systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Jan 22 16:55:37 centos-linux.shared systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Jan 22 16:57:26 centos-linux.shared systemd[1]: Cannot add dependency job for unit firewalld.service, ignoring: Unit is masked.
Warning: firewalld.service changed on disk. Run 'systemctl daemon-reload' to reload units.
并且未安装iptables：
⚡ root@centos-linux > service iptables status
Redirecting to /bin/systemctl status iptables.service
Unit iptables.service could not be found.
Java版本是新的：
✘ ⚡ root@centos-linux > which java
/bin/java
⚡ root@centos-linux > /bin/java -version
openjdk version "1.8.0_191"
OpenJDK Runtime Environment (build 1.8.0_191-b12)
OpenJDK 64-Bit Server VM (build 25.191-b12, mixed mode)
我可以得到这三个update-center.json：
⚡ root@centos-linux > wget http://updates.jenkins.io/update-center.json
--2019-01-22 17:15:05-- http://updates.jenkins.io/update-center.json
Resolving updates.jenkins.io (updates.jenkins.io)... 52.202.51.185
Connecting to updates.jenkins.io (updates.jenkins.io)|52.202.51.185|:80... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
Location: http://updates.jenkins.io/current/update-center.json [following]
--2019-01-22 17:15:06-- http://updates.jenkins.io/current/update-center.json
Reusing existing connection to updates.jenkins.io:80.
HTTP request sent, awaiting response... 302 Found
Location: http://mirrors.jenkins-ci.org/updates/current/update-center.json [following]
--2019-01-22 17:15:06-- http://mirrors.jenkins-ci.org/updates/current/update-center.json
Resolving mirrors.jenkins-ci.org (mirrors.jenkins-ci.org)... 52.202.51.185
Reusing existing connection to updates.jenkins.io:80.
HTTP request sent, awaiting response... 302 Found
Location: http://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json [following]
--2019-01-22 17:15:06-- http://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json
Resolving mirrors.tuna.tsinghua.edu.cn (mirrors.tuna.tsinghua.edu.cn)... 101.6.8.193, 2402:f000:1:408:8100::1
Connecting to mirrors.tuna.tsinghua.edu.cn (mirrors.tuna.tsinghua.edu.cn)|101.6.8.193|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1617663 (1.5M) [application/json]
Saving to: 'update-center.json.8'
100%[===============================================================================================>] 1,617,663 309KB/s in 4.6s
2019-01-22 17:15:11 (344 KB/s) - 'update-center.json.8' saved [1617663/1617663]
⚡ root@centos-linux > wget https://updates.jenkins.io/update-center.json
--2019-01-22 17:08:15-- https://updates.jenkins.io/update-center.json
Resolving updates.jenkins.io (updates.jenkins.io)... 52.202.51.185
Connecting to updates.jenkins.io (updates.jenkins.io)|52.202.51.185|:443... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
Location: https://updates.jenkins.io/current/update-center.json [following]
--2019-01-22 17:08:17-- https://updates.jenkins.io/current/update-center.json
Reusing existing connection to updates.jenkins.io:443.
HTTP request sent, awaiting response... 200 OK
Length: 1617663 (1.5M) [application/json]
Saving to: 'update-center.json.5'
100%[===============================================================================================>] 1,617,663 318KB/s in 5.5s
2019-01-22 17:08:23 (290 KB/s) - 'update-center.json.5' saved [1617663/1617663]
⚡ root@centos-linux > wget https://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json
--2019-01-22 17:08:34-- https://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json
Resolving mirrors.tuna.tsinghua.edu.cn (mirrors.tuna.tsinghua.edu.cn)... 101.6.8.193, 2402:f000:1:408:8100::1
Connecting to mirrors.tuna.tsinghua.edu.cn (mirrors.tuna.tsinghua.edu.cn)|101.6.8.193|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1617663 (1.5M) [application/json]
Saving to: 'update-center.json.6'
100%[===============================================================================================>] 1,617,663 281KB/s in 4.6s
2019-01-22 17:08:39 (342 KB/s) - 'update-center.json.6' saved [1617663/1617663]
是的，我只有java（刚刚通过yum安装）：
⚡ root@centos-linux > which java
/bin/java
⚡ root@centos-linux > whereis java
java: /usr/bin/java /usr/lib/java /etc/java /usr/share/java /usr/share/man/man1/java.1.gz
⚡ root@centos-linux >
⚡ root@centos-linux > l /bin/java
lrwxrwxrwx 1 root root 22 Jan 22 16:06 /bin/java -> /etc/alternatives/java
⚡ root@centos-linux >
⚡ root@centos-linux > l /usr/bin/java
lrwxrwxrwx 1 root root 22 Jan 22 16:06 /usr/bin/java -> /etc/alternatives/java
⚡ root@centos-linux >
⚡ root@centos-linux > diff /bin/java /usr/bin/java
我还在~/.zshrc（我正在使用zsh shell）中添加JAVA_HOME：
export JAVA_HOME=/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.191.b12-1.el7_6.x86_64
export JRE_HOME=$JAVA_HOME/jre
export CLASSPATH=$JAVA_HOME/lib:$JRE_HOME/lib:$CLASSPATH
export PATH=$HOME/bin:/usr/local/bin:$JAVA_HOME/bin:$JRE_HOME/bin:$PATH
我已经知道了来源，没错：
source ~/.zshrc
JAVA_HOME路径：
⚡ root@centos-linux > l /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.191.b12-1.el7_6.x86_64
total 4.0K
drwxr-xr-x 3 root root 16 Jan 22 16:06 .
drwxr-xr-x 3 root root 4.0K Jan 22 16:06 ..
drwxr-xr-x 4 root root 26 Jan 22 16:06 jre
所以，我真的不知道如何在线制作??