(1).安裝或升級java，並配置環境變量
注意：此處分為yum安裝與rpm安裝，區別在於yum安裝省事但配置麻煩，rpm安裝麻煩但配置省事。
1）yum安裝
在安裝前可以使用yum list installed | grep jdk查看已經安裝的JDK文件
[root@xuexi ~]# yum list installed | grep jdk copy-jdk-configs.noarch 3.3-10.el7_5 @updates java-1.8.0-openjdk.x86_64 1:1.8.0.212.b04-0.el7_6 @updates java-1.8.0-openjdk-headless.x86_64 1:1.8.0.212.b04-0.el7_6 @updates
再使用yum list | grep java-1.8查看1.8版本的實際安裝包
[root@xuexi ~]$ yum list | grep java-1.8.0 java-1.8.0-openjdk.x86_64 1:1.8.0.212.b04-0.el7_6 @updates java-1.8.0-openjdk-headless.x86_64 1:1.8.0.212.b04-0.el7_6 @updates java-1.8.0-openjdk.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-accessibility.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-accessibility.x86_64 java-1.8.0-openjdk-accessibility-debug.i686 java-1.8.0-openjdk-accessibility-debug.x86_64 java-1.8.0-openjdk-debug.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-debug.x86_64 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-demo.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-demo.x86_64 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-demo-debug.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-demo-debug.x86_64 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-devel.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-devel.x86_64 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-devel-debug.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-devel-debug.x86_64 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-headless.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-headless-debug.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-headless-debug.x86_64 java-1.8.0-openjdk-javadoc.noarch 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-javadoc-debug.noarch java-1.8.0-openjdk-javadoc-zip.noarch 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-javadoc-zip-debug.noarch java-1.8.0-openjdk-src.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-src.x86_64 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-src-debug.i686 1:1.8.0.212.b04-0.el7_6 updates java-1.8.0-openjdk-src-debug.x86_64 1:1.8.0.212.b04-0.el7_6 updates
差太多了，我這里使用yum -y install java-1.8*安裝下。
在使用yum安裝時是沒有辦法指定文件存放在哪里的，所以jdk文件也不清楚在哪里，環境變量配置上就有一定的麻煩，這一點就沒有rpm好。
我們使用which java看看java腳本最終指向何處
[root@xuexi ~]$ which java /usr/bin/java [root@xuexi ~]$ ll /usr/bin/java lrwxrwxrwx. 1 root root 22 5月 5 16:17 /usr/bin/java -> /etc/alternatives/java [root@xuexi ~]$ ll /etc/alternatives/java lrwxrwxrwx. 1 root root 73 5月 5 16:17 /etc/alternatives/java -> /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64/jre/bin/java [root@xuexi ~]$ ll /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64/jre/bin/java -rwxr-xr-x. 1 root root 7376 4月 18 01:18 /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64/jre/bin/java
這里和Windows就很類似了，我們看看有沒有必要的文件
[root@xuexi ~]# ls /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64 bin demo include jre lib sample src.zip tapset [root@xuexi ~]# ls /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64/lib amd64 dt.jar jconsole.jar orb.idl tools.jar ct.sym ir.idl jexec sa-jdi.jar [root@xuexi ~]# ls /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64/bin appletviewer java jconsole jps native2ascii schemagen xjc clhsdb javac jdb jrunscript orbd serialver extcheck javadoc jdeps jsadebugd pack200 servertool hsdb javah jhat jstack policytool tnameserv idlj javap jinfo jstat rmic unpack200 jar java-rmi.cgi jjs jstatd rmid wsgen jarsigner jcmd jmap keytool rmiregistry wsimport
沒有問題，接下來就可以像Windows一樣開始配置環境變量。注意有四個配置文件加載順序是/etc/profile>>/etc/bashrc>>~/.bashrc>>~/.bash_profile，請根據情況進行文件修改。詳見：
Shell變量的(4).添加環境變量
此處以/etc/profile配置文件為例，在文件末尾添加如下行：
export JAVA_HOME=/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64
export PATH=${JAVA_HOME}/bin:$PATH
export CLASSPATH=.:${JAVA_HOME}/lib/dt.jar:${JAVA_HOME}/lib/tools.jar
接着刷新下配置文件
[root@xuexi ~]# source /etc/profile
最后測試一下
[root@xuexi ~]# java -version openjdk version "1.8.0_212" OpenJDK Runtime Environment (build 1.8.0_212-b04) OpenJDK 64-Bit Server VM (build 25.212-b04, mixed mode) [root@xuexi ~]# echo $JAVA_HOME /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64
2）rpm安裝
首先需要下載一份rpm安裝包並上傳至CentOS7系統，使用rpm -ivh jdk-8u212-linux-x64.rpm安裝或升級java到1.8版本。
[root@xuexi ~]# rpm -ivh jdk-8u212-linux-x64.rpm 警告：jdk-8u212-linux-x64.rpm: 頭V3 RSA/SHA256 Signature, 密鑰 ID ec551f03: NOKEY 准備中... ################################# [100%] 正在升級/安裝... 1:jdk1.8-2000:1.8.0_212-fcs ################################# [100%] Unpacking JAR files... tools.jar... plugin.jar... javaws.jar... deploy.jar... rt.jar... jsse.jar... charsets.jar... localedata.jar...
接着可以使用rpm -pql /root/jdk-8u212-linux-x64.rpm看到安裝包安裝的目錄（會列出所有，請及時停止）。當然也可以使用rpm -ivh --prefix=<dir> jdk-8u212-linux-x64.rpm安裝到指定目錄下。
看一下安裝目錄結構以便配置環境變量
[root@xuexi ~]# ls /usr/java/ default jdk1.8.0_212-amd64 latest [root@xuexi ~]# ls /usr/java/jdk1.8.0_212-amd64/ bin lib src.zip COPYRIGHT LICENSE THIRDPARTYLICENSEREADME-JAVAFX.txt include man THIRDPARTYLICENSEREADME.txt javafx-src.zip README.html jre release
修改/etc/profile文件，在末尾添加如下三行
export JAVA_HOME=/usr/java/jdk1.8.0_212-amd64
export PATH=${JAVA_HOME}/bin:$PATH
export CLASSPATH=.:${JAVA_HOME}/lib/dt.jzr:${JAVA_HOME}/lib/tools.jar
然后刷新配置文件
[root@xuexi ~]# source /etc/profile
最后測試一下
[root@xuexi ~]# echo $JAVA_HOME /usr/java/jdk1.8.0_212-amd64 您在 /var/spool/mail/root 中有新郵件 [root@xuexi ~]# java -version java version "1.8.0_212" Java(TM) SE Runtime Environment (build 1.8.0_212-b10) Java HotSpot(TM) 64-Bit Server VM (build 25.212-b10, mixed mode)