Installing Java
update packages ก่อน
$ sudo apt update
ติดตั้ง Java JDK (openjdk)
$ sudo apt install default-jdk
โปรแกรมจะติดตั้งอยู่ที่ /usr/lib/jvm/java-11-openjdk-amd64/bin/
$ ls -l /usr/lib/jvm/java-11-openjdk-amd64/bin/java* -rwxr-xr-x 1 root root 14560 ม.ค. 20 16:07 /usr/lib/jvm/java-11-openjdk-amd64/bin/java -rwxr-xr-x 1 root root 14608 ม.ค. 20 16:07 /usr/lib/jvm/java-11-openjdk-amd64/bin/javac -rwxr-xr-x 1 root root 14608 ม.ค. 20 16:07 /usr/lib/jvm/java-11-openjdk-amd64/bin/javadoc -rwxr-xr-x 1 root root 14576 ม.ค. 20 16:07 /usr/lib/jvm/java-11-openjdk-amd64/bin/javap
ตรวจสอบการติดตั้งบน Ubuntu 20.04.6
$ java --version openjdk 11.0.18 2023-01-17 OpenJDK Runtime Environment (build 11.0.18+10-post-Ubuntu-0ubuntu120.04.1) OpenJDK 64-Bit Server VM (build 11.0.18+10-post-Ubuntu-0ubuntu120.04.1, mixed mode, sharing)
$ javac --version javac 11.0.18
ตรวจสอบการติดตั้งบน Ubuntu 22.04.2
$ java --version openjdk 11.0.18 2023-01-17 OpenJDK Runtime Environment (build 11.0.18+10-post-Ubuntu-0ubuntu122.04) OpenJDK 64-Bit Server VM (build 11.0.18+10-post-Ubuntu-0ubuntu122.04, mixed mode, sharing)
$ javac --version javac 11.0.18
Managing Java
ใช้คำสั่ง update-alternatives
$ update-alternatives --help
Usage: update-alternatives [<option> ...] <command>
Commands:
--install <link> <name> <path> <priority>
[--slave <link> <name> <path>] ...
add a group of alternatives to the system.
--remove <name> <path> remove <path> from the <name> group alternative.
--remove-all <name> remove <name> group from the alternatives system.
--auto <name> switch the master link <name> to automatic mode.
--display <name> display information about the <name> group.
--query <name> machine parseable version of --display <name>.
--list <name> display all targets of the <name> group.
--get-selections list master alternative names and their status.
--set-selections read alternative status from standard input.
--config <name> show alternatives for the <name> group and ask the
user to select which one to use.
--set <name> <path> set <path> as alternative for <name>.
--all call --config on all alternatives.
<link> is the symlink pointing to /etc/alternatives/<name>.
(e.g. /usr/bin/pager)
<name> is the master name for this link group.
(e.g. pager)
<path> is the location of one of the alternative target files.
(e.g. /usr/bin/less)
<priority> is an integer; options with higher numbers have higher priority in
automatic mode.
Options:
--altdir <directory> change the alternatives directory.
--admindir <directory> change the administrative directory.
--log <file> change the log file.
--force allow replacing files with alternative links.
--skip-auto skip prompt for alternatives correctly configured
in automatic mode (relevant for --config only)
--quiet quiet operation, minimal output.
--verbose verbose operation, more output.
--debug debug output, way more output.
--help show this help message.
--version show the version.
You can have multiple Java installations on one server. You can configure which version is the default for use on the command line by using the update-alternatives command.
$ sudo update-alternatives --config java
ถ้ามี java ตัวเดียวก็จะขึ้นประมาณนี้
$ sudo update-alternatives --config java There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/java-11-openjdk-amd64/bin/java Nothing to configure.
แต่ถ้ามี java หลายตัว ก็จะแสดงให้เราเลือก
javac ก็เหมือนกัน ใช้คำสั่ง
$ sudo update-alternatives --config javac
Setting the JAVA_HOME
$ sudo nano /etc/environment
At the end of this file, add the following line, and to not include the bin/ portion of the path: (หา path ได้ด้วยคำสั่ง update-alternatives)
JAVA_HOME="/usr/lib/jvm/java-11-openjdk-amd64"
Modifying this file will set the JAVA_HOME path for all users on your system.
Save the file and exit the editor.
Now reload this file to apply the changes to your current session:
$ source /etc/environment
Verify that the environment variable is set:
$ echo $JAVA_HOME
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