ติดตั้ง Java JDK บน Ubuntu 20.04

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Installing Java

update packages ก่อน

$ sudo apt update

ติดตั้ง Java JDK 11 (openjdk)

$ sudo apt install default-jdk

แต่ถ้าจะติดตั้ง Java 8 ใช้คำสั่ง

sudo apt install openjdk-8-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"
    JAVA_HOME="/usr/lib/jvm/java-8-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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