Java has a runtime (the “JRE”, or “Java Runtime Environment”) which is required to run Java applications. There’s also a “JDK” (“Java Development Kit”) which is required to develop applications in Java. There are numerous blogs out there that explain how to install Java from repositories. This blog post will detail how to download and install Oracle’s Java SE manually.

Download Oracle Java SE

Oracle Java SE can be downloaded from:

https://www.oracle.com/technetwork/java/javase/downloads/index.html

Click on the Download button for the JDK version you wish to install. There are many Linux options available to download, but the rest of this blog post will assume you downloaded the .tar.gz version.

Extract the Oracle Java SE

Extract the tar file you downloaded to the directory you wish to install Java in. For example, if you downloaded Java 12 (jdk-12_linux-x64_bin.tar.gz) and you wish to install Java to /usr/local/lib:

[syd:z97] Downloads $ sudo tar -zxvf jdk-12_linux-x64_bin.tar.gz -C /usr/local/lib/
 [sudo] password for syd: 
 jdk-12/bin/jaotc
 jdk-12/bin/jar
 jdk-12/bin/jarsigner
 jdk-12/bin/java
 jdk-12/bin/javac
...

[syd:z97] Downloads $ ls -l /usr/local/lib/
 total 12
 drwxr-xr-x 9 root root  4096 Apr  7 23:32 jdk-12
...
 [syd:z97] Downloads $

Set $JAVA_HOME and update $PATH

Create an environment named JAVA_HOME, and set it to point to the directory you installed Java to (in this example, /usr/local/lib/jdk-12). Also add the bin directory (i.e. /usr/local/lib/jdk-12/bin) to your system’s path. I normally make these changes system-wide by creating /etc/profile.d/java.sh that contains:

export JAVA_HOME=/usr/local/lib/jdk-12
export PATH=$PATH:$JAVA_HOME/bin

Run (source) the above using:

$ source /etc/profile.d/java.sh

or just:

$ . /etc/profile.d/java.sh

You could stop there, and Java will work:

$ javac -version
javac 12

But wait! There’s more!!!

You may find yourself in a situation where you need to work on multiple Java projects that use different versions of the JDK. It is possible to quickly switch between different versions of the JDK using update-alternatives.

Register your JDK install with update-alternatives:

$ sudo update-alternatives --install "/usr/bin/java" "java" "/usr/local/lib/jdk-12/bin/java" 1
$ sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/local/lib/jdk-12/bin/javac" 1

You can view how many “alternatives” for javac you have using:

$ update-alternatives --list javac
 /usr/local/lib/jdk-12/bin/javac
 /usr/local/lib/jdk1.8.0_201/bin/javac

To switch between different versions of javac:

$ sudo update-alternatives --config javac
 There are 2 choices for the alternative javac (providing /usr/bin/javac).
 Selection    Path                                Priority   Status
------------------------------------------------------------------------
 0            /usr/local/lib/jdk-12/bin/javac         1         auto mode
 1            /usr/local/lib/jdk-12/bin/javac         1         manual mode
 2            /usr/local/lib/jdk1.8.0_201/bin/javac   1         manual mode 
 Press  to keep the current choice[*], or type selection number:
 

Don’t forget if you switch your javac version, you should also switch your java version too:

$ sudo update-alternatives --config java
...

Uninstalling Java

Assuming I want to uninstall JDK 12 which is installed in /usr/local/lib/jdk-12:

$ sudo update-alternatives --remove "javac" "/usr/local/lib/jdk-12/bin/javac"
$ sudo update-alternatives --remove "java" "/usr/local/lib/jdk-12/bin/java"
$ sudo rm -rf /usr/local/lib/jdk-12

After this, clean up any files that set $JAVA_HOME or add the JDK directory to $PATH (in my case, this is done in /etc/profile.d/java.sh).