Installing Java

When you install Java, it usually means one of two things: you are installing the Java Runtime Environment (JRE) or the Java Development Kit (JDK). The JRE provides the runtime that allows you to run Java programs, whereas the JDK contains the JRE, as well as other tools, that allow you to develop in Java.

We are going to install the JDK here, but to complicate things further, there are different implementations of the JDK—OpenJDK, Oracle Java, IBM Java—and the one we will be using is the default-jdk APT package, which comes with our Ubuntu installation:

$ sudo apt update
$ sudo apt install default-jdk

Next, we need to set a system-wide environment variable so that other programs using Java (for example, Elasticsearch) know where to find it. Run the following command to get a list of Java installations:

$ sudo update-alternatives --config java
There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java
Nothing to configure.

For my machine, there's only a single Java installation, located at /usr/lib/jvm/java-8-openjdk-amd64/. However, if you have multiple versions of Java on your machine, you'll be prompted to select the one you prefer:

$ sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).

  Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1101 auto mode
  1 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1101 manual mode
  2 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1081 manual mode

Press <enter> to keep the current choice[*], or type selection number: 

Next, open /etc/environment and add the path to the JAVA_HOME environment variable:

JAVA_HOME="/usr/lib/jvm/java-8-openjdk-amd64"

JAVA_HOME will be set for any user on login; to apply the changes now, we need to source the file:

$ . /etc/environment
$ echo $JAVA_HOME
/usr/lib/jvm/java-8-openjdk-amd64