Install Java with Apt on Ubuntu 20.04

sudo apt update

java -version

Command 'java' not found, but can be installed with:

sudo apt install default-jre              # version 2:1.11-72, or

sudo apt install openjdk-11-jre-headless  # version 11.0.14+9-0ubuntu2~20.04

sudo apt install openjdk-16-jre-headless  # version 16.0.1+9-1~20.04

sudo apt install openjdk-17-jre-headless  # version 17.0.2+8-1~20.04

sudo apt install openjdk-8-jre-headless   # version 8u312-b07-0ubuntu1~20.04

sudo apt install openjdk-13-jre-headless  # version 13.0.7+5-0ubuntu1~20.04

sudo apt install default-jre

<...>

java -version

openjdk version "11.0.14" 2022-01-18

OpenJDK Runtime Environment (build 11.0.14+9-Ubuntu-0ubuntu2.20.04)

OpenJDK 64-Bit Server VM (build 11.0.14+9-Ubuntu-0ubuntu2.20.04, mixed mode, sharing)

sudo apt install default-jdk

<...>

javac -version

javac 11.0.14

Upgrade java 1.7 to 1.8 in Ubuntu 14.04

java -version
java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer
echo $JAVA_HOME

Set Java home path,

export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
echo $JAVA_HOME
/usr/lib/jvm/java-8-openjdk-amd64

Don't pass MaxPermSize to Java 8+

java -version
openjdk version "1.8.0_91"
OpenJDK Runtime Environment (build 1.8.0_91-8u91-b14-0ubuntu4~14.04-b14)
OpenJDK 64-Bit Server VM (build 25.91-b14, mixed mode)

Set Java MaxPermSize and Min Heap Size

MaxPermSize

The MaxPermSize is the maximum size for the permanent generation heap, a heap that holds the byte code of classes and is kept separated from the object heap containing the actual instances.

Min Heap Size

Min Heap Size (-Xms) is a costly operation. Especially in case of financial applications this could mean delays. With similar real-time requirements, it could be a good idea to set -Xms and -Xmx to the same value.

Example for setting MaxPermSize and MinPermSize in Linux.

Go to bin dir,
cd /opt/apache-tomcat-7.0.61/bin
add a new file with
vi setenv.bat
set JAVA_OPTS=-Dfile.encoding=UTF-8 -Xms512m -Xmx4096m -XX:PermSize=128m -XX:MaxPermSize=512m

Ref: http://stackoverflow.com/questions/6902135/side-effect-for-increasing-maxpermsize-and-max-heap-size