JDK
Installing default JRE/JDK
This is the recommended and easiest option. This will install OpenJDK 6 on Ubuntu 12.04 and earlier and on 12.10+ it will install OpenJDK 7.
Installing Java with apt-get is easy. First, update the package index:
sudo apt-get update
Then, check if Java is not already installed:
java -version
If it returns "The program java can be found in the following packages", Java hasn't been installed yet, so execute the following command:
sudo apt-get install default-jre
This will install the Java Runtime Environment (JRE). If you instead need the Java Development Kit (JDK), which is usually needed to compile Java applications (for example Apache Ant, Apache Maven, Eclipse and IntelliJ IDEA execute the following command:
sudo apt-get install default-jdk
That is everything that is needed to install Java.
All other steps are optional and must only be executed when needed.
Installing OpenJDK 7 (optional)
To install OpenJDK 7, execute the following command:
sudo apt-get install openjdk-7-jre
This will install the Java Runtime Environment (JRE). If you instead need the Java Development Kit (JDK), execute the following command:
sudo apt-get install openjdk-7-jdk
Installing Oracle JDK (optional)
The Oracle JDK is the official JDK; however, it is no longer provided by Oracle as a default installation for Ubuntu.
You can still install it using apt-get. To install any version, first execute the following commands:
sudo apt-get install python-software-properties
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
Then, depending on the version you want to install, execute one of the following commands:
Oracle JDK 6
This is an old version but still in use.
sudo apt-get install oracle-java6-installer
Oracle JDK 7
This is the latest stable version.
sudo apt-get install oracle-java7-installer
Oracle JDK 8
This is a developer preview, the general release is scheduled for March 2014. This external article about Java 8 may help you to understand what it's all about.
sudo apt-get install oracle-java8-installer
Managing Java (optional)
When there are multiple Java installations on your Droplet, the Java version to use as default can be chosen. To do this, execute the following command:
sudo update-alternatives --config java
It will usually return something like this if you have 2 installations (if you have more, it will of course return more):
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 auto mode
1 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 manual mode
2 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 manual mode
Press enter to keep the current choice[*], or type selection number:
You can now choose the number to use as default. This can also be done for the Java compiler (javac):
sudo update-alternatives --config javac
It is the same selection screen as the previous command and should be used in the same way. This command can be executed for all other commands which have different installations. In Java, this includes but is not limited to: keytool, javadoc and jarsigner.
Setting the "JAVA_HOME" environment variable
To set the JAVA_HOME environment variable, which is needed for some programs, first find out the path of your Java installation:
sudo update-alternatives --config java
It returns something like:
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 auto mode
1 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 manual mode
2 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 manual mode
Press enter to keep the current choice[*], or type selection number:
The path of the installation is for each:
- /usr/lib/jvm/java-7-oracle
- /usr/lib/jvm/java-6-openjdk-amd64
- /usr/lib/jvm/java-7-oracle
Copy the path from your preferred installation and then edit the file /etc/environment:
sudo nano /etc/environment
In this file, add the following line (replacing YOUR_PATH by the just copied path):
JAVA_HOME="YOUR_PATH"
That should be enough to set the environment variable. Now reload this file:
source /etc/environment
Test it by executing:
echo $JAVA_HOME
If it returns the just set path, the environment variable has been set successfully. If it doesn't, please make sure you followed all steps correctly.
Ubuntu Oracle JDK
If you're on a fresh installation of Ubuntu with no previous Java installations, this script automates the process outlined above if you don't want to type all that into a console. Remember, you still need to download Java from Oracle's website -- Oracle's links are not wget friendly.
Before using this make sure that this script is in the same directory as the .tar.gz file extension that you downloaded and there are no files that start with jdk-7 in the same folder. If there are, please move them out of the folder temporarily. Remember to make the script executable (chmod +x <script's file>).
#!/bin/sh
tar -xvf jdk-7*
sudo mkdir /usr/lib/jvm
sudo mkdir /usr/lib/jvm/jdk1.7.0
sudo mv ./jdk1.7*/* /usr/lib/jvm/jdk1.7.0
sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jdk1.7.0/bin/java" 1
sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/jdk1.7.0/bin/javac" 1
sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/jdk1.7.0/bin/javaws" 1
sudo chmod a+x /usr/bin/java
sudo chmod a+x /usr/bin/javac
sudo chmod a+x /usr/bin/javaws
If you want to install the plugin for Firefox then add this to the end of the script:
mkdir ~/.mozilla/plugins
ln -s /usr/lib/jvm/jdk1.7.0/jre/lib/amd64/libnpjp2.so ~/.mozilla/plugins/
sudo /etc/init.d/apparmor restart
You can check if the installation succeeded with the following command:
java -version
You should see something like
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)
切換JDK
切換已安裝好的JDK
sudo update-alternatives --config java
選擇已安裝好的JDK
There are 2 choices for the alternative java (providing /usr/bin/java).
SelectionPathPriority Status
0/usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1071 auto mode
1/usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1071 manual mode
2/usr/lib/jvm/jdk1.7.0/bin/java 1 manual mode
Press enter to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/lib/jvm/jdk1.7.0/bin/java to provide /usr/bin/java (java) in manual mode*
檢查是否已切換成功
root@li477-24:~# 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)