If you have installed Ubuntu from scratch, most probably the java package is not installed by default. The java package is necessary to run any java-based app, including running java applets in the browser. While you may not use it it all the time, it is still handy to install it. When you need it, it is there ready to be used.
Sun JDK vs OpenJDK
In the past, Sun JDK was the default java package in Ubuntu. After Sun Microsystem was acquired by Oracle in 2010, Ubuntu (and various distros) switched the java package to the fully open-source OpenJDK as the SUN JDK no longer contain codes that are fully open-sourced.
OpenJDK is community-built and is (almost) 100% similar to the SUN JDK. You can easily install OpenJDK in Ubuntu via the Ubuntu Software Center, by clicking here or running the command:
sudo apt-get install openjdk-7-jre
Personally, other than some minor font rendering issue with OpenJDK, I have not experience any major problem with OpenJDK. That could also because I don’t use many java-based apps in the first place. However, if you have a specific java-based app that don’t run well in OpenJDK, you can switch to the SUN JDK instead. Here is how you do it:
The hard way
This method requires you to install SUN JDK 7 manually. It is a good way for you to learn the trick.
1. Download the SUN JDK 7 here to your home folder.
2. Extract the tar file.
3. Move the extracted file to the “/usr/lib/jvm/” folder:
sudo mv jdk1.7.0_04 /usr/lib/jvm/
4. Install SUN JDK 7 in your system:
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.7.0_04/bin/javac 1 sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0_04/bin/java 1
5. Set the environment variables:
sudo update-alternatives --config javac sudo update-alternatives --config java
You will see an option like the screenshot below:
Enter the option that corresponds to the SUN JDK package. Press Enter.
To test your java:
You should see something like this:
The easy way
If you don’t want to manually install the SUN JDK, you can do it via a PPA. Since there is no official PPA for SUN JDK, you will have to grab a custom PPA that comes with the java package. Note that custom PPAs might add extra software sources to the repository, and cause your system to be bloated or even have conflicts with other apps. So bear in mind the risk involved.
There are several PPAs out there that come with SUN JDK, one that you can use is “webupd8team/java”
sudo add-apt-repository ppa:webupd8team/java sudo apt-get update sudo apt-get install oracle-java7-installer
This should install SUN JDK 7 in your system.
Run Java applet in Browsers (Firefox, Chrome etc)
Even after you have installed the java package, you will find that the java applet in your browser won’t run. If you are using the OpenJDK, here is another package that you need to install:
sudo apt-get install icedtea-7-plugin
For Oracle java, run the commands:
mkdir ~/.mozilla/plugins ln -s /usr/lib/jvm/jdk1.7.0_04/jre/lib/i386/libnpjp2.so ~/.mozilla/plugins
If you are using a 64-bit system, remember to change the “i386″ to “amd64“.
Once install, restart your browser. The java applet should run now. To test if Java is working in your browser, go to http://java.com/en/download/testjava.jsp. If installed correctly, you should see a “Your java is working” message.