OpenJDK Cookbook

To follow this recipe, you will need an installed OpenJDK instance. It will be good if you have an Oracle JDK instance as well, to feel the difference. Also, we will assume that you have a Linux installation and an update-java-alternatives command installed and ready to use. To know how to install OpenJDK on various systems, see the later recipes in this chapter. To know how to switch the system Java version, if you do not have update-alternatives installed (for Fedora, Gentoo, and so on), visit the Configuring OpenJDK on Linux recipe or refer to your distribution documentation/forums.

Please take a look at the following procedures to know the difference between OpenJDK and Oracle JDK:

We can see that the plugin is not from the JDK itself but from a project named IcedTea.

IcedTea is an open source project, whose goal is to replace proprietary parts of the Java ecosystem as much as possible. The plugin itself is from IcedTea-Web, an open source implementation of the Java Web Start and Java browser plugins.

In most distributions, the IcedTea plugin is installed by default. But it is necessary to keep in mind that it's an open source plugin, and definitely not a referenced one. This means that its functionality might be slightly different from the Oracle plugin. It is also possible that some features may not work.

Oracle JDK still has some proprietary components, and the browser plugin is one example. All we need in this chapter is to see the difference between the work of OpenJDK and Oracle JDK components that are different.

Also, the huge difference between OpenJDK and Oracle JDK lies in the license. OpenJDK is open source, while Oracle JDK contains proprietary pieces, and thus it is licensed under the Oracle binary license. The fact that OpenJDK is open source provides a whole new range of benefits (and exciting discoveries) through the ability to study and modify its source code. It is also worth mentioning that more than 90 percent of Oracle JDK is based on OpenJDK source code. This means the OpenJDK quality is not compromised in any way. The browser plugin is not the only thing that is missed in OpenJDK compared to Oracle JDK.

To follow this recipe, we will need an installed Windows system. Windows 7 or Windows 8 will be best, because Windows XP is already officially discontinued by Microsoft.

There is an official build of OpenJDK on Windows, but it exists for referential purposes only. It is official and easy to install, but it doesn't have any security updates or improvements. However, there are unofficial builds, maintained by Alex Casco. We will try to install OpenJDK in both ways:

Congratulations! We've just installed the OpenJDK official binary.

The reference implementation is the only available official binary build of OpenJDK. But it lacks security and is used only for reference purposes. It is a simple archive that needs to be unpacked to use it.

To bypass this unpleasantness and give Windows users an opportunity to install OpenJDK as a binary without building it from source, one of the OpenJDK contributors established a completely unofficial but very useful OpenJDK build set for various platforms.

Moreover, this binary build, unlike the official one, is open source and licensed over GPL. So we can use it even in a completely open source environment without adding any proprietary pieces that will possibly get us in trouble.

Though an official reference binary is outdated, there is an unofficial project that provides OpenJDK builds from sources that are up to date.

Now we will install OpenJDK 7 from unofficial builds:

Although the simplest way to install OpenJDK is to unpack binaries, manually or in an automatic fashion, there is no doubt that working with the source code will give us more flexibility at all possible levels.

To know more, read the following chapters:

To follow this recipe, we will need an OpenJDK instance installed on our Windows system. Windows 7 or Windows 8 will be best, because Windows XP is already officially discontinued by Microsoft.

At first, we need to install our OpenJDK implementation as the default Java instance. This is often necessary for development:

In order to set a newly installed OpenJDK instance as the default JDK, we need to change the system variable. After that change, our Java executables will be visible to the system.

The same procedure is followed to set the CLASSPATH variable. It is not very necessary, and if you are using other libraries such as GNU classpath, you probably know about it.

To follow this recipe, you will need an installed Linux system. It will be better if it has the kernel version 2.6 or higher, though OpenJDK is reported workable on 2.4 kernels as well. Also, if you have the .deb, .rpm, or .ebuild package manager, the recommended way to install any package is to install it using those.

When the installation of various packages is concerned, the process is dependent on our Linux distribution.

For a Debian-based distribution:

For an RPM-based distribution, we'll need to first search for the package names, because package names are varied between different distributions, as shown here:

yum search openjdk

You will see an output like this:

java-1.6.0-openjdk.x86_64 : OpenJDK Runtime Environment
java-1.6.0-openjdk-demo.x86_64 : OpenJDK Demos
java-1.6.0-openjdk-devel.x86_64 : OpenJDK Development Environment
java-1.6.0-openjdk-javadoc.x86_64 : OpenJDK API Documentation
java-1.6.0-openjdk-src.x86_64 : OpenJDK Source Bundle
java-1.7.0-openjdk.x86_64 : OpenJDK Runtime Environment
java-1.7.0-openjdk-demo.x86_64 : OpenJDK Demos
java-1.7.0-openjdk-devel.x86_64 : OpenJDK Development Environment
java-1.7.0-openjdk-javadoc.noarch : OpenJDK API Documentation
java-1.7.0-openjdk-src.x86_64 : OpenJDK Source Bundle

You may install all of the packages that have the desired version. Then, we will run another command, using the package name we've just found:

yum install <a found package name>

This will also trigger an automatic download and installation.

If we have a Gentoo-based distribution, just type the following:

emerge openjdk-1.7

This will, depending on your distribution, unpack and install a binary package or, more probably, automatically build this package from source.

Aside from the recommended ways, there is a generic installation procedure. It is quite simple, though it may do some damage to your operating system, so don't use it unless you really know what you're doing:

To follow this recipe, you will need an OpenJDK instance installed on a Linux system. The deb, rpm, or ebuild distributions will suite us really well, although we will see a method for generic Linux configuration also.

Also, we will need the bash startup files to be installed properly.

First let's check whether your bash startup files are installed. The simplest possible way is to configure your OpenJDK using them. They are system-wide and easy to use, though there are drawbacks in their usage, such as update conflicts:

The preceding code is quite simple—it just appends all JAR files to the classpath.

This script is called during shell initialization, so whenever you perform shell initialization, these variables will be exported. The variables are thus system-wide, so be careful while playing with them, as they can cause your Java to fail permanently if you make some errors in this file.

On Linux, you can see the directory structure of the installed OpenJDK using the tree command.

To do so, install the tree package (use your distribution's documentation if possible) and type:

tree -L 1 <path-to-openjdk> -lah

You will see something like the following:

/usr/lib/jvm/java-7-openjdk-amd64
├── [  22]  ASSEMBLY_EXCEPTION -> jre/ASSEMBLY_EXCEPTION
├── [4.0K]  bin
├── [  41]  docs -> ../../../share/doc/openjdk-7-jre-headless
├── [4.0K]  include
├── [4.0K]  jre
├── [4.0K]  lib
├── [4.0K]  man
├── [  20]  src.zip -> ../openjdk-7/src.zip
└── [  22]  THIRD_PARTY_README -> jre/THIRD_PARTY_README

This is the first-level directory structure in which:

To follow this recipe, you will need an OpenJDK instance installed and an established Internet connection. The recipe is more for the initial understanding of the process rather than for practical use, so if you are familiar with these matters, don't hesitate to skip this recipe entirely.

We will see what OpenJDK consists of:

There's not very much to see, because there is only the basic on-boarding information. Most of the project business is in the bug tracker.

After we read the information about the project on an official site, we will go to JDK JIRA to see what's happening here. We will go to the JDK9 part of JIRA at https://bugs.openjdk.java.net/browse/JDK/fixforversion/14949.

Here we can see various issues related directly to JDK9 and see how the process is going.

Groups are sets of developers who may work on different projects but in one large scope. Developers participate in chosen projects, and projects are sponsored by groups.

There are four major types of projects:

For example, the JDK9 project is a featured one. The graphics rasterizer project is a replacement one, while the Swing group is a whole group that is focused on the improvement of Swing.

Various ports' projects are obviously the portability ones.