Java is different from most of the packages in LFS and BLFS. It is a programming language that works with files of bytecode to obtain instructions and executes them in a Java Virtual Machine (JVM). An introductory java program looks like:
public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello, World");
}
}
This program is saved as HelloWorld.java
. The file name, HelloWorld, must match the class name.
It is then converted into byte code with javac HelloWorld.java. The output
file is HelloWorld.class
. The program
is executed with java
HelloWorld. This creates a JVM and runs the code.
The 'class' extension must not be specified.
Several class files can be combined into one file with the jar command. This is similar to the standard tar command. For instance, the command jar cf myjar.jar *.class will combine all class files in a directory into one jar file. These act as library files.
The JVM can search for and use classes in jar files automatically.
It uses the CLASSPATH
environment
variable to search for jar files. This is a standard list of
colon-separated directory names similar to the PATH
environment variable.
Creating a JVM from source requires a set of circular dependencies. The first thing that's needed is a set of programs called a Java Development Kit (JDK). This set of programs includes java, javac, jar, and several others. It also includes several base jar files.
To start, we set up a binary installation of the JDK created by the
BLFS editors. It is installed in the /opt
directory to allow for multiple
installations, including a source based version.
This package is known to build and work properly using an LFS 12.2 platform.
Binary download (x86): https://anduin.linuxfromscratch.org/BLFS/OpenJDK/OpenJDK-22.0.2/OpenJDK-22.0.2+9-i686-bin.tar.xz
Download MD5 sum: 400033f7e0755c14687afe277a5f5672
Download size (binary): 162 MB
Estimated disk space required: 312 MB
Binary download (x86_64): https://anduin.linuxfromscratch.org/BLFS/OpenJDK/OpenJDK-22.0.2/OpenJDK-22.0.2+9-x86_64-bin.tar.xz
Download MD5 sum: 83f5b325922b3cef357a6e63d368ce91
Download size (binary): 170 MB
Estimated disk space required: 341 MB
alsa-lib-1.2.12, Cups-2.4.10, giflib-5.2.2, and Xorg Libraries
Begin by extracting the appropriate binary tarball for your
architecture and changing to the extracted directory. Install the
binary OpenJDK with the following
commands as the root
user:
install -vdm755 /opt/OpenJDK-22.0.2-bin && mv -v * /opt/OpenJDK-22.0.2-bin && chown -R root:root /opt/OpenJDK-22.0.2-bin
The binary version is now installed. You may create a symlink to
that version by issuing, as the root
user:
ln -sfn OpenJDK-22.0.2-bin /opt/jdk
You may now proceed to Configuring the Java environment, where the instructions assume that the above link exists.