15.5 Java

Java is a compiled language like C, with a simpler syntax and powerful support for object-oriented programming. It has a few niches in Unix systems. For example, it’s often used as a web application environment, and it’s popular for specialized applications. Android applications are usually written in Java. Even though it’s not often seen on a typical Linux desktop, you should know how Java works, at least for standalone applications.

There are two kinds of Java compilers: native compilers for producing machine code for your system (like a C compiler) and bytecode compilers for use by a bytecode interpreter (sometimes called a virtual machine, which is different from the virtual machine offered by a hypervisor, as described in Chapter 17). You’ll practically always encounter bytecode on Linux.

Java bytecode files end in .class. The Java Runtime Environment (JRE) contains all of the programs you need to run Java bytecode. To run a bytecode file, use:

$ java file.class

You might also encounter bytecode files that end in .jar, which are collections of archived .class files. To run a .jar file, use this syntax:

$ java -jar file.jar

Sometimes you need to set the JAVA_HOME environment variable to your Java installation prefix. If you’re really unlucky, you might need to use CLASSPATH to include any directories containing classes that your program expects. This is a colon-delimited set of directories like the regular PATH variable for executables.

If you need to compile a .java file into bytecode, you need the Java Development Kit (JDK). You can run the javac compiler from JDK to create some .class files:

$ javac file.java

JDK also comes with jar, a program that can create and pick apart .jar files. It works like tar.