 Java is a
programming language created by Sun Microsystems that offers
many benefits to professional programmers and
application developers. Java is a byte-compiled
language and is completely portable. You can
execute the same Java code (or Java class) on a
wide range of operating system platforms. Java is
much faster than interpreted languages (TCL,
Perl, et al), but cannot run as fast as fully
compiled languages (C, C++).
Because
of its portability, Java and the World Wide Web
make an excellent match. Web designers can embed
Java applets into their web content for display
in Java-enabled browsers like Netscape Navigator
and Microsoft Internet Explorer. The applets are
downloaded over the Internet within the context
of the web document and are then executed on the
local computer. Applets can add interactivity,
animation, multimedia, or a database interface to
an otherwise dull and listless web site.
The
Java Virtual Machine is at the heart of the Java
programming language. It is the engine that
actually executes Java code. You can't run a Java
class or Java applet without also running an
implementation of the Java Virtual Machine. When
Java code is executed, the instructions are not
executed directly by the hardware of the local
system. Instead the Java Virtual Machine walks
through the instructions step by step and carries
out the action the instruction represents. This
provides a level of protection between your
computer and the software you run on your
computer.
Using
Java on Your Virtual Server
There are several Java tools which are currently
available on your Virtual Server. The tools are
compatible with version 1.0.2 of the Java spec.
The 1.0.2 spec is supported by all Java enabled
browsers.
-
- javac
- Java Bytecode Compiler
javac compiles Java source
code (.java files) into .class
files that contain the Java
bytecode for the class. To
compile an example file called Test.java,
do something like this:
% javac Test.java
The
resulting .class file can
then be embedded into web
content.
- java
- Java Virtual Machine
(Interpreter) and
"Just-In-Time" Compiler
The Java Virtual Machine is an
interpreter for Java bytecode. To
execute the sample Test.class
bytecode compiled using the javac
command above, do something like
this:
% java Test
The Java
Virtual Machine installed on the
servers running FreeBSD is java_X
1.18.
The Java
Virtual Machine installed on the
servers running BSD/OS is Kaffe
0.84. Kaffe version 0.91 (which
is Java 1.1 compliant) is
available as well. The Java
version 1.1 compliant interpreter
can be executed using the java1.1
command.
NOTE:
If your Virtual Server
was ordered after Nov 22,
1999, you are likely
running FreeBSD. To find
out which O/S your
Virtual Server is
running, use the uname
command:
% uname
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These
Java Virtual Machines also
include a
"Just-in-time" (JIT)
code generator. JIT is a
technique for speeding up the
execution of interpreted
programs. Just before a method is
run for the first time, the
machine-independent Java bytecode
for the method is converted into
native machine code. This native
machine code can then be executed
by the computer directly, rather
than via interpreter. JIT code
generator greatly increases the
speed of interpreted bytecode to
nearly the speed of compiled
code.
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