How can programs written in Java not require the JRE?

Question

I had a fresh install and before installing Java was able to run NetBeans and OpenOffice. If these programs were written in Java, how is this possible? I know there exists some .class to .exe converters but I thought these are hacks (not suitable for large applications deployed worldwide). What is the common approach to deploying an application in Java without the JRE? Is there a standard, or most common way?

At first I thought it had to do with Jar files, but from what I red these require an existing Java installation to run.

Answer 1

All Java Applications Run on Top of a JRE. Even NetBeans and OpenOffice.

Most of the Deployable Java Applications have a JRE packaged with it. So that you may see it has no JRE but infact there is one with it and the Application will run on top of it.

Answer 2

There are only two ways in which you can run a program: you either compile it into machine code or you use an interpreter.

The JRE interprets Java bytecode (and does many more things, of course). There are compilers which generate native executables from Java source or byte code. The only one I know about is gcj, and the last time I checked it was only for Java 5 and didn't seem to be very reliable.

Most java applications either require a JRE or have one included in the distribution.

For instance, you can get Oracle SQL Developer with or without the JRE included. About Netbeans, there are distributions requiring a JDK (a JRE is not enough, as it needs the compiler and development tools), and also JDK with Netbeans bundles.

Now, there are three main ways to start a Java application (not counting Java Web Start and the like): you run java command manually, you use a script which does so or you run an executable called "wrapper".

There are several wrappers out there you can customise for any application. The link by PJ Fanning includes a list, if not mistaken. The wrappers use a JRE to run the application and ease starting java applications as services on system startup.

So, unless you find an application which was compiled into native code (and then, ideally, you couldn't tell it was a Java application), you are always using some JRE, be it a preinstalled one, or a bundled one.