How to check JRE version prior to launch?

Question

What's the best way to determine if the version of the JRE installed on a machine is high enough for the application which the user wants to run? Is there a way of doing it using java-only stuff? I'd like the solution to work on Windows/Linux/MacOSX - if the JRE version is too low a message should be displayed. Currently I'm getting an exception if i try to run it on Java 1.5 (the app is built for Java 1.6). If there's no universal solution, what's the best way to do it on Windows?

Answer 1

Generally, we've approached this with a C or (when unix-only) shell wrapper. Not sure this will really work for you.

We also approach this by embedding the JRE in our product. Takes care of 99.9% of the cases (the other 0.1% of the time is a user explicitly changing our configuration to use a different JVM). Again, not sure that this is a reasonable solution for you.

In our case, there is significant amounts of native code (JNI and otherwise), so tailoring an installable image for each platform we support is required anyway. But if you're dealing with a pure-Java solution, you may simply have to document your minimum and tell people to get with the program (no pun intended) if they're to run your stuff. It's sorta like people complaining that my Mac won't run MSVC, or that my Linux box is having problems running World of Warcraft. That's just not the (virtual) machine the software is targeted for - you need to switch. At least in the Java world, we really can call this an upgrade, though, without hurting anyone's OS-religious feelings. (Try telling the Mac user to "upgrade" to Windows XP to run MSVC - there's a beat-down waiting to happen.)

Answer 2

You could do this using reflection and two compilers. Compile a main class with the oldest java version you want to be able to run at all with. It checks the version using System.getProperty("java.version"), or whatever, and then uses reflection to load your real main class if that check passes, possibly even loading the jar directly. The JRE shouldn't load any classes that weren't referenced by your outer main class at compile time.

Answer 3

You might consider Java Webstart. Even if the name implies something like applets, it's about standalone-applications. Webstart is a launcher, that checks a JNLP-file (a simple XML-file, where you configure the download-location of your app, the needed Java-version and some other metadata) and starts your app with the correct JRE. It even updates the application, if a newer version is available. The downside is, you have to write a JNLP-file. Here is an example:

<?xml version="1.0" encoding="utf-8"?>

<!--
###############################################################################
#
# @(#)draw.jnlp 1.6 02/09/11
#
# JNLP File for Draw Demo Application
#
###############################################################################
 -->


<jnlp spec="0.2 1.0"
      codebase="http://java.sun.com/javase/technologies/desktop/javawebstart/apps"
      href="draw.jnlp">
   <information> 
      <title>Draw 4 App</title> 
      <vendor>Sun Microsystems, Inc.</vendor>
      <homepage href="http://java.sun.com/javase/technologies/desktop/javawebstart/demos.html"/>
      <description>A minimalist drawing application along the lines of Illustrator</description>
      <description kind="short">Draw Demo Short Description</description>
      <icon href="images/draw.jpg"/>
      <offline-allowed/> 
   </information> 
   <resources>
      <j2se version="1.3+" href="http://java.sun.com/products/autodl/j2se"/>
      <j2se version="1.3+"/>
      <jar href="draw.jar" main="true" download="eager"/>
   </resources>
   <application-desc main-class="Draw"/>
</jnlp> 

A second possibility is to use a launcher-program. An example is the Apache Commons Launcher. You can also write some launcher app yourself, but that's usually not worth the effort.

Answer 4

Have a launching class compiled for Java 1.2 which invokes the real main() in your 1.6 classes. If an unsupported class exception is thrown them catch it and display a nice error message.

Answer 5

System.getProperties() gives you a listing of JVM properties including the different version ids of the JRE, JVM and specification. This implemented for all versions of Java so should work regardless of version compiled in and version run in, or the implementation.

If you write a basic class to test the version, you can call this first in your main() launching class. It must really be basic functionality though or you might risk breaking it.

Answer 6

All those above is too damn complicated. Just go:

Properties props = System.getProperties()
props.list(System.out)

And you will see everything about your JVM, JRE, JDK and staff. Or get a specific value by using:

//full list of possible props you can see if u run code above
String props = System.getProperty(prop)