Execute php script

I am building a php application. In a script, there is a call to execute a jar file:

?php
exec('java -jar simulations/simulation.jar');
?>

The problem is that this command line executes the jar file:

user@ubuntu: php execSimulation.php

but not the call from the web page. The call is made by AJAX, am I missing something??

<!-- Script to execute the simulation -->
<script type="text/javascript" src="prototype.js"></script>
        <script>

            function sendRequest() {
                new Ajax.Request("ejecutarSimulacion.php", 
                    { 
                    method: 'post', 
                    postBody: 'name='+ $F('name'),
                    onComplete: showResponse 
                    });
                }

            function showResponse(req){
            alert(req.responseText);
            }
        </script>

<form id="test" onsubmit="return false;">
                                <input type="hidden" value="<?php echo $simulacion; ?>" name="name" id="name" >
                                <input type="submit" value="<?php echo $nombre; ?>" onClick="sendRequest()">
                            </form>

When I try to print only the param I send, for instance, the alert shows it, so I am sure that the call is reaching the the server, but I don't know why the jar file is not executed. Any ideas, please?

Thanks in advance.

EDIT

Error trace:

No protocol specified
Exception in thread "main" java.lang.InternalError: Can't connect to X11 window server using ':0.0' as the value of the DISPLAY variable.
    at sun.awt.X11GraphicsEnvironment.initDisplay(Native Method)
    at sun.awt.X11GraphicsEnvironment.access$200(X11GraphicsEnvironment.java:62)
    at sun.awt.X11GraphicsEnvironment$1.run(X11GraphicsEnvironment.java:178)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.awt.X11GraphicsEnvironment.<clinit>(X11GraphicsEnvironment.java:142)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:186)
    at java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(GraphicsEnvironment.java:82)
    at java.awt.Window.init(Window.java:385)
    at java.awt.Window.<init>(Window.java:438)
    at java.awt.Frame.<init>(Frame.java:419)
    at java.awt.Frame.<init>(Frame.java:384)
    at javax.swing.JFrame.<init>(JFrame.java:174)
    at org.opensourcephysics.tools.TranslatorTool.<init>(Unknown Source)
    at org.opensourcephysics.tools.TranslatorTool.<clinit>(Unknown Source)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:186)
    at org.开发者_JAVA百科opensourcephysics.display.OSPRuntime.<clinit>(Unknown Source)
    at org.opensourcephysics.controls.OSPLog.fine(Unknown Source)
    at org.opensourcephysics.tools.ResourceLoader.addSearchPath(Unknown Source)
    at _users.tanqueCalentamiento.TanqueCalentamiento.<clinit>(TanqueCalentamiento.java:18)
Could not find the main class: _users.simulation.Simulation. Program will exit.

It seems your jar requires an X server to be running.

Can't connect to X11 window server using ':0.0' as the value of the DISPLAY variable.

When you run it from command line, do you have X running? If you do, this would explain why it works there, and not from PHP.

You can try to "hijack" the running X session from PHP.

exec('DISPLAY=:0 java -jar simulations/simulation.jar');

You may have to first run xhost +localhost (or xhost +) from the command line, to allow the user PHP is running as to connect to X.

In the immortal words of Jamie Savage, "There's yer problem."

The Java program you're running is trying to get to your X Windows "server" (i.e., the screen) on initialization, which works when you're running from the command line but not from the headless web-server. Talk to the people who wrote org.opensourcephysics.tools.TranslatorTool about how to disable this (grossly dysfunctional) behavior.

Use the full path in exec() : base paths are different between the CLI SAPI and the apache one.

 <?php 
 exec('full/path/to/jar');