Is there a way to know if a Java program was started from the command line or from a jar file?

Question

I want to either display a message in the console or a pop up, so in case a parameter is not specified, I want to know to which should I display

Something like:

if( !file.exists() ) {
    if( fromCommandLine()){
        System.out.println("File doesn't exists");
    }else if ( fromDoubleClickOnJar() ) {
        JOptionPane.showMessage(null, "File doesn't exists");
    }
 }

Answer 1

The straight forward answer is that you cannot tell how the JVM was launched.

But for the example use-case in your question, you don't really need to know how the JVM was launched. What you really need to know is whether the user will see a message written to the console. And the way to do that would be something like this:

if (!file.exists()) {
    Console console = System.console();
    if (console != null) {
        console.format("File doesn't exists%n");
    } else if ( /* we have a GUI Huston */ ) {
        JOptionPane.showMessage(null, "File doesn't exists");
    } else {
        // Put it in the log
    }
 }

The javadoc for Console, while not water tight, strongly hints that a Console object (if it exists) writes to a console and cannot be redirected.

I'll leave it to someone else to fill in a suitable "we have a GUI" predicate ...

EDIT ... and @Stephen Denne provides the answer !GraphicsEnvironment.isHeadless().

Answer 2

From http://java.itags.org/java-essentials/15972/

try {
    GraphicsEnvironment.getLocalGraphicsEnvironment();
} catch(Throwable ex) {
    System.out.println("No graphical environment is available.");
}

Answer 3

You can try with:

if (System.console() != null) {
    // Console attached to the JVM: command prompt output
    System.out.println("...");
} else {
    // No console: use Swing
}

Answer 4

it's true that it is impossible to tell how the JVM was invoked. but... there's a way to side step this. you assumed that when the user double clicked on a JAR, then there's GUI running... ok. so let's extend this assumption. check.. from where the class was invoked, the directory. check that directory.. assuming it's a normal usage, when there's a *.jar file, then the user must've started the app from a jar.. but one flaw is that the user can also click on the main class file. hahahaha

Answer 5

I'm not clear on the question but I'm going to interpret it as you want to differentiate between the following 2

java -jar fred.jar

and

java package.Main

Here is an outline line of the program

import sun.jvmstat.monitor.*;
...
HostIdentifier hostId = new HostIdentifier("localhost");
MonitoredHost monitoredHost = MonitoredHost.getMonitoredHost(hostId);
Set jvms = monitoredHost.activeVms();
for (Object i: jvms) {
   VmIdentifier id = new VmIdentifier("//" + i + "?mode=r");
   MonitoredVm vm = monitoredHost.getMonitoredVm(id, 0);   
   System.out.println(i);
   System.out.println("\t main class: " + MonitoredVmUtil.mainClass(vm, false));
   System.out.println("\t main args: " + MonitoredVmUtil.mainArgs(vm));
   System.out.println("\t jvmArgs: " + MonitoredVmUtil.jvmArgs(vm));
   monitoredHost.detach(vm);
}

The call MonitoredVmUtil.mainClass(vm, false) will either return 'jar' or the name of your main class eg Main.

You have to use $JAVA_HOME/lib/tools.jar to compile and run.

Answer 6

You can get all the input arguments with RuntimeMBean.getInputArguments(), this can be used to detect when debugging is enabled.

EDIT: However, the -jar argument isn't one of them. :(