Develop Reference

Spawn another process on same JVM

From a Java application I want to run another Java application on the same Java installation but in a separate process.
AFAIK for a new process I would use ProcessBuilder to run a command like
java -jar my.jar
but what if java is in a different directory, or should be java.exe since we are on Windows, or java.exe has some other name since the first application was jlinked and jpackaged?
Edit: What I learned meanwhile is that a jpackaged application comes with a native executable that sits in front of the JVM but passes all arguments to the application. That means it is no longer possible to specify an alternative jar to be executed, and some other mechanism is necessary.

If jlink image used within jpackage based apps is built without using the --strip-native-commands flag then the runtime image will contain bin/java (or bin/java.exe and bin/javaw.exe on Windows).
This will mean that you should be able to determine a path to launch a new JVM which works inside or outside of jpackage apps just based on the current runtime's java.home and by appending extra VM arguments in normal way for the classpath and main / module of the other application.
On Windows the environment property echo %PATHEXT% usually contains .EXE so the following should specify the java executable for the current runtime without needing to add the correct file extension on Windows:
String java = Path.of(System.getProperty("java.home"),"bin", "java").toString();
You can test above inside your jpackage and non-jpackaged app with a simple one-liner (note that this is not proper way to use ProcessBuilder):
new ProcessBuilder(java, "-version").start().getErrorStream().transferTo(System.out);
Obviously, the above is no help if you wish to determine whether to use Windows console enabled java.exe versus non-console javaw.exe.

run java jar file on HP-UX as UNIX as "os.name"

I need to execute a jar file on HP-UX that I am not supposed to modify.
I unpacked it using jd-gui and found out that I am failing cause in java there is a condition to check the os, leading to different directions for win, macos, freebds, openbds, gnu and so on.
I am quite sure everything would work if I would be able to make my unix command line reply freebds or openbds to the java call
System.getProperty("os.name")
once executed from a jar file like:
java -jar myjar.jar
is there a way to achieve this? some kind of compatibility mode or a way to preset that parameter.

You can use the -D switch to specify system properties. In my experiment this (unexpectedly) even worked with pre-defined ones like os.name. Therefore this should work:
java -Dos.name=linux -jar myjar.jar

How to run java application file (exe not jar) without installing jdk nor jre

(I already examined the answers here and here, but that doesn't answers my question)
I copied Java's folders (JRE & JDK) to a local folder and I manage to run .jar application by this command:
SomeFolder\Java\jre6\bin\javaw.exe -jar SomeApp.jar
But how can I run .exe application (e.g. eclipse.exe) by a similar way in two cases:
The application required only JRE
The application required also JDK
p.s. unfortunately "portableapps.com" solution is not acceptable.

The problem with .exe files is that they typically search the Java runtime on their own and there is no generic way to tell all of them where it is located.
In case of eclipse you can though e.g. by editing the eclipse.ini file.
Add
-vm
/usr/lib/jvm/java-7-oracle/bin
to the beginning of the file (with your path) and it should now start using the runtime you have specified.
A generic solution would be to figure out how the exe file runs the java code and then run it manually via javaw.exe -jar - that might not be possible if the exe file has the java code included and extracts it on demand. There are on the other hand versions as plain jar (often the version for Linux) that don't need all the exe workarounds.

Long story short...
1) Copy an installed version of the JDK (from another computer) you want to run to the destination computer.
2) Create a batch file to create or modify your environment variables, such as CLASSPATH, JAVA_HOME, and PATH.
#rem CONTENT OF BATCH FILE
#echo off
#cls
SET JAVA_HOME= <-- your JDK location
SET CLASSPATH=.;%JAVA_HOME%
SET PATH=%PATH%;%JAVA_HOME%\bin
3) Run this batch file before you launch Eclipse, or any executable that depends on the Java location. (Don't close the Command window or these transient values will vanish.)
4) You can close the Command window when you no longer need it.
This has worked for me since most Java dependent programs look for the JAVA_HOME variable in your computer's environment the path to Java binaries.
Have fun!

creating 100% standalone executable jar that doesn't require the java command

so apparently if you create an executable jar, in order to run it you still need the java command:
java -jar something.jar
but what if I just want it to run without the java command, so just directly from the command line
something.jar
is there a way to export my java app in eclipse in order to accomplish such

On Unix systems you can append the jar file at the end of an executable script.
On Windows you have to create a batch file.
For instance in Unix:
$cat HelloWorld.java
public class HelloWorld {
public static void main( String ... args ) {
System.out.println("Hola mundo!");
}
}
$cat M.mf
Main-Class: HelloWorld
$cat hello
#!/bin/sh
exec java -jar $0 "$#"
$javac HelloWorld.java
$jar -cmf M.mf hello.jar HelloWorld.class
$cat hello.jar >> hello
$chmod +x hello
$./hello
Hola mundo!
In windows you have to create a batch file like:
::hello.cmd
javaw -jar hello.jar
Which has the same effect.
On Windows and OSX you can double click on the jar to run it, I'm pretty sure you may add a trigger on Linux too.
I hope this help

Excelsior JET - http://www.excelsior-usa.com/jet.html - claims to compile to native code and bring its own runtime support, so it does not require an existing JVM. Commercial product.
I have not tried it myself, but they have spent quite a bit of effort over the years to market JET as a great deployment method for precompiled binaries.
Also note that if you have an executable/runnable jar which works fine with "java -jar someting.jar" and you just want to be able to invoke it in a more convenient way, this is the job of the program accepting your command and launching the java command.
For Linux you can frequently add an alias saying that "something" expands to "java -jar something.jar", and some command interpreters allow for saying that all commands ending with jars should be executed specially. The exact details depend on which shell (command line interpreter) you are using.

What you need is a tool called 'Java Executable Wrapper'.You can use it to Pack all your class files to a Single Executable Package.
The One i recomment is launch4j,you can download it from sourceforge launch4j.sourceforge.net
Launch4J can be used to create standalone Executables (.exe) from a jar file for windows Environment.

The thing is, that Java gets interpreted by the JVM, so you'll at least need to ship it with your app.
To be a little more specific about this, Java gets kind of compiled to byte-code so it can be interpreted faster. But the Byte-Code can't run without the JVM. This is the nice side of Java: You don't need to recompile your Apps to run on other platforms like Linux or OS X, the JVM takes care of that (as it is written in native code and is recompiled for those platforms).
There are some compilers out there which can convert your Java code to something native like C which can then be executed without the JVM. But this isn't the idea behind Java and most of those tools suck at what they do.
If you want your App to run without any interpreter, you'll need to use a compiled language like C or C++

Java program runs on a JVM, for the first question I don't think there's a compiler that can do the job well. For the second question since a jar file is not an executable per se, there must be some sort of settings in the target machine, "executing" a jar file without providing the java command is a matter of convenience for the user. On Windows every file extension has a program associated with it, so .doc documents have (usually) Word as the program associated -that setting is set by the office installer, the java runtime also sets the setting for .jar files when you install it, but behind the scenes, java command will be used by the system. So the short answer to the second question is: depends on the target machine.

Jar file of java

I have created a java application and packed it into a jar file on a Windows platform. Now I wants to deploy that jar file on Debian Linux.
Will the same jar file work for Debian Linux?
Is the command, used in windows for executing a jar file from the command prompt, same for Debain Linux?
i.e.
java -jvm "MyJar.jar"
Will the above command work for Debian Linux?

Generally, it should. However this depends on a few conditions:
If you use native code (JNI) you must make sure that the native library is available for the target platform
You must make sure you have no paths hardcoded which are Windows specific (in fact you should even watch out for special characters like the Path seperator : vs. ;)
You cannot use Runtime specific code

Yes.
Jar files are portable across platforms, and the syntax of the jar command is the same on both Linux and Windows.
EDIT: You should use the latest version of Sun Java unless there is a very good reason not to. Installation instructions: http://wiki.debian.org/Java/Sun

1. Will the same jar file work for Debian Linux?
Yes. Hence the nature of Java (portable code)
2. Is the command, used in windows for executing a jar file from the command prompt, same for Debain Linux?
java -jar "MyJar.jar"

yes, the main idea of java is that it (should) run on different operating systems, as long as a java runtime is installed.
though i have never heard of the -jvm flag.
if you want to start a jar file you should use the -jar flag.
java -jar "MyJar.jar"
you can also read up on the Write once run anywhere principle.

I do my development on a mac but run in linux and windows environments without any problem. Key is not to use JNI, As everyone else have mentioned I would use java -jar "MyJar.jar"

Almost. Use:
java -jar "MyJar.jar"
And of course you shouldn't have used anything such JNI or runtime stuff

Yes, although you might want to do, in Linux:
java -jar YourJar.jar
Instead of:
java -jvm YourJar.jar