First off, let me say that anything this tricky is probably hard for a reason.
This approach that may work for you if you really need it. As written, it assumes "java" is on the caller's path.
Overview:
Declare a Bootstrapper class as the main class in the jar's manifest.
The bootstrapper spawns another process in which we call java (passing in any command-line arguments you want) on the "real" main class.
Redirect the child processes System.out and System.err to the bootstrapper's respective streams
Wait for the child process to finish
Here's a good background article.
src/main/java/scratch/Bootstrap.java - this class is defined in pom.xml as
the jar's mainclass: <mainClass>scratch.Bootstrap</mainClass>
package scratch;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintStream;
public class Bootstrap {
class StreamProxy extends Thread {
final InputStream is;
final PrintStream os;
StreamProxy(InputStream is, PrintStream os) {
this.is = is;
this.os = os;
}
public void run() {
try {
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line = null;
while ((line = br.readLine()) != null) {
os.println(line);
}
} catch (IOException ex) {
throw new RuntimeException(ex.getMessage(), ex);
}
}
}
private void go(){
try {
/*
* Spin up a separate java process calling a non-default Main class in your Jar.
*/
Process process = Runtime.getRuntime().exec("java -cp scratch-1.0-SNAPSHOT-jar-with-dependencies.jar -Xmx500m scratch.App");
/*
* Proxy the System.out and System.err from the spawned process back to the user's window. This
* is important or the spawned process could block.
*/
StreamProxy errorStreamProxy = new StreamProxy(process.getErrorStream(), System.err);
StreamProxy outStreamProxy = new StreamProxy(process.getInputStream(), System.out);
errorStreamProxy.start();
outStreamProxy.start();
System.out.println("Exit:" + process.waitFor());
} catch (Exception ex) {
System.out.println("There was a problem execting the program. Details:");
ex.printStackTrace(System.err);
if(null != process){
try{
process.destroy();
} catch (Exception e){
System.err.println("Error destroying process: "+e.getMessage());
}
}
}
}
public static void main(String[] args) {
new Bootstrap().go();
}
}
src/main/java/scratch/App.java - this is the normal entry point for your program
package scratch;
public class App
{
public static void main( String[] args )
{
System.out.println( "Hello World! maxMemory:"+Runtime.getRuntime().maxMemory() );
}
}
Calling: java -jar scratch-1.0-SNAPSHOT-jar-with-dependencies.jar
Returns:
Hello World! maxMemory:520290304
Exit:0
