Unable to fetch random port when server.port is 0 - spring-boot

Question

I need to fetch port number on which undertow was Started by my Sprint boot app. I have defined server.port=0 in application.properties. I cannot use fix port numbers like 8080.
package com.aggregate.application;
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Properties;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.context.event.ApplicationReadyEvent;
import org.springframework.boot.web.servlet.support.SpringBootServletInitializer;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationListener;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.env.Environment;
#Configuration
#ComponentScan(basePackages = {"com.aggregate"})
#EnableAutoConfiguration
public class GServiceApplication extends SpringBootServletInitializer
implements ApplicationListener<ApplicationReadyEvent> {
#Autowired
private ApplicationContext applicationContext;
#Override
public void onApplicationEvent(ApplicationReadyEvent event) {
try {
String ip = InetAddress.getLocalHost().getHostAddress();
String port = applicationContext.getBean(Environment.class).getProperty("server.port");
System.out.printf("ip:port=" +ip+ ":"+port);
} catch (UnknownHostException e) {
e.printStackTrace();
}
}
public static void main(String[] args) throws UnknownHostException
{
SpringApplication application = new SpringApplication(GServiceApplication.class);
Properties properties = new Properties();
properties.put("server.port", 0);
properties.put("server.address", InetAddress.getLocalHost().getHostAddress());
application.setDefaultProperties(properties);
application.run(args);
}
}
Undertow started:- o.s.b.w.e.u.UndertowServletWebServer : Undertow started on port(s) 55646 (http) with context path '' as printed in console
Expected result:- ip:port=xx.xx.x.1x1:55646
Actual result:- ip:port=xx.xx.x.1x1:0

Answer 1

Passing in the port number 0 is a trick that the Java core ServerSocket class can do. Undertow isn't aware of this; it just assumes that the port is always fixed. So there's no official API to read the port number that is actually used; but if you find the Undertow object, you can do:
// assuming you only have one:
ListenerInfo listenerInfo = undertow.getListenerInfo().iterator().next();
InetSocketAddress socketAddress = (InetSocketAddress) listenerInfo.getAddress();
URI uri = URI.create(listenerInfo.getProtcol() + "://" + socketAddress.getHostString() + ":" + socketAddress.getPort());
HTH