Helidon SE provides a web server using Java virtual threads. When we configure the web server we can specify a specific port number the server will listen on for incoming request. If we want to use a random port number we must specify the value 0. Helidon will then start the web server on a random port number that is available on our machine.

The following example shows how to start a web server on a random port number. We use Helidon SE to write our code:

package mrhaki.helidon;

import io.helidon.logging.common.LogConfig;
import io.helidon.webserver.WebServer;

public class Application {
    public static void main(String[] args) {
        // Load logging configuration.
        LogConfig.configureRuntime();

        // Configure web server on a random port number.
        WebServer server = WebServer.builder()
            .port(0)  // Use random port number
            .build()
            .start();

        // Print port number the server is listening on.
        System.out.println("WEB server is up at http://localhost:" + server.port());
    }
}

When we start our application we see the following output:

2024.10.11 17:27:36.005 Logging at runtime configured using classpath: /logging.properties
2024.10.11 17:27:36.606 Helidon SE 4.1.2 features: [Config, Encoding, Media, WebServer]
2024.10.11 17:27:36.621 [0x2326f965] http://0.0.0.0:61685 bound for socket '@default'
2024.10.11 17:27:36.639 Started all channels in 28 milliseconds. 863 milliseconds since JVM startup. Java 21.0.4+7-LTS
WEB server is up at http://localhost:61685

The next time we start our application we see a different port number:

2024.10.11 17:28:11.283 Logging at runtime configured using classpath: /logging.properties
2024.10.11 17:28:11.852 Helidon SE 4.1.2 features: [Config, Encoding, Media, WebServer]
2024.10.11 17:28:11.873 [0x28b386dd] http://0.0.0.0:61698 bound for socket '@default'
2024.10.11 17:28:11.892 Started all channels in 41 milliseconds. 835 milliseconds since JVM startup. Java 21.0.4+7-LTS
WEB server is up at http://localhost:61698

We can also use the Helidon Configuration API to configure the web server to use a random port number. We can for example set the port number to 0 in the application.yaml file. In the following example we initialize standard configuration and use it configure the webserver:

package mrhaki.helidon;

import io.helidon.config.Config;
import io.helidon.logging.common.LogConfig;
import io.helidon.webserver.WebServer;

public class Application {
    public static void main(String[] args) {
        // Load logging configuration.
        LogConfig.configureRuntime();

        // Initialize the configuration.
        Config config = Config.create();
        Config.global(config);

        // Configure web server on a random port number.
        WebServer server = WebServer.builder()
            .config(config.get("server"))  // Get port number from configuration.
            .build()
            .start();

        // Print port number the server is listening on.
        System.out.println("WEB server is up at http://localhost:" + server.port());
    }
}

With our new configuration we can use an environment variable SERVER_PORT to set the port number to 0 for our web server. The configuration could also be defined in an application.yaml file:

server:
  port: 0

Written with Helidon SE 4.2.1.

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