SpringBoot: How to get the port number from the code

 When you set server.port=0 in your Spring Boot application, the server will start on a random available port. Here's how to get the actual port being used:

Best Ways to Get the Random Port

1. Using ServletWebServerApplicationContext (Recommended)

java

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import org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext;
import org.springframework.stereotype.Component;

@Component
public class RandomPortProvider {
    
    private final ServletWebServerApplicationContext serverContext;
    
    public RandomPortProvider(ServletWebServerApplicationContext serverContext) {
        this.serverContext = serverContext;
    }
    
    public int getAssignedPort() {
        return serverContext.getWebServer().getPort();
    }
}

2. Using @LocalServerPort

java

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import org.springframework.boot.web.server.LocalServerPort;
import org.springframework.stereotype.Component;

@Component
public class RandomPortProvider {
    
    @LocalServerPort
    private int actualPort;
    
    public int getPort() {
        return actualPort;
    }
}

3. In Tests (Using Spring Boot Test Utilities)

If you're writing tests:

java

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import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.web.server.LocalServerPort;
import org.junit.jupiter.api.Test;

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class MyIntegrationTest {
    
    @LocalServerPort
    private int randomPort;
    
    @Test
    public void testWithRandomPort() {
        System.out.println("Server is running on port: " + randomPort);
        // Your test code here
    }
}

Important Notes

1. The @Value("${server.port}") approach won't work for random ports - it will return 0, not the actual assigned port.

2. These methods will give you the actual port:

   • After the application context has started

   • After the server is initialized

3. If you need the port very early in the startup process (like in a CommandLineRunner or ApplicationRunner), make sure to get it after the server has started.

Example Usage in Application Startup

java

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import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext;
import org.springframework.stereotype.Component;

@Component
public class PortLogger implements CommandLineRunner {
    
    private final ServletWebServerApplicationContext webServerAppCtxt;
    
    public PortLogger(ServletWebServerApplicationContext webServerAppCtxt) {
        this.webServerAppCtxt = webServerAppCtxt;
    }
    
    @Override
    public void run(String... args) {
        int port = webServerAppCtxt.getWebServer().getPort();
        System.out.println("Server is running on port: " + port);
    }
}

This approach is reliable whether you're using a fixed port or server.port=0.

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Below is a sample how to get it

package com.example.greet_service_with_eureka.config;

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext;
import org.springframework.stereotype.Component;

@Component
public class PortLogger implements CommandLineRunner {
    
    private final ServletWebServerApplicationContext webServerAppCtxt;
    private int port;
    
    public PortLogger(ServletWebServerApplicationContext webServerAppCtxt) {
        this.webServerAppCtxt = webServerAppCtxt;
    }

    public int getPort() {
        return port;
    }

    public void setPort(int port) {
        this.port = port;
    }

    @Override
    public void run(String... args) {
        setPort(webServerAppCtxt.getWebServer().getPort());
        System.out.println("Server is running on port: " + port);
    }
}

package com.example.greet_service_with_eureka.config;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.web.ServerProperties;
import org.springframework.stereotype.Component;

@Component
public class MyComponent {

    @Autowired
    private ServerProperties serverProperties;

    public int getServerPort() {
        return serverProperties.getPort();
    }
}

package com.example.greet_service_with_eureka.controller;


import com.example.greet_service_with_eureka.config.MyComponent;
import com.example.greet_service_with_eureka.config.PortLogger;
import com.example.greet_service_with_eureka.model.Greet;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;


@RestController
public class GreetController {

    private String message = "Demo with RestTemplate";

    @Value("${server.port}")
    private String serverPort;

    @Autowired
    MyComponent myComponent;

    @Autowired
    PortLogger portLogger;

    @RequestMapping("/greet")
    public Greet greet() {
        System.out.println("### code 101 Inside greet");
        return new Greet(message +"="+serverPort +"   /from code="+myComponent.getServerPort() +"  /fromCommandLiner="+portLogger.getPort());
    }
}

Test greet with

http://localhost:62410/greet

Note:port is random

Yo can see only from  you can get the port CommandLineRunner

output would like

{"message": "Demo with RestTemplate=0   /from code=0  /fromCommandLiner=62410"}