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Learn how to retrieve the actual server port in a Spring Boot 3 application started with a port set to `0`. This guide breaks down the solution and provides tips for success.
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How to Obtain the Actual Server Port in a Spring Boot 3 Application

If you've recently upgraded to Spring Boot 3 and found yourself grappling with how to retrieve the actual server port for your application, you're not alone. When starting a Spring Boot application with the server port set to 0, the application dynamically binds to an available port. This can cause some confusion when trying to determine which port was actually assigned upon startup. In this post, we will explore how to retrieve this information in a programmatic sense and address some common issues that might arise during this process.

The Problem

When you start a Spring Boot application with a command like this:

[[See Video to Reveal this Text or Code Snippet]]

the application binds to a random, available port on your server. However, you often need to know what port has been assigned for various reasons, such as logging, debugging, or configuring other services that interact with your Spring Boot application.

Previous Approach in Spring Boot 2

In earlier versions of Spring Boot (like 2.x), you could easily retrieve the assigned port using the following code snippet:

[[See Video to Reveal this Text or Code Snippet]]

However, this does not seem to work in Spring Boot 3, leading many developers to search for a new solution.

Solution Overview

Retaining the ability to grab the actual server port in Spring Boot 3 is important. Fortunately, the solution aligns closely with the previous methods from version 2. Here are the steps you can follow to accomplish this.

Steps to Retrieve the Server Port

Start Your Application: Begin by launching your Spring Boot application with the server port set to 0.

Access the Application Context: Use ConfigurableApplicationContext to hold your application context. This is where you'd typically get access to your application’s beans and properties.

Retrieve the Server Port: Use the same approach that worked in Spring Boot 2.x, with a slight adjustment. The local.server.port should still be accessible from the environment properties.

Here’s a modified version that can work in Spring Boot 3:

[[See Video to Reveal this Text or Code Snippet]]

Ensuring Proper Configuration

If you encounter issues retrieving the port, ensure that:

Your application is indeed running a web server.

Logging is enabled for Spring classes, allowing you to see any potential error messages or problems during startup.

Conclusion

Although you might have faced challenges when transitioning to Spring Boot 3, retrieving the actual server port after starting your application with a port set to 0 is still possible. By leveraging the application context and accessing the right properties, you can easily obtain this essential piece of information for your Spring Boot application.

Final Note

If you ever encounter instances where you cannot retrieve the port, check your application’s setup. Sometimes, the unexpected may occur due to misconfiguration, such as starting without the web server or having restricted logging. Being aware of these pitfalls will help you troubleshoot more efficiently.

With this understanding, you can confidently manage your Spring applications and integrate them into your overall architecture.