Spring Boot application default port

When you develop long enough with Spring Boot, you know the error very well. You work on your Spring Boot application, and you use a console for running the application. Or you use your favourite IDE runner for starting your application (my preferred IDE is IntelliJ IDEA). Your Spring Boot application in the running embedded in servlet container when suddenly you need to do something else. Instead of adequately terminating the embedded servlet container with your application, you detach from the servlet by a closing terminal window or switching to a different project in IDE. However, because you did not correctly terminate your IDE runner and did not correctly kill the terminal’s application process, the servlet with the application is still running. After that, you might, of course, start developing something else. Surprise, surprise, you cannot. Any Spring Boot application port, by default number 8080, is still occupied with the unterminated process of your Spring Boot servlet container. (In the tutorial, we assume you are using the Spring Boot application port on default port number 8080).

I guess you might be a little bit confused by all the wording describing the issue. Let’s take a look first at some terminology to understand the problem here fully:

What is process?

We are aware that readers of this blog are pretty skilled in computers. So, we will provide a brief description of what is computer process without going to further details.

Every time you start a program on your computer, it requires a certain amount of resource. Your Central Processor Unit, known as CPU, add to this program its process.

If you open task manager on your Windows, you will see a list of all process your CPU is trying to manage at once.

What is process ID?

The process ID, also known as PID, is the number assigned by the operating system to each new process.

What is port number?

A port number is how to identify individual processes for network message when they arrive on a server. In a real sense, a port number is the logical address of each application or process that helps identify the sender and receiver of processed messages.

You can imagine port numbers as doors in the hallway into which message flows through the networks. It leaves one door and enters another.

What is Spring Boot server container?

Spring Boot is a Spring project which makes Spring development fast. Without going too much into the Spring Boot project details, your Spring Boot application runs in a server container, better known as a servlet. This servlet runs in one computer process and lister on port 8080 for traffic.

Restart computer

One way how to solve this is to restart your computer. During the restart of Windows OS, start all the fresh, and port 8080 will be empty. But this is not the fastest and definitely not the right way for developers to solve this.

Set different port number

Another way is to solve this issue by assigning the Spring Boot application a different port number. When we set a different port number for the application, we will not kill the existing process with the application servlet container. However, we will create a new-different process for the servlet container, which will be listening on a different port. In this way, port 8080 will be still occupied, but we will at least be able to continue to develop.

Go to Spring Boot application.properties file and set different port number for server.port .

Despite this quick workaround, there is a much more mature way to solve this problem. I would like you to show a better way to kill a Spring Boot servlet container listening on port 8080.

Kill the process listening at port 8080

To kill the process listening at port 8080 in the Windows environment, we need only go through a few steps. First, check the image below to get the idea.

Find the process occupying port 8080

On the Windows, open your CMD terminal. With simple command netstat -ano | findstr PORT_NUMBER, where for PORT_NUMBER add port number 8080, and on command prompt output (hit enter), we will get the process id of process listening on port 8080.

Kill the process with command prompt

Now, all we need to do is just type the following command taskkill /F /PID PROCESS_ID and replace PROCESS_ID by the process id we got from the previous command result.

If you have typed everything correctly, process listening on port 8080 should be terminated.

How to install Azure CosmosDB Emulator

First of all, we need to install Azure CosmosDB Emulator locally. Go to https://docs.microsoft.com/en-us/azure/cosmos-db/local-emulator#installation, download the windows installation package and install Azure Cosmos DB Emulator.

When installing Azure Cosmos DB Emulator passed successfully on your Windows machine, start the emulator (if it did not start automatically).

If it is already running you can find it minimized in the system hidden in the running application system and either click Open Data Explorer … or visit https://localhost:8081/_explorer/index.html page. You can now get finally into the Azure CosmosDB Emulator.

Exporting security certificates

Azure Cosmos DB Emulator uses a secure connection with your application. We need to extract and register your certificate for communication with the emulator.

Let’s type “Manage user certificates” into Windows Start console or open Windows Certificate Manager (running certlm.msc in the run prompt). Go to Personal->Certificates folder and export certificate with DocumentDbEmulatorCertificate name. The whole tutorial how to do it can be found at Microsoft Azure website explaining how to export the Azure Cosmos DB SSL certificate.

You can name your file when exporting arbitrary. However, we will name it for our example documentdbemulatorcert.cer.

Now when I have extracted the SSL certificate, I need to upload it into the correct Java SDK location.

Check which version of Java do you use in your project. In IntelliJ idea you can use either keyword shortcut [ctrl] + [shift] + [alt] + [s] or go to File->Project Structure->SDKs and check which version your Spring Boot application uses or where SDK is located.

Copy certificate into the Java SDK directory. It should be copied into the %JAVA_HOME%/jre/lib/security location.

If you use OpenJDK, OpenJDK does not contain jre folder. Just copy it into %JAVA_HOME%/lib/security location.

Registering certificate

The nearly last thing we need to do is to register the certificate into the system.

We need to open the Windows Command prompt as an administrator and navigate at JDK’s folder at jdk\jre\lib\security.

Hint: A little hint if you are more familiar with Linux based systems than Windows. If you open a folder in Windows, copy the path to the folder and place it into quotation marks in the command prompt. Executing the command changes the directory instantly.

Then, upon the folder, we need just to run the following command to register the certificate into the keystore on your system:

keytool -keystore cacerts -importcert -alias documentdbemulator -file documentdbemulatorcert.cer

Hint: Keytool is a tool located in your Java installation, so you need to have jdk/bin on your PATH for this to work.

If you need a hint with registration, check this tutorial at Microsoft Azure website where you learn how to add a root certificate to the certification authority certificates storage. However instead of bc2025.cer name in tutorial use name of your certificate.

Otherwise, when the prompt asks for a password, type your password. More likely you didn’t change it, so default password is changeit. Following, you get a question, if you trust this certificate. Answer it simply yes.

That is it. The certificate is registered, and Azure Cosmos DB Emulator should be now created securely and with ease.

Using emulator with local Spring Boot app development

And all we need to do now is just run the Spring Boot application. If you implemented a connection to Cosmos DB and registered connection properties to application.properties or application.yml you can successfully run the application.

azure:
   cosmosdb:
    uri: https://localhost:8081
    key: VERY_LONG_KEY
    database: YOUR_DB_NAME

To fill up application.yml you will need to implement your own data:

You can find VERY_LONG_KEY in your Azure Cosmos DB Emulator Data Explorer under the Quick start tab.

And all we need to do now is just run the Spring Boot application. If you check now Data Explorer under Explorer tab, you can find now your database with all the defined structures.

Troubleshooting

In case you already successfully installed the Azure CosmosDB Emulator and you want to restart the database, you have an option to reset data. Data reset comes handy when you get an authentication issue over HTTPS with your Azure CosmosDB certificate on the application run.

Pick up “Reset data” from the Azure CosmosDB Emulator mini-panel dialogue window from your start button. Operation is not reversible; it will clean up all your caches and the internal state of the Azure CosmosDB Emulator database and reset the certificate.

Then it would be best if you continued extracting the certificate from the list of certificates according to the description in Exporting Security Certificates. However, before registering the certificate back to Java keytool, you need to remove the old alias on the certificate with a command like this:

keytool -delete -alias documentdbemulator -keystore cacerts -storepass changeit

Notes: behind -alias you place the name of the certificate, -keystore is the name of a file containing the certificates and behind -storepass is your password (default is changeit).