1. Overview
This tutorial will discuss a feature added in Spring Boot 2.5 that enables the possibility to specify a prefix for system environment variables. This way, we can run multiple different Spring Boot applications in the same environment as all properties will expect a prefixed version.
2. Environment Variable Prefixes
We might need to run multiple Spring Boot applications in the same environment and often face the problem of environment variable names to assign to different properties.
We could use Spring Boot properties which, in a way, can be similar, but we also might want to set a prefix at the application level to leverage on the environment side.
Let's set up, as an example, a simple Spring Boot application, and modify an application property, for example, the tomcat server port, by setting this prefix.
2.1. Our Spring Boot Application
Let's create a Spring Boot application to demonstrate this feature. First, let's add a prefix to the application. We call it “prefix” to keep it simple:
@SpringBootApplication
public class PrefixApplication {
public static void main(String[] args) {
SpringApplication application = new SpringApplication(PrefixApplication.class);
application.setEnvironmentPrefix("prefix");
application.run(args);
}
}We can't use, as a prefix, a word that already contains the underscore character (_). Otherwise, the application will throw an error.
We also want to make an endpoint to check at what port our application is listening to:
@Controller
public class PrefixController {
@Value(value = "${server.port}")
private int serverPort;
@GetMapping("/prefix")
public String getServerPortInfo(final Model model) {
model.addAttribute("serverPort", serverPort);
return "prefix";
}
}In this case, we are using Thymeleaf to resolve our template while setting our server port, with a simple body like:
<html>
// ...
<body>
It is working as we expected. Your server is running at port : <b th:text="${serverPort}"></b>
</body>
</html>2.2. Setting Environment Variables
We can now set our environment variable like prefix_server_port to, for example, 8085. We can see how to set system environment variables, for instance, in Linux.
Once we set the environment variable, we expect the application to create properties based on that prefix.
In the case of running from an IDE, we need to edit the launch configuration and add the environment variable or pick it from environment variables that are already loaded.
2.3. Running the Application
We can now start the application from the command line or with our favorite IDE.
If we load with our browser the URL http://localhost:8085/prefix, we can see that the server is running and at the port we prefixed earlier:
It is working as we expected. Your server is running at port : 8085The application won't start if we do not prefix the environment variable, as the property ${server.port} defined in the controller is not resolvable.
3. Conclusion
In this tutorial, we have seen how to use a prefix for environment variables with Spring Boot. It can help, for example, if we want to run multiple Spring Boot applications in the same environment and assign different values to a property with the same name, such as the server port.
As always, the code presented in this article is available over on GitHub.
