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Table of Contents

Configure CORS in a Micronaut application

Learn how to configure Cross-Origin Resource Sharing (CORS) in a Micronaut application.

Authors: Sergio del Amo

Micronaut Version: 3.9.2

1. Getting Started

In this guide, we will create a Micronaut application written in Java.

In this guide, you will configure Cross-Origin Resource Sharing (CORS) in a Micronaut application:

Cross-Origin Resource Sharing (CORS) is an HTTP-header based mechanism that allows a server to indicate any origins (domain, scheme, or port) other than its own from which a browser should permit loading resource

2. What you will need

To complete this guide, you will need the following:

  • Some time on your hands

  • A decent text editor or IDE

  • JDK 1.8 or greater installed with JAVA_HOME configured appropriately

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

4. Writing the Application

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide --build=maven --lang=java
If you don’t specify the --build argument, Gradle is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

Setup IntelliJ IDEA to develop Micronaut Applications.

5. Static Website

In a different folder, create a static website. It makes an HTTP Get Request via Javascript, and it updates an HTML element with the response.

src/main/../../index.html
<!DOCTYPE html>
<html>
        <head>
                <script>
                function request(method, url) {
                        const xhr = new XMLHttpRequest();
                        xhr.open(method, url);
                        xhr.onreadystatechange = function () {
                                document.getElementById("message").innerHTML = this.responseText;
                        };
                        xhr.send();                                        
                }
                request("GET","http://localhost:8080/hello");                
                </script>                
        </head>
        <body>
        <h1 id="message">CORS not setup</h1>
        </body>
</html>

Add a Controller, which the static website consumes via Javascript.

5.1. Controller

In order to create a microservice that responds with "Hello World" you first need a controller.

Create a Controller:

src/main/java/example/micronaut/HelloController.java
package example.micronaut;

import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.annotation.Produces;

@Controller("/hello") (1)
public class HelloController {
    @Get (2)
    @Produces(MediaType.TEXT_PLAIN) (3)
    public String index() {
        return "Hello World"; (4)
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /hello.
2 The @Get annotation maps the index method to an HTTP GET request on /hello.
3 By default, a Micronaut response uses application/json as Content-Type. We are returning a String, not a JSON object, so we set it to text/plain.
4 A String "Hello World" is returned as the result

6. Running the Application

To run the application, use the ./mvnw mn:run command, which starts the application on port 8080.

7. Run the static website

Serve the static website with jwebserver

The jwebserver tool provides a minimal HTTP server, designed to be used for prototyping, testing, and debugging. It serves a single directory hierarchy, and only serves static files. Only HTTP/1.1 is supported; HTTP/2 and HTTPS are not supported.

To use jwebserver, use Java 18 or greater in a terminal window. You can use SDKMan to get a list of available Java versions for your computer via sdk list java

For example:

% sdk use java 19.0.2-amzn
Using java version 19.0.2-amzn in this shell.
% jwebserver
Binding to loopback by default. For all interfaces use "-b 0.0.0.0" or "-b ::".
Serving ... and subdirectories on 127.0.0.1 port 8000
URL http://127.0.0.1:8000/

jwebserver serves the UI in port 8000

7.1. CORS Error

Open http://127.0.0.1:8000/, and you will see an error such as:

cors

7.2. Enable CORS Configuration

Enable CORS configuration

src/main/resources/application.yml
---
micronaut:
  server:
    cors:
      enabled: true

Restart the Micronaut application.

Refresh the UI in your browser, and you should see Hello World and no errors in the console.

7.3. Fine-grained CORS Configuration

The previous setup is a "wide open" configuration. It is best to enable CORS access only to a specific list of allowed origins.

You can do that via:

src/main/resources/application.yml
---
micronaut:
  server:
    cors:
      enabled: true

      configurations:
        ui:
          allowed-origins:
            - http://127.0.0.1:8000

You can start the static server in a different port. For example, You could run jwebserver -p 8001 and verify that it is not allowed. The application’s configuration only allows requests with origin 127.0.0.1:8000

8. Next steps

Explore more features with Micronaut Guides.

Learn more about configuring CORS with the Micronaut Framework.

9. Help with the Micronaut Framework

The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.