How do you handle cross-origin requests in Spring Boot?

Table of Contents

• Introduction
• 1. Understanding CORS in Spring Boot
• 2. Global CORS Configuration in Spring Boot
  • Example: Global CORS Configuration
  • Explanation:
  • Global CORS Example in Action:
• 3. Controller-Level CORS Configuration
  • Example: Controller-Level CORS Configuration
  • Explanation:
  • Controller-Level CORS Example in Action:
• 4. Configuring CORS for Specific HTTP Methods
  • Example: Specific Methods with @CrossOrigin
  • Explanation:
• 5. Handling Preflight Requests
  • Example: Handling Preflight with @CrossOrigin
• 6. Using Spring Security with CORS
  • Example: Configuring CORS with Spring Security
• Conclusion

Introduction

Handling cross-origin requests is an essential part of modern web development, especially when your backend and frontend applications are hosted on different domains or ports. Cross-Origin Resource Sharing (CORS) is a mechanism that allows a web page from one origin to request resources from another origin.

In Spring Boot, CORS can be configured to allow or restrict cross-origin requests based on your security and functional requirements. In this guide, we’ll explore how to handle CORS in Spring Boot applications at both global and controller levels.

1. Understanding CORS in Spring Boot

CORS is a protocol that allows or denies HTTP requests made from one origin to access resources from another origin. By default, browsers block such requests for security reasons. When the CORS policy is configured correctly, the browser will allow the cross-origin request based on the rules set by the server.

Spring Boot provides several ways to handle CORS, from global configuration to controller-specific settings. Let’s look at how to configure CORS in your Spring Boot application.

2. Global CORS Configuration in Spring Boot

The simplest way to configure CORS globally in a Spring Boot application is by using a @Configuration class and defining a CorsMapping to specify which domains are allowed to make requests to your server.

Example: Global CORS Configuration

You can configure global CORS in Spring Boot by defining a WebMvcConfigurer bean in your configuration class:

Explanation:

• addMapping("/**"): This applies the CORS configuration to all endpoints in the application.
• allowedOrigins(...): Specifies which domains are allowed to make cross-origin requests.
• allowedMethods(...): Specifies which HTTP methods (e.g., GET, POST) are allowed for cross-origin requests.
• allowedHeaders("*"): Allows all headers in the request.
• allowCredentials(true): Allows sending cookies and authentication information with requests.

Global CORS Example in Action:

Once configured, any cross-origin requests to your Spring Boot backend from the specified domains (localhost:4200 and example.com) will be allowed, ensuring the proper headers are included in the response.

3. Controller-Level CORS Configuration

In some cases, you may want to allow cross-origin requests only for specific controllers or methods. Spring Boot makes this easy by using the @CrossOrigin annotation at the controller or method level.

Example: Controller-Level CORS Configuration

Explanation:

• @CrossOrigin(origins = "http://localhost:4200"): This annotation enables CORS for the /users endpoint, allowing requests from http://localhost:4200.
• The CORS configuration is only applied to the specific method (getUsers()), so other endpoints won't be affected unless you apply @CrossOrigin to them as well.

Controller-Level CORS Example in Action:

This method will now allow cross-origin requests from the frontend application running on http://localhost:4200, but other methods in the controller will not be affected unless they also have the @CrossOrigin annotation.

4. Configuring CORS for Specific HTTP Methods

You can fine-tune the CORS configuration for different HTTP methods (GET, POST, PUT, DELETE, etc.) by setting the allowed methods for specific controller actions or globally.

Example: Specific Methods with @CrossOrigin

Explanation:

• The @CrossOrigin annotation on the getProducts() method allows only GET requests from the specified origin.
• The @CrossOrigin annotation on the addProduct() method allows only POST requests from the same origin.

This way, you can restrict which methods are allowed for CORS requests from specific origins.

5. Handling Preflight Requests

When a browser sends a cross-origin request with custom headers or certain HTTP methods (like PUT or DELETE), it sends an OPTIONS request first, known as the preflight request, to check if the server allows the cross-origin request.

Spring Boot automatically handles preflight OPTIONS requests when you configure CORS. However, you can also customize how OPTIONS requests are handled using @CrossOrigin or the global configuration.

Example: Handling Preflight with @CrossOrigin

In this example:

• The @CrossOrigin annotation ensures the correct headers are sent in the preflight response.

6. Using Spring Security with CORS

If your Spring Boot application is secured with Spring Security, you need to configure CORS at the security level as well. The following code demonstrates how to configure CORS within a Spring Security configuration:

Example: Configuring CORS with Spring Security

This configuration integrates CORS support directly into the Spring Security filter chain, ensuring that CORS headers are properly applied to the responses.

Conclusion

Handling cross-origin requests is essential when developing modern web applications with separate frontend and backend services. Spring Boot provides several flexible ways to configure CORS:

1. Global CORS Configuration: Set up CORS for the entire application using a WebMvcConfigurer.
2. Controller-Level CORS: Apply CORS to specific controllers or methods using the @CrossOrigin annotation.
3. Preflight Request Handling: Spring Boot automatically handles preflight requests, but you can customize them if needed.
4. Spring Security Integration: Ensure CORS is configured alongside Spring Security for secured applications.

By using these methods, you can safely enable cross-origin resource sharing in your Spring Boot application, ensuring that your frontend and backend can communicate securely and effectively.