Prevent loadbalancer filter to wrap exception into RuntimeException

[ComputerDaddyGuy]

Current behavior

• In order to allow loadbalancer lifecycle processors to handle failed requests, the lb filter catches any Exception thrown from subsequent filters. After processing, the exception is rethrown but wrapped into a RuntimeException
• This wrap is (I suppose) originally due to usage of lambda, which does not allow to throw checked exceptions.

return (request, next) -> {
	// ...
	try {
		// Call subsequent filters
	}
	catch (Exception e) {
		supportedLifecycleProcessors.forEach(...); // Handle CompletionContext.Status.FAILED
		throw new RuntimeException(e); // <-- wrap and rethrow
	}
};

Issue

• Wrapping into a RuntimeException prevents error handlers to catch the original exception

var builder = RouterFunctions.route();

// Not working because of RuntimeException wrap
builder = builder.onError(MyCustomException.class::isInstance, (ex, request) -> ...);

// Custom exception is catched here
builder = builder.onError(_ -> true, (ex, request) -> ...);

Proposed solution in this PR

• The lb filter has been adapted to use an anonymous class instead of a lambda
• The original exception is rethrown as is, without any change or wrap
• A test has been added to ensure the lb filter does not wrap the original exception

[ComputerDaddyGuy]

Any chance that maybe @ryanjbaxter or @spencergibb (or any one else) review this PR? Is it missing something, or maybe should I abandon it?
Thank you in advance!

[spencergibb]

Please be patient

[Copilot]

Pull request overview

This PR updates the Spring Cloud Gateway Server MVC load balancer (lb) filter so that exceptions from downstream filters are rethrown without being wrapped in a RuntimeException, enabling router error handlers to match on the original exception type.

Changes:

• Refactors the lb(...) filter implementation to rethrow the original exception instead of wrapping it.
• Adds an integration test route and assertion verifying the original exception type/message are preserved.
• Introduces a test-only filter chain that captures exception details into response headers for verification.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
spring-cloud-gateway-server-webmvc/src/main/java/org/springframework/cloud/gateway/server/mvc/filter/LoadBalancerFilterFunctions.java	Stops wrapping downstream exceptions in RuntimeException and rethrows the original exception after lifecycle completion processing.
spring-cloud-gateway-server-webmvc/src/test/java/org/springframework/cloud/gateway/server/mvc/ServerMvcIntegrationTests.java	Adds a new integration test and router function to verify exceptions are not wrapped by the load balancer filter.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[ryanjbaxter]

I also cherrypicked this into the 4.3.x branch 0420992

[ComputerDaddyGuy]

Thank you! ☺️