Optimize Request Predicate attribute merging

[poutsma]

When using router functions, the bulk of RequestPredicate execution is spent in attribute map merging. We should consider ways to improve this, for instance by using composite maps instead of allocating new ones.

[injae-kim]

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/function/RouterFunctions.java

Line 1247 in 6e9607c

return Collections.unmodifiableMap(new LinkedHashMap<>(attributes));

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/function/RouterFunctions.java

Lines 1267 to 1269 in 6e9607c

	Map<String, Object> attributes = new LinkedHashMap<>(this.attributes);
	attributes.put(name, value);
	return new AttributesRouterFunction<>(this.delegate, attributes);

What an interesting issue! I think we can use some CompositeMap instead of UnmodifiableMap & deep copy on every withAttributes() on AttributesRouterFunction.
(I'm not sure above is what this issue wants)

I fixed similar issue on another opensource before, so if you busy I can create PR :) thank you!

[poutsma]

@injae-kim There are a lot of intrinsic details that can go wrong here, which has caused many regressions in the past. So I'd rather tackle this one myself, thanks!

[injae-kim]

which has caused many regressions in the past.

Aha thanks for the explanation! I got it.
Please share on this issue if there's something I can help 😃