Unexpected query parameter order with UriComponentsBuilder

With spring-web 6.2.5, I get the following behavior :

System.out.println(UriComponentsBuilder.fromUriString(“http://myhost?a={p1}&b={p2}&a={p3}”).buildAndExpand(“a1”, “b1”, “a2”));

Output :

http://myhost?a=a1&a=b1&b=a2

I was expecting the following output :

http://myhost?a=a1&b=b1&a=a2

It seems that query params are reordered.

Even if it’s not a good idea to rely on query parameter order, it seems to be legal (see stackoverflow > Is it legal or safe to depend on the ordering of URL query parameters?)… So I think that things like RestClient may require proper query parameter order handling.

Furthermore, query params reordering seems to happen before uri template variables are expanded so values get swapped when given from an array (ie not from map) as the order of variables is then significant.

Start by reproducing the example with UriComponentsBuilder and the three repeated query parameters. Trace how query parameters are stored and how template variables are expanded, paying attention to the ordering change before expansion. Done means the expected a=a1, b=b1, a=a2 order is preserved for array-based expansion.