MetricsRestTemplateCustomizer and LocalHostUriTemplateHandler cannot be used together when building a RestTemplate with RestTemplateBuilder

[jhorstmann]

The changes in #25744 break some usages of LocalHostUriTemplateHandler. The following code fails after updating from 2.4.4 to 2.4.6

restTemplate = restTemplateBuilder.defaultMessageConverters()
        .uriTemplateHandler(LocalHostUriTemplateHandler(context.environment))
        .errorHandler(DefaultResponseErrorHandler())
        .build()

with

java.lang.IllegalArgumentException: RootUri must not be null

at org.springframework.util.Assert.notNull(Assert.java:201)
at org.springframework.boot.web.client.RootUriTemplateHandler.<init>(RootUriTemplateHandler.java:61)
at org.springframework.boot.web.client.RootUriTemplateHandler.withHandlerWrapper(RootUriTemplateHandler.java:96)
at org.springframework.boot.actuate.metrics.web.client.MetricsClientHttpRequestInterceptor.createUriTemplateHandler(MetricsClientHttpRequestInterceptor.java:105)
at org.springframework.boot.actuate.metrics.web.client.MetricsRestTemplateCustomizer.customize(MetricsRestTemplateCustomizer.java:61)
at org.springframework.boot.web.client.RestTemplateBuilder.configure(RestTemplateBuilder.java:634)
at org.springframework.boot.web.client.RestTemplateBuilder.build(RestTemplateBuilder.java:589)

The LocalHostUriTemplateHandler extends RootUriTemplateHandler and so gets the special handling introduced here. However, it does not have the rootUri field set and instead override the getRootUri method, which causes this line to pass a null rootUri

[wilkinsona]

Thanks for the report, @jhorstmann.

Interestingly, the auto-configured TestRestTemplate isn't affected by this despite it using both MetricsClientHttpRequestInterceptor and LocalHostUriTemplateHandler. I think that's due to ordering. The factory overwrites the URI template handler with a LocalHostUriTemplateHandler:

spring-boot/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/TestRestTemplateContextCustomizer.java

Lines 138 to 140 in c521437

	LocalHostUriTemplateHandler handler = new LocalHostUriTemplateHandler(applicationContext.getEnvironment(),
	sslEnabled ? "https" : "http");
	template.setUriTemplateHandler(handler);

This leaves MetricsClientHttpRequestInterceptor without its CapturingUriTemplateHandler so it won't see any URI templates. I'm not sure that this matters for TestRestTemplate as metrics are unlikely to be important for requests made in test code but it does leave things in a bit of an odd state.