Polish "Provide access to root URI from TestRestTemplate"

See gh-10641

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

@@ -166,10 +166,14 @@ public class TestRestTemplate {
 		this.restTemplate.setUriTemplateHandler(handler);
 	}
 
+	/**
+	 * Returns the root URI applied by a {@link RootUriTemplateHandler} or {@code ""} if
+	 * the root URI is not available.
+	 * @return the root URI
+	 */
 	public String getRootUri() {
 		UriTemplateHandler uriTemplateHandler = this.restTemplate.getUriTemplateHandler();
-		if (RootUriTemplateHandler.class
-				.isAssignableFrom(uriTemplateHandler.getClass())) {
+		if (uriTemplateHandler instanceof RootUriTemplateHandler) {
 			return ((RootUriTemplateHandler) uriTemplateHandler).getRootUri();
 		}
 		return "";

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

@@ -86,7 +86,6 @@ public class TestRestTemplateTests {
 	public void getRootUriRootUriSetViaRestTemplateBuilder() {
 		String rootUri = "http://example.com";
 		RestTemplate delegate = new RestTemplateBuilder().rootUri(rootUri).build();
-
 		assertThat(new TestRestTemplate(delegate).getRootUri()).isEqualTo(rootUri);
 	}
 
@@ -98,7 +97,6 @@ public class TestRestTemplateTests {
 				LocalHostUriTemplateHandler.class);
 		given(templateHandler.getRootUri()).willReturn(rootUri);
 		template.setUriTemplateHandler(templateHandler);
-
 		assertThat(template.getRootUri()).isEqualTo(rootUri);
 	}