Polish OAuth2TokenIntrospectionClient

Placed URI.create in constructor so that the code doesn't do that
processing on each request. Also moved the construction helper methods
up by the constructor for added readability.

Issue: gh-6798

oauth2/oauth2-resource-server/src/main/java/org/springframework/security/oauth2/server/resource/introspection/NimbusOAuth2TokenIntrospectionClient.java

@@ -74,7 +74,7 @@ public class NimbusOAuth2TokenIntrospectionClient implements OAuth2TokenIntrospe
 		Assert.notNull(clientId, "clientId cannot be null");
 		Assert.notNull(clientSecret, "clientSecret cannot be null");
 
-		this.requestEntityConverter = this.defaultRequestEntityConverter(introspectionUri);
+		this.requestEntityConverter = this.defaultRequestEntityConverter(URI.create(introspectionUri));
 		RestTemplate restTemplate = new RestTemplate();
 		restTemplate.getInterceptors().add(new BasicAuthenticationInterceptor(clientId, clientSecret));
 		this.restOperations = restTemplate;
@@ -93,10 +93,30 @@ public class NimbusOAuth2TokenIntrospectionClient implements OAuth2TokenIntrospe
 		Assert.notNull(introspectionUri, "introspectionUri cannot be null");
 		Assert.notNull(restOperations, "restOperations cannot be null");
 
-		this.requestEntityConverter = this.defaultRequestEntityConverter(introspectionUri);
+		this.requestEntityConverter = this.defaultRequestEntityConverter(URI.create(introspectionUri));
 		this.restOperations = restOperations;
 	}
 
+	private Converter<String, RequestEntity<?>> defaultRequestEntityConverter(URI introspectionUri) {
+		return token -> {
+			HttpHeaders headers = requestHeaders();
+			MultiValueMap<String, String> body = requestBody(token);
+			return new RequestEntity<>(body, headers, HttpMethod.POST, introspectionUri);
+		};
+	}
+
+	private HttpHeaders requestHeaders() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON_UTF8));
+		return headers;
+	}
+
+	private MultiValueMap<String, String> requestBody(String token) {
+		MultiValueMap<String, String> body = new LinkedMultiValueMap<>();
+		body.add("token", token);
+		return body;
+	}
+
 	/**
 	 * {@inheritDoc}
 	 */
@@ -127,26 +147,6 @@ public class NimbusOAuth2TokenIntrospectionClient implements OAuth2TokenIntrospe
 		this.requestEntityConverter = requestEntityConverter;
 	}
 
-	private Converter<String, RequestEntity<?>> defaultRequestEntityConverter(String introspectionUri) {
-		return token -> {
-			HttpHeaders headers = requestHeaders();
-			MultiValueMap<String, String> body = requestBody(token);
-			return new RequestEntity<>(body, headers, HttpMethod.POST, URI.create(introspectionUri));
-		};
-	}
-
-	private HttpHeaders requestHeaders() {
-		HttpHeaders headers = new HttpHeaders();
-		headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON_UTF8));
-		return headers;
-	}
-
-	private MultiValueMap<String, String> requestBody(String token) {
-		MultiValueMap<String, String> body = new LinkedMultiValueMap<>();
-		body.add("token", token);
-		return body;
-	}
-
 	private ResponseEntity<String> makeRequest(RequestEntity<?> requestEntity) {
 		try {
 			return this.restOperations.exchange(requestEntity, String.class);