RestTempalte customizer can return an instance of rest template; fixes gh-1440 (#1446)

spring-cloud-sleuth-zipkin/src/main/java/org/springframework/cloud/sleuth/zipkin2/DefaultZipkinRestTemplateCustomizer.java

@@ -28,7 +28,7 @@ import org.springframework.web.client.RestTemplate;
 
 /**
  * Default {@link ZipkinRestTemplateCustomizer} that provides the GZip compression if
- * {@link ZipkinProperties#compression} is enabled.
+ * {@link ZipkinProperties#getCompression()} is enabled.
  *
  * @author Marcin Grzejszczak
  * @since 1.1.0
@@ -42,10 +42,11 @@ public class DefaultZipkinRestTemplateCustomizer implements ZipkinRestTemplateCu
 	}
 
 	@Override
-	public void customize(RestTemplate restTemplate) {
+	public RestTemplate customizeTemplate(RestTemplate restTemplate) {
 		if (this.zipkinProperties.getCompression().isEnabled()) {
 			restTemplate.getInterceptors().add(0, new GZipInterceptor());
 		}
+		return restTemplate;
 	}
 
 	private class GZipInterceptor implements ClientHttpRequestInterceptor {

spring-cloud-sleuth-zipkin/src/main/java/org/springframework/cloud/sleuth/zipkin2/ZipkinRestTemplateCustomizer.java

@@ -31,6 +31,25 @@ import org.springframework.web.client.RestTemplate;
  */
 public interface ZipkinRestTemplateCustomizer {
 
-	void customize(RestTemplate restTemplate);
+	/**
+	 * Customizes the {@link RestTemplate}.
+	 * @deprecated use
+	 * {@link ZipkinRestTemplateCustomizer#customizeTemplate(RestTemplate)}
+	 * @param restTemplate object to customize
+	 */
+	@Deprecated
+	default void customize(RestTemplate restTemplate) {
+		customizeTemplate(restTemplate);
+	};
+
+	/**
+	 * Customizes the {@link RestTemplate} instance. Might return a new one if necessary.
+	 * @param restTemplate default object to customize
+	 * @return customized {@link RestTemplate} or a new object
+	 * @since 2.2.0
+	 */
+	default RestTemplate customizeTemplate(RestTemplate restTemplate) {
+		return restTemplate;
+	}
 
 }

spring-cloud-sleuth-zipkin/src/main/java/org/springframework/cloud/sleuth/zipkin2/sender/ZipkinRestTemplateSenderConfiguration.java

@@ -58,7 +58,7 @@ class ZipkinRestTemplateSenderConfiguration {
 	public Sender restTemplateSender(ZipkinProperties zipkin,
 			ZipkinRestTemplateCustomizer zipkinRestTemplateCustomizer) {
 		RestTemplate restTemplate = new ZipkinRestTemplateWrapper(zipkin, this.extractor);
-		zipkinRestTemplateCustomizer.customize(restTemplate);
+		restTemplate = zipkinRestTemplateCustomizer.customizeTemplate(restTemplate);
 		return new RestTemplateSender(restTemplate, zipkin.getBaseUrl(),
 				zipkin.getEncoder());
 	}