Improve null-safety of module/spring-boot-tomcat

See gh-46926

module/spring-boot-tomcat/src/main/java/org/springframework/boot/tomcat/metrics/TomcatMetricsBinder.java

@@ -61,8 +61,9 @@ public class TomcatMetricsBinder implements ApplicationListener<ApplicationStart
 	public void onApplicationEvent(ApplicationStartedEvent event) {
 		ApplicationContext applicationContext = event.getApplicationContext();
 		Manager manager = findManager(applicationContext);
-		this.tomcatMetrics = new TomcatMetrics(manager, this.tags);
-		this.tomcatMetrics.bindTo(this.meterRegistry);
+		TomcatMetrics tomcatMetrics = new TomcatMetrics(manager, this.tags);
+		tomcatMetrics.bindTo(this.meterRegistry);
+		this.tomcatMetrics = tomcatMetrics;
 	}
 
 	private @Nullable Manager findManager(ApplicationContext applicationContext) {
@@ -89,8 +90,9 @@ public class TomcatMetricsBinder implements ApplicationListener<ApplicationStart
 
 	@Override
 	public void destroy() {
-		if (this.tomcatMetrics != null) {
-			this.tomcatMetrics.close();
+		TomcatMetrics tomcatMetrics = this.tomcatMetrics;
+		if (tomcatMetrics != null) {
+			tomcatMetrics.close();
 		}
 	}
 

module/spring-boot-tomcat/src/main/java/org/springframework/boot/tomcat/servlet/NestedJarResourceSet.java

@@ -119,17 +119,20 @@ class NestedJarResourceSet extends AbstractSingleArchiveResourceSet {
 
 	@Override
 	protected boolean isMultiRelease() {
-		if (this.multiRelease == null) {
+		Boolean multiRelease = this.multiRelease;
+		if (multiRelease == null) {
 			synchronized (this.archiveLock) {
-				if (this.multiRelease == null) {
+				multiRelease = this.multiRelease;
+				if (multiRelease == null) {
 					// JarFile.isMultiRelease() is final so we must go to the manifest
 					Manifest manifest = getManifest();
 					Attributes attributes = (manifest != null) ? manifest.getMainAttributes() : null;
-					this.multiRelease = (attributes != null) && attributes.containsKey(MULTI_RELEASE);
+					multiRelease = (attributes != null) && attributes.containsKey(MULTI_RELEASE);
+					this.multiRelease = multiRelease;
 				}
 			}
 		}
-		return this.multiRelease;
+		return multiRelease;
 	}
 
 	@Override