Polish "Use StackWalker to deduce main application class"

See gh-31701

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot;
 
+import java.lang.StackWalker.StackFrame;
 import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -26,9 +27,11 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Properties;
 import java.util.Set;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -275,12 +278,15 @@ public class SpringApplication {
 	}
 
 	private Class<?> deduceMainApplicationClass() {
-		return StackWalker.getInstance(StackWalker.Option.RETAIN_CLASS_REFERENCE)
-				.walk((s) -> s.filter(e -> Objects.equals(e.getMethodName(), "main")).findFirst()
-						.map(StackWalker.StackFrame::getDeclaringClass))
+		return StackWalker.getInstance(StackWalker.Option.RETAIN_CLASS_REFERENCE).walk(this::findMainClass)
 				.orElse(null);
 	}
 
+	private Optional<Class<?>> findMainClass(Stream<StackFrame> stack) {
+		return stack.filter((frame) -> Objects.equals(frame.getMethodName(), "main")).findFirst()
+				.map(StackWalker.StackFrame::getDeclaringClass);
+	}
+
 	/**
 	 * Run the Spring application, creating and refreshing a new
 	 * {@link ApplicationContext}.

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -23,7 +23,6 @@ import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
@@ -1315,35 +1314,6 @@ class SpringApplicationTests {
 						.accepts(hints);
 	}
 
-	@Test
-	void deduceMainApplicationClass() {
-		assertThat(
-				Objects.equals(deduceMainApplicationClassByStackWalker(), deduceMainApplicationClassByThrowException()))
-						.isTrue();
-	}
-
-	private Class<?> deduceMainApplicationClassByThrowException() {
-		try {
-			StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
-			for (StackTraceElement stackTraceElement : stackTrace) {
-				if ("main".equals(stackTraceElement.getMethodName())) {
-					return Class.forName(stackTraceElement.getClassName());
-				}
-			}
-		}
-		catch (ClassNotFoundException ex) {
-			// Swallow and continue
-		}
-		return null;
-	}
-
-	private Class<?> deduceMainApplicationClassByStackWalker() {
-		return StackWalker.getInstance(StackWalker.Option.RETAIN_CLASS_REFERENCE)
-				.walk((s) -> s.filter(e -> Objects.equals(e.getMethodName(), "main")).findFirst()
-						.map(StackWalker.StackFrame::getDeclaringClass))
-				.orElse(null);
-	}
-
 	private <S extends AvailabilityState> ArgumentMatcher<ApplicationEvent> isAvailabilityChangeEventWithState(
 			S state) {
 		return (argument) -> (argument instanceof AvailabilityChangeEvent<?>)