Use absolute path when showing classpath where main class was not found

Closes gh-40463

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/JavaPluginAction.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -174,7 +174,7 @@ final class JavaPluginAction implements PluginApplicationAction {
 				.provider(() -> (String) bootJar.getManifest().getAttributes().get("Start-Class"));
 			bootJar.getMainClass()
 				.convention(resolveMainClassName.flatMap((resolver) -> manifestStartClass.isPresent()
-						? manifestStartClass : resolveMainClassName.get().readMainClassName()));
+						? manifestStartClass : resolver.readMainClassName()));
 			bootJar.getTargetJavaVersion()
 				.set(project.provider(() -> javaPluginExtension(project).getTargetCompatibility()));
 			bootJar.resolvedArtifacts(runtimeClasspath.getIncoming().getArtifacts().getResolvedArtifacts());

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/ResolveMainClassName.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -153,9 +153,8 @@ public class ResolveMainClassName extends DefaultTask {
 		String classpath = getClasspath().filter(File::isDirectory)
 			.getFiles()
 			.stream()
-			.map((directory) -> getProject().getProjectDir().toPath().relativize(directory.toPath()))
-			.map(Path::toString)
-			.collect(Collectors.joining(","));
+			.map(File::getAbsolutePath)
+			.collect(Collectors.joining(File.pathSeparator));
 		return this.outputFile.map(new ClassNameReader(classpath));
 	}
 

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/WarPluginAction.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -96,7 +96,7 @@ class WarPluginAction implements PluginApplicationAction {
 					.provider(() -> (String) bootWar.getManifest().getAttributes().get("Start-Class"));
 				bootWar.getMainClass()
 					.convention(resolveMainClassName.flatMap((resolver) -> manifestStartClass.isPresent()
-							? manifestStartClass : resolveMainClassName.get().readMainClassName()));
+							? manifestStartClass : resolver.readMainClassName()));
 				bootWar.getTargetJavaVersion()
 					.set(project.provider(() -> javaPluginExtension(project).getTargetCompatibility()));
 				bootWar.resolvedArtifacts(runtimeClasspath.getIncoming().getArtifacts().getResolvedArtifacts());

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/run/BootTestRunIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -113,7 +113,7 @@ class BootTestRunIntegrationTests {
 		else {
 			assertThat(result.getOutput())
 				.contains("Main class name has not been configured and it could not be resolved from classpath "
-						+ String.join(File.separator, "build", "classes", "java", "test"));
+						+ canonicalPathOf("build/classes/java/test"));
 		}
 	}