Limit use of deprecated mainClassName in Gradle plugin tests

Closes gh-27320

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-build-image-env-runtime.gradle

@@ -4,7 +4,7 @@ plugins {
 }
 
 bootJar {
-	mainClassName 'com.example.ExampleApplication'
+	mainClass = 'com.example.ExampleApplication'
 }
 
 // tag::env-runtime[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-build-image-publish.gradle

@@ -4,7 +4,7 @@ plugins {
 }
 
 bootJar {
-	mainClassName 'com.example.ExampleApplication'
+	mainClass = 'com.example.ExampleApplication'
 }
 
 // tag::publish[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-build-image-publish.gradle.kts

@@ -7,7 +7,7 @@ plugins {
 }
 
 tasks.getByName<BootJar>("bootJar") {
-	mainClassName = "com.example.ExampleApplication"
+	mainClass.set("com.example.ExampleApplication")
 }
 
 // tag::publish[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/bundling/AbstractBootArchiveIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -66,6 +66,14 @@ abstract class AbstractBootArchiveIntegrationTests {
 				.isEqualTo(TaskOutcome.SUCCESS);
 	}
 
+	@Deprecated
+	@TestTemplate
+	void basicBuildUsingDeprecatedMainClassName()
+			throws InvalidRunnerConfigurationException, UnexpectedBuildFailure, IOException {
+		assertThat(this.gradleBuild.build(this.taskName).task(":" + this.taskName).getOutcome())
+				.isEqualTo(TaskOutcome.SUCCESS);
+	}
+
 	@TestTemplate
 	void reproducibleArchive()
 			throws InvalidRunnerConfigurationException, UnexpectedBuildFailure, IOException, InterruptedException {

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests-basicBuildUsingDeprecatedMainClassName.gradle

@@ -0,0 +1,8 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootJar {
+	mainClass = 'com.example.Application'
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests-upToDateWhenBuiltTwiceWithLaunchScriptIncluded.gradle

@@ -4,6 +4,6 @@ plugins {
 }
 
 bootJar {
-	mainClassName = 'com.example.Application'
+	mainClass = 'com.example.Application'
 	launchScript()
 }

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootWarIntegrationTests-basicBuildUsingDeprecatedMainClassName.gradle

@@ -0,0 +1,8 @@
+plugins {
+	id 'war'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootWar {
+	mainClassName = 'com.example.Application'
+}