Add mainClassName to springBoot DSL in Gradle plugin

This commit introduces a new mainClassName property on the springBoot
DSL provided by the Gradle plugin. It can be used to explicitly
configure the mainClassName of the default bootRun, bootJar, and
bootWar tasks in a single place. Previously, the only mechanism
provided to do so was the mainClassName property that's only available
when the application plugin has been applied.

Closes gh-10623

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/packaging.adoc

@@ -93,8 +93,16 @@ property:
 include::../gradle/packaging/boot-jar-main-class.gradle[tags=main-class]
 ----
 
-Alternatively, if the {application-plugin}[`application` plugin] has been applied
-its `mainClassName` project property can be used:
+Alternatively, the main class name can be configured project-wide using the
+`mainClassName` property of the Spring Boot DSL:
+
+[source,groovy,indent=0,subs="verbatim"]
+----
+include::../gradle/packaging/spring-boot-dsl-main-class.gradle[tags=main-class]
+----
+
+If the {application-plugin}[`application` plugin] has been applied its `mainClassName`
+project property can be used for the same purpose:
 
 [source,groovy,indent=0,subs="verbatim"]
 ----

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/running.adoc

@@ -19,8 +19,16 @@ classpath. The main class can also be configured explicitly using the task's
 include::../gradle/running/boot-run-main.gradle[tags=main]
 ----
 
-Alternatively, if the {application-plugin}[`application` plugin] has been applied
-its `mainClassName` project property can be used:
+Alternatively, the main class name can be configured project-wide using the
+`mainClassName` property of the Spring Boot DSL:
+
+[source,groovy,indent=0,subs="verbatim"]
+----
+include::../gradle/running/spring-boot-dsl-main-class-name.gradle[tags=main-class]
+----
+
+If the {application-plugin}[`application` plugin] has been applied its `mainClassName`
+project property can be used for the same purpose:
 
 [source,groovy,indent=0,subs="verbatim"]
 ----

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/gradle/packaging/spring-boot-dsl-main-class.gradle

@@ -0,0 +1,14 @@
+buildscript {
+	dependencies {
+		classpath files(pluginClasspath.split(','))
+	}
+}
+
+apply plugin: 'org.springframework.boot'
+apply plugin: 'java'
+
+// tag::main-class[]
+springBoot {
+	mainClassName = 'com.example.ExampleApplication'
+}
+// end::main-class[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/gradle/running/spring-boot-dsl-main-class-name.gradle

@@ -0,0 +1,21 @@
+buildscript {
+	dependencies {
+		classpath files(pluginClasspath.split(','))
+	}
+}
+
+apply plugin: 'org.springframework.boot'
+apply plugin: 'java'
+apply plugin: 'application'
+
+// tag::main-class[]
+springBoot {
+	mainClassName = 'com.example.ExampleApplication'
+}
+// end::main-class[]
+
+task configuredMainClass {
+	doLast {
+		println bootRun.mainClassName
+	}
+}

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

@@ -39,6 +39,8 @@ public class SpringBootExtension {
 
 	private final Project project;
 
+	private String mainClassName;
+
 	/**
 	 * Creates a new {@code SpringBootPluginExtension} that is associated with the given
 	 * {@code project}.
@@ -49,6 +51,24 @@ public class SpringBootExtension {
 		this.project = project;
 	}
 
+	/**
+	 * Returns the main class name of the application.
+	 *
+	 * @return the name of the application's main class
+	 */
+	public String getMainClassName() {
+		return this.mainClassName;
+	}
+
+	/**
+	 * Sets the main class name of the application.
+	 *
+	 * @param mainClassName the name of the application's main class
+	 */
+	public void setMainClassName(String mainClassName) {
+		this.mainClassName = mainClassName;
+	}
+
 	/**
 	 * Creates a new {@link BuildInfo} task named {@code bootBuildInfo} and configures the
 	 * Java plugin's {@code classes} task to depend upon it.

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

@@ -25,6 +25,7 @@ import java.util.function.Supplier;
 import org.gradle.api.Project;
 import org.gradle.api.file.FileCollection;
 
+import org.springframework.boot.gradle.dsl.SpringBootExtension;
 import org.springframework.boot.loader.tools.MainClassFinder;
 
 /**
@@ -47,6 +48,12 @@ final class MainClassConvention implements Callable<String> {
 
 	@Override
 	public String call() throws Exception {
+		SpringBootExtension springBootExtension = this.project.getExtensions()
+				.findByType(SpringBootExtension.class);
+		if (springBootExtension != null
+				&& springBootExtension.getMainClassName() != null) {
+			return springBootExtension.getMainClassName();
+		}
 		if (this.project.hasProperty("mainClassName")) {
 			Object mainClassName = this.project.property("mainClassName");
 			if (mainClassName != null) {