Ignore mainClassName property when its value is null

Closes gh-9892

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

@@ -48,7 +48,10 @@ final class MainClassConvention implements Callable<Object> {
 	@Override
 	public Object call() throws Exception {
 		if (this.project.hasProperty("mainClassName")) {
-			return this.project.property("mainClassName");
+			Object mainClassName = this.project.property("mainClassName");
+			if (mainClassName != null) {
+				return mainClassName;
+			}
 		}
 		return resolveMainClass();
 	}

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

@@ -44,10 +44,7 @@ public class BootRunIntegrationTests {
 
 	@Test
 	public void basicExecution() throws IOException {
-		File output = new File(this.gradleBuild.getProjectDir(),
-				"src/main/java/com/example");
-		output.mkdirs();
-		FileSystemUtils.copyRecursively(new File("src/test/java/com/example"), output);
+		copyApplication();
 		new File(this.gradleBuild.getProjectDir(), "src/main/resources").mkdirs();
 		BuildResult result = this.gradleBuild.build("bootRun");
 		assertThat(result.task(":bootRun").getOutcome()).isEqualTo(TaskOutcome.SUCCESS);
@@ -58,10 +55,7 @@ public class BootRunIntegrationTests {
 
 	@Test
 	public void sourceResourcesCanBeUsed() throws IOException {
-		File output = new File(this.gradleBuild.getProjectDir(),
-				"src/main/java/com/example");
-		output.mkdirs();
-		FileSystemUtils.copyRecursively(new File("src/test/java/com/example"), output);
+		copyApplication();
 		BuildResult result = this.gradleBuild.build("bootRun");
 		assertThat(result.task(":bootRun").getOutcome()).isEqualTo(TaskOutcome.SUCCESS);
 		assertThat(result.getOutput()).contains("1. " + urlOf("src/main/resources"));
@@ -78,6 +72,16 @@ public class BootRunIntegrationTests {
 				.contains("Main class name = com.example.CustomMainClass");
 	}
 
+	@Test
+	public void applicationPluginMainClassNameIsNotUsedWhenItIsNull() throws IOException {
+		copyApplication();
+		BuildResult result = this.gradleBuild.build("echoMainClassName");
+		assertThat(result.task(":echoMainClassName").getOutcome())
+				.isEqualTo(TaskOutcome.SUCCESS);
+		assertThat(result.getOutput())
+				.contains("Main class name = com.example.BootRunApplication");
+	}
+
 	@Test
 	public void applicationPluginJvmArgumentsAreUsed() throws IOException {
 		BuildResult result = this.gradleBuild.build("echoJvmArguments");
@@ -87,6 +91,13 @@ public class BootRunIntegrationTests {
 				.contains("JVM arguments = [-Dcom.foo=bar, -Dcom.bar=baz]");
 	}
 
+	private void copyApplication() throws IOException {
+		File output = new File(this.gradleBuild.getProjectDir(),
+				"src/main/java/com/example");
+		output.mkdirs();
+		FileSystemUtils.copyRecursively(new File("src/test/java/com/example"), output);
+	}
+
 	private String urlOf(String path) throws IOException {
 		return new File(this.gradleBuild.getProjectDir().getCanonicalFile(), path).toURI()
 				.toURL().toString();

spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/run/BootRunIntegrationTests-applicationPluginMainClassNameIsNotUsedWhenItIsNull.gradle

@@ -0,0 +1,15 @@
+buildscript {
+	dependencies {
+		classpath files(pluginClasspath.split(','))
+	}
+}
+
+apply plugin: 'application'
+apply plugin: 'org.springframework.boot'
+
+task echoMainClassName {
+	dependsOn compileJava
+	doLast {
+		println 'Main class name = ' + bootRun.main
+	}
+}