diff --git a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/MainClassConvention.java b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/MainClassConvention.java
index 85711944e0..c98c97e672 100644
--- a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/MainClassConvention.java
+++ b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/MainClassConvention.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -22,6 +22,7 @@ import java.util.Objects;
 import java.util.concurrent.Callable;
 import java.util.function.Supplier;
 
+import org.gradle.api.InvalidUserDataException;
 import org.gradle.api.Project;
 import org.gradle.api.file.FileCollection;
 
@@ -66,7 +67,8 @@ final class MainClassConvention implements Callable<Object> {
 	private String resolveMainClass() {
 		return this.classpathSupplier.get().filter(File::isDirectory).getFiles().stream()
 				.map(this::findMainClass).filter(Objects::nonNull).findFirst()
-				.orElse(null);
+				.orElseThrow(() -> new InvalidUserDataException(
+						"Main class name has not been configured and it could not be resolved"));
 	}
 
 	private String findMainClass(File file) {
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootArchive.java b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootArchive.java
index 69bfe894af..96d7362874 100644
--- a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootArchive.java
+++ b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootArchive.java
@@ -39,7 +39,6 @@ public interface BootArchive extends Task {
 	 * @return the main class name
 	 */
 	@Input
-	@Optional
 	String getMainClassName();
 
 	/**
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootJar.java b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootJar.java
index da15b24780..e63e83dd25 100644
--- a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootJar.java
+++ b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootJar.java
@@ -42,10 +42,10 @@ public class BootJar extends Jar implements BootArchive {
 
 	private final CopySpec bootInf;
 
-	private FileCollection classpath;
-
 	private String mainClassName;
 
+	private FileCollection classpath;
+
 	/**
 	 * Creates a new {@code BootJar} task.
 	 */
@@ -76,6 +76,13 @@ public class BootJar extends Jar implements BootArchive {
 
 	@Override
 	public String getMainClassName() {
+		if (this.mainClassName == null) {
+			String manifestStartClass = (String) getManifest().getAttributes()
+					.get("Start-Class");
+			if (manifestStartClass != null) {
+				setMainClassName(manifestStartClass);
+			}
+		}
 		return this.mainClassName;
 	}
 
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootWar.java b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootWar.java
index 36c1a61852..e6be12492f 100644
--- a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootWar.java
+++ b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootWar.java
@@ -68,6 +68,13 @@ public class BootWar extends War implements BootArchive {
 
 	@Override
 	public String getMainClassName() {
+		if (this.mainClassName == null) {
+			String manifestStartClass = (String) getManifest().getAttributes()
+					.get("Start-Class");
+			if (manifestStartClass != null) {
+				setMainClassName(manifestStartClass);
+			}
+		}
 		return this.mainClassName;
 	}
 
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/JavaPluginActionIntegrationTests.java b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/JavaPluginActionIntegrationTests.java
index 753e18cbef..d69b6ae046 100644
--- a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/JavaPluginActionIntegrationTests.java
+++ b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/JavaPluginActionIntegrationTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -104,6 +104,14 @@ public class JavaPluginActionIntegrationTests {
 		assertThat(result.task(":jar").getOutcome()).isEqualTo(TaskOutcome.SKIPPED);
 	}
 
+	@Test
+	public void errorMessageIsHelpfulWhenMainClassCannotBeResolved() {
+		BuildResult result = this.gradleBuild.buildAndFail("build", "-PapplyJavaPlugin");
+		assertThat(result.task(":bootJar").getOutcome()).isEqualTo(TaskOutcome.FAILED);
+		assertThat(result.getOutput()).contains(
+				"Main class name has not been configured and it could not be resolved");
+	}
+
 	@Test
 	public void jarAndBootJarCanBothBeBuilt() {
 		BuildResult result = this.gradleBuild.build("assemble");
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/WarPluginActionIntegrationTests.java b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/WarPluginActionIntegrationTests.java
index 0f3f4c6434..7392860e57 100644
--- a/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/WarPluginActionIntegrationTests.java
+++ b/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/WarPluginActionIntegrationTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -72,4 +72,12 @@ public class WarPluginActionIntegrationTests {
 						this.gradleBuild.getProjectDir().getName() + "-boot.war"));
 	}
 
+	@Test
+	public void errorMessageIsHelpfulWhenMainClassCannotBeResolved() {
+		BuildResult result = this.gradleBuild.buildAndFail("build", "-PapplyWarPlugin");
+		assertThat(result.task(":bootWar").getOutcome()).isEqualTo(TaskOutcome.FAILED);
+		assertThat(result.getOutput()).contains(
+				"Main class name has not been configured and it could not be resolved");
+	}
+
 }