Improve error message with Maven build-image and war packaging

Fixes gh-24522

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/docs/asciidoc/packaging-oci-image.adoc

@@ -1,11 +1,13 @@
 [[build-image]]
 == Packaging OCI Images
-The plugin can create an https://github.com/opencontainers/image-spec[OCI image] using https://buildpacks.io/[Cloud Native Buildpacks] (CNB).
+The plugin can create an https://github.com/opencontainers/image-spec[OCI image] from an executable jar file using https://buildpacks.io/[Cloud Native Buildpacks] (CNB).
 Images can be built using the `build-image` goal.
 
 NOTE: For security reasons, images build and run as non-root users.
 See the {buildpacks-reference}/reference/spec/platform-api/#users[CNB specification] for more details.
 
+NOTE: The `build-image` goal is not supported with projects using <<repackage, war packaging>>.
+
 The easiest way to get started is to invoke `mvn spring-boot:build-image` on a project.
 It is possible to automate the creation of an image whenever the `package` phase is invoked, as shown in the following example:
 

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/java/org/springframework/boot/maven/BuildImageTests.java

@@ -152,6 +152,12 @@ void failsWhenBuilderFails(MavenBuild mavenBuild) {
 						.containsPattern("Builder lifecycle '.*' failed with status code"));
 	}
 
+	@TestTemplate
+	void failsWithWarPackaging(MavenBuild mavenBuild) {
+		mavenBuild.project("build-image-war-packaging").goals("package").executeAndFail(
+				(project) -> assertThat(buildLog(project)).contains("Executable jar file required for building image"));
+	}
+
 	private void writeLongNameResource(File project) {
 		StringBuilder name = new StringBuilder();
 		new Random().ints('a', 'z' + 1).limit(128).forEach((i) -> name.append((char) i));

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/projects/build-image-war-packaging/pom.xml

@@ -0,0 +1,55 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>org.springframework.boot.maven.it</groupId>
+	<artifactId>build-image-war</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<packaging>war</packaging>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+		<maven.compiler.source>@java.version@</maven.compiler.source>
+		<maven.compiler.target>@java.version@</maven.compiler.target>
+	</properties>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>@project.groupId@</groupId>
+				<artifactId>@project.artifactId@</artifactId>
+				<version>@project.version@</version>
+				<executions>
+					<execution>
+						<goals>
+							<goal>build-image</goal>
+						</goals>
+					</execution>
+				</executions>
+			</plugin>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-war-plugin</artifactId>
+				<version>@maven-war-plugin.version@</version>
+				<configuration>
+					<archive>
+						<manifestEntries>
+							<Not-Used>Foo</Not-Used>
+						</manifestEntries>
+					</archive>
+				</configuration>
+			</plugin>
+		</plugins>
+	</build>
+	<dependencies>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-context</artifactId>
+			<version>@spring-framework.version@</version>
+		</dependency>
+		<dependency>
+			<groupId>jakarta.servlet</groupId>
+			<artifactId>jakarta.servlet-api</artifactId>
+			<version>@jakarta-servlet.version@</version>
+			<scope>provided</scope>
+		</dependency>
+	</dependencies>
+</project>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/projects/build-image-war-packaging/src/main/java/org/test/SampleApplication.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2012-2020 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.test;
+
+public class SampleApplication {
+
+	public static void main(String[] args) throws Exception {
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/BuildImageMojo.java

@@ -180,7 +180,11 @@ private File getJarFile() {
 			name.append("-").append(this.classifier);
 		}
 		name.append(".jar");
-		return new File(this.sourceDirectory, name.toString());
+		File jarFile = new File(this.sourceDirectory, name.toString());
+		if (!jarFile.exists()) {
+			throw new IllegalStateException("Executable jar file required for building image");
+		}
+		return jarFile;
 	}
 
 	private BuildRequest customize(BuildRequest request) {