Merge pull request #1023 from bono007

* pr/1023:
  Polish "Keep track of original ProjectDescription"
  Keep track of original ProjectDescription

Closes gh-1023

initializr-docs/src/main/asciidoc/configuration-guide.adoc

@@ -129,7 +129,9 @@ include::{code-examples}/doc/generator/project/SampleContributor.java[tag=code]
 ==== Project Generation Lifecycle
 When a `ProjectGenerator` is instructed to generate a project, the specified
 `ProjectDescription` can be customized using available `ProjectDescriptionCustomizer`
-beans and can be ordered using Spring's `Ordered` interface.
+beans and can be ordered using Spring's `Ordered` interface. A `ProjectDescriptionDiff`
+bean is available for extensions that wish to know if an attribute of the original
+`ProjectDescription` was modified.
 
 Once the description has been customized based on the available
 ``ProjectDescriptionCustomizer``s, the generator uses a `ProjectAssetGenerator` to

initializr-generator/src/main/java/io/spring/initializr/generator/project/DefaultProjectDescriptionDiffFactory.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2012-2019 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 io.spring.initializr.generator.project;
+
+/**
+ * A default {@link ProjectDescriptionDiffFactory} implementation that creates default
+ * {@link ProjectDescriptionDiff} instances.
+ *
+ * @author Chris Bono
+ */
+public class DefaultProjectDescriptionDiffFactory implements ProjectDescriptionDiffFactory {
+
+	@Override
+	public ProjectDescriptionDiff create(ProjectDescription description) {
+		return new ProjectDescriptionDiff(description);
+	}
+
+}

initializr-generator/src/main/java/io/spring/initializr/generator/project/MutableProjectDescription.java

@@ -61,6 +61,35 @@ public class MutableProjectDescription implements ProjectDescription {
 
 	private String baseDirectory;
 
+	public MutableProjectDescription() {
+	}
+
+	/**
+	 * Create a new instance with the state of the specified {@code source}.
+	 * @param source the source description to initialize this instance with
+	 */
+	protected MutableProjectDescription(MutableProjectDescription source) {
+		this.platformVersion = source.getPlatformVersion();
+		this.buildSystem = source.getBuildSystem();
+		this.packaging = source.getPackaging();
+		this.language = source.getLanguage();
+		this.requestedDependencies.putAll(source.getRequestedDependencies());
+		this.groupId = source.getGroupId();
+		this.artifactId = source.getArtifactId();
+		this.version = source.getVersion();
+		this.name = source.getName();
+		this.description = source.getDescription();
+		this.applicationName = source.getApplicationName();
+		this.packageName = source.getPackageName();
+		this.baseDirectory = source.getBaseDirectory();
+	}
+
+	@Override
+	public MutableProjectDescription createCopy() {
+		return new MutableProjectDescription(this);
+	}
+
+	@Override
 	public Version getPlatformVersion() {
 		return this.platformVersion;
 	}
@@ -69,6 +98,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.platformVersion = platformVersion;
 	}
 
+	@Override
 	public BuildSystem getBuildSystem() {
 		return this.buildSystem;
 	}
@@ -77,6 +107,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.buildSystem = buildSystem;
 	}
 
+	@Override
 	public Packaging getPackaging() {
 		return this.packaging;
 	}
@@ -85,6 +116,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.packaging = packaging;
 	}
 
+	@Override
 	public Language getLanguage() {
 		return this.language;
 	}
@@ -101,10 +133,12 @@ public class MutableProjectDescription implements ProjectDescription {
 		return addDependency(id, builder.build());
 	}
 
+	@Override
 	public Map<String, Dependency> getRequestedDependencies() {
 		return Collections.unmodifiableMap(this.requestedDependencies);
 	}
 
+	@Override
 	public String getGroupId() {
 		return this.groupId;
 	}
@@ -113,6 +147,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.groupId = groupId;
 	}
 
+	@Override
 	public String getArtifactId() {
 		return this.artifactId;
 	}
@@ -121,6 +156,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.artifactId = artifactId;
 	}
 
+	@Override
 	public String getVersion() {
 		return this.version;
 	}
@@ -129,6 +165,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.version = version;
 	}
 
+	@Override
 	public String getName() {
 		return this.name;
 	}
@@ -137,6 +174,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.name = name;
 	}
 
+	@Override
 	public String getDescription() {
 		return this.description;
 	}
@@ -145,6 +183,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.description = description;
 	}
 
+	@Override
 	public String getApplicationName() {
 		return this.applicationName;
 	}
@@ -153,6 +192,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.applicationName = applicationName;
 	}
 
+	@Override
 	public String getPackageName() {
 		if (StringUtils.hasText(this.packageName)) {
 			return this.packageName;
@@ -167,6 +207,7 @@ public class MutableProjectDescription implements ProjectDescription {
 		this.packageName = packageName;
 	}
 
+	@Override
 	public String getBaseDirectory() {
 		return this.baseDirectory;
 	}

initializr-generator/src/main/java/io/spring/initializr/generator/project/ProjectDescription.java

@@ -32,6 +32,15 @@ import io.spring.initializr.generator.version.Version;
  */
 public interface ProjectDescription {
 
+	/**
+	 * Create a full copy of this description so that any additional changes made on this
+	 * instance are not reflected on the returned copy.
+	 * @return a clone of this instance
+	 */
+	default ProjectDescription createCopy() {
+		throw new UnsupportedOperationException();
+	}
+
 	/**
 	 * Return a immutable mapping of requested {@link Dependency dependencies}.
 	 * @return the requested dependencies

initializr-generator/src/main/java/io/spring/initializr/generator/project/ProjectDescriptionDiff.java

@@ -0,0 +1,199 @@
+/*
+ * Copyright 2012-2019 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 io.spring.initializr.generator.project;
+
+import java.util.Objects;
+import java.util.function.BiConsumer;
+
+import io.spring.initializr.generator.buildsystem.BuildSystem;
+import io.spring.initializr.generator.language.Language;
+import io.spring.initializr.generator.packaging.Packaging;
+import io.spring.initializr.generator.version.Version;
+
+/**
+ * Provides a convenient API for determining if certain fields on a
+ * {@link ProjectDescription} were modified.
+ *
+ * @author Chris Bono
+ * @author Stephane Nicoll
+ */
+public class ProjectDescriptionDiff {
+
+	private final ProjectDescription original;
+
+	/**
+	 * Create a {@link ProjectDescriptionDiff} that uses a copy of the specified
+	 * description as its source.
+	 * @param original the description to copy as the source
+	 */
+	public ProjectDescriptionDiff(ProjectDescription original) {
+		this.original = original.createCopy();
+	}
+
+	/**
+	 * Return the original {@link ProjectDescription} that is being tracked.
+	 * @return the original description
+	 */
+	public ProjectDescription getOriginal() {
+		return this.original;
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code platformVersion} is different on the
+	 * original source project description than the specified project description.
+	 * @param current the project description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifPlatformVersionChanged(ProjectDescription current, BiConsumer<Version, Version> consumer) {
+		if (!Objects.equals(this.original.getPlatformVersion(), current.getPlatformVersion())) {
+			consumer.accept(this.original.getPlatformVersion(), current.getPlatformVersion());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code buildSystem} is different on the
+	 * original source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifBuildSystemChanged(ProjectDescription current, BiConsumer<BuildSystem, BuildSystem> consumer) {
+		if (!Objects.equals(this.original.getBuildSystem(), current.getBuildSystem())) {
+			consumer.accept(this.original.getBuildSystem(), current.getBuildSystem());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code packaging} is different on the original
+	 * source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifPackagingChanged(ProjectDescription current, BiConsumer<Packaging, Packaging> consumer) {
+		if (!Objects.equals(this.original.getPackaging(), current.getPackaging())) {
+			consumer.accept(this.original.getPackaging(), current.getPackaging());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code language} is different on the original
+	 * source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifLanguageChanged(ProjectDescription current, BiConsumer<Language, Language> consumer) {
+		if (!Objects.equals(this.original.getLanguage(), current.getLanguage())) {
+			consumer.accept(this.original.getLanguage(), current.getLanguage());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code groupId} is different on the original
+	 * source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifGroupIdChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getGroupId(), current.getGroupId())) {
+			consumer.accept(this.original.getGroupId(), current.getGroupId());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code artifactId} is different on the original
+	 * source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifArtifactIdChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getArtifactId(), current.getArtifactId())) {
+			consumer.accept(this.original.getArtifactId(), current.getArtifactId());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code version} is different on the original
+	 * source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifVersionChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getVersion(), current.getVersion())) {
+			consumer.accept(this.original.getVersion(), current.getVersion());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code name} is different on the original
+	 * source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifNameChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getName(), current.getName())) {
+			consumer.accept(this.original.getName(), current.getName());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code description} is different on the
+	 * original source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifDescriptionChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getDescription(), current.getDescription())) {
+			consumer.accept(this.original.getDescription(), current.getDescription());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code applicationName} is different on the
+	 * original source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifApplicationNameChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getApplicationName(), current.getApplicationName())) {
+			consumer.accept(this.original.getApplicationName(), current.getApplicationName());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code packageName} is different on the
+	 * original source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifPackageNameChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getPackageName(), current.getPackageName())) {
+			consumer.accept(this.original.getPackageName(), current.getPackageName());
+		}
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code baseDirectory} is different on the
+	 * original source project description than the specified project description.
+	 * @param current the description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	public void ifBaseDirectoryChanged(ProjectDescription current, BiConsumer<String, String> consumer) {
+		if (!Objects.equals(this.original.getBaseDirectory(), current.getBaseDirectory())) {
+			consumer.accept(this.original.getBaseDirectory(), current.getBaseDirectory());
+		}
+	}
+
+}

initializr-generator/src/main/java/io/spring/initializr/generator/project/ProjectDescriptionDiffFactory.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2012-2019 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 io.spring.initializr.generator.project;
+
+/**
+ * A factory for {@link ProjectDescriptionDiff}.
+ *
+ * @author Chris Bono
+ */
+public interface ProjectDescriptionDiffFactory {
+
+	/**
+	 * Create a {@link ProjectDescriptionDiff} for the specified
+	 * {@link ProjectDescription}. Any change on the specified {@code description} is
+	 * tracked by the returned instance.
+	 * @param description the project description to use as the source of the diff
+	 * @return a diff instance using the current state of the specified description as its
+	 * source
+	 */
+	ProjectDescriptionDiff create(ProjectDescription description);
+
+}

initializr-generator/src/main/java/io/spring/initializr/generator/project/ProjectGenerator.java

@@ -120,8 +120,12 @@ public class ProjectGenerator {
 	private Supplier<ProjectDescription> resolve(ProjectDescription description, ProjectGenerationContext context) {
 		return () -> {
 			if (description instanceof MutableProjectDescription) {
+				MutableProjectDescription mutableDescription = (MutableProjectDescription) description;
+				ProjectDescriptionDiffFactory diffFactory = context.getBeanProvider(ProjectDescriptionDiffFactory.class)
+						.getIfAvailable(DefaultProjectDescriptionDiffFactory::new);
+				context.registerBean(ProjectDescriptionDiff.class, () -> diffFactory.create(mutableDescription));
 				context.getBeanProvider(ProjectDescriptionCustomizer.class).orderedStream()
-						.forEach((customizer) -> customizer.customize((MutableProjectDescription) description));
+						.forEach((customizer) -> customizer.customize(mutableDescription));
 			}
 			return description;
 		};

initializr-generator/src/test/java/io/spring/initializr/generator/project/ProjectDescriptionDiffTests.java

@@ -0,0 +1,226 @@
+/*
+ * Copyright 2012-2019 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 io.spring.initializr.generator.project;
+
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+
+import io.spring.initializr.generator.buildsystem.BuildSystem;
+import io.spring.initializr.generator.buildsystem.gradle.GradleBuildSystem;
+import io.spring.initializr.generator.buildsystem.maven.MavenBuildSystem;
+import io.spring.initializr.generator.language.Language;
+import io.spring.initializr.generator.language.java.JavaLanguage;
+import io.spring.initializr.generator.packaging.Packaging;
+import io.spring.initializr.generator.packaging.jar.JarPackaging;
+import io.spring.initializr.generator.version.Version;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.fail;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+/**
+ * Tests for {@link ProjectDescriptionDiff}.
+ *
+ * @author Chris Bono
+ * @author Stephane Nicoll
+ */
+class ProjectDescriptionDiffTests {
+
+	@Test
+	void projectDescriptionDiffCopySource() {
+		ProjectDescription original = createFullProjectDescription();
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(original);
+		assertThat(diff.getOriginal()).usingRecursiveComparison().isEqualTo(original);
+		assertThat(diff.getOriginal()).isNotSameAs(original);
+	}
+
+	@Test
+	void projectDescriptionDiffWithUnmodifiedDescriptionDoesNotInvokeConsumer() {
+		ProjectDescription description = createFullProjectDescription();
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		diff.ifPlatformVersionChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifBuildSystemChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifPackagingChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifLanguageChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifGroupIdChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifArtifactIdChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifVersionChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifNameChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifDescriptionChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifApplicationNameChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifPackageNameChanged(description, (original, actual) -> fail("Values should not have changed"));
+		diff.ifBaseDirectoryChanged(description, (original, actual) -> fail("Values should not have changed"));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedPlatformVersionInvokesConsumer() {
+		Version original = Version.parse("2.1.0.RELEASE");
+		Version actual = Version.parse("2.2.0.RELEASE");
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setPlatformVersion(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setPlatformVersion(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifPlatformVersionChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedBuildSystemInvokesConsumer() {
+		BuildSystem original = BuildSystem.forId(MavenBuildSystem.ID);
+		BuildSystem actual = BuildSystem.forId(GradleBuildSystem.ID);
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setBuildSystem(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setBuildSystem(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifBuildSystemChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedPackagingInvokesConsumer() {
+		Packaging original = Packaging.forId(JarPackaging.ID);
+		Packaging actual = Packaging.forId("war");
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setPackaging(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setPackaging(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifPackagingChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedLanguageInvokesConsumer() {
+		Language original = Language.forId(JavaLanguage.ID, "11");
+		Language actual = Language.forId(JavaLanguage.ID, "13");
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setLanguage(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setLanguage(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifLanguageChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedGroupIdInvokesConsumer() {
+		String original = "com.example";
+		String actual = "com.example.app";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setGroupId(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setGroupId(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifGroupIdChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedArtifactIdInvokesConsumer() {
+		String original = "demo";
+		String actual = "app";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setArtifactId(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setArtifactId(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifArtifactIdChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedVersionInvokesConsumer() {
+		String original = "1.0.0";
+		String actual = "1.1.0";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setVersion(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setVersion(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifVersionChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedNameInvokesConsumer() {
+		String original = "demo";
+		String actual = "application";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setName(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setName(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifNameChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedDescriptionInvokesConsumer() {
+		String original = "Demo Application";
+		String actual = "Application";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setDescription(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setDescription(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifDescriptionChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedApplicationNameInvokesConsumer() {
+		String original = "DemoApplication";
+		String actual = "Application";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setApplicationName(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setApplicationName(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifApplicationNameChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedPackageNameInvokesConsumer() {
+		String original = "com.example";
+		String actual = "com.example.app";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setPackageName(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setPackageName(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifPackageNameChanged(description, consumer));
+	}
+
+	@Test
+	void projectDescriptionDiffWithModifiedBaseDirectoryInvokesConsumer() {
+		String original = null;
+		String actual = "demo-app";
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setBaseDirectory(original);
+		ProjectDescriptionDiff diff = new ProjectDescriptionDiff(description);
+		description.setBaseDirectory(actual);
+		validateConsumer(original, actual, (consumer) -> diff.ifBaseDirectoryChanged(description, consumer));
+	}
+
+	private MutableProjectDescription createFullProjectDescription() {
+		MutableProjectDescription description = new MutableProjectDescription();
+		description.setBuildSystem(BuildSystem.forId(MavenBuildSystem.ID));
+		description.setLanguage(Language.forId(JavaLanguage.ID, "11"));
+		description.setPlatformVersion(Version.parse("2.2.0.RELEASE"));
+		description.setGroupId("com.example");
+		description.setArtifactId("demo");
+		description.setName("demo");
+		description.setVersion("0.0.8");
+		description.setApplicationName("DemoApplication");
+		description.setPackageName("com.example.demo");
+		description.setPackaging(Packaging.forId("jar"));
+		description.setBaseDirectory(".");
+		return description;
+	}
+
+	@SuppressWarnings("unchecked")
+	private <T> void validateConsumer(T original, T actual, Consumer<BiConsumer<T, T>> test) {
+		BiConsumer<T, T> consumer = mock(BiConsumer.class);
+		test.accept(consumer);
+		verify(consumer).accept(original, actual);
+	}
+
+}

initializr-generator/src/test/java/io/spring/initializr/generator/project/ProjectGeneratorTests.java

@@ -32,6 +32,7 @@ import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.inOrder;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoInteractions;
 
 /**
@@ -53,6 +54,33 @@ public class ProjectGeneratorTests {
 		assertThat(description).isSameAs(beanDescription);
 	}
 
+	@Test
+	void generateProvideDefaultProjectDescriptionDiff() {
+		ProjectGenerator generator = new ProjectGenerator(mockContextInitializr());
+		MutableProjectDescription description = new MutableProjectDescription();
+		ProjectDescriptionDiff diff = generator.generate(description, (context) -> {
+			assertThat(context.getBeansOfType(ProjectDescriptionDiff.class)).hasSize(1);
+			return context.getBean(ProjectDescriptionDiff.class);
+		});
+		assertThat(diff).isInstanceOf(ProjectDescriptionDiff.class);
+	}
+
+	@Test
+	void generateUseAvailableProjectDescriptionDiffFactory() {
+		ProjectDescriptionDiff diff = mock(ProjectDescriptionDiff.class);
+		ProjectDescriptionDiffFactory diffFactory = mock(ProjectDescriptionDiffFactory.class);
+		MutableProjectDescription description = new MutableProjectDescription();
+		given(diffFactory.create(description)).willReturn(diff);
+		ProjectGenerator generator = new ProjectGenerator(
+				(context) -> context.registerBean(ProjectDescriptionDiffFactory.class, () -> diffFactory));
+		ProjectDescriptionDiff actualDiff = generator.generate(description, (context) -> {
+			assertThat(context.getBeansOfType(ProjectDescriptionDiff.class)).hasSize(1);
+			return context.getBean(ProjectDescriptionDiff.class);
+		});
+		assertThat(actualDiff).isSameAs(diff);
+		verify(diffFactory).create(description);
+	}
+
 	@Test
 	void generateInvokeContextInitializerBeforeContextIsRefreshed() {
 		ProjectGenerator generator = new ProjectGenerator((context) -> {

initializr-web/src/test/java/io/spring/initializr/web/controller/custom/CustomProjectDescription.java

@@ -28,6 +28,19 @@ class CustomProjectDescription extends MutableProjectDescription {
 
 	private boolean customFlag;
 
+	CustomProjectDescription() {
+	}
+
+	CustomProjectDescription(CustomProjectDescription source) {
+		super(source);
+		this.customFlag = source.isCustomFlag();
+	}
+
+	@Override
+	public CustomProjectDescription createCopy() {
+		return new CustomProjectDescription(this);
+	}
+
 	boolean isCustomFlag() {
 		return this.customFlag;
 	}

initializr-web/src/test/java/io/spring/initializr/web/controller/custom/CustomProjectDescriptionDiff.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2012-2019 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 io.spring.initializr.web.controller.custom;
+
+import java.util.Objects;
+import java.util.function.BiConsumer;
+
+import io.spring.initializr.generator.project.ProjectDescriptionDiff;
+
+/**
+ * Extends the base {@link ProjectDescriptionDiff} to provide convenient diff methods on
+ * {@link CustomProjectDescription}.
+ *
+ * @author Chris Bono
+ * @author Stephane Nicoll
+ */
+class CustomProjectDescriptionDiff extends ProjectDescriptionDiff {
+
+	private final CustomProjectDescription original;
+
+	CustomProjectDescriptionDiff(CustomProjectDescription original) {
+		super(original);
+		this.original = original;
+	}
+
+	/**
+	 * Calls the specified consumer if the {@code customFlag} is different on the original
+	 * source project description than the specified project description.
+	 * @param current the project description to test against
+	 * @param consumer to call if the property has changed
+	 */
+	void ifCustomFlagChanged(CustomProjectDescription current, BiConsumer<Boolean, Boolean> consumer) {
+		if (!Objects.equals(this.original.isCustomFlag(), current.isCustomFlag())) {
+			consumer.accept(this.original.isCustomFlag(), current.isCustomFlag());
+		}
+	}
+
+}

initializr-web/src/test/java/io/spring/initializr/web/controller/custom/ProjectGenerationControllerCustomRequestIntegrationTests.java

@@ -16,7 +16,10 @@
 
 package io.spring.initializr.web.controller.custom;
 
+import io.spring.initializr.generator.project.MutableProjectDescription;
 import io.spring.initializr.generator.project.ProjectDescription;
+import io.spring.initializr.generator.project.ProjectDescriptionCustomizer;
+import io.spring.initializr.generator.project.ProjectDescriptionDiffFactory;
 import io.spring.initializr.generator.test.project.ProjectStructure;
 import io.spring.initializr.metadata.InitializrMetadata;
 import io.spring.initializr.metadata.InitializrMetadataProvider;
@@ -33,6 +36,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
 import org.springframework.test.context.ActiveProfiles;
+import org.springframework.util.Assert;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -44,8 +48,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  */
 @ActiveProfiles("test-default")
 @Import(CustomProjectGenerationConfiguration.class)
-public class ProjectGenerationControllerCustomRequestIntegrationTests
-		extends AbstractInitializrControllerIntegrationTests {
+class ProjectGenerationControllerCustomRequestIntegrationTests extends AbstractInitializrControllerIntegrationTests {
 
 	@Test
 	void createProjectWithCustomFlagEnabled() {
@@ -79,6 +82,16 @@ public class ProjectGenerationControllerCustomRequestIntegrationTests
 			return new CustomProjectGenerationController(metadataProvider, projectGenerationInvoker);
 		}
 
+		@Bean
+		CustomProjectDescriptionCustomizer customProjectDescriptionCustomizer() {
+			return new CustomProjectDescriptionCustomizer();
+		}
+
+		@Bean
+		CustomProjectDescriptionDiffFactory customProjectDescriptionDiffFactory() {
+			return new CustomProjectDescriptionDiffFactory();
+		}
+
 	}
 
 	static class CustomProjectRequestToDescriptionConverter
@@ -91,10 +104,28 @@ public class ProjectGenerationControllerCustomRequestIntegrationTests
 			description.setCustomFlag(request.isCustomFlag());
 			// Override attributes for test purposes
 			description.setPackageName("org.example.custom");
-			description.setApplicationName("CustomApp");
 			return description;
 		}
 
 	}
 
+	static class CustomProjectDescriptionCustomizer implements ProjectDescriptionCustomizer {
+
+		@Override
+		public void customize(MutableProjectDescription description) {
+			description.setApplicationName("CustomApp");
+		}
+
+	}
+
+	static class CustomProjectDescriptionDiffFactory implements ProjectDescriptionDiffFactory {
+
+		@Override
+		public CustomProjectDescriptionDiff create(ProjectDescription description) {
+			Assert.isInstanceOf(CustomProjectDescription.class, description);
+			return new CustomProjectDescriptionDiff((CustomProjectDescription) description);
+		}
+
+	}
+
 }