Add indication that the JVM version was downgraded

This commit adds a warning when it detects that the JVM version was
downgraded. While not providing the exact reason, it provides a link to
the supported Java versions.

See gh-250

start-site/src/main/java/io/spring/start/site/extension/description/DescriptionProjectGenerationConfiguration.java

@@ -0,0 +1,40 @@
+/*
+ * 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.start.site.extension.description;
+
+import io.spring.initializr.generator.project.ProjectDescription;
+import io.spring.initializr.generator.project.ProjectDescriptionDiff;
+import io.spring.initializr.generator.project.ProjectGenerationConfiguration;
+
+import org.springframework.context.annotation.Bean;
+
+/**
+ * {@link ProjectGenerationConfiguration} for customizations relevant to the
+ * {@link ProjectDescription}.
+ *
+ * @author Stephane Nicoll
+ */
+@ProjectGenerationConfiguration
+public class DescriptionProjectGenerationConfiguration {
+
+	@Bean
+	public InvalidJvmVersionHelpDocumentCustomizer invalidJvmVersionHelpDocumentCustomizer(ProjectDescriptionDiff diff,
+			ProjectDescription description) {
+		return new InvalidJvmVersionHelpDocumentCustomizer(diff, description);
+	}
+
+}

start-site/src/main/java/io/spring/start/site/extension/description/InvalidJvmVersionHelpDocumentCustomizer.java

@@ -0,0 +1,56 @@
+/*
+ * 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.start.site.extension.description;
+
+import java.util.Objects;
+
+import io.spring.initializr.generator.project.ProjectDescription;
+import io.spring.initializr.generator.project.ProjectDescriptionDiff;
+import io.spring.initializr.generator.spring.documentation.HelpDocument;
+import io.spring.initializr.generator.spring.documentation.HelpDocumentCustomizer;
+
+/**
+ * A {@link HelpDocumentCustomizer} that adds a warning when the JVM level was changed.
+ *
+ * @author Stephane Nicoll
+ */
+public class InvalidJvmVersionHelpDocumentCustomizer implements HelpDocumentCustomizer {
+
+	private final ProjectDescriptionDiff diff;
+
+	private final ProjectDescription description;
+
+	public InvalidJvmVersionHelpDocumentCustomizer(ProjectDescriptionDiff diff, ProjectDescription description) {
+		this.diff = diff;
+		this.description = description;
+	}
+
+	@Override
+	public void customize(HelpDocument document) {
+		this.diff.ifLanguageChanged(this.description, (original, current) -> {
+			String originalJvmVersion = original.jvmVersion();
+			String currentJvmVersion = current.jvmVersion();
+			if (!Objects.equals(originalJvmVersion, currentJvmVersion)) {
+				document.getWarnings().addItem(String.format(
+						"The JVM level was changed from '%s' to '%s', review the [JDK Version Range](%s) on the wiki for more details.",
+						originalJvmVersion, currentJvmVersion,
+						"https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Versions#jdk-version-range"));
+			}
+		});
+	}
+
+}

start-site/src/main/java/io/spring/start/site/extension/description/package-info.java

@@ -0,0 +1,20 @@
+/*
+ * 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.
+ */
+
+/**
+ * Extensions for customization of the project description.
+ */
+package io.spring.start.site.extension.description;

start-site/src/main/resources/META-INF/spring.factories

@@ -8,4 +8,5 @@ io.spring.start.site.extension.dependency.springcloud.SpringCloudProjectGenerati
 io.spring.start.site.extension.dependency.springdata.SpringDataProjectGenerationConfiguration,\
 io.spring.start.site.extension.dependency.springintegration.SpringIntegrationProjectGenerationConfiguration,\
 io.spring.start.site.extension.dependency.springrestdocs.SpringRestDocsProjectGenerationConfiguration,\
+io.spring.start.site.extension.description.DescriptionProjectGenerationConfiguration,\
 io.spring.start.site.extension.code.kotin.KotlinProjectGenerationConfiguration

start-site/src/test/java/io/spring/start/site/extension/description/InvalidJvmVersionHelpDocumentCustomizerTests.java

@@ -0,0 +1,51 @@
+/*
+ * 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.start.site.extension.description;
+
+import io.spring.initializr.generator.test.io.TextAssert;
+import io.spring.initializr.generator.test.project.ProjectStructure;
+import io.spring.initializr.web.project.ProjectRequest;
+import io.spring.start.site.extension.AbstractExtensionTests;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for {@link InvalidJvmVersionHelpDocumentCustomizer}.
+ *
+ * @author Stephane Nicoll
+ */
+class InvalidJvmVersionHelpDocumentCustomizerTests extends AbstractExtensionTests {
+
+	@Test
+	void warningAddedWithSpringBoot21AndJava13() {
+		assertHelpDocument("2.1.0.RELEASE", "13").lines().containsSubsequence("# Read Me First",
+				"* The JVM level was changed from '13' to '11', review the [JDK Version Range](https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Versions#jdk-version-range) on the wiki for more details.");
+	}
+
+	@Test
+	void warningNotAddedWithCompatibleVersion() {
+		assertHelpDocument("2.1.0.RELEASE", "11").doesNotContain("# Read Me First");
+	}
+
+	private TextAssert assertHelpDocument(String platformVersion, String jvmVersion) {
+		ProjectRequest request = createProjectRequest("web");
+		request.setBootVersion(platformVersion);
+		request.setJavaVersion(jvmVersion);
+		ProjectStructure project = generateProject(request);
+		return new TextAssert(project.getProjectDirectory().resolve("HELP.md"));
+	}
+
+}