Preserve property descriptions during Gradle incremental compilation

configuration-metadata/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessor.java

@@ -21,6 +21,7 @@
 import java.time.Duration;
 import java.util.ArrayDeque;
 import java.util.Deque;
+import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Locale;
@@ -77,6 +78,8 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 
 	static final String ADDITIONAL_METADATA_LOCATIONS_OPTION = "org.springframework.boot.configurationprocessor.additionalMetadataLocations";
 
+	static final String DESCRIPTION_CACHE_LOCATION_OPTION = "org.springframework.boot.configurationprocessor.descriptionCacheLocation";
+
 	static final String CONFIGURATION_PROPERTIES_ANNOTATION = "org.springframework.boot.context.properties.ConfigurationProperties";
 
 	static final String CONFIGURATION_PROPERTIES_SOURCE_ANNOTATION = "org.springframework.boot.context.properties.ConfigurationPropertiesSource";
@@ -113,7 +116,8 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 
 	static final String ENDPOINT_ACCESS_ENUM = "org.springframework.boot.actuate.endpoint.Access";
 
-	private static final Set<String> SUPPORTED_OPTIONS = Set.of(ADDITIONAL_METADATA_LOCATIONS_OPTION);
+	private static final Set<String> SUPPORTED_OPTIONS = Set.of(ADDITIONAL_METADATA_LOCATIONS_OPTION,
+			DESCRIPTION_CACHE_LOCATION_OPTION);
 
 	private MetadataStore metadataStore;
 
@@ -123,6 +127,10 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 
 	private MetadataGenerationEnvironment metadataEnv;
 
+	private DescriptionCache descriptionCache;
+
+	private final Set<String> classFileBackedTypes = new HashSet<>();
+
 	protected String configurationPropertiesAnnotation() {
 		return CONFIGURATION_PROPERTIES_ANNOTATION;
 	}
@@ -189,6 +197,10 @@ public synchronized void init(ProcessingEnvironment env) {
 				configurationPropertiesSourceAnnotation(), nestedConfigurationPropertyAnnotation(),
 				deprecatedConfigurationPropertyAnnotation(), constructorBindingAnnotation(), autowiredAnnotation(),
 				defaultValueAnnotation(), endpointAnnotations(), readOperationAnnotation(), nameAnnotation());
+		String cacheLocation = env.getOptions().get(DESCRIPTION_CACHE_LOCATION_OPTION);
+		if (cacheLocation != null) {
+			this.descriptionCache = new DescriptionCache(cacheLocation);
+		}
 	}
 
 	@Override
@@ -289,6 +301,9 @@ private void processTypeElement(String prefix, TypeElement element, ExecutableEl
 			Deque<TypeElement> seen) {
 		if (!seen.contains(element)) {
 			seen.push(element);
+			if (this.descriptionCache != null && !this.metadataEnv.hasSourceTree(element)) {
+				this.classFileBackedTypes.add(this.metadataEnv.getTypeUtils().getQualifiedName(element));
+			}
 			new PropertyDescriptorResolver(this.metadataEnv).resolve(element, source).forEach((descriptor) -> {
 				this.metadataCollector.add(descriptor.resolveItemMetadata(prefix, this.metadataEnv));
 				ItemHint itemHint = descriptor.resolveItemHint(prefix, this.metadataEnv);
@@ -405,14 +420,38 @@ protected void writeSourceMetadata() throws Exception {
 	protected ConfigurationMetadata writeMetadata() throws Exception {
 		ConfigurationMetadata metadata = this.metadataCollector.getMetadata();
 		metadata = mergeAdditionalMetadata(metadata, () -> this.metadataStore.readAdditionalMetadata());
+		fillCachedDescriptions(metadata);
 		removeIgnored(metadata);
 		if (!metadata.getItems().isEmpty()) {
 			this.metadataStore.writeMetadata(metadata);
+			updateDescriptionCache(metadata);
 			return metadata;
 		}
 		return null;
 	}
 
+	private void fillCachedDescriptions(ConfigurationMetadata metadata) {
+		if (this.descriptionCache == null) {
+			return;
+		}
+		for (ItemMetadata item : metadata.getItems()) {
+			if (item.isOfItemType(ItemMetadata.ItemType.PROPERTY) && item.getDescription() == null
+					&& item.getSourceType() != null && this.classFileBackedTypes.contains(item.getSourceType())) {
+				String cached = this.descriptionCache.getDescription(item.getName());
+				if (cached != null) {
+					item.setDescription(cached);
+				}
+			}
+		}
+	}
+
+	private void updateDescriptionCache(ConfigurationMetadata metadata) {
+		if (this.descriptionCache == null) {
+			return;
+		}
+		this.descriptionCache.update(metadata);
+	}
+
 	private void removeIgnored(ConfigurationMetadata metadata) {
 		for (ItemIgnore itemIgnore : metadata.getIgnored()) {
 			metadata.removeMetadata(itemIgnore.getType(), itemIgnore.getName());

configuration-metadata/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/DescriptionCache.java

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2012-present 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.springframework.boot.configurationprocessor;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+
+import org.springframework.boot.configurationprocessor.metadata.ConfigurationMetadata;
+import org.springframework.boot.configurationprocessor.metadata.ItemMetadata;
+import org.springframework.boot.configurationprocessor.metadata.JsonMarshaller;
+
+/**
+ * Cache for property descriptions that persists across incremental builds. Stores
+ * metadata in a location outside of {@code CLASS_OUTPUT} so that Gradle's incremental
+ * compilation does not delete it.
+ *
+ * @author Agustin Palazzo
+ * @see ConfigurationMetadataAnnotationProcessor
+ */
+class DescriptionCache {
+
+	private final File cacheFile;
+
+	private ConfigurationMetadata cachedMetadata;
+
+	DescriptionCache(String cacheFilePath) {
+		this.cacheFile = new File(cacheFilePath);
+		this.cachedMetadata = read();
+	}
+
+	/**
+	 * Look up a cached description for the given property name.
+	 * @param propertyName the fully qualified property name
+	 * @return the cached description or {@code null}
+	 */
+	String getDescription(String propertyName) {
+		if (this.cachedMetadata == null) {
+			return null;
+		}
+		for (ItemMetadata item : this.cachedMetadata.getItems()) {
+			if (item.isOfItemType(ItemMetadata.ItemType.PROPERTY) && propertyName.equals(item.getName())) {
+				return item.getDescription();
+			}
+		}
+		return null;
+	}
+
+	/**
+	 * Replace the cache with the given metadata. After
+	 * {@link ConfigurationMetadataAnnotationProcessor#fillCachedDescriptions} has
+	 * restored descriptions from the cache, the metadata is the complete picture of the
+	 * current build. Replacing (instead of merging) ensures that entries for deleted or
+	 * de-annotated types are automatically pruned.
+	 * @param metadata the current build's metadata with descriptions already filled
+	 */
+	void update(ConfigurationMetadata metadata) {
+		write(metadata);
+		this.cachedMetadata = new ConfigurationMetadata(metadata);
+	}
+
+	private ConfigurationMetadata read() {
+		if (!this.cacheFile.exists()) {
+			return null;
+		}
+		try (FileInputStream in = new FileInputStream(this.cacheFile)) {
+			return new JsonMarshaller().read(in);
+		}
+		catch (Exception ex) {
+			return null;
+		}
+	}
+
+	private void write(ConfigurationMetadata metadata) {
+		this.cacheFile.getParentFile().mkdirs();
+		try (FileOutputStream out = new FileOutputStream(this.cacheFile)) {
+			new JsonMarshaller().write(metadata, out);
+		}
+		catch (IOException ex) {
+			// Cache write failure is non-fatal
+		}
+	}
+
+}

configuration-metadata/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/MetadataGenerationEnvironment.java

@@ -137,6 +137,10 @@ TypeUtils getTypeUtils() {
 		return this.typeUtils;
 	}
 
+	boolean hasSourceTree(TypeElement element) {
+		return this.fieldValuesParser.hasSourceTree(element);
+	}
+
 	Messager getMessager() {
 		return this.messager;
 	}

configuration-metadata/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/FieldValuesParser.java

@@ -30,13 +30,19 @@
  * @since 1.1.2
  * @see JavaCompilerFieldValuesParser
  */
-@FunctionalInterface
 public interface FieldValuesParser {
 
 	/**
 	 * Implementation of {@link FieldValuesParser} that always returns an empty result.
 	 */
-	FieldValuesParser NONE = (element) -> Collections.emptyMap();
+	FieldValuesParser NONE = new FieldValuesParser() {
+
+		@Override
+		public Map<String, Object> getFieldValues(TypeElement element) {
+			return Collections.emptyMap();
+		}
+
+	};
 
 	/**
 	 * Return the field values for the given element.
@@ -46,4 +52,16 @@ public interface FieldValuesParser {
 	 */
 	Map<String, Object> getFieldValues(TypeElement element) throws Exception;
 
+	/**
+	 * Return whether the given element has a source tree available. Elements loaded from
+	 * {@code .class} files during incremental compilation will not have a source tree,
+	 * meaning javadoc comments are unavailable.
+	 * @param element the element to check
+	 * @return {@code true} if the element has source, {@code false} if backed by
+	 * {@code .class}
+	 */
+	default boolean hasSourceTree(TypeElement element) {
+		return true;
+	}
+
 }

configuration-metadata/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/JavaCompilerFieldValuesParser.java

@@ -57,6 +57,16 @@ public Map<String, Object> getFieldValues(TypeElement element) throws Exception
 		return Collections.emptyMap();
 	}
 
+	@Override
+	public boolean hasSourceTree(TypeElement element) {
+		try {
+			return this.trees.getTree(element) != null;
+		}
+		catch (Exception ex) {
+			return true;
+		}
+	}
+
 	/**
 	 * {@link TreeVisitor} to collect fields.
 	 */