Update the metadata annotation processor to support incremental builds

This commit udpdates the metadata annotation processor so that change
data from an incremental build is merged with the metadata from the
previous build.

Closes gh-2321

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

@@ -25,7 +25,9 @@ import java.io.OutputStream;
 import java.io.PrintWriter;
 import java.io.StringWriter;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
@@ -62,11 +64,14 @@ import org.springframework.boot.configurationprocessor.metadata.JsonMarshaller;
  *
  * @author Stephane Nicoll
  * @author Phillip Webb
+ * @author Kris De Volder
  * @since 1.2.0
  */
 @SupportedAnnotationTypes({ "*" })
 public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor {
 
+	public static final String METADATA_PATH = "META-INF/spring-configuration-metadata.json";
+
 	static final String CONFIGURATION_PROPERTIES_ANNOTATION = "org.springframework.boot."
 			+ "context.properties.ConfigurationProperties";
 
@@ -85,6 +90,17 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 
 	private ConfigurationMetadata metadata;
 
+	/**
+	 * On incremental builds, holds the 'old' metadata (created by the previous build).
+	 */
+	private ConfigurationMetadata oldmetadata;
+
+	/**
+	 * On incremental builds, keeps track of the types that where presented to the
+	 * processor. This includes types that are not annotated.
+	 */
+	protected Set<String> processedSourceTypes;
+
 	private TypeUtils typeUtils;
 
 	private FieldValuesParser fieldValuesParser;
@@ -109,6 +125,10 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 		super.init(env);
 		this.metadata = new ConfigurationMetadata();
 		this.typeUtils = new TypeUtils(env);
+		this.oldmetadata = readMetadata();
+		if (isIncremental()) {
+			this.processedSourceTypes = new HashSet<String>();
+		}
 		try {
 			this.fieldValuesParser = new JavaCompilerFieldValuesParser(env);
 		}
@@ -119,10 +139,37 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 		}
 	}
 
+	protected boolean isIncremental() {
+		return this.oldmetadata != null;
+	}
+
+	protected boolean isDeleted(String sourceType) {
+		return this.processingEnv.getElementUtils().getTypeElement(sourceType) == null;
+	}
+
+	protected boolean isProcessed(String sourceType) {
+		return this.processedSourceTypes.contains(sourceType);
+	}
+
+	/**
+	 * Called during incremental build on all the 'root elements' that are being presented
+	 * to the processor.
+	 */
+	protected void markAsProcessed(Element element) {
+		if (element instanceof TypeElement) {
+			this.processedSourceTypes.add(this.typeUtils.getType(element));
+		}
+	}
+
 	@Override
 	public boolean process(Set<? extends TypeElement> annotations,
 			RoundEnvironment roundEnv) {
 		Elements elementUtils = this.processingEnv.getElementUtils();
+		if (isIncremental()) {
+			for (Element element : roundEnv.getRootElements()) {
+				markAsProcessed(element);
+			}
+		}
 		for (Element element : roundEnv.getElementsAnnotatedWith(elementUtils
 				.getTypeElement(configurationPropertiesAnnotation()))) {
 			processElement(element);
@@ -328,16 +375,19 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 		return values;
 	}
 
-	protected void writeMetaData(ConfigurationMetadata metadata) {
+	protected ConfigurationMetadata writeMetaData(ConfigurationMetadata metadata) {
 		metadata = mergeAdditionalMetadata(metadata);
+		if (isIncremental()) {
+			mergeOldMetadata(metadata);
+		}
 		if (!metadata.getItems().isEmpty()) {
 			try {
 				FileObject resource = this.processingEnv.getFiler().createResource(
-						StandardLocation.CLASS_OUTPUT, "",
-						"META-INF/spring-configuration-metadata.json");
+						StandardLocation.CLASS_OUTPUT, "", METADATA_PATH);
 				OutputStream outputStream = resource.openOutputStream();
 				try {
 					new JsonMarshaller().write(metadata, outputStream);
+					return metadata;
 				}
 				finally {
 					outputStream.close();
@@ -347,6 +397,42 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 				throw new IllegalStateException(ex);
 			}
 		}
+		return null;
+	}
+
+	protected ConfigurationMetadata readMetadata() {
+		try {
+			FileObject resource = this.processingEnv.getFiler().getResource(
+					StandardLocation.CLASS_OUTPUT, "", METADATA_PATH);
+			InputStream in = resource.openInputStream();
+			try {
+				ConfigurationMetadata data = new ConfigurationMetadata();
+				data.addAll(new JsonMarshaller().read(in));
+				if (!data.getItems().isEmpty()) {
+					return data;
+				}
+			}
+			finally {
+				in.close();
+			}
+		}
+		catch (IOException e) {
+			// no 'old' metadata
+		}
+		return null;
+	}
+
+	private void mergeOldMetadata(ConfigurationMetadata metadata) {
+		List<ItemMetadata> items = this.oldmetadata.getItems();
+		for (ItemMetadata oldItem : items) {
+			String sourceType = oldItem.getSourceType();
+			if (sourceType == null || isProcessed(sourceType) || isDeleted(sourceType)) {
+				// skip
+			}
+			else {
+				metadata.add(oldItem);
+			}
+		}
 	}
 
 	private ConfigurationMetadata mergeAdditionalMetadata(ConfigurationMetadata metadata) {