DATACMNS-1020 - Improvements to revision API.

RevisionRepository now returns Optional for methods that could previously return null. Revision exposes additional methods to lookup required revision numbers and dates. Revisions now implements Streamable and exposes a ….none() factory method to create an empty instance.

AnnotationBasedRevisionMetadata now uses Lazy to lookup the fields with revision annotations. AnnotationDetectionFieldCallback now also uses Optional in places it previously returned null. StreamUtils now exposes factory methods for Collector instances producing unmodifiable List and Set instances.

Related ticket: DATACMNS-867.

src/main/java/org/springframework/data/util/AnnotationDetectionFieldCallback.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 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.
@@ -17,6 +17,7 @@ package org.springframework.data.util;
 
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Field;
+import java.util.Optional;
 
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.util.Assert;
@@ -33,7 +34,7 @@ import org.springframework.util.ReflectionUtils.FieldCallback;
 public class AnnotationDetectionFieldCallback implements FieldCallback {
 
 	private final Class<? extends Annotation> annotationType;
-	private Field field;
+	private Optional<Field> field = Optional.empty();
 
 	/**
 	 * Creates a new {@link AnnotationDetectionFieldCallback} scanning for an annotation of the given type.
@@ -43,6 +44,7 @@ public class AnnotationDetectionFieldCallback implements FieldCallback {
 	public AnnotationDetectionFieldCallback(Class<? extends Annotation> annotationType) {
 
 		Assert.notNull(annotationType, "AnnotationType must not be null!");
+
 		this.annotationType = annotationType;
 	}
 
@@ -52,16 +54,14 @@ public class AnnotationDetectionFieldCallback implements FieldCallback {
 	 */
 	public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
 
-		if (this.field != null) {
+		if (this.field.isPresent()) {
 			return;
 		}
 
-		Annotation annotation = AnnotatedElementUtils.findMergedAnnotation(field, annotationType);
+		if (AnnotatedElementUtils.findMergedAnnotation(field, annotationType) != null) {
 
-		if (annotation != null) {
-
-			this.field = field;
-			ReflectionUtils.makeAccessible(this.field);
+			ReflectionUtils.makeAccessible(field);
+			this.field = Optional.of(field);
 		}
 	}
 
@@ -70,8 +70,20 @@ public class AnnotationDetectionFieldCallback implements FieldCallback {
 	 * 
 	 * @return
 	 */
-	public Class<?> getType() {
-		return field == null ? null : field.getType();
+	public Optional<Class<?>> getType() {
+		return field.map(Field::getType);
+	}
+
+	/**
+	 * Returns the type of the field or throws an {@link IllegalArgumentException} if no field could be found.
+	 * 
+	 * @return
+	 * @throws IllegalStateException
+	 */
+	public Class<?> getRequiredType() {
+
+		return getType().orElseThrow(() -> new IllegalStateException(
+				String.format("Unable to obtain type! Didn't find field with annotation %s!", annotationType)));
 	}
 
 	/**
@@ -81,9 +93,10 @@ public class AnnotationDetectionFieldCallback implements FieldCallback {
 	 * @return
 	 */
 	@SuppressWarnings("unchecked")
-	public <T> T getValue(Object source) {
+	public <T> Optional<T> getValue(Object source) {
 
 		Assert.notNull(source, "Source object must not be null!");
-		return field == null ? null : (T) ReflectionUtils.getField(field, source);
+
+		return field.map(it -> (T) ReflectionUtils.getField(it, source));
 	}
 }

src/main/java/org/springframework/data/util/StreamUtils.java

@@ -15,9 +15,15 @@
  */
 package org.springframework.data.util;
 
+import static java.util.stream.Collectors.*;
+
+import java.util.Collections;
 import java.util.Iterator;
+import java.util.List;
+import java.util.Set;
 import java.util.Spliterator;
 import java.util.Spliterators;
+import java.util.stream.Collector;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
@@ -53,4 +59,22 @@ public interface StreamUtils {
 				? stream.onClose(() -> ((CloseableIterator<T>) iterator).close()) //
 				: stream;
 	}
+
+	/**
+	 * Returns a {@link Collector} to create an unmodifiable {@link List}.
+	 * 
+	 * @return will never be {@literal null}.
+	 */
+	public static <T> Collector<T, ?, List<T>> toUnmodifiableList() {
+		return collectingAndThen(toList(), Collections::unmodifiableList);
+	}
+
+	/**
+	 * Returns a {@link Collector} to create an unmodifiable {@link Set}.
+	 * 
+	 * @return will never be {@literal null}.
+	 */
+	public static <T> Collector<T, ?, Set<T>> toUnmodifiableSet() {
+		return collectingAndThen(toSet(), Collections::unmodifiableSet);
+	}
 }