Add query Explain Support.

src/main/java/org/springframework/data/elasticsearch/core/RequestFactory.java

@@ -1144,6 +1144,8 @@ private SearchRequest prepareSearchRequest(Query query, @Nullable Class<?> clazz
 			sourceBuilder.timeout(timeout);
 		}
 
+		sourceBuilder.explain(query.getExplain());
+
 		request.source(sourceBuilder);
 		return request;
 	}
@@ -1224,6 +1226,8 @@ private SearchRequestBuilder prepareSearchRequestBuilder(Query query, Client cli
 			searchRequestBuilder.setTimeout(timeout);
 		}
 
+		searchRequestBuilder.setExplain(query.getExplain());
+
 		return searchRequestBuilder;
 	}
 

src/main/java/org/springframework/data/elasticsearch/core/SearchHit.java

@@ -23,6 +23,7 @@
 import java.util.Map;
 import java.util.stream.Collectors;
 
+import org.springframework.data.elasticsearch.core.document.Explanation;
 import org.springframework.data.elasticsearch.core.document.NestedMetaData;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -44,16 +45,18 @@ public class SearchHit<T> {
 	private final Map<String, List<String>> highlightFields = new LinkedHashMap<>();
 	private final Map<String, SearchHits<?>> innerHits = new LinkedHashMap<>();
 	@Nullable private final NestedMetaData nestedMetaData;
-	@Nullable private String routing;
+	@Nullable private final String routing;
+	@Nullable private final Explanation explanation;
 
 	public SearchHit(@Nullable String index, @Nullable String id, @Nullable String routing, float score,
 			@Nullable Object[] sortValues, @Nullable Map<String, List<String>> highlightFields, T content) {
-		this(index, id, routing, score, sortValues, highlightFields, null, null, content);
+		this(index, id, routing, score, sortValues, highlightFields, null, null, null, content);
 	}
 
 	public SearchHit(@Nullable String index, @Nullable String id, @Nullable String routing, float score,
 			@Nullable Object[] sortValues, @Nullable Map<String, List<String>> highlightFields,
-			@Nullable Map<String, SearchHits<?>> innerHits, @Nullable NestedMetaData nestedMetaData, T content) {
+			@Nullable Map<String, SearchHits<?>> innerHits, @Nullable NestedMetaData nestedMetaData,
+			@Nullable Explanation explanation, T content) {
 		this.index = index;
 		this.id = id;
 		this.routing = routing;
@@ -69,7 +72,7 @@ public SearchHit(@Nullable String index, @Nullable String id, @Nullable String r
 		}
 
 		this.nestedMetaData = nestedMetaData;
-
+		this.explanation = explanation;
 		this.content = content;
 	}
 
@@ -176,4 +179,13 @@ public String toString() {
 	public String getRouting() {
 		return routing;
 	}
+
+	/**
+	 * @return the explanation for this SearchHit.
+	 * @since 4.2
+	 */
+	@Nullable
+	public Explanation getExplanation() {
+		return explanation;
+	}
 }

src/main/java/org/springframework/data/elasticsearch/core/SearchHitMapping.java

@@ -113,6 +113,7 @@ SearchHit<T> mapHit(SearchDocument searchDocument, T content) {
 				getHighlightsAndRemapFieldNames(searchDocument), //
 				mapInnerHits(searchDocument), //
 				searchDocument.getNestedMetaData(), //
+				searchDocument.getExplanation(), //
 				content); //
 	}
 
@@ -196,6 +197,7 @@ private SearchHits<?> mapInnerDocuments(SearchHits<SearchDocument> searchHits, C
 							searchDocument.getHighlightFields(), //
 							searchHit.getInnerHits(), //
 							persistentEntityWithNestedMetaData.nestedMetaData, //
+							searchHit.getExplanation(), //
 							targetObject));
 				});
 

src/main/java/org/springframework/data/elasticsearch/core/document/DocumentAdapters.java

@@ -161,24 +161,20 @@ public static SearchDocument from(SearchHit source) {
 		Map<String, SearchHits> sourceInnerHits = source.getInnerHits();
 
 		if (sourceInnerHits != null) {
-			sourceInnerHits.forEach((name, searchHits) -> {
-				innerHits.put(name, SearchDocumentResponse.from(searchHits, null, null));
-			});
+			sourceInnerHits
+					.forEach((name, searchHits) -> innerHits.put(name, SearchDocumentResponse.from(searchHits, null, null)));
 		}
 
-		NestedMetaData nestedMetaData = null;
-
-		if (source.getNestedIdentity() != null) {
-			 nestedMetaData = from(source.getNestedIdentity());
-		}
+		NestedMetaData nestedMetaData = from(source.getNestedIdentity());
+		Explanation explanation = from(source.getExplanation());
 
 		BytesReference sourceRef = source.getSourceRef();
 
 		if (sourceRef == null || sourceRef.length() == 0) {
 			return new SearchDocumentAdapter(
 					source.getScore(), source.getSortValues(), source.getFields(), highlightFields, fromDocumentFields(source,
 							source.getIndex(), source.getId(), source.getVersion(), source.getSeqNo(), source.getPrimaryTerm()),
-					innerHits, nestedMetaData);
+					innerHits, nestedMetaData, explanation);
 		}
 
 		Document document = Document.from(source.getSourceAsMap());
@@ -192,25 +188,40 @@ public static SearchDocument from(SearchHit source) {
 		document.setPrimaryTerm(source.getPrimaryTerm());
 
 		return new SearchDocumentAdapter(source.getScore(), source.getSortValues(), source.getFields(), highlightFields,
-				document, innerHits, nestedMetaData);
+				document, innerHits, nestedMetaData, explanation);
 	}
 
-	private static NestedMetaData from(SearchHit.NestedIdentity nestedIdentity) {
+	@Nullable
+	private static Explanation from(@Nullable org.apache.lucene.search.Explanation explanation) {
 
-		NestedMetaData child = null;
+		if (explanation == null) {
+			return null;
+		}
 
-		if (nestedIdentity.getChild() != null) {
-			child = from(nestedIdentity.getChild());
+		List<Explanation> details = new ArrayList<>();
+		for (org.apache.lucene.search.Explanation detail : explanation.getDetails()) {
+			details.add(from(detail));
 		}
 
+		return new Explanation(explanation.isMatch(), explanation.getValue().doubleValue(), explanation.getDescription(),
+				details);
+	}
+
+	@Nullable
+	private static NestedMetaData from(@Nullable SearchHit.NestedIdentity nestedIdentity) {
+
+		if (nestedIdentity == null) {
+			return null;
+		}
+		NestedMetaData child = from(nestedIdentity.getChild());
 		return NestedMetaData.of(nestedIdentity.getField().string(), nestedIdentity.getOffset(), child);
 	}
 
 	/**
 	 * Create an unmodifiable {@link Document} from {@link Iterable} of {@link DocumentField}s.
 	 *
 	 * @param documentFields the {@link DocumentField}s backing the {@link Document}.
-	 * @param index
+	 * @param index the index where the Document was found
 	 * @return the adapted {@link Document}.
 	 */
 	public static Document fromDocumentFields(Iterable<DocumentField> documentFields, String index, String id,
@@ -458,10 +469,11 @@ static class SearchDocumentAdapter implements SearchDocument {
 		private final Map<String, List<String>> highlightFields = new HashMap<>();
 		private final Map<String, SearchDocumentResponse> innerHits = new HashMap<>();
 		@Nullable private final NestedMetaData nestedMetaData;
+		@Nullable private final Explanation explanation;
 
 		SearchDocumentAdapter(float score, Object[] sortValues, Map<String, DocumentField> fields,
 				Map<String, List<String>> highlightFields, Document delegate, Map<String, SearchDocumentResponse> innerHits,
-				@Nullable NestedMetaData nestedMetaData) {
+				@Nullable NestedMetaData nestedMetaData, @Nullable Explanation explanation) {
 
 			this.score = score;
 			this.sortValues = sortValues;
@@ -470,6 +482,7 @@ static class SearchDocumentAdapter implements SearchDocument {
 			this.highlightFields.putAll(highlightFields);
 			this.innerHits.putAll(innerHits);
 			this.nestedMetaData = nestedMetaData;
+			this.explanation = explanation;
 		}
 
 		@Override
@@ -646,6 +659,12 @@ public Set<Entry<String, Object>> entrySet() {
 			return delegate.entrySet();
 		}
 
+		@Override
+		@Nullable
+		public Explanation getExplanation() {
+			return explanation;
+		}
+
 		@Override
 		public boolean equals(Object o) {
 			if (this == o) {

src/main/java/org/springframework/data/elasticsearch/core/document/Explanation.java

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2021 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.data.elasticsearch.core.document;
+
+import java.util.List;
+import java.util.Objects;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
+/**
+ * class that holds explanations returned from an Elasticsearch search.
+ * 
+ * @author Peter-Josef Meisch
+ */
+public class Explanation {
+	private final boolean match;
+	private final Double value;
+	@Nullable private final String description;
+	private final List<Explanation> details;
+
+	public Explanation(boolean match, Double value, @Nullable String description, List<Explanation> details) {
+
+		Assert.notNull(value, "value must not be null");
+		Assert.notNull(details, "details must not be null");
+
+		this.match = match;
+		this.value = value;
+		this.description = description;
+		this.details = details;
+	}
+
+	public boolean isMatch() {
+		return match;
+	}
+
+	public Double getValue() {
+		return value;
+	}
+
+	@Nullable
+	public String getDescription() {
+		return description;
+	}
+
+	public List<Explanation> getDetails() {
+		return details;
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		Explanation that = (Explanation) o;
+
+		if (match != that.match)
+			return false;
+		if (!value.equals(that.value))
+			return false;
+		if (!Objects.equals(description, that.description))
+			return false;
+		return details.equals(that.details);
+	}
+
+	@Override
+	public int hashCode() {
+		int result = (match ? 1 : 0);
+		result = 31 * result + value.hashCode();
+		result = 31 * result + (description != null ? description.hashCode() : 0);
+		result = 31 * result + details.hashCode();
+		return result;
+	}
+
+	@Override
+	public String toString() {
+		return "Explanation{" + //
+				"match=" + match + //
+				", value=" + value + //
+				", description='" + description + '\'' + //
+				", details=" + details + //
+				'}'; //
+	}
+}

src/main/java/org/springframework/data/elasticsearch/core/document/SearchDocument.java

@@ -21,7 +21,7 @@
 import org.springframework.lang.Nullable;
 
 /**
- * Extension to {@link Document} exposing a search response related data.
+ * Extension to {@link Document} exposing search response related data.
  *
  * @author Mark Paluch
  * @author Peter-Josef Meisch
@@ -98,4 +98,11 @@ default NestedMetaData getNestedMetaData() {
 	default String getRouting() {
 		return getFieldValue("_routing");
 	}
+
+	/**
+	 * @return the explanation for the SearchHit.
+	 * @since 4.2
+	 */
+	@Nullable
+	Explanation getExplanation();
 }

src/main/java/org/springframework/data/elasticsearch/core/document/package-info.java

@@ -1,3 +1,6 @@
+/**
+ * Classes related to the Document structure of Elasticsearch documents and search responses.
+ */
 @org.springframework.lang.NonNullApi
 @org.springframework.lang.NonNullFields
 package org.springframework.data.elasticsearch.core.document;

src/main/java/org/springframework/data/elasticsearch/core/query/AbstractQuery.java

@@ -61,6 +61,7 @@ abstract class AbstractQuery implements Query {
 	@Nullable private Integer trackTotalHitsUpTo;
 	@Nullable private Duration scrollTime;
 	@Nullable private TimeValue timeout;
+	private boolean explain = false;
 
 	@Override
 	@Nullable
@@ -270,4 +271,16 @@ public TimeValue getTimeout() {
 	public void setTimeout(@Nullable TimeValue timeout) {
 		this.timeout = timeout;
 	}
+
+	@Override
+	public boolean getExplain() {
+		return explain;
+	}
+
+	/**
+	 * @param explain the explain flag on the query.
+	 */
+	public void setExplain(boolean explain) {
+		this.explain = explain;
+	}
 }

src/main/java/org/springframework/data/elasticsearch/core/query/Query.java

@@ -285,4 +285,12 @@ default boolean hasScrollTime() {
 	 */
 	@Nullable
 	TimeValue getTimeout();
+
+	/**
+	 * @return {@literal true} when the query has the eplain parameter set, defaults to {@literal false}
+	 * @since 4.2
+	 */
+	default boolean getExplain() {
+		return false;
+	}
 }