DATAMONGO-1183 - Add support for Hashed Indexes.

We now support hashed index definitions via IndexOperations. Reading index information back allows to identify a hashed index via isHashed().

Original pull request: spring-projects#750.

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/HashIndexed.java

@@ -0,0 +1,71 @@
+/*
+ * Copyright 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 org.springframework.data.mongodb.core.index;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Marker interface for a property that should be used as key for a
+ * <a href="https://docs.mongodb.com/manual/core/index-hashed/">Hashed Index</a>. If used on a simple property the index
+ * uses a hashing function to compute the hash of the value of the index field. Added to a property of complex type the
+ * embedded document is collapsed and the hash computed for the entire object.
+ * <p />
+ * 
+ * <pre>
+ *     <code>
+ *
+ * &#64;Document
+ * public class DomainType {
+ *
+ *   &#64;HashIndexed @Id String id;
+ * }
+ *     </code>
+ * </pre>
+ *
+ * {@link HashIndexed} can also be used as meta {@link java.lang.annotation.Annotation} to create composed annotations.
+ * 
+ * <pre>
+ *     <code>
+ *
+ * &#64;Indexed
+ * &#64;HashIndexed
+ * &#64;Retention(RetentionPolicy.RUNTIME)
+ * public @interface IndexAndHash {
+ *
+ *   &#64;AliasFor(annotation = Indexed.class, attribute = "name")
+ *   String name() default "";
+ * }
+ *
+ * &#64;Document
+ * public class DomainType {
+ *
+ *   &#64;ComposedHashIndexed(name = "idx-name") String value;
+ * }
+ *     </code>
+ * </pre>
+ *
+ * @author Christoph Strobl
+ * @since 2.2
+ * @see HashedIndex
+ */
+@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD })
+@Retention(RetentionPolicy.RUNTIME)
+public @interface HashIndexed {
+
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/HashedIndex.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 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 org.springframework.data.mongodb.core.index;
+
+import org.bson.Document;
+import org.springframework.util.Assert;
+
+/**
+ * {@link IndexDefinition} implementation for MongoDB
+ * <a href="https://docs.mongodb.com/manual/core/index-hashed/">Hashed Indexes</a> maintaining entries with hashes of
+ * the values of the indexed field.
+ *
+ * @author Christoph Strobl
+ * @since 2.2
+ */
+public class HashedIndex implements IndexDefinition {
+
+	private final String field;
+
+	private HashedIndex(String field) {
+
+		Assert.hasText(field, "Field must not be null nor empty!");
+		this.field = field;
+	}
+
+	/**
+	 * Creates a new {@link HashedIndex} for the given field.
+	 *
+	 * @param field must not be {@literal null} nor empty.
+	 * @return
+	 */
+	public static HashedIndex hashed(String field) {
+		return new HashedIndex(field);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.index.IndexDefinition#getIndexKeys()
+	 */
+	@Override
+	public Document getIndexKeys() {
+		return new Document(field, "hashed");
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.index.IndexDefinition#getIndexOptions()
+	 */
+	@Override
+	public Document getIndexOptions() {
+		return new Document();
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/IndexField.java

@@ -30,7 +30,7 @@
 public final class IndexField {
 
 	enum Type {
-		GEO, TEXT, DEFAULT;
+		GEO, TEXT, DEFAULT, HASH;
 	}
 
 	private final String key;
@@ -49,7 +49,9 @@ private IndexField(String key, @Nullable Direction direction, @Nullable Type typ
 		if (Type.GEO.equals(type) || Type.TEXT.equals(type)) {
 			Assert.isNull(direction, "Geo/Text indexes must not have a direction!");
 		} else {
-			Assert.notNull(direction, "Default indexes require a direction");
+			if (!Type.HASH.equals(type)) {
+				Assert.notNull(direction, "Default indexes require a direction");
+			}
 		}
 
 		this.key = key;
@@ -65,6 +67,17 @@ public static IndexField create(String key, Direction order) {
 		return new IndexField(key, order, Type.DEFAULT);
 	}
 
+	/**
+	 * Creates a {@literal hashed} {@link IndexField} for the given key.
+	 *
+	 * @param key must not be {@literal null} or empty.
+	 * @return new instance of {@link IndexField}.
+	 * @since 2.2
+	 */
+	static IndexField hashed(String key) {
+		return new IndexField(key, null, Type.HASH);
+	}
+
 	/**
 	 * Creates a geo {@link IndexField} for the given key.
 	 *
@@ -120,6 +133,16 @@ public boolean isText() {
 		return Type.TEXT.equals(type);
 	}
 
+	/**
+	 * Returns whether the {@link IndexField} is a {@literal hashed}.
+	 *
+	 * @return {@literal true} if {@link IndexField} is hashed.
+	 * @since 2.2
+	 */
+	public boolean isHashed() {
+		return Type.HASH.equals(type);
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see java.lang.Object#equals(java.lang.Object)

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/IndexInfo.java

@@ -96,12 +96,17 @@ public static IndexInfo indexInfoOf(Document sourceDocument) {
 
 			} else {
 
-				Double keyValue = new Double(value.toString());
+				if (ObjectUtils.nullSafeEquals("hashed", value)) {
+					indexFields.add(IndexField.hashed(key));
+				} else {
 
-				if (ONE.equals(keyValue)) {
-					indexFields.add(IndexField.create(key, ASC));
-				} else if (MINUS_ONE.equals(keyValue)) {
-					indexFields.add(IndexField.create(key, DESC));
+					Double keyValue = new Double(value.toString());
+
+					if (ONE.equals(keyValue)) {
+						indexFields.add(IndexField.create(key, ASC));
+					} else if (MINUS_ONE.equals(keyValue)) {
+						indexFields.add(IndexField.create(key, DESC));
+					}
 				}
 			}
 		}
@@ -206,6 +211,14 @@ public Optional<Duration> getExpireAfter() {
 		return Optional.ofNullable(expireAfter);
 	}
 
+	/**
+	 * @return {@literal true} if a hashed index field is present.
+	 * @since 2.2
+	 */
+	public boolean isHashed() {
+		return getIndexFields().stream().anyMatch(IndexField::isHashed);
+	}
+
 	@Override
 	public String toString() {