Polishing.

Introduce default exists(…) method on KeyValueAdapter to allow for store-specific optimizations of the exists method. Lazily construct a single PartTree instance for reuse.

Refine generics.

See #386
Original pull request: #386.

src/main/java/org/springframework/data/keyvalue/core/KeyValueAdapter.java

@@ -184,4 +184,16 @@ public interface KeyValueAdapter extends DisposableBean {
 	 * @return
 	 */
 	long count(KeyValueQuery<?> query, String keyspace);
+
+	/**
+	 * Determine whether result of given {@link KeyValueQuery} within {@literal keyspace} contains at least one element.
+	 *
+	 * @param query must not be {@literal null}.
+	 * @param keyspace must not be {@literal null}.
+	 * @return
+	 * @since 2.7
+	 */
+	default boolean exists(KeyValueQuery<?> query, String keyspace) {
+		return count(query, keyspace) > 0;
+	}
 }

src/main/java/org/springframework/data/keyvalue/core/KeyValueOperations.java

@@ -177,6 +177,16 @@ public interface KeyValueOperations extends DisposableBean {
 	 */
 	long count(KeyValueQuery<?> query, Class<?> type);
 
+	/**
+	 * Determine whether result of given {@link KeyValueQuery} contains at least one element.
+	 *
+	 * @param query
+	 * @param type
+	 * @return
+	 * @since 2.7
+	 */
+	boolean exists(KeyValueQuery<?> query, Class<?> type);
+
 	/**
 	 * @return mapping context in use.
 	 */

src/main/java/org/springframework/data/keyvalue/core/KeyValueTemplate.java

@@ -445,6 +445,15 @@ public class KeyValueTemplate implements KeyValueOperations, ApplicationEventPub
 		return executeRequired(adapter -> adapter.count(query, resolveKeySpace(type)));
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.keyvalue.core.KeyValueOperations#count(org.springframework.data.keyvalue.core.query.KeyValueQuery, java.lang.Class)
+	 */
+	@Override
+	public boolean exists(KeyValueQuery<?> query, Class<?> type) {
+		return executeRequired(adapter -> adapter.exists(query, resolveKeySpace(type)));
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.keyvalue.core.KeyValueOperations#getMappingContext()

src/main/java/org/springframework/data/keyvalue/repository/query/KeyValuePartTreeQuery.java

@@ -34,6 +34,7 @@ import org.springframework.data.repository.query.ResultProcessor;
 import org.springframework.data.repository.query.parser.AbstractQueryCreator;
 import org.springframework.data.repository.query.parser.PartTree;
 import org.springframework.data.spel.EvaluationContextProvider;
+import org.springframework.data.util.Lazy;
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.spel.standard.SpelExpression;
 import org.springframework.lang.Nullable;
@@ -50,9 +51,10 @@ import org.springframework.util.ClassUtils;
  */
 public class KeyValuePartTreeQuery implements RepositoryQuery {
 
-	private final QueryMethodEvaluationContextProvider evaluationContextProvider;
+	private final Lazy<PartTree> partTree;
 	private final QueryMethod queryMethod;
 	private final KeyValueOperations keyValueOperations;
+	private final QueryMethodEvaluationContextProvider evaluationContextProvider;
 	private final QueryCreatorFactory<AbstractQueryCreator<KeyValueQuery<?>, ?>> queryCreatorFactory;
 
 	/**
@@ -83,13 +85,16 @@ public class KeyValuePartTreeQuery implements RepositoryQuery {
 	 * @since 2.0
 	 */
 	public KeyValuePartTreeQuery(QueryMethod queryMethod, QueryMethodEvaluationContextProvider evaluationContextProvider,
-			KeyValueOperations keyValueOperations, QueryCreatorFactory queryCreatorFactory) {
+			KeyValueOperations keyValueOperations,
+			QueryCreatorFactory<AbstractQueryCreator<KeyValueQuery<?>, ?>> queryCreatorFactory) {
 
 		Assert.notNull(queryMethod, "Query method must not be null!");
 		Assert.notNull(evaluationContextProvider, "EvaluationContextprovider must not be null!");
 		Assert.notNull(keyValueOperations, "KeyValueOperations must not be null!");
 		Assert.notNull(queryCreatorFactory, "QueryCreatorFactory type must not be null!");
 
+		this.partTree = Lazy
+				.of(() -> new PartTree(queryMethod.getName(), queryMethod.getEntityInformation().getJavaType()));
 		this.queryMethod = queryMethod;
 		this.keyValueOperations = keyValueOperations;
 		this.evaluationContextProvider = evaluationContextProvider;
@@ -130,17 +135,15 @@ public class KeyValuePartTreeQuery implements RepositoryQuery {
 					: keyValueOperations.count(query, queryMethod.getEntityInformation().getJavaType());
 
 			return new PageImpl(IterableConverter.toList(result), page, count);
-
 		} else if (queryMethod.isCollectionQuery()) {
 
 			return this.keyValueOperations.find(query, queryMethod.getEntityInformation().getJavaType());
-
 		} else if (queryMethod.isQueryForEntity()) {
 
 			Iterable<?> result = this.keyValueOperations.find(query, queryMethod.getEntityInformation().getJavaType());
 			return result.iterator().hasNext() ? result.iterator().next() : null;
-		} else if (new PartTree(queryMethod.getName(), queryMethod.getEntityInformation().getJavaType()).isExistsProjection()) {
-			return keyValueOperations.count(query, queryMethod.getEntityInformation().getJavaType()) > 0;
+		} else if (partTree.get().isExistsProjection()) {
+			return keyValueOperations.exists(query, queryMethod.getEntityInformation().getJavaType());
 		}
 
 		throw new UnsupportedOperationException("Query method not supported.");
@@ -206,8 +209,7 @@ public class KeyValuePartTreeQuery implements RepositoryQuery {
 	 */
 	public KeyValueQuery<?> createQuery(ParameterAccessor accessor) {
 
-		PartTree tree = new PartTree(getQueryMethod().getName(), getQueryMethod().getEntityInformation().getJavaType());
-
+		PartTree tree = this.partTree.get();
 		AbstractQueryCreator<? extends KeyValueQuery<?>, ?> queryCreator = queryCreatorFactory.queryCreatorFor(tree,
 				accessor);
 
@@ -235,7 +237,7 @@ public class KeyValuePartTreeQuery implements RepositoryQuery {
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	public interface QueryCreatorFactory<T extends AbstractQueryCreator> {
+	public interface QueryCreatorFactory<T extends AbstractQueryCreator<?, ?>> {
 
 		T queryCreatorFor(PartTree partTree, ParameterAccessor accessor);
 	}
@@ -260,6 +262,7 @@ public class KeyValuePartTreeQuery implements RepositoryQuery {
 		}
 
 		@Override
+		@SuppressWarnings("unchecked")
 		public AbstractQueryCreator<KeyValueQuery<?>, ?> queryCreatorFor(PartTree partTree, ParameterAccessor accessor) {
 
 			Assert.state(constructor != null,