DATACMNS-1637 - Add default stream() method to CloseableIterator.

CloseableIterator now allows construction of a Stream that is associated with a close hook to release resources after its use.

CloseableIterator<Person> iterator = …;

try (Stream<Person> stream = iterator.stream()) {
	assertThat(stream.count()).isEqualTo(3);
}

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

@@ -17,13 +17,18 @@ package org.springframework.data.util;
 
 import java.io.Closeable;
 import java.util.Iterator;
+import java.util.Spliterator;
+import java.util.Spliterators;
+import java.util.stream.Stream;
+import java.util.stream.StreamSupport;
 
 /**
  * A {@link CloseableIterator} serves as a bridging data structure for the underlying data store specific results that
- * can be wrapped in a Java 8 {@link java.util.stream.Stream}. This allows implementations to clean up any resources
- * they need to keep open to iterate over elements.
+ * can be wrapped in a Java 8 {@link #stream() java.util.stream.Stream}. This allows implementations to clean up any
+ * resources they need to keep open to iterate over elements.
  *
  * @author Thomas Darimont
+ * @author Mark Paluch
  * @param <T>
  * @since 1.10
  */
@@ -35,4 +40,35 @@ public interface CloseableIterator<T> extends Iterator<T>, Closeable {
 	 */
 	@Override
 	void close();
+
+	/**
+	 * Create a {@link Spliterator} over the elements provided by this {@link Iterator}. Implementations should document
+	 * characteristic values reported by the spliterator. Such characteristic values are not required to be reported if
+	 * the spliterator reports {@link Spliterator#SIZED} and this collection contains no elements.
+	 * <p>
+	 * The default implementation should be overridden by subclasses that can return a more efficient spliterator. To
+	 * preserve expected laziness behavior for the {@link #stream()} method, spliterators should either have the
+	 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be late-binding.
+	 *
+	 * @return a {@link Spliterator} over the elements in this {@link Iterator}.
+	 * @since 2.4
+	 */
+	default Spliterator<T> spliterator() {
+		return Spliterators.spliterator(this, 0, 0);
+	}
+
+	/**
+	 * Return a sequential {@code Stream} with this {@link Iterator} as its source. The resulting stream calls
+	 * {@link #clone()} when {@link Stream#close() closed}. The resulting {@link Stream} must be closed after use, it can
+	 * be declared as a resource in a {@code try}-with-resources statement.
+	 * <p>
+	 * This method should be overridden when the {@link #spliterator()} method cannot return a spliterator that is
+	 * {@code IMMUTABLE}, {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} for details.)
+	 *
+	 * @return a sequential {@code Stream} over the elements in this {@link Iterator}.
+	 * @since 2.4
+	 */
+	default Stream<T> stream() {
+		return StreamSupport.stream(spliterator(), false).onClose(this::close);
+	}
 }

src/test/java/org/springframework/data/util/CloseableIteratorUnitTests.java

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2020 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.util;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link CloseableIterator}.
+ *
+ * @author Mark Paluch
+ */
+class CloseableIteratorUnitTests {
+
+	@Test // DATACMNS-1637
+	void shouldCreateStream() {
+
+		CloseableIteratorImpl<String> iterator = new CloseableIteratorImpl<>(Arrays.asList("1", "2", "3").iterator());
+
+		List<String> collection = iterator.stream().map(it -> "hello " + it).collect(Collectors.toList());
+
+		assertThat(collection).contains("hello 1", "hello 2", "hello 3");
+		assertThat(iterator.closed).isFalse();
+	}
+
+	@Test // DATACMNS-1637
+	void closeStreamShouldCloseIterator() {
+
+		CloseableIteratorImpl<String> iterator = new CloseableIteratorImpl<>(Arrays.asList("1", "2", "3").iterator());
+
+		try (Stream<String> stream = iterator.stream()) {
+			assertThat(stream.findFirst()).hasValue("1");
+		}
+
+		assertThat(iterator.closed).isTrue();
+	}
+
+	static class CloseableIteratorImpl<T> implements CloseableIterator<T> {
+
+		private final Iterator<T> delegate;
+		private boolean closed = false;
+
+		CloseableIteratorImpl(Iterator<T> delegate) {
+			this.delegate = delegate;
+		}
+
+		@Override
+		public void close() {
+			closed = true;
+		}
+
+		@Override
+		public boolean hasNext() {
+			return delegate.hasNext();
+		}
+
+		@Override
+		public T next() {
+			return delegate.next();
+		}
+	}
+}