Close streams fetched from data-providers

flow-data/src/main/java/com/vaadin/flow/data/provider/DataCommunicator.java

@@ -610,8 +610,10 @@ public T getItem(int index) {
              * because the backend can have the item on that index (we simply
              * not yet fetched this item during the scrolling).
              */
-            return (T) getDataProvider().fetch(buildQuery(index, 1)).findFirst()
-                    .orElse(null);
+            try (Stream<T> stream = getDataProvider()
+                    .fetch(buildQuery(index, 1))) {
+                return stream.findFirst().orElse(null);
+            }
         }
     }
 
@@ -1010,18 +1012,20 @@ protected Stream<T> fetchFromProvider(int offset, int limit) {
                 int page = 0;
                 do {
                     final int newOffset = offset + page * pageSize;
-                    Stream<T> dataProviderStream = doFetchFromDataProvider(
-                            newOffset, pageSize);
-                    // Stream.Builder is not thread safe, so for parallel stream
-                    // we need to first collect items before adding them
-                    if (dataProviderStream.isParallel()) {
-                        getLogger().debug(
-                                "Data provider {} has returned parallel stream on 'fetch' call",
-                                getDataProvider().getClass());
-                        dataProviderStream.collect(Collectors.toList())
-                                .forEach(addItemAndCheckConsumer);
-                    } else {
-                        dataProviderStream.forEach(addItemAndCheckConsumer);
+                    try (Stream<T> dataProviderStream = doFetchFromDataProvider(
+                            newOffset, pageSize)) {
+                        // Stream.Builder is not thread safe, so for parallel
+                        // stream we need to first collect items before adding
+                        // them
+                        if (dataProviderStream.isParallel()) {
+                            getLogger().debug(
+                                    "Data provider {} has returned parallel stream on 'fetch' call",
+                                    getDataProvider().getClass());
+                            dataProviderStream.collect(Collectors.toList())
+                                    .forEach(addItemAndCheckConsumer);
+                        } else {
+                            dataProviderStream.forEach(addItemAndCheckConsumer);
+                        }
                     }
                     page++;
                 } while (page < pages
@@ -1040,8 +1044,10 @@ protected Stream<T> fetchFromProvider(int offset, int limit) {
             getLogger().debug(
                     "Data provider {} has returned parallel stream on 'fetch' call",
                     getDataProvider().getClass());
-            stream = stream.collect(Collectors.toList()).stream();
-            assert !stream.isParallel();
+            try (Stream<T> parallelStream = stream) {
+                stream = parallelStream.collect(Collectors.toList()).stream();
+                assert !stream.isParallel();
+            }
         }
 
         SizeVerifier verifier = new SizeVerifier<>(limit);
@@ -1476,17 +1482,20 @@ private Activation activate(Range range) {
 
         // XXX Explicitly refresh anything that is updated
         List<String> activeKeys = new ArrayList<>(range.length());
-        fetchFromProvider(range.getStart(), range.length()).forEach(bean -> {
-            boolean mapperHasKey = keyMapper.has(bean);
-            String key = keyMapper.key(bean);
-            if (mapperHasKey) {
-                // Ensure latest instance from provider is used
-                keyMapper.refresh(bean);
-                passivatedByUpdate.values().stream()
-                        .forEach(set -> set.remove(key));
-            }
-            activeKeys.add(key);
-        });
+        try (Stream<T> stream = fetchFromProvider(range.getStart(),
+                range.length())) {
+            stream.forEach(bean -> {
+                boolean mapperHasKey = keyMapper.has(bean);
+                String key = keyMapper.key(bean);
+                if (mapperHasKey) {
+                    // Ensure latest instance from provider is used
+                    keyMapper.refresh(bean);
+                    passivatedByUpdate.values().stream()
+                            .forEach(set -> set.remove(key));
+                }
+                activeKeys.add(key);
+            });
+        }
         boolean needsSizeRecheck = activeKeys.size() < range.length();
         return new Activation(activeKeys, needsSizeRecheck);
     }

flow-data/src/main/java/com/vaadin/flow/data/provider/DataView.java

@@ -59,6 +59,22 @@ public interface DataView<T> extends Serializable {
     /**
      * Get the full data available to the component. Data is filtered and sorted
      * the same way as in the component.
+     * <p>
+     * Consumers of the returned stream are responsible for closing it when all
+     * the stream operations are done to ensure that any resources feeding the
+     * stream are properly released. Failure to close the stream might lead to
+     * resource leaks.
+     * <p>
+     * It is strongly recommended to use a try-with-resources block to
+     * automatically close the stream after its terminal operation has been
+     * executed. Below is an example of how to properly use and close the
+     * stream:
+     *
+     * <pre>{@code
+     * try (Stream<T> stream = dataView.getItems()) {
+     *     stream.forEach(System.out::println); // Example terminal operation
+     * }
+     * }</pre>
      *
      * @return filtered and sorted data set
      */

flow-data/src/main/java/com/vaadin/flow/data/provider/hierarchy/HierarchicalCommunicationController.java

@@ -255,16 +255,18 @@ private List<String> activate(Range range) {
         // XXX Explicitly refresh anything that is updated
         List<String> activeKeys = new ArrayList<>(range.length());
 
-        fetchItems.apply(parentKey, range).forEach(bean -> {
-            boolean mapperHasKey = keyMapper.has(bean);
-            String key = keyMapper.key(bean);
-            if (mapperHasKey) {
-                // Ensure latest instance from provider is used
-                keyMapper.refresh(bean);
-                passivatedByUpdate.values().forEach(set -> set.remove(key));
-            }
-            activeKeys.add(key);
-        });
+        try (Stream<T> stream = fetchItems.apply(parentKey, range)) {
+            stream.forEach(bean -> {
+                boolean mapperHasKey = keyMapper.has(bean);
+                String key = keyMapper.key(bean);
+                if (mapperHasKey) {
+                    // Ensure latest instance from provider is used
+                    keyMapper.refresh(bean);
+                    passivatedByUpdate.values().forEach(set -> set.remove(key));
+                }
+                activeKeys.add(key);
+            });
+        }
         return activeKeys;
     }
 

flow-data/src/main/java/com/vaadin/flow/data/provider/hierarchy/HierarchyMapper.java

@@ -534,8 +534,9 @@ private Stream<T> getFlatChildrenStream(T parent) {
     private Stream<T> getFlatChildrenStream(T parent, boolean includeParent) {
         List<T> childList = Collections.emptyList();
         if (isExpanded(parent)) {
-            childList = doFetchDirectChildren(parent)
-                    .collect(Collectors.toList());
+            try (Stream<T> stream = doFetchDirectChildren(parent)) {
+                childList = stream.collect(Collectors.toList());
+            }
             if (childList.isEmpty()) {
                 removeChildren(parent == null ? null
                         : getDataProvider().getId(parent));
@@ -563,8 +564,9 @@ private Stream<T> getChildrenStream(T parent, Range range,
             boolean includeParent) {
         List<T> childList = Collections.emptyList();
         if (isExpanded(parent)) {
-            childList = doFetchDirectChildren(parent, range)
-                    .collect(Collectors.toList());
+            try (Stream<T> stream = doFetchDirectChildren(parent, range)) {
+                childList = stream.collect(Collectors.toList());
+            }
             if (childList.isEmpty()) {
                 removeChildren(parent == null ? null
                         : getDataProvider().getId(parent));

flow-data/src/test/java/com/vaadin/flow/data/provider/DataCommunicatorTest.java

@@ -996,6 +996,18 @@ public void getItem_withUndefinedSizeAndSorting() {
                 dataCommunicator.getItem(2));
     }
 
+    @Test
+    public void getItem_streamIsClosed() {
+        AtomicBoolean streamIsClosed = new AtomicBoolean();
+        dataCommunicator.setDataProvider(createDataProvider(streamIsClosed),
+                null);
+
+        fakeClientCommunication();
+        dataCommunicator.getItem(0);
+
+        Assert.assertTrue(streamIsClosed.get());
+    }
+
     @Test
     public void itemCountEstimateAndStep_defaults() {
         Assert.assertEquals(dataCommunicator.getItemCountEstimate(),
@@ -1353,6 +1365,18 @@ public void fetchFromProvider_itemCountLessThanTwoPages_correctItemsReturned() {
 
     }
 
+    @Test
+    public void fetchFromProvider_streamIsClosed() {
+        AtomicBoolean streamIsClosed = new AtomicBoolean();
+        dataCommunicator.setDataProvider(createDataProvider(streamIsClosed),
+                null);
+        dataCommunicator.setRequestedRange(0, 50);
+
+        fakeClientCommunication();
+
+        Assert.assertTrue(streamIsClosed.get());
+    }
+
     @Test
     public void fetchEnabled_getItemCount_stillReturnsItemsCount() {
         dataCommunicator.setFetchEnabled(false);
@@ -1737,6 +1761,11 @@ public Stream<Item> fetch(Query<Item, Object> query) {
     }
 
     private AbstractDataProvider<Item, Object> createDataProvider() {
+        return createDataProvider(new AtomicBoolean());
+    }
+
+    private AbstractDataProvider<Item, Object> createDataProvider(
+            AtomicBoolean streamIsClosed) {
         return new AbstractDataProvider<Item, Object>() {
             @Override
             public boolean isInMemory() {
@@ -1752,7 +1781,8 @@ public int size(Query<Item, Object> query) {
             public Stream<Item> fetch(Query<Item, Object> query) {
                 return asParallelIfRequired(IntStream.range(query.getOffset(),
                         query.getLimit() + query.getOffset()))
-                        .mapToObj(Item::new);
+                        .mapToObj(Item::new)
+                        .onClose(() -> streamIsClosed.set(true));
             }
         };
     }

flow-data/src/test/java/com/vaadin/flow/data/provider/hierarchy/HierarchicalCommunicatorDataTest.java

@@ -18,7 +18,9 @@
 import java.io.Serializable;
 import java.util.Arrays;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.IntStream;
+import java.util.stream.Stream;
 
 import org.junit.Assert;
 import org.junit.Before;
@@ -221,6 +223,33 @@ public void uniqueKeyProviderIsNotSet_keysGeneratedByKeyMapper() {
                         communicator.getKeyMapper().get(i)));
     }
 
+    @Test
+    public void expandRoot_streamIsClosed() {
+        AtomicBoolean streamIsClosed = new AtomicBoolean();
+
+        dataProvider = new TreeDataProvider<>(treeData) {
+
+            @Override
+            public Stream<Item> fetchChildren(
+                    HierarchicalQuery<Item, SerializablePredicate<Item>> query) {
+                return super.fetchChildren(query)
+                        .onClose(() -> streamIsClosed.set(true));
+            }
+        };
+
+        communicator.setDataProvider(dataProvider, null);
+
+        communicator.expand(ROOT);
+        fakeClientCommunication();
+
+        communicator.setParentRequestedRange(0, 50, ROOT);
+        fakeClientCommunication();
+
+        communicator.reset();
+
+        Assert.assertTrue(streamIsClosed.get());
+    }
+
     @Test
     public void expandRoot_filterOutAllChildren_clearCalled() {
         parentClearCalled = false;

flow-data/src/test/java/com/vaadin/flow/data/provider/hierarchy/HierarchyMapperWithDataTest.java

@@ -21,6 +21,7 @@
 import java.util.Comparator;
 import java.util.List;
 import java.util.Objects;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
@@ -288,6 +289,42 @@ public void getExpandedItems_tryToAddItemsToCollection_shouldThrowException() {
         expandedItems.add(new TreeNode("third-1"));
     }
 
+    @Test
+    public void fetchHierarchyItems_streamIsClosed() {
+        AtomicBoolean streamIsClosed = new AtomicBoolean();
+        mapper = new HierarchyMapper<>(new TreeDataProvider<>(data) {
+            @Override
+            public Stream<Node> fetchChildren(
+                    HierarchicalQuery<Node, SerializablePredicate<Node>> query) {
+                return super.fetchChildren(query)
+                        .onClose(() -> streamIsClosed.set(true));
+            }
+        });
+        Node rootNode = testData.get(0);
+        mapper.expand(rootNode);
+        mapper.fetchHierarchyItems(rootNode, Range.between(0, 10)).count();
+
+        Assert.assertTrue(streamIsClosed.get());
+    }
+
+    @Test
+    public void fetchChildItems_streamIsClosed() {
+        AtomicBoolean streamIsClosed = new AtomicBoolean();
+        mapper = new HierarchyMapper<>(new TreeDataProvider<>(data) {
+            @Override
+            public Stream<Node> fetchChildren(
+                    HierarchicalQuery<Node, SerializablePredicate<Node>> query) {
+                return super.fetchChildren(query)
+                        .onClose(() -> streamIsClosed.set(true));
+            }
+        });
+        Node rootNode = testData.get(0);
+        mapper.expand(rootNode);
+        mapper.fetchChildItems(rootNode, Range.between(0, 10));
+
+        Assert.assertTrue(streamIsClosed.get());
+    }
+
     private void expand(Node node) {
         insertRows(mapper.expand(node, mapper.getIndexOf(node).orElse(null)));
     }