CollectionToCollectionConverter avoids collection copy for untyped collection when simply returning source anyway

Also uses addAll instead of iteration over untyped collection now, supporting optimized addAll in target collection type, and avoids repeated getElementTypeDescriptor calls.

Issue: SPR-11479

spring-core/src/main/java/org/springframework/core/convert/support/CollectionToCollectionConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2014 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.
@@ -34,16 +34,19 @@ import org.springframework.core.convert.converter.ConditionalGenericConverter;
  * parameterized type to the target collection's parameterized type if necessary.
  *
  * @author Keith Donald
+ * @author Juergen Hoeller
  * @since 3.0
  */
 final class CollectionToCollectionConverter implements ConditionalGenericConverter {
 
 	private final ConversionService conversionService;
 
+
 	public CollectionToCollectionConverter(ConversionService conversionService) {
 		this.conversionService = conversionService;
 	}
 
+
 	@Override
 	public Set<ConvertiblePair> getConvertibleTypes() {
 		return Collections.singleton(new ConvertiblePair(Collection.class, Collection.class));
@@ -60,27 +63,34 @@ final class CollectionToCollectionConverter implements ConditionalGenericConvert
 		if (source == null) {
 			return null;
 		}
-		boolean copyRequired = !targetType.getType().isInstance(source);
 		Collection<?> sourceCollection = (Collection<?>) source;
+
+		// Shortcut if possible...
+		boolean copyRequired = !targetType.getType().isInstance(source);
 		if (!copyRequired && sourceCollection.isEmpty()) {
-			return sourceCollection;
+			return source;
 		}
+		TypeDescriptor elementDesc = targetType.getElementTypeDescriptor();
+		if (elementDesc == null && !copyRequired) {
+			return source;
+		}
+
+		// At this point, we need a collection copy in any case, even if just for finding out about element copies...
 		Collection<Object> target = CollectionFactory.createCollection(targetType.getType(), sourceCollection.size());
-		if (targetType.getElementTypeDescriptor() == null) {
+		if (elementDesc == null) {
-			for (Object element : sourceCollection) {
+			target.addAll(sourceCollection);
-				target.add(element);
-			}
 		}
 		else {
 			for (Object sourceElement : sourceCollection) {
 				Object targetElement = this.conversionService.convert(sourceElement,
-						sourceType.elementTypeDescriptor(sourceElement), targetType.getElementTypeDescriptor());
+						sourceType.elementTypeDescriptor(sourceElement), elementDesc);
 				target.add(targetElement);
 				if (sourceElement != targetElement) {
 					copyRequired = true;
 				}
 			}
 		}
+
 		return (copyRequired ? target : source);
 	}
 

spring-core/src/test/java/org/springframework/core/convert/support/CollectionToCollectionConverterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2014 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.
@@ -44,16 +44,19 @@ import static org.junit.Assert.*;
 /**
  * @author Keith Donald
  * @author Juergen Hoeller
+ * @author Stephane Nicoll
  */
 public class CollectionToCollectionConverterTests {
 
 	private GenericConversionService conversionService = new GenericConversionService();
 
+
 	@Before
 	public void setUp() {
 		conversionService.addConverter(new CollectionToCollectionConverter(conversionService));
 	}
 
+
 	@Test
 	public void scalarList() throws Exception {
 		List<String> list = new ArrayList<String>();
@@ -77,8 +80,6 @@ public class CollectionToCollectionConverterTests {
 		assertEquals(37, result.get(1));
 	}
 
-	public ArrayList<Integer> scalarListTarget;
-
 	@Test
 	public void emptyListToList() throws Exception {
 		conversionService.addConverter(new CollectionToCollectionConverter(conversionService));
@@ -90,8 +91,6 @@ public class CollectionToCollectionConverterTests {
 		assertEquals(list, conversionService.convert(list, sourceType, targetType));
 	}
 
-	public List<Integer> emptyListTarget;
-
 	@Test
 	public void emptyListToListDifferentTargetType() throws Exception {
 		conversionService.addConverter(new CollectionToCollectionConverter(conversionService));
@@ -106,8 +105,6 @@ public class CollectionToCollectionConverterTests {
 		assertTrue(result.isEmpty());
 	}
 
-	public LinkedList<Integer> emptyListDifferentTarget;
-
 	@Test
 	public void collectionToObjectInteraction() throws Exception {
 		List<List<String>> list = new ArrayList<List<String>>();
@@ -149,8 +146,6 @@ public class CollectionToCollectionConverterTests {
 		assertEquals((Integer) 23, result.get(1).get(1).get(0));
 	}
 
-	public List<List<List<Integer>>> objectToCollection;
-
 	@Test
 	@SuppressWarnings("unchecked")
 	public void stringToCollection() throws Exception {
@@ -165,10 +160,46 @@ public class CollectionToCollectionConverterTests {
 		TypeDescriptor targetType = new TypeDescriptor(getClass().getField("objectToCollection"));
 		assertTrue(conversionService.canConvert(sourceType, targetType));
 		List<List<List<Integer>>> result = (List<List<List<Integer>>>) conversionService.convert(list, sourceType, targetType);
-		assertEquals((Integer)9, result.get(0).get(0).get(0));
+		assertEquals((Integer) 9, result.get(0).get(0).get(0));
-		assertEquals((Integer)12, result.get(0).get(0).get(1));
+		assertEquals((Integer) 12, result.get(0).get(0).get(1));
-		assertEquals((Integer)37, result.get(1).get(0).get(0));
+		assertEquals((Integer) 37, result.get(1).get(0).get(0));
-		assertEquals((Integer)23, result.get(1).get(0).get(1));
+		assertEquals((Integer) 23, result.get(1).get(0).get(1));
+	}
+
+	@Test
+	public void convertEmptyVector_shouldReturnEmptyArrayList() {
+		Vector<String> vector = new Vector<String>();
+		vector.add("Element");
+		testCollectionConversionToArrayList(vector);
+	}
+
+	@Test
+	public void convertNonEmptyVector_shouldReturnNonEmptyArrayList() {
+		Vector<String> vector = new Vector<String>();
+		vector.add("Element");
+		testCollectionConversionToArrayList(vector);
+	}
+
+	@Test
+	public void testCollectionsEmptyList() throws Exception {
+		CollectionToCollectionConverter converter = new CollectionToCollectionConverter(new GenericConversionService());
+		TypeDescriptor type = new TypeDescriptor(getClass().getField("list"));
+		converter.convert(list, type, TypeDescriptor.valueOf(Class.forName("java.util.Collections$EmptyList")));
+	}
+
+	@SuppressWarnings("rawtypes")
+	private void testCollectionConversionToArrayList(Collection<String> aSource) {
+		Object myConverted = (new CollectionToCollectionConverter(new GenericConversionService())).convert(
+				aSource, TypeDescriptor.forObject(aSource), TypeDescriptor.forObject(new ArrayList()));
+		assertTrue(myConverted instanceof ArrayList<?>);
+		assertEquals(aSource.size(), ((ArrayList<?>) myConverted).size());
+	}
+
+	@Test
+	public void listToCollectionNoCopyRequired() throws NoSuchFieldException {
+		List<?> input = new ArrayList<String>(Arrays.asList("foo", "bar"));
+		assertSame(input, conversionService.convert(input, TypeDescriptor.forObject(input),
+				new TypeDescriptor(getClass().getField("wildCardCollection"))));
 	}
 
 	@Test
@@ -210,8 +241,6 @@ public class CollectionToCollectionConverterTests {
 		assertEquals(resources, conversionService.convert(resources, sourceType, new TypeDescriptor(getClass().getField("resources"))));
 	}
 
-	public List<String> strings;
-
 	@Test(expected=ConversionFailedException.class)
 	public void nothingInCommon() throws Exception {
 		List<Object> resources = new ArrayList<Object>();
@@ -221,8 +250,24 @@ public class CollectionToCollectionConverterTests {
 		assertEquals(resources, conversionService.convert(resources, sourceType, new TypeDescriptor(getClass().getField("resources"))));
 	}
 
+
+	public ArrayList<Integer> scalarListTarget;
+
+	public List<Integer> emptyListTarget;
+
+	public LinkedList<Integer> emptyListDifferentTarget;
+
+	public List<List<List<Integer>>> objectToCollection;
+
+	public List<String> strings;
+
+	public List list = Collections.emptyList();
+
+	public Collection<?> wildCardCollection = Collections.emptyList();
+
 	public List<Resource> resources;
 
+
 	public static abstract class BaseResource implements Resource {
 
 		@Override
@@ -286,39 +331,8 @@ public class CollectionToCollectionConverterTests {
 		}
 	}
 
-	public static class TestResource extends BaseResource {
-
-	}
-
-	@Test
-	public void convertEmptyVector_shouldReturnEmptyArrayList() {
-		Vector<String> vector = new Vector<String>();
-		vector.add("Element");
-		testCollectionConversionToArrayList(vector);
-	}
 
-	@Test
+	public static class TestResource extends BaseResource {
-	public void convertNonEmptyVector_shouldReturnNonEmptyArrayList() {
-		Vector<String> vector = new Vector<String>();
-		vector.add("Element");
-		testCollectionConversionToArrayList(vector);
-	}
-
-	@Test
-	public void testCollectionsEmptyList() throws Exception {
-		CollectionToCollectionConverter converter = new CollectionToCollectionConverter(new GenericConversionService());
-		TypeDescriptor type = new TypeDescriptor(getClass().getField("list"));
-		converter.convert(list, type, TypeDescriptor.valueOf(Class.forName("java.util.Collections$EmptyList")));
-	}
-
-	public List list = Collections.emptyList();
-
-	@SuppressWarnings("rawtypes")
-	private void testCollectionConversionToArrayList(Collection<String> aSource) {
-		Object myConverted = (new CollectionToCollectionConverter(new GenericConversionService())).convert(
-				aSource, TypeDescriptor.forObject(aSource), TypeDescriptor.forObject(new ArrayList()));
-		assertTrue(myConverted instanceof ArrayList<?>);
-		assertEquals(aSource.size(), ((ArrayList<?>) myConverted).size());
 	}
 
 }