DATAREST-1273 - Properly support adding to collections via indexes in JsonPatch expressions.

We now support adding a to a collection via JsonPatch expressions by using the collection's size as target index. Contrary to paths pointing to existing collection elements we cannot determine the type of the object to be created. Thus, we fall back to the common element type of all existing collection elements. We also reject indexes pointing to elements with an index greater than the current collection's size.

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/json/patch/SpelPath.java

@@ -36,9 +36,11 @@ import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.Expression;
 import org.springframework.expression.ExpressionException;
 import org.springframework.expression.spel.SpelEvaluationException;
+import org.springframework.expression.spel.SpelMessage;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.expression.spel.support.SimpleEvaluationContext;
 import org.springframework.util.Assert;
+import org.springframework.util.CollectionUtils;
 import org.springframework.util.ConcurrentReferenceHashMap;
 
 /**
@@ -210,6 +212,7 @@ class SpelPath {
 	static class TypedSpelPath extends SpelPath {
 
 		private static final String INVALID_PATH_REFERENCE = "Invalid path reference %s on type %s (from source %s)!";
+		private static final String INVALID_COLLECTION_INDEX = "Invalid collection index %s for collection of size %s. Use '…/-' or the collection's actual size as index to append to it!";
 		private static final Map<CacheKey, TypedSpelPath> TYPED_PATHS = new ConcurrentReferenceHashMap<>(32);
 		private static final EvaluationContext CONTEXT = SimpleEvaluationContext.forReadWriteDataBinding().build();
 
@@ -286,7 +289,24 @@ class SpelPath {
 
 			Assert.notNull(root, "Root object must not be null!");
 
-			return expression.getValueType(CONTEXT, root);
+			try {
+
+				return expression.getValueType(CONTEXT, root);
+
+			} catch (SpelEvaluationException o_O) {
+
+				if (!SpelMessage.COLLECTION_INDEX_OUT_OF_BOUNDS.equals(o_O.getMessageCode())) {
+					throw o_O;
+				}
+
+				Object collectionOrArray = getParent().getValue(root);
+
+				if (Collection.class.isInstance(collectionOrArray)) {
+					return CollectionUtils.findCommonElementType(Collection.class.cast(collectionOrArray));
+				}
+			}
+
+			throw new IllegalArgumentException(String.format("Cannot obtain type for path %s on %s!", path, root));
 		}
 
 		/**
@@ -384,6 +404,11 @@ class SpelPath {
 			} else {
 
 				List<Object> list = parentPath.getValue(target);
+
+				if (listIndex > list.size()) {
+					throw new PatchException(String.format(INVALID_COLLECTION_INDEX, listIndex, list.size()));
+				}
+
 				list.add(listIndex >= 0 ? listIndex.intValue() : list.size(), value);
 			}
 		}

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/json/patch/AddOperationUnitTests.java

@@ -15,6 +15,7 @@
  */
 package org.springframework.data.rest.webmvc.json.patch;
 
+import static org.assertj.core.api.Assertions.*;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.Assert.*;
 
@@ -111,4 +112,29 @@ public class AddOperationUnitTests {
 
 		assertThat(todo.getUninitialized()).containsExactly("Text");
 	}
+
+	@Test // DATAREST-1273
+	public void addsItemToTheEndOfACollectionViaIndex() {
+
+		List<Todo> todos = new ArrayList<Todo>();
+		todos.add(new Todo(1L, "A", false));
+
+		Todo todo = new Todo(2L, "B", true);
+		AddOperation.of("/1", todo).perform(todos, Todo.class);
+
+		assertThat(todos).element(1).isEqualTo(todo);
+	}
+
+	@Test // DATAREST-1273
+	public void rejectsAdditionBeyondEndOfList() {
+
+		List<Todo> todos = new ArrayList<Todo>();
+		todos.add(new Todo(1L, "A", false));
+
+		assertThatExceptionOfType(PatchException.class) //
+				.isThrownBy(() -> AddOperation.of("/2", new Todo(2L, "B", true)).perform(todos, Todo.class)) //
+				.withMessageContaining("index") //
+				.withMessageContaining("2") //
+				.withMessageContaining("1");
+	}
 }