Introduce null-safe index operator in SpEL

See spring-projectsgh-29847

Author: Trympyrym

spring-expression/src/main/java/org/springframework/expression/spel/ast/Indexer.java

@@ -103,12 +103,12 @@ private enum IndexedType {ARRAY, LIST, MAP, STRING, OBJECT}
 	private PropertyAccessor cachedWriteAccessor;
 
 
-	/**
-	 * Create an {@code Indexer} with the given start position, end position, and
-	 * index expression.
-	 */
-	public Indexer(int startPos, int endPos, SpelNodeImpl indexExpression) {
-		super(startPos, endPos, indexExpression);
+	private final boolean nullSafe;
+
+
+	public Indexer(boolean nullSafe, int startPos, int endPos, SpelNodeImpl expr) {
+		super(startPos, endPos, expr);
+		this.nullSafe = nullSafe;
 	}
 
 
@@ -161,6 +161,9 @@ protected ValueRef getValueRef(ExpressionState state) throws EvaluationException
 
 		// Raise a proper exception in case of a null target
 		if (target == null) {
+			if (this.nullSafe) {
+				return ValueRef.NullValueRef.INSTANCE;
+			}
 			throw new SpelEvaluationException(getStartPosition(), SpelMessage.CANNOT_INDEX_INTO_NULL_VALUE);
 		}
 

spring-expression/src/main/java/org/springframework/expression/spel/standard/InternalSpelExpressionParser.java

@@ -399,7 +399,7 @@ private SpelNodeImpl eatNode() {
 	@Nullable
 	private SpelNodeImpl eatNonDottedNode() {
 		if (peekToken(TokenKind.LSQUARE)) {
-			if (maybeEatIndexer()) {
+			if (maybeEatIndexer(false)) {
 				return pop();
 			}
 		}
@@ -419,7 +419,8 @@ private SpelNodeImpl eatDottedNode() {
 		Token t = takeToken();  // it was a '.' or a '?.'
 		boolean nullSafeNavigation = (t.kind == TokenKind.SAFE_NAVI);
 		if (maybeEatMethodOrProperty(nullSafeNavigation) || maybeEatFunctionOrVar() ||
-				maybeEatProjection(nullSafeNavigation) || maybeEatSelection(nullSafeNavigation)) {
+				maybeEatProjection(nullSafeNavigation) || maybeEatSelection(nullSafeNavigation) ||
+				maybeEatIndexer(nullSafeNavigation)) {
 			return pop();
 		}
 		if (peekToken() == null) {
@@ -537,7 +538,8 @@ else if (maybeEatTypeReference() || maybeEatNullReference() || maybeEatConstruct
 		else if (maybeEatBeanReference()) {
 			return pop();
 		}
-		else if (maybeEatProjection(false) || maybeEatSelection(false) || maybeEatIndexer()) {
+		else if (maybeEatProjection(false) || maybeEatSelection(false) ||
+				maybeEatIndexer(false)) {
 			return pop();
 		}
 		else if (maybeEatInlineListOrMap()) {
@@ -699,7 +701,7 @@ else if (peekToken(TokenKind.COLON, true)) {  // map!
 		return true;
 	}
 
-	private boolean maybeEatIndexer() {
+	private boolean maybeEatIndexer(boolean nullSafeNavigation) {
 		Token t = peekToken();
 		if (t == null || !peekToken(TokenKind.LSQUARE, true)) {
 			return false;
@@ -709,7 +711,7 @@ private boolean maybeEatIndexer() {
 			throw internalException(t.startPos, SpelMessage.MISSING_SELECTION_EXPRESSION);
 		}
 		eatToken(TokenKind.RSQUARE);
-		this.constructedNodes.push(new Indexer(t.startPos, t.endPos, expr));
+		this.constructedNodes.push(new Indexer(nullSafeNavigation, t.startPos, t.endPos, expr));
 		return true;
 	}
 

spring-expression/src/test/java/org/springframework/expression/spel/IndexingTests.java

@@ -376,6 +376,20 @@ void listOfMaps() {
 		assertThat(expression.getValue(this, String.class)).isEqualTo("apple");
 	}
 
+	@Test
+	void nullSafeIndex() {
+		ContextWithNullCollections testContext = new ContextWithNullCollections();
+		StandardEvaluationContext context = new StandardEvaluationContext(testContext);
+		Expression expr = new SpelExpressionParser().parseRaw("nullList?.[4]");
+		assertThat(expr.getValue(context)).isNull();
+
+		expr = new SpelExpressionParser().parseRaw("nullArray?.[4]");
+		assertThat(expr.getValue(context)).isNull();
+
+		expr = new SpelExpressionParser().parseRaw("nullMap:?.[4]");
+		assertThat(expr.getValue(context)).isNull();
+	}
+
 
 	@Target({ElementType.FIELD})
 	@Retention(RetentionPolicy.RUNTIME)
@@ -436,4 +450,11 @@ public Class<?>[] getSpecificTargetClasses() {
 
 	}
 
+
+	static class ContextWithNullCollections {
+		public List nullList = null;
+		public String[] nullArray = null;
+		public Map nullMap = null;
+	}
+
 }