Allow auto grow for entries w/o default constructor in SpEL

Prior to this commit, The "auto grow" feature in SpEL expressions only
worked for element types with a default constructor. For example, auto
grow did not work for a list of BigDecimal elements.

This commit inserts a null value in the list when no default
constructor can be found for the element type.

Closes gh-25367

Author: Martin Knopf

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

@@ -712,10 +712,10 @@ private void growCollectionIfNecessary() {
 				}
 				TypeDescriptor elementType = this.collectionEntryDescriptor.getElementTypeDescriptor();
 				try {
-					Constructor<?> ctor = ReflectionUtils.accessibleConstructor(elementType.getType());
+					Constructor<?> ctor = getConstructor(elementType.getType());
 					int newElements = this.index - this.collection.size();
 					while (newElements >= 0) {
-						this.collection.add(ctor.newInstance());
+						this.collection.add(ctor == null ? null : ctor.newInstance());
 						newElements--;
 					}
 				}
@@ -725,6 +725,15 @@ private void growCollectionIfNecessary() {
 			}
 		}
 
+		Constructor<?> getConstructor(Class<?> type) {
+			try {
+				return ReflectionUtils.accessibleConstructor(type);
+			}
+			catch (Throwable ex) {
+				return null;
+			}
+		}
+
 		@Override
 		public boolean isWritable() {
 			return true;

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

@@ -20,6 +20,7 @@
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
+import java.math.BigDecimal;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashMap;
@@ -199,6 +200,27 @@ public void setGenericPropertyContainingListAutogrow() {
 		}
 	}
 
+	public List<BigDecimal> decimals;
+
+	@Test
+	public void autoGrowWithoutDefaultConstructor() {
+		this.decimals = new ArrayList<>();
+		SpelExpressionParser parser = new SpelExpressionParser(new SpelParserConfiguration(true, true));
+		parser.parseExpression("decimals[0]").setValue(this, "123.4");
+		assertThat(decimals.get(0)).isEqualTo(BigDecimal.valueOf(123.4));
+	}
+
+	@Test
+	public void indexIntoPropertyContainingNullList() {
+		this.decimals = new ArrayList<>();
+		this.decimals.add(null);
+		this.decimals.add(BigDecimal.ONE);
+		SpelExpressionParser parser = new SpelExpressionParser(new SpelParserConfiguration(true, true));
+		parser.parseExpression("decimals[0]").setValue(this, "9876.5");
+		assertThat(decimals.get(0)).isEqualTo(BigDecimal.valueOf(9876.5));
+		assertThat(decimals.get(1)).isEqualTo(BigDecimal.ONE);
+	}
+
 	@Test
 	public void indexIntoPropertyContainingList() {
 		List<Integer> property = new ArrayList<>();

src/docs/asciidoc/core/core-expressions.adoc

@@ -339,8 +339,12 @@ index into an array or collection and the element at the specified index is `nul
 you can automatically create the element. This is useful when using expressions made up of a
 chain of property references. If you index into an array or list
 and specifying an index that is beyond the end of the current size of the array or
-list, you can automatically grow the array or list to accommodate that index. The following
-example demonstrates how to automatically grow the list:
+list, you can automatically grow the array or list to accommodate that index. In order to add
+an element at the specified index, SpEL will try to create the element using a default
+constructor before setting the specified value. If the element type does not have a default
+constructor, `null` will be added. Note if there is no built-in or custom converter, that knows
+how to set the value, `null` will remain in the array or list at the specified index.
+The following example demonstrates how to automatically grow the list:
 
 [source,java,indent=0,subs="verbatim,quotes",role="primary"]
 .Java