Allow auto grow for entries without default constructor in SpEL

[MartinKnopf]

Hi,

I wanted to add a BigDecimal to a list using SpEL. Auto grow only works for element types with a default constructor and since BigDecimal does not have a default constructor auto grow does not work with my list.

Here is an example:

public class SpelTest {

    public List<BigDecimal> values;
    
    SpelExpressionParser parser;

    @Before
    public void setup() {
        values = new ArrayList<>();
        parser = new SpelExpressionParser(new SpelParserConfiguration(true, true));
    }

    @Test
    public void shouldChangeValue() {
        values.add(BigDecimal.ONE);

        parser.parseExpression("values[0]").setValue(this, "123.4");

        assertThat(values.get(0)).isEqualTo(BigDecimal.valueOf(123.4)); // passes
    }

    @Test
    public void shouldAddValue() {
        parser.parseExpression("values[0]").setValue(this, "123.4");

        assertThat(values.get(0)).isEqualTo(BigDecimal.valueOf(123.4)); // fails
    }
}

Changing the first entry passes but adding an entry fails with

Caused by: java.lang.NoSuchMethodException: java.math.BigDecimal.<init>()
    at java.base/java.lang.Class.getConstructor0(Class.java:3349)
    at java.base/java.lang.Class.getDeclaredConstructor(Class.java:2553)
    at org.springframework.util.ReflectionUtils.accessibleConstructor(ReflectionUtils.java:185)
    at org.springframework.expression.spel.ast.Indexer$CollectionIndexingValueRef.growCollectionIfNecessary(Indexer.java:715)
    ... 55 more

My solution inserts null when no default constructor can be found for the element type. Not sure if this is a good solution but at least it does not seem to break any tests. Please be nice :-)

[sbrannen]

@aclement, would you mind taking a quick look at this to assess the implementation within the context of SpEL?

[aclement]

I think I'm ok with that change. Might be worth a line in the section in the docs around Parser configuration to mention you might get nulls in the array if it is expanded and there is no default ctor for that element type. As I recall SpEL compilation doesn't currently support collection growing so there is no impact on that mode with changes in this area.

[sbrannen]

I think I'm ok with that change.

Thanks for the quick review, @aclement. Much appreciated!

Might be worth a line in the section in the docs around Parser configuration to mention you might get nulls in the array if it is expanded and there is no default ctor for that element type. As I recall SpEL compilation doesn't currently support collection growing so there is no impact on that mode with changes in this area.

@HorseD, would you mind adding such documentation to the reference manual (core-expressions.adoc) and including that in this PR?

[MartinKnopf]

Hey,

I added a note to the reference manual plus one more unit test.

Have a good day!

[sbrannen]

I added a note to the reference manual plus one more unit test.

Thanks

Have a good day!

You, too!

[sbrannen]

This has been merged into master in 35c0ae7 and slightly revised in b0570cd.

Thanks