How to add an object without a constructor to a list using Spring Expression Language

问题

I want to add a BigDecimal to a list using Spring Expression Language.

public class SpelTest {

    public List<BigDecimal> values;
    
    StandardEvaluationContext context;
    SpelExpressionParser parser;

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

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

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

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

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

Not sure why SpEL isn't able to properly initialize a BigDecimal when the list is empty. Surprisingly I found nothing about this problem.

Thanks for helping!

回答1:

You could avoid this problem by setting the whole list instead of single (not initialized) elements. Instead of

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

use:

parser.parseExpression("values").setValue(context, "123.4");

This also works for multiple elements, quite neat:

parser.parseExpression("values").setValue(context, "123.4, 456.7");

回答2:

The problem is that you activated autoGrowCollections on the SpelParserConfiguraion. Therefore it tries to create an element with a default constructor if you try to access a non-existing element of the collection with the index operator []. BigDecimal has no default constructor and because of this, it fails.

What you could do is to create the object in the SpEL itself. E.G.:

    @Test
    public void shouldAddValue() {
        parser.parseExpression("values.add(0, new java.math.BigDecimal(\"123.4\"))").getValue(context);

        assertThat(values.size() > 0);
        assertThat(values.get(0)).isEqualTo(BigDecimal.valueOf(123.4)); 
    }

Or you could create a subclass of BigDecimal witch has a default constructor and use this class.

回答3:

As @Nirud pointed out, the problem is, that BigDecimal does not have a default constructor. I extended SpEL to add null to the list, when there is no default constructor. See this pull request: https://github.com/spring-projects/spring-framework/pull/25367

来源：https://stackoverflow.com/questions/62771421/how-to-add-an-object-without-a-constructor-to-a-list-using-spring-expression-lan

标签

spring-el

spring-expression-language

spring-expression