Using nested annotations for key-value pairs in a custom annotation

Introduction

In my previous article "Using HashMap in Custom Annotations", I explained how to use HashMap in annotations using enumeration constants.

Nested annotations can also be used to map key-value pairs.

List of types supported in annotations

Annotations

Requires two custom annotations. The first annotation (such as MapItem) contains a key-value pair, and the second annotation (such as MapItems) contains a list of MapItem annotations.

Custom annotation @MapItem

The annotation @MapItem represents a single key-value pair.

@Target(ElementType.FIELD)
public @interface MapItem {

    String key();
    String value();
}

Custom annotation @MapItems

The annotation @MapItems defines a MapItem list.

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface MapItems {

    MapItem[] items();
}

Functional testing

The @MapItem annotation list is set in the @MapItems annotation.

class ExampleDto {

    @MapItems(items = {
        @MapItem(key = "1", value = "MALE"),
        @MapItem(key = "2", value = "FEMALE"),
        @MapItem(key = "6", value = "DIVERS")
    })
    public String salutation;
}

MapItemsTest tests MapItems annotations. The test is performed on the salutation field.

To demonstrate how to use a @MapItem list, I create a HashMap from the @MapItem and compare it to the expected HashMap.

class MapItemsTest {

    @Test
    void testMapItems() throws NoSuchFieldException {

        Field field = ExampleDto.class.getDeclaredField("salutation");
        field.setAccessible(true);

        MapItems annotation = field.getAnnotation(MapItems.class);

        Map<String, String> mappingItems = Arrays
                .stream(annotation.items())
                .collect(
                    Collectors.toMap(
                        MapItem::key,
                        MapItem::value
                    )
                );

        assertEquals(
            new HashMap<>() {{
                put("1", "MALE");
                put("2", "FEMALE");
                put("6", "DIVERS");
            }},
            mappingItems
        );
    }
}

Conclusion

Advantages

This is a neat solution and easy to implement.

Disadvantages

For example, if key-value pairs are to be used in a validator, they must be obtained indirectly.

Full example

https://www.php.cn/link/164710e8521a5b39302f816392f05bc2

Related articles

• Use HashMap in custom annotations
• Create custom Jackson JsonSerializer and JsonDeserializer for mapped values

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