Using a HashMap in a custom Annotation

Introduction

There may be a requirement to define key-value pairs in an annotation in order to perform a mapping based on keys or/and values.

In this case a HashMap would be the first choice. However, annotations do not support the type HashMap. A HashMap can be used with the Enum type, which is supported by annotations.

List of supported Types in Annotations

In my article "Creating a custom Jackson JsonSerializer and JsonDeserializer for mapping values", I create the custom annotation @MappingTable to use key-value pairs for bidirectional mapping in a custom JsonSerializer and JsonDeserializer.

Enum Constant

The Maps enumeration type contains the constant SALUTATION_MAP. The HashMaps are defined in these constants using Map.of(). Alternatively, new HashMap<>() can be used.

import java.util.Map; public enum Maps { SALUTATION_MAP(Map.of("1", "MALE", "2", "FEMALE", "6", "DIVERS")); private final Map<String, String> map; Maps(Map<String, String> map) { this.map = map; } public Map<String, String> getMap() { return this.map; } } 

Custom Annotation @MapItems

The enumeration type Maps is set as the type in the annotation.

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface MapItems { Maps map(); } 

Functional Test

The enum constants are referenced in the @MapItems annotation.

class ExampleDto { @MapItems(map = Maps.SALUTATION_MAP) public String salutation; } 

The MapItemsTest tests the MapItems annotation. The test is done on the salutation field.

class MapItemsTest { @Test void testEnumConstants() throws NoSuchFieldException { Field field = ExampleDto.class.getDeclaredField("salutation"); field.setAccessible(true); MapItems annotation = field.getAnnotation(MapItems.class); assertSame(Maps.SALUTATION_MAP.getMap(), annotation.map().getMap()); } } 

Conclusion

Pro

• Simple implementation
• Reuse of the HashMap, for example, when validating a property of a payload, validating fields in a DTO or collecting key-value pairs in a response.

Contra

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Full example

https://github.com/alaugks/article-annotation-key-value-pairs