Java Jpa 自定义实体属性转换器及规范

Question

In my Java code, I have a JPA entity define as below

public class Entity {

    @Id
    private Long id;

    ... other attributes;

    @Column(name = "ctx")
    @Convert(converter = MapToJsonStringConverter.class)
    private Map<String, String> ctx;
}

@Convert
public class MapToJsonStringConverter implements AttributeConverter<Map<String, String>, String> {

    @Autowired
    private JsonParserService jsonParserService;

    @Override
    public String convertToDatabaseColumn(final Map<String, String> map) {
        return Optional.ofNullable(map)
                .map(jsonParserService::writeValueAsString)
                .orElse(null);
    }

    @Override
    public Map<String, String> convertToEntityAttribute(final String string) {
        return Optional.ofNullable(string)
                .map(jsonParserService::readValueAsMap)
                .orElse(null);
    }
}

A Jpa attribute converter is used to convert the ctx attribute to/ from a JSON string. I need to create a JPA specification that allows me to query the ctx field in the database using a LIKE clause:

ctx LIKE '%test-value%'

Of course, when I try to create such specification the criteria builder cannot match the type of the attribute as it is expecting a map when instead I only supply a string

cb.like(root.get(Entity_.ctx), string))  <--- compile error Cannot resolve method 'like(javax.persistence.criteria.Expression<M>, java.lang.String)'

Is there a way to make this work instead of using a native query? Consider that the final specification also involves other attributes of the same entity.

Answer 1

As mentionned in commentaries

cb.like(root.<String>get("ctx"), clause);

Does not work as root.get(Entity_.ctx) or its equivalent does return a Map<String, String> and explicitly typing the generic ( <String>get(...) ) won't help as Map is not instanceof String . The solution as mentionned here is to use as provided by Criteria.

cb.like(root.get(Entity_.ctx).as(String.class), string)

It does work thanks to Map overriding Object.toString() . Otherwise you would need to create your own class extending Map and custom the override to toString() to return your custom representation (which is used by default by Json libs).