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Json deserialization of enum, forbid int
I have a DTO that contains an enum field:
@Getter
@Setter
static class Foo {
Bar bar;
}
enum Bar {
X, Y
}
When I deserialize a JSON, it allows int as values:
var objectMapper = new ObjectMapper();
Foo foo = objectMapper.readValue("""
{"bar": 1}"
""",
Foo.class
);
I expect an error here, but get the value Bar.Y.
How can I validate a JSON so only string values for enum are allowed?
Here's the JSON library dependency:
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.17.1</version>
</dependency>
2 answers
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One solution is to create a method in the enum that takes a String and converts it to the correspondent value. Then we annotate this method with @JsonCreator:
import com.fasterxml.jackson.annotation.JsonCreator;
enum Bar {
X, Y;
@JsonCreator
public static Bar fromString(String value) {
return Bar.valueOf(value);
}
}
Now only valid strings are accepted. In this case, {"bar": "X"} and {"bar": "Y"}. Any other value (such as {"bar": "A"} or {"bar": 1}) will raise an exception.
Another solution is to create your own deserializer:
import com.fasterxml.jackson.core.JacksonException;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonMappingException;
import java.io.IOException;
public class BarDeserializer extends JsonDeserializer<Bar> {
@Override
public Bar deserialize(JsonParser jp, DeserializationContext dc) throws IOException, JacksonException {
JsonToken jsonToken = jp.getCurrentToken();
if (jsonToken == JsonToken.VALUE_STRING) { // value is a string
try {
return Bar.valueOf(jp.getValueAsString());
} catch (IllegalArgumentException | NullPointerException e) {
throw new JsonMappingException(jp, "Invalid value for enum Bar", e);
}
}
// value is not a string
throw new JsonMappingException(jp, "Value must be a string");
}
}
Then you annotate the enum with it:
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
@JsonDeserialize(using = BarDeserializer.class)
enum Bar {
X, Y;
}
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The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| talex | (no comment) | Sep 24, 2024 at 19:11 |
This is something that could go really wrong. I remember using this, and it was fine initially, until the enum in question evolved, and the ordinals changed, so the values didn't correspond anymore to what the initial assumption was.
Anyway, long story short, if you would like to prevent this system-wide (and I recommend it), configure ObjectMapper to prevent using the Enum's ordinals with DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS.
Something like this should work:
final var mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS, true);
If you are using Spring Boot you can achieve this modifying application.yaml:
spring:
jackson:
deserialization:
fail-on-numbers-for-enums: true
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