#Jackson Mapper - String[] conversion possible?

28 messages · Page 1 of 1 (latest)

tawny salmon
#

This should be trivial, but I cannot figure out how to do it. In looking at custom annotations and additional registered serializers, I must be missing something.

`import com.fasterxml.jackson.databind.annotation.JsonSerialize;
import com.fasterxml.jackson.databind.ser.impl.StringArraySerializer;

public class ExampleDto {
@JsonSerialize(using = StringArraySerializer.class)
public String[] field;
}

public class ExampleDtoTest {
@Test
public void testConvertToMap() {
HashMap<String, Object> expectedMap = new HashMap<>() {{
put("field", (new String[]{"1","2","3"}));
}};

    ExampleDto exampleDto = new ExampleDto();
    exampleDto.field = new String[]{"1","2","3"};

    ObjectMapper mapper = new ObjectMapper();
    HashMap result = mapper.convertValue(exampleDto, HashMap.class);
    assertEquals(String[].class, result.get("field").getClass()); // fails
}

}`

worthy heartBOT
#

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gaunt wren
#

Use a List<String>. In Java 11 you can use List.of to make one

shut valve
#

Wait, what are you testing? Why do you care what convert(object, HashMap.cass) does?

tawny salmon
#

The test is to demonstrate the conversion to ArrayList. The question is how to convert a String[] property into a Hashmap<String, Object> with a <String, String[]> entry instead of the coerced <String, ArrayList<String>> entry.

#

I believe what is happening is that the serializer is called to make json, which is then tokenized with a json parser. Then the jackson json parser is used to make the HashMap.class, which has a default jackson serializer. In order to ensure type safety of each element of an array, it uses an ArrayList. So the String[] type disappears.
Making a ~50ln deserializer works to coerce the type back to String[] - but that seems overkill. Is there any other way? I would imagine this has come up before for Jackson.

shut valve
#

But... Jackson is for converting to JSON, not to arrays.

tawny salmon
#

[1,2,3] is simpler datatype that exists in JSON than ArrayList(){{ 1,2,3 }}.

The question remains, other than a fairly robust deserializer to a mapper as a module, is there a simpler way?

shut valve
#

You just refuse to develop your thoughts regarding my observation?

tawny salmon
#

I don't understand.

shut valve
#

Jackson is for converting to JSON, not to arrays. What can possibly make it so you care whether that convert above will give out an array or a List?

tawny salmon
#

This isn't relevant. The question is about methodology, not circumstance. Why I care is that I am trying to figure out a way to do it, beyond my current solution. Let's just assume I can't control the source datastructure (it's part of another API in jar), does that matter? No.

#

I have another API that needs a HashMap and it can only handle primitive members and primitive arrays.

shut valve
#

You do understand how much this is guaranteed to be an X/Y problem?

tawny salmon
#

@JsonSerialize addresses this same X/Y problem, from a different direction. I don't understand the avoidance.

shut valve
#

.... No. Jackson does the job it is expected to do. Whether internally arrays or lists are involved is none of the business of whoever isn't making Jackson itself.

tawny salmon
#

This is incorrect. Jackson provides a conversion API via convertValue.

#

They implement a defaultSerializer which does not involve json

shut valve
#

That it provides that changes none of the previous observations.

tawny salmon
#

This was a long-winded way of saying "I don't know and don't care". GL with whatever.

shut valve
#

This was a long-winded way of confirming you're guaranteed to be in a X/Y problem situation, which experts on the domain may be uninterested in. But you do you.

tawny salmon
#

The following seems overkill, but it works (more or less, no float etc) added as:
ObjectMapper mapper = new ObjectMapper(); SimpleModule module = new SimpleModule(); module.addDeserializer(HashMap.class, new CustomDeSerializer()); mapper.registerModule(module); HashMap<String,Object> result = mapper.convertValue(exampleDto, new TypeReference<>(){});

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tawny salmon
#

If you can make due with the bytes as an Object[], rather than a String[] ,
mapper.configure(DeserializationFeature.USE_JAVA_ARRAY_FOR_JSON_ARRAY, true);

tawny salmon
#

ObjectMapper mapper = new ObjectMapper(); MapType collectionType = mapper.getTypeFactory().constructMapType(HashMap.class, String.class, String[].class); HashMap<String,Object> result = mapper.convertValue(exampleDto, collectionType);
This is an initialization that is needed to pass the test. It does not survive additional field types, but it does avoid the conversion to ArrayList succinctly.

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