Serialize and Deserialize Java object having vavr list

Question

I have a Java object with vavr list.

@Value
@Builder(toBuilder = true)
public class Customer {

    private final List<Remark> remarks;

}

When I serialize the object objectwriter.writeValueAsString(currentDossier) and print the values then in the JSON output I see,

    { "remarks" : {
        "empty" : true,
        "lazy" : false,
        "async" : false,
        "traversableAgain" : true,
        "sequential" : true,
        "ordered" : false,
        "singleValued" : false,
        "distinct" : false,
        "orNull" : null,
        "memoized" : false
      } 
    }

where remarks is a vavr list field inside the object.

Now if I try to deserialize the same value into the object objectMapper.readValue(json, Customer.class) then I get error:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `io.vavr.collection.List` 
(no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (String)"{"remarks" : {

Why is the seralized JSON text not working when trying to deserialze?

  @Test
  public void mapperForVavrList() throws JsonProcessingException {
    Customer cus = Customer.builder().remarks(List.empty()).build();

    ObjectWriter ow = new ObjectMapper()
      .writer().withDefaultPrettyPrinter();

    String json1 = ow.writeValueAsString(cus);

    log.info(json1);

    ObjectMapper objectMapper = new ObjectMapper();
    Customer cus2 = objectMapper.readValue(json1, Customer.class);

    log.info(cus2.toString());

  }

Is there any other way in the Java code to print vavr object in textual format and convert it back to POJO?

https://stackoverflow.com/questions/46285615/serializer-deserializer-for-vavr-objects — firstpostcommenter, May 26 '21 at 13:48
https://stackoverflow.com/questions/38238712/how-to-convert-string-value-to-custom-model-object-in-java — firstpostcommenter, May 26 '21 at 16:03
Is there any other way in the Java code to print vavr object in textual format(be it json, string, etc) and change it back to POJO using that textual value? I tried GSON but it also does not seem to print List values correctly when converting vavr object to text — firstpostcommenter, May 26 '21 at 17:07
Jackson has this functionality, what you need to do is to register the vavr-module. That's all. — Opal, May 27 '21 at 05:27
Here you go: https://github.com/vavr-io/vavr-jackson: `mapper.registerModule(new VavrModule());`. — Opal, May 27 '21 at 09:48
https://stackoverflow.com/questions/20837856/can-not-deserialize-instance-of-java-util-arraylist-out-of-start-object-token — firstpostcommenter, May 27 '21 at 13:02

Opal · Accepted Answer · 2021-05-27T13:59:20.050

You can easily serialize/deserialize vavr objects to/from JSON with jackson. What you need to do is to register VavrModule on the instance of the ObjectMapper:

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new VavrModule());

To get VavrModule you need to add the dependency to vavr-jackson

EDIT

Here you can find a complete working example:

import com.fasterxml.jackson.databind.ObjectMapper;
import io.vavr.collection.List;
import io.vavr.jackson.datatype.VavrModule;
import org.apache.commons.lang3.builder.ToStringBuilder;

import java.io.IOException;

class Lol2 {

    public static void main(String[] args) throws IOException {
        Customer c = new Customer();
        Remark r = new Remark();
        r.whatever = "whatever";
        c.remarks = List.of(r);

        ObjectMapper ow = new ObjectMapper();
        ow.registerModule(new VavrModule());

        System.out.println(c);
        String json = ow.writeValueAsString(c);
        System.out.println(json);
        Customer c2 = ow.readValue(json, Customer.class);
        System.out.println(c2);

    }
}

class Customer {

    List<Remark> remarks;

    public List<Remark> getRemarks() {
        return remarks;
    }

    public void setRemarks(List<Remark> remarks) {
        this.remarks = remarks;
    }

    @Override
    public String toString() {
        return new ToStringBuilder(this)
            .append("remarks", remarks)
            .toString();
    }
}

class Remark {

    String whatever;

    public String getWhatever() {
        return whatever;
    }

    public void setWhatever(String whatever) {
        this.whatever = whatever;
    }

    @Override
    public String toString() {
        return new ToStringBuilder(this)
            .append("whatever", whatever)
            .toString();
    }
}

Now I get the error `Can not deserialize instance of io.vavr.collection.List out of START_OBJECT token`. I can understand if I get this error when I am getting the json input from some external source but here I am just serializing and de-serializing the object using jackson so not sure why I get the error — firstpostcommenter, May 27 '21 at 12:55
Can you please provide the code that reproduces this problem: http://sscce.org/ ?w — Opal, May 27 '21 at 13:16
I am using the same code present in `mapperForVavrList()` in the question. Just added the line `objectMapper.registerModule(new VavrModule());`. And then I get the error at the line `Customer cus2 = objectMapper.readValue(json1, Customer.class);` — firstpostcommenter, May 27 '21 at 13:46
@firstpostcommenter please view my edited answer. Also, this is not the code you're using. There's no `Remark` class provided. — Opal, May 27 '21 at 13:59
Thanks, it works. Since I was setting empty list I did not add remark class details in my original question. — firstpostcommenter, May 27 '21 at 15:04
@firstpostcommenter, when you are asking for help you should respect that others are helping you for free on their free time. Dave their time with a good, replicable example. — Opal, May 27 '21 at 19:06

Serialize and Deserialize Java object having vavr list

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