Unable to deserialize when using new Record classes

Question

I am trying to see if I can replace my existing Pojos with the new Record classes in Java 14. But unable to do so. Getting following error:

com.faste

Answer 1

It is possible with some Jackson Annotations, which cause Jackson to use fields instead of getters. Still far less verbose than a pre-Java 14 class (without Lombok or similar solutions).

record Foo(@JsonProperty("a") int a, @JsonProperty("b") int b){
}

This probably works because according to https://openjdk.java.net/jeps/359:

Declaration annotations are permitted on record components if they are applicable to record components, parameters, fields, or methods. Declaration annotations that are applicable to any of these targets are propagated to implicit declarations of any mandated members.

See also: When is the @JsonProperty property used and what is it used for?

It is also possible to make use @JsonAutoDetect

@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
record Bar(int a, int b){
}

If configuring the Objectmapper to use field Visibility globally, this annotation on class level is not needed.

See also: How to specify jackson to only use fields - preferably globally

Example:

public class Test {
    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper om = new ObjectMapper();
        System.out.println(om.writeValueAsString(new Foo(1, 2)));  //{"a":1,"b":2}
        System.out.println(om.writeValueAsString(new Bar(3, 4)));  //{"a":3,"b":4} 
    }

    record Foo(@JsonProperty("a") int a, @JsonProperty("b") int b){
    }

    @JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
    record Bar(int a, int b){
    }
}

There is also a Github issue for that feature: https://github.com/FasterXML/jackson-future-ideas/issues/46

Answer 2

• Use the parameter names module for jackson, https://github.com/FasterXML/jackson-modules-java8/tree/master/parameter-names (make sure the compiler sets -parameters) or add `@JsonProperty("name") to each field in the record
• add @JsonCreator to the constructor. I can't tell if the inheritance will work properly, so you might have to explicitly declare the constructor and annotate it.

If a public accessor method or (non-compact) canonical constructor is declared explicitly, then it only has the annotations which appear on it directly; nothing is propagated from the corresponding record component to these members.

From https://openjdk.java.net/jeps/384

So add

new ObjectMapper().registerModules(new ParameterNamesModule())

and try

@JsonCreator record Value(String x);

or something like

record Value(String x) {

@JsonCreator
public Value(String x) {
this.x = x;
}
}

or all the way to

record Value(@JsonProperty("x") String x) {

@JsonCreator
public Value(@JsonProperty("x") String x) {
this.x = x;
}
}

This is how I get immutable pojos with lombok and jackson to work, and I don't see why records wouldn't work under the same format. My setup is Jackson parameter names module, -parameters compiler flag for java 8 (I don't think this is required for like jdk9+), @JsonCreator on the constructor. Example of a real class working with this setup.

@Value
@AllArgsConstructor(onConstructor_ = @JsonCreator)
public final class Address {

  private final String line1;

  private final String line2;

  private final String city;

  private final String region;

  private final String postalCode;

  private final CountryCode country;
}

Answer 3

The compiler generates the constructor and other accessor method for a Record.

In your case,

  public final class Post extends java.lang.Record {  
  public Post(int, int java.lang.String, java.lang.String);
  public java.lang.String toString();
  public final int hashCode();
  public final boolean equals(java.lang.Object);
  public int userId();
  public int id();
  public java.lang.String title();
  public java.lang.String body();
}

Here you can see that there is not default constructor which is needed got Jackson. The constructor you used is a compact constructor,

public Post {
 }

You can define a default/no args constructor as,

public record Post(int userId, int id, String title, String body) {
    public Post() {
        this(0,0, null, null);
    }
}

But Jackson uses Getter and Setters to set values. So in short, you can not use Record for mapping the response.

Answer 4

This is slated for jackson 2.12

https://github.com/FasterXML/jackson-future-ideas/issues/46