How do you specify default values for Jackson deserialization

Question

@ResponseBody
@RequestMapping(value="/getUser")
public JSONObject getContent(@ReqeustBody User user) 

Up here is my Controller code.

@Data
public class User{
    private String username = "administrator";
    private String password = "123456";   
    private Integer age = 18;
}

Up here is my User class code.

{
    "username":"admin",
    "password":"000",
    "age":""
}

When I POST the JSON above, I get the age property to be null.

I want Jackson to deserialize the empty fields ("" or null) in JSON with default values.

Like this:

{
    "username":"admin",
    "password":"000",
    "age":18
}

What should I do?

Solution

You can define a custom getter property where setting your default value in case of null.

   public Integer getAge() {
        if (age == null) {
            return 18;
        } else {
            return this.age;
        }
    }

Note that you can't change the setAge method because it is not invoked in this case, infact there is no age field that will inform Jackson to call that method.

---

An alternative is to use a custom constructor and use the JsonSetter annotation with the value Nulls.SKIP

Value that indicates that an input null value should be skipped and no assignment is to be made; this usually means that the property will have its default value.

as follow:

 public class User {
    @JsonSetter(nulls = Nulls.SKIP)
    private Integer age;

    public User() {
        this.age = 18;
    }

    ...
}

---

The @JsonSetter is present in package com.fasterxml.jackson.annotation and can be imported as dependency in maven using

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>YOURVERSION</version>
</dependency>

Comments

[Arthur Eirich]

See here

[Echoinacup]

It is strange that the age field is initialized but as far as I understand in the response it is missing

[Davide Lorenzo MARINO]

The setAge doesn't work because there is no age field. If there was an age field with a null value it could work, but not in absence of the field age.

[Michael]

I didn't find the JsonSetter annotation that I could write on the field, com.fasterxml.jackson.annotation.JsonSetter is only allowed on class and method .

[Davide Lorenzo MARINO]

@Michael it is present in the package com.fasterxml.jackson.annotation. Check the updated answer for details

[Michael]

Thank you Davide, my version is 2.8.0, it can't be written on a field.

[kevinjom]

@Michael correct, as the comment in the source code suggests // ^^^ allowed on Fields, (constructor) parameters since 2.9

[kevinjom]

@DavideLorenzoMARINO Ah I just saw the @Data annotation from lombok ( I assume), so there is no setter there. This is actually a bit tricky, but its java's nature, even there is no setter, the client code can still change the object's field value by using reflection which I assume thats how jackson does it. I think your answer makes sense in the context that you still wanna use @Data.