如果没有字段，Jackson将无法进行反序列化

Question

I have a pojo like

class Pojo {
    private String string;

    public String getString() { return string; }

    @JsonSetter(nulls = Nulls.FAIL)
    public void setString(String string) {
        this.string = string;
    }
}

I want to make jackson fail when deserializing if the string field is null or absent. (ie {"string":null} or {} )

As you can see, I've succeeded in the first goal with the JsonSetter annotation. What I am hoping for now is something like that but for a missing property. I found a few other questions asking similar things but they were quite old and referenced features that might be implemented in the future. With the recent release of jackson 2.9, I was hoping maybe this is now possible.

Answer 1

@JsonProperty has a required element that can be used

to ensure existence of property value in JSON

Unfortunately, Jackson currently (2.9) only supports it for use with @JsonCreator annotated constructors or factory methods. Since @JsonSetter only works with setters, you'll have to do the null validation yourself.

For example, you'd define a constructor like

@JsonCreator
public Pojo(@JsonProperty(value = "string", required = true) String string) {
    if (string == null) {
        throw new IllegalArgumentException("string cannot be null");
    }
    this.string = string;
}

If the property is present, but set to null , Jackson would throw an InvalidDefinitionException that wraps the IllegalArgumentException thrown in the constructor.

If the property is absent, Jackson would throw a MismatchedInputException stating that a property is missing.

Both of these exceptions are subtypes of JsonMappingException , so you can easily deal with them the same way.

---

With this solution, you could also get rid of the setter altogether and make the field final if that suited your design better.

Answer 2

You may perform bean validation here by annotating the field of interest with @NotNull . You may remove the annotation from your setter.

class Pojo {

    @NotNull
    private String string;

    public String getString() { return string; }

    public void setString(String string) {
        this.string = string;
    }

}

Similarly if you want to fail the validation for other constraints like size, pattern etc, you may use similar equivalent annotations available here .