Jackson deserialization circumventing final fields

Question

Here's the code

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.Data;
import lombok.ToString;

public class Main {
    public static void main(String[] args) throws Exception {
        Fields f1 = new Fields(1);
        System.out.println(f1);

        ObjectMapper mapper = new ObjectMapper();
        String str = mapper.writeValueAsString(f1);
        System.out.println(str);

        Fields f2 = mapper.readValue(str, Fields.class);
        System.out.println(f2);
    }

    @Data
    @ToString
    public static class Fields {
        private final long value1;
        private final long value2;

        public Fields(@JsonProperty("blah") long value) {
            this.value1 = value++;
            this.value2 = value++;
            System.out.println(this);
        }
    }
}

Output

Main.Fields(value1=1, value2=2)
Main.Fields(value1=1, value2=2)
{"value1":1,"value2":2}
Main.Fields(value1=0, value2=1)
Main.Fields(value1=1, value2=2)

My questions are:

• Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?
• If jackson is able to set fields directly, why is it required that I annotate the constructor with @JsonProperty? (Removing @JsonProperty from Fields results in error; and I didn't even need to annotate with correct properties)

Thank you

Answer 1

Why is it required that I annotate the constructor with @JsonProperty?

It's not. What is required is an accessible constructor. You can either have a parameterless constructor

public Fields() {
    this.value1 = 0;
    this.value2 = 0;
    System.out.println("cons: " + this);
}

(that necessarily initializes the fields since they are final) or you can have a constructor that Jackson will try to resolve based on the declared @JsonProperty name. Note that JsonProperty#required is false by default.

Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?

Because it can. It thus allows you to use immutable types with deserialization. There is no built-in way that I know of through which you can disable this feature.

Answer 2

Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?

You can set MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS property to false (it is true by default) when configuring your mapper.

Example:

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.MapperFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Temp {

private static final String RAW = "{\"value1\": \"aabbcc\",\"value2\":\"zzzzz\"}";

public static void main(String[] args) throws Exception {
    System.out.println(new ObjectMapper().readValue(RAW, TestClass.class));

    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); // you will receive UnrecognizedPropertyException without this line
    mapper.configure(MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS, false);

    System.out.println(mapper.readValue(RAW, TestClass.class));
}

public static class TestClass {

    private final String value1;
    private final String value2;

    @JsonCreator
    public static TestClass createTestClass(
            @JsonProperty("value1") String value1,
            @JsonProperty("blah") String value2) {

        return new TestClass(value1, value2);
    }

    private TestClass(String value1, String value2) {
        this.value1 = value1;
        this.value2 = value2;
    }

    public String getValue1() {
        return value1;
    }

    public String getValue2() {
        return value2;
    }

    @Override
    public String toString() {
        return "TestClass{" + "value1=" + value1 + ", value2=" + value2 + '}';
    }
}

}

Output:

TestClass{value1=aabbcc, value2=zzzzz}
TestClass{value1=aabbcc, value2=null}