Final in Java

Posted on 2021-12-15. Approx 300 words. Approx 2 minutes read.
Tags: #java #json #serialisation

In java, normally you'd think if a variable is defined as final, its value will not change.

For example, the content of Main.java:

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public class Main {
  public static void main(String... args) {
    Example example = new Example();
    example.printI();
  }
}

class Example {
  final private Integer i = 10;
  public void printI() {
    System.out.println(i);
  }
}

The above code should always print 10, right? Well, that's not the case. At least in this case:

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import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;

public class Main {
    public static void main(String... args) throws IOException {
        Example example1 = new Example();
        example1.printI();

        ObjectMapper mapper = new ObjectMapper();
        String jsonInString = "{\"i\": 11}";
        Example example2 = mapper.reader().withType(Example.class).readValue(jsonInString);
        example2.printI();
    }
}

class Example {
    public final Integer i = 10;

    public void printI() {
        System.out.println(i);
    }
}

This will print:

10
11

Reason for this?

Deserialization is implemented by the JVM on a level below the basic language constructs.1

So, language rules do not apply to JVM. The object was initialized with i = 10 and it was updated while deserialization despite being final.

Code sample on GitHub: https://github.com/njkevlani/final-in-java-code.

  1. https://stackoverflow.com/a/2904589/6645446 ↩︎