CodeGym/Help with Java Tasks/Serialisation in a custom code example -- NotActiveExce...

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Szekesfehervar

09.12.2021
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Serialisation in a custom code example -- NotActiveException

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I wanted to try and test a lot of things about serialization/deserialization with a made-up code, just for myself. Unfortunately, I got stuck at the very beginning. Here is the code:

import java.io.*;

public class Solution implements Serializable { //

    private int age;
    private int weight;
    private String name;
    private int length;

    public Solution(int age, int weight, String name, int length) {
        this.age = age;
        this.weight = weight;
        this.name = name;
        this.length = length;
    }

    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
    }

    public static void main(String[] args) throws Exception {
        Solution s1 = new Solution(3,6,"Tiger",50);
        System.out.println(s1);

        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(baos);

        s1.writeObject(oos);
    }
}

I got this: "NotActiveException: not in call to writeObject" Unfortunately, I couldn't figure out what this means. What I need to do to serialize the s1 object?

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Lisa

Level 41

9 December 2021, 22:34

useful

You need to call writeObject on the output stream and pass it the object you want to serialize

oos.writeObject(s1);

Gellert Varga

Level 23 , Szekesfehervar, Hungary

9 December 2021, 23:22

Yeah... it really works now.:) Thank you! And we shouldn't use the s1.writeObject() method directly (i mean it's forbidden), it's only used by the reflection mechanism mentioned by you?

Lisa

Level 41

11 December 2021, 09:20

useful

Yes, that's how it is meant to work. Keep in mind that there are three ways of serialization. Fully automatic (you just have the transient keyword) Some control but still automatic (when you implement the private void writeObject() method) And the manual version when you use the Externalizable interface The difference is, that the latter overrides methods. Means that these methods are accessible and you can call those methods on the object from outside the class (the usual use case). Here your above approach would work but I won't recommend it. Just keep things consistent. The ObjectOutputStream writeObject delegates to the objects writeExternal method. So you can use it exactly like for a Serializable implementation and have the possibility to switch between both (if needed). So both is working for Externalizable

out.writeObject(a); // as already said, I'd use that
a.writeExternal(out);

Gellert Varga

Level 23 , Szekesfehervar, Hungary

11 December 2021, 20:51

Very clear explanation, thank you very much!! But I would have a further question... about the second version ("some control, but still automatic"), based on a specific example:

import java.io.*;

public class Solution implements Serializable {

    private int age;
    private int weight;
    transient private String name;

    public Solution(int age, int weight, String name) {
        this.age = age;
        this.weight = weight;
        this.name = name;
    }

    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject(); // it loads non-transient and non-static fields
        name = "overrode";
     //   age = in.readInt();
     //   weight = in.readInt();
    }

    public static void main(String[] args) throws Exception {
        Solution s1 = new Solution(3,6,"Tiger");

        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(baos);

        oos.writeObject(s1);

        byte[] savedObject = baos.toByteArray();

        ByteArrayInputStream bais = new ByteArrayInputStream(savedObject);
        ObjectInputStream ois = new ObjectInputStream(bais);
        Solution s2 = (Solution) ois.readObject();

        System.out.println(s1);
        System.out.println(s1.age);
        System.out.println(s1.weight);
        System.out.println(s1.name +"\n");

        System.out.println(s2);
        System.out.println(s2.age);
        System.out.println(s2.weight);
        System.out.println(s2.name);
    }
}

Gellert Varga

Level 23 , Szekesfehervar, Hungary

11 December 2021, 20:52

When I use this program as shown above, everything works perfectly well. That's fine, there's no question about it. But take the following case: I make disused (comment out) lines 20 and 21, and I try to put lines 22 and 23 into use (in the readObject method). In this case, I get an extremely long 18-line Exception that I can’t decipher. The question is what is causing the error? 1) maybe it is mandatory to use defaultReadObject() inside readObject()? 2) maybe it is forbidden to use readInt() inside readObject()?

Lisa

Level 41

11 December 2021, 21:27

useful

import java.io.*;
import java.util.Objects;

public class Solution implements Serializable {

    private int age;
    private int weight;
    transient private String name;
    private static long id = 101; // I love dogs so much

    public Solution(int age, int weight, String name) {
        this.age = age;
        this.weight = weight;
        this.name = name;
    }

    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        out.writeObject(name); // semi-manual serialization eg. allows to serialize transient fields
        out.writeLong(id); // possible to serialize static fields
        // let's change the id so we later can see if reading works
        id = -7;
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject(); // it loads non-transient and non-static fields
        // defaultReadObject already read those fields... it's doing the default automatic serialization.
        // so if you try to read them again... nothing will be found.
        //   age = in.readInt();
        //   weight = in.readInt();
        // but instead we saved name and id
        name = (String) in.readObject();
        id = in.readLong();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Solution solution = (Solution) o;
        return age == solution.age && weight == solution.weight && Objects.equals(name, solution.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(age, weight, name);
    }
}

Lisa

Level 41

11 December 2021, 21:27

the main method to test it...

public static void main(String[] args) throws Exception {
    Solution s1 = new Solution(3,6,"Tiger");

    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ObjectOutputStream oos = new ObjectOutputStream(baos);

    oos.writeObject(s1);

    byte[] savedObject = baos.toByteArray();

    ByteArrayInputStream bais = new ByteArrayInputStream(savedObject);
    ObjectInputStream ois = new ObjectInputStream(bais);
    Solution s2 = (Solution) ois.readObject();

    if (s2.equals(s1))
        System.out.println("all good and the id is " + id);
}

Lisa

Level 41

11 December 2021, 21:31

useful

and here how it'll look if you want to use automatic (semi) serialization without defaultRead-WriteObject (but I'd go for Externalizable in that case)

private void writeObject(ObjectOutputStream out) throws IOException {
    out.writeObject(name);
    out.writeInt(age);
    out.writeInt(weight);

}

private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
    name = (String) in.readObject();
    age = in.readInt();
    weight = in.readInt();
}

Lisa

Level 41

11 December 2021, 21:34

Ahhh... it's a book, I wrote a lil book... uihh, hihihi 😜🤪

Gellert Varga

Level 23 , Szekesfehervar, Hungary

11 December 2021, 23:01

Thank you very much for this book!:) Now it is clear. So it is not mandatory to use defaultWrite(Read)Object(). The whole thing can be done manually. The only thing is: the writeObject() and readObject() methods must be completely parallel. If I use the 'default' method in one, I must use it in the other, too. Or I should not use it in either. The reason I got that horrible long Exception is because I used defaultWriteObject method when saving the object, and when reading it I wanted to use readInt() methods only (without using the defaultReadObject method). I assume they are not compatible. - And I also got a good tip on the possibility to save static fields.:)