How to Assign Object Reference IDs During Custom Serialization in Java

What are the best practices for assigning object reference IDs while implementing custom serialization in Java?

public class CustomObject implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private transient int referenceId;

    public CustomObject(String name, int referenceId) {
        this.name = name;
        this.referenceId = referenceId;
    }

    private void writeObject(ObjectOutputStream oos) throws IOException {
        // Assign a reference ID here
        oos.defaultWriteObject();
        oos.writeInt(referenceId);
    }

    private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
        ois.defaultReadObject();
        referenceId = ois.readInt(); // Read the reference ID back
    }
}

Custom serialization in Java allows developers to define how objects are serialized and deserialized, including the customization of reference IDs. This can be particularly useful when you want to maintain object identity across different serialized forms, ensuring that object graphs retain their structure, particularly when objects reference each other.

public class CustomObject implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private transient int referenceId;

    public CustomObject(String name, int referenceId) {
        this.name = name;
        this.referenceId = referenceId;
    }

    private void writeObject(ObjectOutputStream oos) throws IOException {
        oos.defaultWriteObject();
        oos.writeInt(referenceId);
    }

    private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
        ois.defaultReadObject();
        referenceId = ois.readInt();
    }
}

Causes

• Inappropriately handling object reference IDs can lead to issues such as duplicate objects or loss of object identity.
• Not using transient variables appropriately may result in unintended data retention or loss during serialization.

Solutions

• Implement the Serializable interface and provide custom readObject and writeObject methods to handle object state explicitly.
• Use a transient variable to store reference IDs that need to be assigned and processed manually during serialization.

Mistake: Forgetting to implement the Serializable interface in custom classes.

Solution: Always ensure that all classes that need serialization implement Serializable.

Mistake: Not handling the class structure change after serialization (e.g. adding/removing fields).

Solution: Use serialVersionUID to manage backward compatibility in your serialized classes.