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Ensure readResolve Methods are Inheritable

High
maintainabilityreliability

What is it?​

This practice refers to the correct use of the readResolve method in Java's serialization mechanism, ensuring that it can be accessed by subclasses in a class hierarchy.

Why apply it?​

Developers may need to add logic to handle deserialized objects. A readResolve method should be inheritable so that subclasses can have access to this functionality. Making it protected instead of private ensures it can be used effectively in inheritance chains.

How to fix it?​

Make readResolve methods protected rather than private to allow subclasses to inherit and possibly override the behavior as needed.

Examples​

Example 1:​

Negative​

The negative example uses a private readResolve method, preventing subclass access to the method.

import java.io.ObjectStreamException;
import java.io.Serializable;

public class Fruit implements Serializable {
    private static final long serialVersionUID = 1L;

    private Object readResolve() throws ObjectStreamException {  // Noncompliant
        // Custom logic for deserialization
        return this;
    }

    // Additional fields and methods...
}

public class Raspberry extends Fruit {
    // No access to parent's readResolve method
}

Example 2:​

Positive​

The positive example demonstrates a protected readResolve method allowing subclass access to this method for customization.

import java.io.ObjectStreamException;
import java.io.Serializable;

public class Fruit implements Serializable {
    private static final long serialVersionUID = 1L;

    protected Object readResolve() throws ObjectStreamException {
        // Custom logic for deserialization
        return this;
    }

    // Additional fields and methods...
}

public class Raspberry extends Fruit {
    // Access to parent's readResolve method
}

Negative​

The negative example demonstrates a private readResolve implementation wrongly preventing subclasses from leveraging any deserialization logic.

import java.io.ObjectStreamException;
import java.io.Serializable;

abstract class ElectronicDevice implements Serializable {
    private static final long serialVersionUID = 2L;

    private Object readResolve() throws ObjectStreamException {  // Noncompliant
        // Restoration work omitted
        return this;
    }

    // Device-related fields and methods...
}

public class Smartphone extends ElectronicDevice {
    // Cannot access parent's readResolve method
}

Example 3:​

Positive​

Here, a hierarchy of classes correctly uses readResolve with proper visibility, ensuring readResolve is accessible by all subclasses.

import java.io.ObjectStreamException;
import java.io.Serializable;

abstract class ElectronicDevice implements Serializable {
    private static final long serialVersionUID = 2L;

    protected Object readResolve() throws ObjectStreamException {
        // Restore object invariant
        return this;
    }

    // Device-related fields and methods...
}

public class Smartphone extends ElectronicDevice {
    // Benefits from parent's readResolve method
}