Externalizable vs. Serializable

When it comes to persisting objects in Java, two commonly used interfaces are Externalizable and Serializable. Both interfaces provide a way to convert Java objects into a stream of bytes, allowing them to be saved into a file, sent over a network, or stored in memory. However, there are some notable differences between these interfaces that developers should be aware of when deciding which one to use.

Serializable

The Serializable interface is the simpler of the two. It is a marker interface, meaning it does not declare any methods that need to be implemented. When a class implements Serializable, all of its non-transient member variables are automatically saved and restored by the Java serialization mechanism.

To make a class Serializable, you simply need to add the implements Serializable clause to your class declaration. For example:

public class MyClass implements Serializable {
    // class implementation
}

However, it's important to note that not all objects of a Serializable class hierarchy may be serializable. If a parent class is not serializable, it must have a no-argument constructor for its subclass to be serialized successfully.

Serialization has the advantage of being easy to use, as it requires minimal code changes. Additionally, it supports a wide range of object graphs and can handle complex data structures. However, the downside is that it can be slower and result in larger serialized files compared to Externalizable.

Externalizable

The Externalizable interface, on the other hand, provides more control over the serialization process. Unlike Serializable, it requires the implementation of two methods: writeExternal and readExternal. These methods define how an object should be serialized or deserialized.

To make a class Externalizable, you need to implement the Externalizable interface and provide implementations for the writeExternal and readExternal methods. For example:

public class MyClass implements Externalizable {
    // class implementation
    
    @Override
    public void writeExternal(ObjectOutput out) throws IOException {
        // serialization logic
    }
    
    @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        // deserialization logic
    }
}

By implementing Externalizable, you have full control over which attributes should be serialized and how they should be serialized. This allows for more efficient serialization and deserialization, as you can skip unnecessary fields or perform custom serialization logic.

However, this additional control comes at the cost of increased complexity. You have to manually handle the serialization and deserialization process, which requires more code. Additionally, Externalizable can be less flexible when dealing with changes to the object's class structure, as it requires manual adjustments to handle the changes.

Conclusion

In summary, the choice between Externalizable and Serializable depends on your specific requirements. If you need a simple and efficient way to serialize objects without much manual intervention, Serializable is the way to go. On the other hand, if you require fine-grained control over the serialization process or need to optimize performance, Externalizable offers more flexibility.

Consider your application's needs, performance considerations, and maintainability when deciding which interface to use. Each interface has its strengths and weaknesses, and understanding them will help you make an informed choice in handling object serialization in Java.