What is a serialVersionUID and Why Should You Use It?

In Java, the serialVersionUID is a unique value for a serializable class. It’s a field that the developers can declare within a class during serialization and deserialization. When an object is serialized, the serialVersionUID is also written to the stream of bytes along with the object’s data.

In this blog, we will discuss what serialVersionUID is and why you should use it.

Table of contents:

• What is a serialVersionUID in Java?
• Serialization and Deserialization in Java
• Why should I use serialVersionUID?
• Different Methods of Serialization in Java
  1. Default Serialization
  2. Custom Serialization
  3. Externalizable Interface
  4. JSON Serialization
  5. XML Serialization
• Conclusion

What is a serialVersionUID in Java?

The serialVersionUID is a unique value used to ensure compatibility between the serialized and deserialized objects of a Serializable class in Java. It serves as a number to verify that the class used during deserialization matches the one used during serialization. If the serialVersionUID values differ between the sender’s and receiver’s classes, then an InvalidClassException will be thrown. 

The serialVersionUID can be declared by the class as a static final long field. The different classes can have different serialVersionUID values, and these values do not need to be unique across all the classes. The serialver tool in the JDK is used to generate the serialVersionUID for a class.

Syntax:

ANY-ACCESS-MODIFIER static final long serialVersionUID = <value>L;

Where:

• ANY-ACCESS-MODIFIER: any access modifier, such as public, protected, or private
• static: this keyword ensures that the serialVersionUID belongs to the class only, not to the object of that class.
• final: It ensures that the value of serialVersionUID cannot be changed once it is set.
• long: The type of the serialVersionUID, which must be long.
• serialVersionUID: The name of the field, it is a standard name used by Java for knowing the version of a class during serialization and deserialization.
• <value>: This is the numeric value of the serialVersionUID.
• L: The L at the end indicates that the value is of type long. If not specified, it will be assumed as an integer.

Example:

Output:

Explanation: The above code explains the serialization and deserialization in Java using the serialVersionUID. A Person object is created and serialized to a file named person.ser using ObjectOutputStream. Then, the object is deserialized from the file using ObjectInputStream, and its values are printed. The serialVersionUID makes sure that the serialized and deserialized versions of both the classes match each other and prevents errors like InvalidClassException if the structure of the class changes.

Serialization and Deserialization in Java

Serialization is the process of converting an object into a byte stream.

The ObjectOutputStream class provides the writeObject() method for serializing an object.

Deserialization is the reverse process of it, where the byte stream is converted into the actual Java object in memory. This process allows the object's state to be persisted and restored later..

The ObjectInputStream class contains the readObject() method for deserialization.

The byte stream that is produced by serialization is platform-independent, i.e. an object that is serialized on one platform can be deserialized on another platform

Let’s take an example. Imagine you have a toy car. And you have taken a photo of that car, now you can keep that photo and look at it afterward to remember what the car looked like before. In this example, taking the photo of the car is like serialization, and looking at the photo afterward is like deserialization.

To make a Java object serializable, we use the java.io.Serializable interface.  

Where objects of classes that implement the Serializable interface can be serialized. The Serializable interface is a marker interface, meaning it has no data members or methods

Some Important Points:

• If a parent class is implementing the Serializable interface, then its child class doesn't need to implement it. 
• Only the non-static instance variables are serialized.
• The constructor of an object is not called during the process of deserialization.
• Any associated objects must also implement the Serializable interface.

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Why should I use serialVersionUID?

serialVersionUID should be used in Java for the following key reasons:

• Ensures Compatibility: It makes sure that the class that is used during the deserialization is the same as the one used during the serialization. This prevents errors if the class structure is changed over time.
• Prevents InvalidClassException: If the serialVersionUID values get changed between the sender and receiver's class, an InvalidClassException is thrown. By using serialVersionUID, you can avoid this error.
• Helps Version Control: If you change a class, i.e. add or remove the fields, serialVersionUID helps you to manage the compatibility between the different versions of the class.
• Required for Serializable Classes: When you are serializing an object, Java suggests that you should define the serialVersionUID to maintain the integrity of the serialized data across the different versions of the class.

Different Methods of Serialization in Java

There are mainly 5 methods of serialization in Java. These are as follows:

1. Default Serialization in Java

It is the automatic serialization provided by Java, where the runtime environment serializes the object by default when the object of the class is implemented in the Serializable interface. 

2. Custom Serialization in Java

By overriding the writeObject() and readObject() methods, custom Serialization allows more control over the serialization process. These methods allow you to change the way an object’s fields are serialized and deserialized. 

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3. Externalizable Interface in Java

The Externalizable interface is another way to get custom serialization. It requires you to implement two methods: writeExternal and readExternal. These methods give more control than the Serializable interface, but they are also more complex to apply.

4. JSON Serialization in Java

JSON Serialization involves converting Java objects into JSON (JavaScript Object Notation) format, which is a lightweight data-interchange format. This is done by using third-party libraries like Jackson. It also allows you to serialize the Java objects to JSON and deserialize JSON to Java objects.

5. XML Serialization in Java

XML serialization in Java converts Java objects into XML strings and vice versa. This method is useful for data interchange in applications that require an XML format. 

Conclusion

The SerialVersionUID is a unique number in Java that helps your objects stay properly matched when they are saved and later loaded by the memory. It prevents errors if the class has changed over time. By using it, you can make sure objects are correctly matched during loading, even after the program is updated. This makes it easier to manage the saved data and share it between the different systems 

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What is a serialVersionUID and Why Should you use it - FAQs