What is a serialVersionUID in Java

The serialVersionUID in Java 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 the system serializes an object, it also writes this ID to the byte stream along with the object’s data.

In this blog, we will discuss what is a serialVersionUID, why you should use it, serialization and deserialization in Java, how to generate them in Java efficiently, and best practices for using it in Java.

Table of contents:

• What is a serialVersionUID in Java?
• How to Generate serialVersionUID Automatically in Java
• Serialization and Deserialization in Java
• Why should you use serialVersionUID?
• Different Methods of Serialization in Java
  • Default Serialization in Java
  • Custom Serialization in Java Using writeObject() & readObject()
  • Serialization Using Externalizable Interface in Java
  • JSON Serialization in Java with Jackson
  • XML Serialization in Java: How to Convert Objects to XML
• What Happens If You Don’t Use serialVersionUID?
• Best Practices for Using serialVersionUID in Java
• Common Serialization Pitfalls
• Conclusion

What is a serialVersionUID in Java?

The serialVersionUID in Java 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 values differ between the sender’s and receiver’s classes, then the system throws an InvalidClassException.

The class declares this special ID as a static final long field. The different classes can have different serial values, and these values do not need to be unique across all the classes. The serialver tool in the JDK is used to generate this value for a class.

serialVersionUID syntax in Java:

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 value belongs to the class only, not to the object of that class.
• final: It ensures that the value cannot be changed once it is set.
• long: This sets the data type of the value, which should be long.
• serialVersionUID: The field uses a standard name that Java recognizes to determine the version of a class during serialization and deserialization.
• <value>: This is the numeric value of the serial number.
• L: The L at the end indicates that the value is of type long. If you don’t specify it, the system assumes it as an integer.

Example in Java:

Output:

Explanation: The above code explains Java serialization and deserialization 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 version stamp ensures that the serialized and deserialized versions of both classes match each other, preventing errors such as InvalidClassException if the class structure changes.

How to Generate serialVersionUID Automatically in Java

To generate this value in Java, you can use one of the following methods:

1. Using IntelliJ IDEA (code editor)

• Open your Java class.
• Click inside the class body.
• Press Alt + Enter (or right-click -> Refactor -> Add).
• IntelliJ will add this line automatically:

private static final long serialVersionUID = 1L;

2. Using Eclipse

• Open your Java class.
• Click inside the class name.
• Press Ctrl + 1 and choose "Add".
• Eclipse can create a simple one or calculate a unique one for you.

3. Using the Command Line Tool

• First, compile your class:

javac MyClass.java

• Then run:

serialver MyClass

• It will show:

private static final long serialVersionUID = 1234567890123456789L;

Note: When you save and load objects (e.g., to a file), this ID helps Java to know that the class has not changed too much. If the ID is missing or doesn’t match, you will get errors.

Before we move any further, it is important that you have a brief knowledge of serialization and deserialization in Java.

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

Example:

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 in Java is a marker interface, meaning it has no data members or methods

Some Important Points to Remember While Implementing Java Serialization and Deserialization:

• If a parent class is implementing the Serializable interface in Java, 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 in Java.

Why should you use serialVersionUID?

Here are the key reasons why you should use a serialVersionUID in Java:

• 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 values get changed between the sender and receiver's classes, an InvalidClassException is thrown. By using this serialization number, you can avoid this error.
• Helps Version Control: If you change a class, i.e., add or remove the fields, the unique serialization ID 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 this special ID to maintain the integrity of the serialized data across different versions of the class.

Different Methods of Serialization in Java

Now that you are aware of why you should use serialVersionUID in Java, let us look at the various methods of Serialization in Java. There are mainly five 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 implements the Serializable interface. 

2. Custom Serialization in Java Using writeObject() & readObject()

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. Serialization Using 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 with Jackson

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 Java objects to JSON and deserialize JSON to Java objects.

5. XML Serialization in Java: How to Convert Objects to XML

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. 

What Happens If You Don't Use serialVersionUID?

If you don't use this special ID in a Java class that implements Serializable, Java will automatically generate one at runtime based on the class's details (like its fields and methods).

Here are the points that will help you to understand what can happen if you don't use the unique serialization ID in Java:

1. Compatibility Issues: If you make changes to the class, such as adding or removing a field, and try to deserialize an old object, the generated ID might not match, which causes an InvalidClassException.
2. Unpredictable Errors: Since the ID is generated by the JVM, it can vary between compiler versions or platforms, making the deserialization unreliable.
3. Harder Debugging: Without a fixed ID, it is difficult to track why serialization-related errors are occurring after the class changes.

Best Practices for Using serialVersionUID in Java

In order to avoid unpredictable errors in Java, you need to follow the best practices given below:

• Always define the class version identifier in a class that implements Serializable.
• Use the private static final long serialVersionUID keyword and assign a long number to it.
• If you don’t define a special ID, Java will generate one automatically, but this can change if you modify the class, causing errors when reading old serialized objects.
• Set this identifier field to 1L if you are not planning to change the class often.
• If your class changes but should stay compatible with older saved objects, keep the same unique serialization ID.
• Let your IDE (like Eclipse or IntelliJ) generate a compatibility ID for you because it is safer and faster.

Common Serialization Pitfalls

Let us now look at some common pitfalls that you must avoid while performing serialization in Java.

• Missing Class Identifier: Lack of a unique class identifier can cause an InvalidClassException if the class changes, making previously stored objects potentially incompatible with the updated class definition.
• Class Structure Changes: Changing class structure without updating the identifier can cause deserialization failures, as old objects might remain serialized while new ones refer to incompatible versions.
• Non-Serializable Fields: Serializing non-serializable fields can trigger NotSerializableException, halting the process and risking loss of critical application data.
• Large Object Graphs: Handling large object graphs slows serialization, increases memory usage, and impacts performance in high-load or distributed environments.
• Untrusted Data Sources: Deserializing untrusted data may allow malicious code execution during deserialization, risking security breaches or unauthorized system access.

Conclusion

The SerialVersionUID is a unique number in Java that helps your objects stay properly matched when they are saved and later loaded into 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.

To master Java serialization and prepare for related interview questions, check out our comprehensive Java course with dedicated serialization modules and real-world interview prep.

Useful Resources:

• Encapsulation in Java With Examples
• Java File I/O - Input/Output in Java with Examples
• Exception Handling in Java
• Java User Input - Scanner, BufferedReader, and Console

What is a serialVersionUID in Java - FAQs