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 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 a serialVersionUID is, why you should use it, serialization and deserialization in Java, how to generate serialVersionUID in Java efficiently, and best practices for using serialVersionUID 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 I use serialVersionUID?
• Different Methods of Serialization in Java
  1. Default Serialization in Java
  2. Custom Serialization in Java Using writeObject() & readObject()
  3. Serialization Using Externalizable Interface in Java
  4. JSON Serialization in Java with Jackson
  5. 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
• 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 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.

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 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 to know 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.

serialVersionUID 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 serialVersionUID makes sure that the serialized and deserialized versions of both classes match each other and prevents errors like InvalidClassException if the structure of the class changes.

How to Generate serialVersionUID Automatically in Java

To generate a serialVersionUID 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 serialVersionUID).
• 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 serialVersionUID".
• 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.

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 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 I use serialVersionUID?

Here are the key reasons why use 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 serialVersionUID values get changed between the sender and receiver's classes, 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 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 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 serialVersionUID 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 serialVersionUID 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 the unpredictable errors in Java, you need to follow the serialVersionUID best practices given below:

• Always define serialVersionUID 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 serialVersionUID, Java will generate one automatically, but this can change if you modify the class, causing errors when reading old serialized objects.
• Set serialVersionUID = 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 serialVersionUID.
• Let your IDE (like Eclipse or IntelliJ) generate a serialVersionUID for you because it is safer and faster.

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 

What is a serialVersionUID in Java - FAQs