How to Fix Java.IO.NotSerializableException in Java

1. Reproduce the java.io.NotSerializableException in Java
2. Causes of the java.io.NotSerializableException in Java
3. Fix the java.io.NotSerializableException in Java

Today, we will reproduce the java.io.NotSerializableException while writing a program in Java. We will also understand what this error means, leading to its causes and solutions.

Reproduce the java.io.NotSerializableException in Java

Example Code (Student.java file):

package com.serializertest;

class Student {
  private String studentId;

  public String getId() {
    return studentId;
  }
  public void setId(String id) {
    this.studentId = id;
  }
}

The Student class is a helper class with a member variable named studentId. It also has member methods named getId() and setId() to get and set the id of a student.

Example Code (SerializerTest.java file):

package com.serializertest;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

public class SerializerTest {
  public static void main(String[] args) throws IOException {
    FileOutputStream fileOutputStream = new FileOutputStream("students.txt");
    ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);

    Student student = new Student();
    student.setId("0001");

    objectOutputStream.writeObject(student);
    objectOutputStream.close();
  }
}

The SerializerTest is our main class where we have the main() driver method. Inside the main(), we create an instance of the FileOutputStream class to create a file.

Similarly, we create another instance of the ObjectOutputStream class.

Then, we create an object of the Student class and call its setId() method by passing a string argument. Next, we use the object of the ObjectOutputStream class to write an object to the stream.

For that, we are using the writeObject() method.

After that, we close the stream using the close() method and run the program, but it gives us the following exception in the program’s output console.

Exception in thread "main" java.io.NotSerializableException: com.serializertest.Student

Why are we facing this? Let’s understand the error to find out its causes below.

Causes of the java.io.NotSerializableException in Java

It is essential to understand NotSerializableException and Serialization to find the causes.

Serialization is a mechanism in Java programming that we use to write an object’s state into the byte stream. In contrast, the reverse operation of it is called Deserialization.

The Serialization and Deserialization are platform-independent. It means both processes can be performed on different platforms.

We use Serialization in Remote Method Invocation (RMI) and Enterprise Java Beans (EJB). It is also used in Java Persistence API (JPA), Hibernate, etc.

The NotSerializableException is an exception that extends the ObjectStreamException class, defined as a superclass of all other exceptions specific to the Object Stream classes.

Additionally, the ObjectStreamException extends the IOException, which further signals that an I/O exception has been generated.

Now, we know the Serialization and NotSerializableException in Java. This discussion takes us to discover the reasons for NotSerializableException in Java.

1. The NotSerializableException occurs when an instance of a class must implement a Serializable interface.
2. We also get this exception either by an instance of a class or the serialization runtime. Remember, the argument of NotSerializableException is a name of a class.
3. As per the documentation, the complete object graph requires to be serializable. The NotSerializableException also occurs if at least one of the fields does not implement the Serializable interface while attempting to serialize an instance of a class.

Fix the java.io.NotSerializableException in Java

We can fix the java.io.NotSerializableException in Java using the following solutions.

1. Find the class that throws this exception and make it Serializable by implementing the Serializable interface. Remember, this solution might not be a good choice if a class that throws the NotSerializableException belongs to the third-party library.

2. We can declare the objects as transient if the class refers to the non-serializable objects where these objects must not be serializable. The question is, why declare it as transient?

   It is because the fields of the class declared as transient would be ignored by serializable runtime, and we will not get any exceptions.

3. If we need its data and its third party, we can consider other means of serialization. For instance, XML, JSON, etc., where we can get the third-party objects serialized without changing their definitions.

In our case, implementing the Serializable fixed the java.io.NotSerializableException. See the following example code.

Example Code (Students.java file):

package com.serializertest;

import java.io.Serializable;

class Student implements Serializable {
  private String studentId;

  public String getId() {
    return studentId;
  }
  public void setId(String id) {
    this.studentId = id;
  }
}

Example Code (SerializerTest.java file):

package com.serializertest;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

public class SerializerTest {
  public static void main(String[] args) throws IOException {
    FileOutputStream fileOutputStream = new FileOutputStream("students.txt");
    ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);

    Student student = new Student();
    student.setId("0001");

    objectOutputStream.writeObject(student);
    objectOutputStream.close();
  }
}