Why do POJO classes need to implement the Serializable interface?

Question

Why do POJO Java classes have to implement the Serializable interface? What will happen if I do not implement Serializable?

@Entity
@Table(name="Customer")
public class Customer implements Serializable {

    private static final long serialVersionUID = -5294188737237640015L;
    /**
     * Holds Customer id of the customer
     */
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "cust_id")
    private int Custid;

    /** Holds first Name of the customer*/
    @Column(name = "first_name")
    private String firstName;

    /** Holds last Name of the customer */
    @Column(name = "last_name")
    private String lastName;

    /** Holds Customer mobile number of customer*/
    @Column(name = "cust_mobile")
    private long MobileNo;

    /**Holds customer address of customer*/
    @Column(name = "cust_address")

Answer 1

First this is no longer a Plain Old Java Object, Because it has annotations.

But staying on your premise, The Serializable is required in POJOs if those are intended to be used in Distributed Systems and Systems which use caching and flushing into files and reading back.

Mostly JPA implementations do run in Distributed manner and Use caching, thus this POJO is required to implement Serializable.

For more information read this discussion