Database.Stateful in Batch Apex in Salesforce

 Batch Apex is stateless.  Each execution of a batch Apex job is considered a discrete transaction. If we implements Database.Stateful we can maintained state across transactions. Using Database.Stateful only instance variable holds values static members does not hold values. If we want to count records as batch proceeds maintaining state is important as after one transaction new transaction will start and members will loose their values.

Let's understand with an example,

public class UpdateContactAddresses implements

    Database.Batchable<sObject>, Database.Stateful {

 

    public Integer recordsProcessed = 0;

    public Database.QueryLocator start(Database.BatchableContext bc) {

         String query = 'SELECT Id,Name,Phone,(SELECT id,Phone from Contacts) FROM Account';

        return Database.getQueryLocator(query);

    }

    public void execute(Database.BatchableContext bc, List<Account> scope){

        // process each batch of records

        List<Contact> contacts = new List<Contact>();

        for (Account account : scope) {

            for (Contact contact : account.contacts) {

                contact.phone= account.phone;

              

                contacts.add(contact);

             

                recordsProcessed = recordsProcessed + 1;

            }

        }

        update contacts;

    }

    public void finish(Database.BatchableContext bc){

 

        AsyncApexJob job = [SELECT Id, Status, NumberOfErrors,

            JobItemsProcessed,

            TotalJobItems, CreatedBy.Email

            FROM AsyncApexJob

            WHERE Id = :bc.getJobId()];

        // call some utility to send email

          Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();

        String[] toAddresses = new String[] {'Testabc@gmail.com'};

        mail.setToAddresses(toAddresses);

        mail.setSubject('Batch Status ' + job.Status + 'Record Processed ' + recordsProcessed );

        mail.setPlainTextBody('Total Jobs Processed: ' + job .TotalJobItems +   'with '+ job .NumberOfErrors + ' failures.');

        Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });

    }

}