Problem

You want to execute more than one set of batch Apex processing, but the default Force.com setup doesn't allow you to call one batch Apex class from another.

Description

In Force.com you cannot call a batch Apex class from another batch Apex class because batch Apex is a future call. However, you can use Force.com email services to serialize these batch processes.

To put this into action, you need to create an email service to send an email after a batch is complete, modify the first batch class to call this service after completion, and create an inbound email handler to intercept the email and start the second batch.

Create a class "EmailHandler" . The purpose of this class is to send email to the Force.com email service address.

public class EmailHandler
{
    public static void sendEmail(string body)
    {
       Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
       // email address of the salesforce email service
       String[] toAddresses = new String[] {'myhandler@x4m9vxsizy32oug74yh6rhj7e.in.salesforce.com'};
       mail.setToAddresses(toAddresses);
       mail.setSubject('Test Batch' );
       mail.setPlainTextBody(body);
       Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
    }
    
}

You'll want to use the email address of the email service configured later in this recipe, in the toAddresses array.

Now create two Batch Apex classes. These are the sample batch classes:

 global class Batch1 implements Database.Batchable<Sobject>{
        //constructor
        global Batch1(){}
            
        
        //Method to get the data to be proceesed  
        global database.Querylocator start(Database.BatchableContext context){
            string query = 'select id, name from Account limit 1000';
            return Database.getQueryLocator(query);
        }
            
         
        //Method to execute the batch
        global void execute(Database.BatchableContext context , Sobject[] scope){
            for(Sobject s : scope){ 
                Account a = (Account)s;
                // TO DO
                // add your logic 
            }
        }
         
         //Method to be called after the excute
         global void finish(Database.BatchableContext context){
             // send an email
             EmailHandler.sendEmail('start second batch');
         }
}

global class Batch2 implements Database.Batchable<Sobject>{
        //constructor
        global Batch2(){}
            
        
        //Method to get the data to be proceesed  
        global database.Querylocator start(Database.BatchableContext context){
            string query = 'select id, name from Contact limit 1000';
            return Database.getQueryLocator(query);
        }
            
         
        //Method to execute the batch
        global void execute(Database.BatchableContext context , Sobject[] scope){
            for(Sobject s : scope){ 
                Contact c = (Contact)s;
                // TO DO
                // add your logic 
            }
        }
         
         //Method to be called after the excute
         global void finish(Database.BatchableContext context){
         
         }
}

Notice how the finish() method in Batch1 calls the email service.

Finally, create a class which implements Messaging.InboundEmailHandler.

global class EmailServiceHandler implements Messaging.InboundEmailHandler 
{
        global Messaging.InboundEmailResult handleInboundEmail(Messaging.InboundEmail email,Messaging.InboundEnvelope envelope) 
        {
    if (email.plainTextBody== 'start second batch')
           {
              // create the instance of second batch class
              Batch2 obj = new Batch2();
              Database.executeBatch(obj);
           } 
           Messaging.InboundEmailResult result = new Messaging.InboundEmailresult();
    return result;
 }
}

Make sure that the email address assigned to this service is used in the EmailHandler class.

Now execute the Batch1 class. After execution, it will call the email service, which will result in an email being sent and intercepted by the EmailServiceHandler, which will execute the second batch.

 Batch1 obj = new Batch1();
 Database.executeBatch(obj);

References

• An Introduction to Email Services