How can I add the RestContext.response in the batch class finish method?

Question

I am creating a REST class wherein all the processing is done by a batch class. I would want the RestContext.response line to be made after the execute method. But it returns this error: "System.NullPointerException: Attempt to de-reference a null object". This error occurs even if I put the line of code in the execute method.

Is it possible to put the RestContext.response in a batch class? Or is there any better way to do it? Hope you can help me...

This is my code:

global class sample_API {
@HttpPost
  global static void getAccounts() {
    
    sampleAPIBatch runAPIBatch = new sampleAPIBatch();
    Id batchJobId = Database.executeBatch(runAPIBatch);
  }
}    

/batch class/

global class sampleAPIBatch implements Database.Batchable <sObject>, Database.Stateful, Database.AllowsCallouts {
...

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

global void finish(Database.BatchableContext bc) {  
    //here results to : System.NullPointerException: Attempt to de-reference a null object  
    RestContext.response.responseBody = Blob.valueOf(JSON.serialize(theResult));
}

Answer 1

You can do the following (assuming theResult is a stateful batch class variable)

RestContext.response resp = new RestResponse();
resp.responseBody = Blob.valueOf(JSON.serialize(theResult)); 

Note it is a bit weird for the finish() to construct a RestResponse but presumably you are passing this to some other method that is expecting this.