How to use @HttpGet annotation in salesforce?

 The @HttpGet annotation is used at the method level and enables us to expose an Apex method as a REST resource. To use the @HttpGet annotation our apex method must be defined as global static.To create a custom webservice the first step is to create an apex class annoted with @RestResource annotation.

In a class annoted with @RestResource we can create 5 methods each with different annotations like GET,DELETE,POST,PUT,PATCH. Multiple methods cannot be annoted with same notation, for example in a class there can be only one method annoted with GET,DELETE,POST,PUT,PATCH at a time.

Note: Methods annotated with @HttpGet are also called if the HTTP request uses the HEAD request method.

Below is an example of webservice which need id or any unique identifier as a parameter in request and it will return record associated with it.

@RestResource(urlMapping='/getAccountOnExternalIdtofetchsinglerecord/*')

global with sharing class getAccounttoSingleRecord {

      @Httpget

      global static String fettringchAccount(){

      Account obj=new Account();

      RestRequest req = RestContext.request;

      RestResponse res = Restcontext.response;

      string accId = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);

      obj=[Select id , name from Account where id=:accId];

      string serializeResponse=JSON.Serialize(obj);

      return serializeResponse;

      }

   }

• The above web service will be accessible at below URL with GET method from external system after successful authentication.

'https://instance.salesforce.com/services/apexrest/getAccountOnExternalIdtofetchsinglerecord/' + externalAccId;

• The method "fetchAccount()" will be automatically called once the request is made at above URL with GET method.

• The externalAccId mentioned in above URL can be any unique parameter using which we can identify the correct record, 

      In our example we are expecting id of the account record.

• We use the RestRequest class to access incoming request.

           For more details visit: What is RestRequest class in salesforce?

• We use the RestResponce class and it is used to pass data from an apex RESTful web service method to an HTTP response.

          For more details visit: What is RestResponse class in salesforce?

Now, let us try to this test our webservice from workbench.

 /services/apexrest/getAccountOnExternalIdtofetchsinglerecord/0017F00004eD8XzQAK

         Click "Execute" button and now click "Show Raw Response".

"{\"attributes\":{\"type\":\"Account\",\"url\":\"/services/data/v55.0/sobjects/Account/0017F00004eD8XzQAK\"},

  \"Id\":\"0017F00004eD8XzQAK\",\"Name\":\"Test 105 Account\"}"

As we can see the response we are returning here is of string type, if we want to return JSON format we need to modify the above webservice class as shown below,

@RestResource(urlMapping='/getAccountOnExternalIdtofetchsinglerecord/*')

global with sharing class getAccounttoSingleRecord {

      @Httpget

      global static Account fettringchAccount(){

      Account obj=new Account();

      RestRequest req = RestContext.request;

      RestResponse res = Restcontext.response;

      string accId = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);

      obj=[Select id , name from Account where id=:accId];

      // string serializeResponse=JSON.Serialize(obj);

      return obj;

      }

   }

Now, Click "Execute" button and now click "Show Raw Response".

{

  "attributes" : {

    "type" : "Account",

    "url" : "/services/data/v49.0/sobjects/Account/0017F00004eD8XzQAK"

  },

  "Id" : "0017F00004eD8XzQAK",

  "Name" : "Test 105 Account"

}