Add a Custom Event Handler

1. Create the Java package, by creating a new folder called handlers under srv/src/main/java/com/sap/cap/productsservice.

   

2. Create the Java class file AdminService.java in the created handlers folder, with the following content and make sure you Save the file:

 package com.sap.cap.productsservice.handlers;

 import java.util.HashMap;
 import java.util.Map;

 import org.springframework.stereotype.Component;

 import com.sap.cds.services.cds.CdsCreateEventContext;
 import com.sap.cds.services.cds.CdsReadEventContext;
 import com.sap.cds.services.cds.CdsService;
 import com.sap.cds.services.handler.EventHandler;
 import com.sap.cds.services.handler.annotations.On;
 import com.sap.cds.services.handler.annotations.ServiceName;

 @Component
 @ServiceName("AdminService")
 public class AdminService implements EventHandler {

     private Map<Object, Map<String, Object>> products = new HashMap<>();

     @On(event = CdsService.EVENT_CREATE, entity = "AdminService.Products")
     public void onCreate(CdsCreateEventContext context) {
         context.getCqn().entries().forEach(e -> products.put(e.get("ID"), e));
         context.setResult(context.getCqn().entries());
     }

     @On(event = CdsService.EVENT_READ, entity = "AdminService.Products")
     public void onRead(CdsReadEventContext context) {
         context.setResult(products.values());
     }

 }

This class now handles the READ and CREATE events that target the Products entity of the AdminService.

• The READ operation just returns all entities kept in memory.

• The CREATE event extracts the payload from the CQN representation and stores it in memory.

CDS Query Notation (CQN) is the common language in CAP to run queries against services. It can be used to talk to the services defined by your model, but also remote services, such as the database.

The event handler uses the following APIs, which are available for service providers in CAP Java:

• Event handler classes have to implement the marker interface EventHandler and register themselves as Spring Beans (@Component). The marker interface is important, because it enables the CAP Java runtime to identify these classes among all Spring Beans.
• Event handler methods are registered with @On, @Before, or @After annotations. Every event, such as an entity creation, runs through these three phases. Each phase has a slightly different semantic. You’ll learn more about these semantics in the subsequent tutorial.
• The annotation @ServiceName specifies the default service name all event handler methods apply to. Here this is AdminService, as this was also the name when defining the service in the CDS model.
• Event handler methods get an event-specific event context parameter, which provides access to the input parameters of the event and the ability to set the result. For example, let’s look at the CdsCreateEventContext context parameter. The event we’re extending is the CREATE event. The type of the context variable is specific to this extended CREATE event. The onCreate method returns void, as the result is set by running: context.setResult(…).

1. Restart the application by running the following command in the terminal:

   cd ~/projects/products-service && mvn clean spring-boot:run
   

2. Choose Open in New Tab when prompted.

   

   A new Browser tab should be opened with your application.

   

Try to insert some data into the running application. For example, use cURL from a new terminal to run the following request.

1. You can open terminals by choosing Terminal > New Terminal from the main menu.

2. Execute the following command in the terminal to insert some data into the running application (while the process for the application is still running in the other terminal window):

   curl -X POST http://localhost:8080/odata/v4/AdminService/Products \
   -H "Content-Type: application/json" \
   -d '{"ID": 42, "title": "My Tutorial Product", "descr": "You are doing an awesome job!"}'
   

The POST request causes an OData Insert on the entity Products of the service AdminService. The properties of the record to be created are passed in the body of the request (-d argument in the curl command) as JSON (argument -H Content-Type: application/json in the curl command).

The response will be the created record and should look similar to this output:

`{"@context":"$metadata#Products/$entity","ID":42,"title":"My Tutorial Product","descr":"You are doing an awesome job!"}`.