First Java Cap Application

[bruce_wang3]

Setting Up Local Development

Install Java & Maven & Node.js

Install the cds-dk

npm i -g @sap/cds-dk

Install the VSCode & Plugin

VSCode Pluging : Java Extension Pack



VSCode Pluging : Language support for Java ™ for Visual Studio Code



VSCode Pluging: SAP Cloud Platform core data services plug-in for Visual Studio Code



VSCode Pluging: vscode-sqlite



VSCode Pluging: Debugger for Java

Verify the Setup

java --version

mvn -version

node -version

 

Create First Cap Application

Step1: Generate initial java project

Enter your java project workspace in terminal, and run below command to create initial java project

mvn -B archetype:generate \

-DarchetypeArtifactId=cds-services-archetype \

-DarchetypeGroupId=com.sap.cds \

-DarchetypeVersion=RELEASE \

-DgroupId=com.sap.cap \

-DartifactId=products-service \

-Dpackage=com.sap.cap.productsservice

After run success, the initial project will be generated in your folder. Regarding the case, the folder named products-service

Step2: Check project in VSCode

Drag the project folder into VSCode, the project structure looks like below picture

• The project is named products-service


• The db folder stores database-related artifacts


• The srv folder stores your Java application

 

Step3: Create first CDS service

1. In srv folder root, create a file, named as admin-service.cds


2. Add following service definition to the file.

It means define a service AdminService contains domain model Products

service AdminService {

    entity Products {

        key ID : Integer;

        title  : String(111);

        descr  : String(1111);

    }

}

Step4: Compile model definition

In VSCode, open a terminal window and run below command to compile project

mvn clean install

After running this command, some files are generated and added to the srv/src/main/resources/edmx folder. This is the default path, where CAP Java runtime looks for the model definitions.

Step5: Run application locally

The created project is based on spring boot, then you could run the application with below command in terminal.

After run success, the application will be started at default port 8080

mvn clean spring-boot:run

Step6: Inspect running result

Open the link in chrome: http://localhost:8080

Create java class for custom event handler

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

 

• Create the Java class file AdminService.java in the created handlers folder, with the following content

 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(…).

Restart the application

Before running the command, make sure the application already stopped, otherwise there is an error of port 8080 was used

mvn clean spring-boot: run

Test create product

open a new terminal to run curl like below

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!"}'

Read data from products entity page

Debug in VSCode

We also could start the application with vscode

1. stop the running application first with ctrl+c


2. select the run in vscode to create a vscode running configuration


3. You could click the run button to run application, then you could set break point in your java code

Change the project as a Reusable Service

Step1: Define the domain model

Previously, we defined a service which contains a cds entity. Normally when modeling with CDS the best practice is to separate services from the domain model

Therefore, we will define the complete domain model in the db folder

1. Go to your db folder and create a file named schema.cds


2. Add the following code to the file

    namespace sap.capire.products;



    using { Currency, cuid, managed, sap.common.CodeList } from '@sap/cds/common';



    entity Products : cuid, managed {

        title    : localized String(111);

        descr    : localized String(1111);

        stock    : Integer;

        price    : Decimal(9,2);

        currency : Currency;

        category : Association to Categories;

    }



    entity Categories : CodeList {

        key ID   : Integer;

        parent   : Association to Categories;

        children : Composition of many Categories on children.parent = $self;

    }

Step2: Understand keywords

The file means to create two domain model: Products, Categories with CDS Definition Language (CDL)

It also imports various common definitions from the @Sap/cds/common package (a globally available reuse package):

• Currency


• cuid


• managed


• CodeList

The localized Keyword

The localized keyword can be used to mark elements, which require translation. The ability to store translations for different languages and to store a default fallback translation is automatically handled by CDS for you. You will see this in action in more detail in the next tutorial.

Associations and Compositions

Associations and compositions can be used to define relationships between entities. They often allow you to define these relationships without explicitly working with foreign keys.

While associations define a rather loose coupling between the entities, compositions define a containment relationship. Compositions can also be thought of as defining deep structures. You can perform deep inserts and upserts along these structures.

In your domain model, the Categories entities define a parent and children element. This enables a hierarchy of categories. The children of a category are modelled as a composition. A category with all of its children defines a deep nested structure. Deleting a category would automatically delete all of its children. However, the parent of a category is modelled as an association. Deleting a category obviously shouldn’t delete its parent.

The cuid and managed Aspects

Both cuid and managed are aspects. Aspects extend an entity with additional elements. The cuid aspect adds a key element ID of type UUID to the entity.

The managed aspect adds four additional elements to the entity. These capture the time of the creation and last update of the entity, and the user, which performed the creation and last update.

The CodeList Aspect and the Currency Type

CodeLists can be used to store global, translatable definitions based on codes, such as currencies, countries, or languages. Especially for UIs, a CodeList can be useful to provide a value help for certain input fields.

The Currency definition is a type. It defines an association to a Currencies entity. The Currencies entity is based on ISO 4217 and uses three-letter alpha codes as keys such as EUR or USD and provides the possibility to store the corresponding currency symbol such as € or $.

Step3: Rewrite the AdminService

Previously we defined a simple service, called AdminService, which directly defined the entity Products. As you now have defined the Products entity in your domain model, the AdminService just needs to expose it. In addition, you defined the Categories entity, which should also be part of your service.

1. Go to srv folder  and open the admin-service.cds file


2. Replace the content with the following code

using { sap.capire.products as db } from '../db/schema';



service AdminService {

    entity Products   as projection on db.Products;

    entity Categories as projection on db.Categories;

}

Step4: Deploy domain model to sqlite

First, install SQLite to project, in VSCode open a terminal to run below command

npm install --save-dev sqlite3

To initialize the database with the defined domain model, execute the following command in the terminal

cds deploy --to sqlite

After run successfully, there is a sqlite.db file generated in your project folder

Step5: Connect to sqlite within VSCode

open VSCode with your keyborad F1, select SQLite: open database, next select your project sqlite db

check deployed content in sqlitedb

Step5: Configure CAP application to use SQLite database

Go to srv/src/main/resources, locate, and open the application.yaml file. This file was created when you initialized the application.

At the end of the file, add below content, replace /Users/{youruser}/Documents/workspace to your actual folder path

---

spring:

  profiles: sqlite

  datasource:

    url: "jdbc:sqlite:/Users/{youruser}/Documents/workspace/products-service/sqlite.db"

    driver-class-name: org.sqlite.JDBC

    initialization-mode: never

    hikari:

      maximum-pool-size: 1

Step6: Use CAP's generic persistence handling

The CAP Java SDK has a Persistence Service that provides out-of-the-box capabilities to store and retrieve entities from a database. Therefore, no custom coding is required for this. The entities defined in your AdminService will be automatically served via OData and you can just delete the AdminService.java file that was created earlier.

Delete the AdminService.java file in the handlers folder.

Step 7: Run and test your application

Run with memory database with below command

mvn spring-boot:run

Run with SQLite database with below command

mvn spring-boot:run -Dspring-boot.run.profiles=sqlite

In terminal, run below curl to insert categories

curl -X POST http://localhost:8080/odata/v4/AdminService/Categories \

-H "Content-Type: application/json" \

-d '{"ID": 1, "name": "TechEd", "descr": "TechEd related topics", "children": [{"ID": 10, "name": "CAP Java", "descr": "Run on Java"}, {"ID": 11, "name": "CAP Node.js", "descr": "Run on Node.js"}]}'

Check inserted data via browser with the URL:

http://localhost:8080/odata/v4/AdminService/Categories

Step 7: Set up for reuse

The reuse of models can be achieved by publishing NPM modules with the models and defining dependencies to these NPM modules in other applications. There are a two steps we need to perform to prepare the products-service application for reuse.

• The name of the products-service reuse module, should be @Sap/capire-products

• To make it easier to reuse the module, an index.cds file can be added to the products-service. This ensures a better decoupling from other applications.
  
  Create a new file index.cds in the ~/projects/products-service folder and place the following content inside this file

using from './db/schema';

using from './srv/admin-service';