Skip to Content

Reuse a CAP Java Service

Create a new application and reuse the existing CAP Java service.
You will learn
  • How to reuse a CAP project through NPM packages
  • How to load sample data using CSV files
  • How to use the localized keyword
renejeglinskyRené JeglinskyJanuary 17, 2023
Created by
April 14, 2020

Now, that your products service is ready to be reused, you will build a bookstore application upon it.

In this tutorial you will create the model and the services of the bookstore application. After that you will initialize the database of your bookstore application with localized example data coming from CSV files.

You will then run your application – still without any custom coding required – and see the localization features of CAP in action.

  • Step 1

    From the products service that you created in the previous tutorial, we just want to reuse the domain and service models. For the bookstore, which you will develop in this tutorial, we need to create and initialize a bookstore project first.

    1. From the main menu of SAP Business Application Studio, choose Terminal New Terminal.

    2. Before adding the bookstore project, we need to make sure that you are in the projects folder. Both projects (products-service and bookstore) should be placed next to each other. Run the following command in the newly created terminal to go back to the projects folder:

      cd ~/projects
    3. Now that you are in the correct folder, run the following command:

      mvn -B archetype:generate -DarchetypeArtifactId=cds-services-archetype \
      -DarchetypeVersion=RELEASE \ -DartifactId=bookstore
    4. To open the bookstore project in a new workspace go to File Open Folder.

    5. Choose bookstore from the project list and then OK.

      open the bookstore application

      If you see a notification asking if you want to synchronize the Java classpath/configuration, choose Always.

      If you have any problem indication for any of the pom.xml files yet, don’t worry and ignore them for now.

  • Step 2

    As the product-service should be reused for the bookstore, you need to add a dependency between those two projects. Reusable models can be published as NPM modules and imported through dependencies in the package.json of a project.

    Make sure that you have followed all the sub-steps in the previous tutorial Set up for reuse before continuing.

    First, we need to simulate a release of the product-service module, and consume this release in the bookstore application.

    1. From the main menu of SAP Business Application Studio, choose Terminal New Terminal. Change into the bookstore directory by executing the following command in the terminal:

      cd ~/projects/bookstore

      This step is optional if you are already in the right directory.

    2. Install the reusable service project as npm dependency:

      npm install $(npm pack ../products-service -s)

      npm pack creates a tarball from the products-service, which is then directly used as a dependency in the bookstore application. More about npm pack:

      You will find a sap-capire-products-1.0.0.tgz in the root folder of the bookstore project, which is the tarball file of the products-service project.

    3. Install all other packages and simplify the overall dependency structure npm dedupe:

      npm install && npm dedupe

      If you open the package.json of your bookstore project, you will see a dependency to @sap/capire-products.

      dependency to tutorial products as tarball in package.json

    What technology is used to reuse services between different applications in CAP?

  • Step 3

    Now that you have created your bookstore project, you need to define the domain model.

    1. Within the ~/projects/bookstore/db folder, create a file called schema.cds.

    2. Add the following code to your newly created schema.cds file and make sure you Save the file:

      namespace sap.capire.bookstore;
      using { Currency, cuid, managed }      from '@sap/cds/common';
      using { sap.capire.products.Products } from '@sap/capire-products';
      entity Books as projection on Products; extend Products with {
          // Note: we map Books to Products to allow reusing AdminService as is
          author : Association to Authors;
      entity Authors : cuid {
          firstname : String(111);
          lastname  : String(111);
          books     : Association to many Books on = $self;
      @Capabilities.Updatable: false
      entity Orders : cuid, managed {
          items    : Composition of many OrderItems on items.parent = $self;
          total    : Decimal(9,2) @readonly;
          currency : Currency;
      @Capabilities.Updatable: false
      entity OrderItems : cuid {
          parent    : Association to Orders not null;
          book_ID   : UUID;
          amount    : Integer;
          netAmount : Decimal(9,2) @readonly;

    The domain model defines four entities:

    • Books

    • Authors

    • Orders

    • OrderItems

    Again the Currency, cuid and managed types and aspects are imported, as described in the previous tutorial.

    In addition, it imports the Products entity, which is reused for the Books entity. To establish the relation between books and authors, the Products entity is extended with an additional association to Authors.

    The total element of the Orders entity and the netAmount element of the OrderItems entity are annotated with @readonly. This means the value of these elements cannot be set by a client. The value is calculated by custom code. You will implement this custom code in a later tutorial. Both of these entities are also annotated with @Capabilities.Updatable: false, which means that they cannot be updated, only created and deleted.

  • Step 4

    You will now define the services, that should expose the entities you have defined in your domain model:

    1. Within the ~/projects/bookstore/srv folder, create a file called services.cds.

    2. Add the following code to the services.cds file and make sure you Save the file:

      using { sap.capire.bookstore as db } from '../db/schema';
      // Define Books Service
      service BooksService {
          @readonly entity Books as projection   on db.Books { *, category as genre } excluding { category, createdBy, createdAt, modifiedBy, modifiedAt };
          @readonly entity Authors as projection on db.Authors;
      // Define Orders Service
      service OrdersService {
          entity Orders as projection on db.Orders;
          entity OrderItems as projection on db.OrderItems;
      // Reuse Admin Service
      using { AdminService } from '@sap/capire-products';
      extend service AdminService with {
          entity Authors as projection on db.Authors;

    The services.cds file defines three services:

    • BooksService

    • OrdersService

    • AdminService

    The BooksService is used to provide a read-only view on the Books and Authors data. Modifications of these entities isn’t possible via this service.

    The OrdersService allows to view, create, and delete orders.

    The AdminService is reused from the products service. But we’ve added the Authors entity to it. It can be used to create, update, and delete products and authors.

    It’s considered best practice to define services with a single use-case in mind. For example, the AdminService is meant for administrating Products, Authors and Categories, while the BooksService is meant for exposing a catalog of books and authors, hiding administrative data such as creation and modification times from the end user.

  • Step 5

    After defining the domain model and the services that you want to expose, you could already start your application. But first, we’d like to add some sample data to your database. To do so, we’ll need to use some CSV files:

    1. From your bookstore project, right-click the db folder and choose New Folder. Call the folder data.

    2. Go to the data folder by running the following command in the terminal:

      cd ~/projects/bookstore/db/data
    3. Download CSV data for the Authors entity by running the following command in the terminal:

      curl -O
    4. Download CSV data for the Books entity by running the following command in the terminal:

      curl -O
    5. Rename it with the following command:

      mv sap.capire.bookstore-Books.csv sap.capire.products-Products.csv
    6. Download translated CSV data for the Books entity by running the following command in the terminal:

      curl -O
    7. Rename it with the following command:

      mv sap.capire.bookstore-Books_texts.csv sap.capire.products-Products_texts.csv
    8. Download CSV data for the Categories entity by running the following command in the terminal:

      curl -O

      You should now have 4 CSV files with sample data. The files deliver initial data for the service that we reuse and for one entity that we created in the bookstore service:

      • sap.capire.products-Categories.csv
      • sap.capire.products-Products.csv
      • sap.capire.products-Products_texts.csv
      • sap.capire.bookstore-Authors.csv

      The name of the CSV has to match the pattern [namespace]-[entity name] exactly otherwise the the application will fail to start.

  • Step 6
    1. In the terminal, go to the root of the bookstore project:

      cd ~/projects/bookstore
    2. Ensure that you have stopped all previously running applications (including the products-service application) by using CTRL+C.

      In case you forgot to stop the products-service application and the tab in which it was started is no longer opened you can run killall mvn in the terminal to ensure it’s terminated.

    3. Start the application by running:

      mvn spring-boot:run
    4. You will see a pop-up in the SAP Business Application Studio. Choose Open in a New Tab.

    5. When you open the URL in a new tab, you will see a welcome page. To see the books data click on Books directly from the welcome page.

      Instead of clicking on the Books entry on the welcome page, you could simple add odata/v4/BooksService/Books to the URL.

      data for books entity
    6. To read the localized German example data, append the query parameter ?sap-language=de to the URL. For example, <APP_URL>/odata/v4/BooksService/Books?sap-language=de. Try to switch the language between German (de) and English (en).

    Great Job! You have successfully developed the bookstore application and reused your products service from the previous tutorial.

    In the next tutorial you will extend this application with custom code, to calculate the total and netAmount elements of the Orders and OrderItems entity. In addition, you will add custom code that will decrease the stock in the Books entity whenever an order is created.

Back to top