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Reuse a CAP Java Service

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Reuse a CAP Java Service

Reuse a CAP Java Service

October 14, 2021
Created by
April 14, 2020
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 profiles
  • How to use SQL Tools in the SAP Business Application Studio
  • How to use the localized keyword
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Now, that your products service is ready to be reused, you’ll build a bookstore application upon it.

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

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

Step 1: Create bookstore project skeleton

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’ll 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=1.17.0 \ -DartifactId=bookstore
  4. To open the bookstore project in a new workspace go to File > Open Workspace

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

    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.

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Step 2: Install reusable service as npm dependency

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 of step 9 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. 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’ll 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’ll see a dependency to @sap/capire-products.

    dependency to tutorial products as tarball in package.json
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Step 3: Define bookstore domain model

Now that you’ve 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’ll 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.

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Step 4: Define bookstore service

You’ll now define the services, that should expose the entities you’ve 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.

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Step 5: Load sample data using CSV files

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 SQLite 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. Download translated CSV data for the Books entity by running the following command in the terminal:

    curl -O
  6. 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. Have a look at the content of these files.

    4 new downloaded files

    The name of the CSV has to match the pattern [namespace]-[entity name] exactly otherwise the file will be ignored.

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Step 6: Deploy bookstore domain model

Let’s deploy the bookstore domain model again to a SQLite database.

  1. First of all install the SQLite node packages to the project. Therefore, open the terminal and go to the root of your project:

    cd ~/projects/bookstore

  2. To install sqlite3 node packages, run:

    npm install --save-dev sqlite3
  3. To initialize the bookstore database with the defined domain model and sample data, run:

    cds deploy --to sqlite

This will create a file called sqlite.db in your project root. The name of this database, is defined by an entry in your package.json.

Why do you need the SQLite Node.js packages?
Step 7: Configure database usage

To configure your Java application to use the sqlite.db database:

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

  2. For the field url replace the string "jdbc:sqlite::memory:?cache=shared" with a reference to your local database "jdbc:sqlite:/home/user/projects/bookstore/sqlite.db"

  3. Set the value of initialization-mode from always to never.

    You can update this value to never, because you have already initialized the database when running cds deploy --to sqlite.

    state of application yaml
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Step 8: Use SQL tools in SAP Business Application Studio

To have a look at the content of the database, you are going to use the SQL Tools as part of the SAP Business Application Studio.

  1. From the main menu of SAP Application Studio, go to File > Settings > Open Preferences

    open preference settings
  2. Add the following content to the JSON structure in the User file and make you sure you Save the file:

    IMPORTANT: Don’t forget to add a comma at the end of the existing line.

         "sqltools.connections": [
                 "name": "sqlite",
                 "dialect": "SQLite",
                 "database": "/home/user/projects/bookstore/sqlite.db"

    Your User file should looks like this:
    content of user preferences file

  3. Open the SQLTools pane from the side menu:

    content of user preferences file
  4. You’ll see a new connection called sqlite. Choose the power cable icon to connect.

  5. Most likely you’ll prompted to install SQLite3 Dependencies. Click on Install now.

  6. Click on Connect to SQLite.

    connect after installation
  7. Click on the triangle next to the database on the left-hand side of the connection name sqlite to expand the view. Do the same for tables and open, for example, the Products data by clicking on the magnifying glass as shown below:

    display products

    Sometimes the first table selected doesn’t load properly. If that is the case select a different table. You can then switch back to see the Products data:

    products table
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Step 9: Run and test the bookstore application
  1. In the terminal, go to the root of the bookstore project:

    cd ~/projects/bookstore

  2. Ensure that you’ve 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’ll 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’ll 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).

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

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

Next Steps

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