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Exercise_8_Part1_ConfigurePersistence.md

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Exercise 8 (Part 1): Configure Persistence

Learning Goal

After this exercise you will know how to configure the persistence layer of a Spring application using Spring Data JPA. EclipseLink is used as the JPA implementation. While we want to connect to a PostgreSQL database in the cloud when running the application in production mode, the same method of configuring the database connection should be used when testing the persistence layer using a local database within your VM.

The task of this exercise is to setup the persistence layer in such a way that it uses the PostgreSQL database connection information from the VCAP_SERVICES environment variables. To access our database, we make use of a convenience CRUD repository interface. This repository uses the EntityManager provided by EclipseLink.

Used dependencies:

In this exercise we provide the CloudDatabaseConfig class which sets up a CRUD repository based on the information provided in the VCAP_SERVICES environment variable. For this exercise it suffices to regard this class as a "black box".

Prerequisite

Continue with your solution of the last exercise. If this does not work, you can checkout the branch origin/solution-6-Deploy-Ads-On-CF.

Step 1: Add Maven dependencies

Add the following dependencies to your pom.xml using the XML view of Eclipse at the end of the list:

  • Add spring cloud connector dependencies:
<!-- Spring Cloud Connector -->
<dependency>
	<groupId>org.springframework.cloud</groupId>
	<artifactId>spring-cloud-spring-service-connector</artifactId>
	<version>1.2.1.RELEASE</version>
	<exclusions>
		<exclusion>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
		</exclusion>
	</exclusions>
</dependency>
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-cloudfoundry-connector</artifactId>
    <version>1.2.2.RELEASE</version>
    <scope>runtime</scope>
</dependency>
  • Add spring-data-jpa dependency:
<dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-jpa</artifactId>
    <version>1.9.2.RELEASE</version>
    <exclusions>
        <exclusion>
            <!-- We need spring-core 4.2 or later, but spring-data includes 4.1.9 -->
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
        </exclusion>
    </exclusions>
</dependency>
  • Add EclipseLink (JPA implementation) dependencies:
<!-- EclipseLink as JPA implementation -->
<dependency>
    <groupId>org.eclipse.persistence</groupId>
    <artifactId>org.eclipse.persistence.jpa</artifactId>
    <version>2.6.2</version>
</dependency>
  • Add postgresql dependency:
<!-- PostgreSQL database implementations -->
<dependency>
    <groupId>org.postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>9.4-1206-jdbc41</version>
    <scope>runtime</scope>
</dependency>
  • Add commons-dbcp2 dependency:
<!-- connection pooling -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-dbcp2</artifactId>
    <version>2.1.1</version>
    <scope>runtime</scope>
</dependency>

Note: After you've changed the Maven settings, don't forget to update your Eclipse project! To do so right click on your Eclipse project and select Maven - Update Project ... (ALT-F5)

Step 2: Define a simple entity

In order to use the existing Advertisement class as a JPA entity, you need to

  • annotate the Advertisement class with @Entity and
  • specify an identifier by adding a field id of type Long (Note primitive type long does not work!), which is annotated with @Id and @GeneratedValue(strategy = GenerationType.AUTO).
  • if you decide to add a setter for the id field, make sure that its argument also is of type Long not long

Use the annotations in the package javax.persistence (corresponding to the JPA API).

Step 3: Define a Repository Using Spring Data JPA

  • Create an interface AdvertisementRepository in the com.sap.bulletinboard.ads.models package that extends the CrudRepository interface. Use Advertisement as the entity type and Long as the id type.
public interface AdvertisementRepository extends CrudRepository<Advertisement, Long> {

}
  • Have a look at the methods provided by the CrudRepository interface. By extending CrudRepository, AdvertisementRepository inherits several methods for saving, deleting, and finding entities.
  • Note: Internally Spring uses reflection to generate a proxy class, which contains the desired implementation details of the specified interface.

Step 4: Configure Repository

  • Create a class EntityManagerFactoryProvider in the package com.sap.bulletinboard.ads.util and copy the code from here.
  • Create a class CloudDatabaseConfig in the package com.sap.bulletinboard.ads.config and copy the code from here.

Some explanations to the code:

Based on the VCAP_SERVICES environment variable the spring-cloud connector instantiates a DataSource instance. Note: Locally this environment variable is set so that a local PostgreSQL database named test is used.

In order to persist the data using JPA, we provide (EclipseLink) implementations of EntityManager and TransactionManager using a logical database connection, which is represented by the Data Source.

Finally the CRUD repository, which uses the EntityManager and TransactionManager, is instantiated by using the @EnableJpaRepositories annotation.

Step 5: Build and Run Microservice Locally

  • In Eclipse, in the "Servers" view, open the Tomcat server. In the opened detail view (Overview tab), click on "Open launch configuration". Change to the "Environment" tab, and inspect the VCAP_SERVICES environment variable. You should see that this allows your application to connect to a local database.
  • Build and run the microservice.
  • Ensure that the advertisement table is created with columns id and title. You can use the DBeaver Eclipse Perspective, where the table should be visible within the Database Navigator view under Schemas - public - Tables.
  • Also note that a sequence table is created. This table is used by EclipseLink to determine which (unused) ID should be used for a created entity.

Notes:

  • When running the service in the terminal ensure that you've executed localEnvironmentSetup.sh as explained here.
  • If you want to run the tomcat7:run Maven goal in Eclipse, you need to configure the environment variables (in the corresponding run configuration) starting with this exercise. Please have a look at the localEnvironmentSetup.sh file for this. Alternatively, use the Tomcat server in Eclipse (without Maven), or run Maven in the console.
  • Note, that the data is still read from / written into the HashMap. We will change that in the second part of the exercise.

[Optional] Step 6: Specify the Table / Field Names Using JPA Annotations

  • Annotate the Advertisement class with @Table(name = "advertisements") to specify the name of the database table.
  • Annotate the title field with @Column(name = "mytitle") to specify the name of the column. Note that names starting with an underscore might be reserved, and actually cause problems in HSQLDB.
  • Build and run the microservice.
  • Ensure that the advertisements table is created with columns id and mytitle. You can use the DBeaver Eclipse Perspective, where the table should be visible within the Database Navigator view under Schemas - public - Tables. You probably need to do a refresh to visualize them. If the new table is still not shown, you need to stop, clean... and restart your Tomcat server.
  • Take a look at the annotations offered by JPA API.

Used Frameworks and Tools

Further Reading

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