Lean example of Tomcat 8 + Guice 4 + EclipseLink 2.6.0

I wrote a Lean example of Tomcat 8 + Guice 4 in previous entry. this time, I’ll try JPA(EclipseLink) integration with automatic transaction management.

The entire project which based on Maven can be obtained from My GitHub repository.

Prerequisites

For database connection, we’ll use a DataSource which is defined on Tomcat server. to define a Embedded Derby DataSource, This entry would help.

Dependencies

<dependencies>
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>3.1.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>org.eclipse.persistence</groupId>
        <artifactId>eclipselink</artifactId>
        <version>2.6.0</version>
    </dependency>
    <dependency>
        <groupId>com.google.inject</groupId>
        <artifactId>guice</artifactId>
        <version>4.0</version>
    </dependency>
    <dependency>
        <groupId>com.google.inject.extensions</groupId>
        <artifactId>guice-servlet</artifactId>
        <version>4.0</version>
    </dependency>
    <dependency>
        <groupId>com.google.inject.extensions</groupId>
        <artifactId>guice-persist</artifactId>
        <version>4.0</version>
    </dependency>
</dependencies>

web.xml

No changes against previous entry except addition of resource-ref element.

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">

    <!-- taken from https://github.com/google/guice/wiki/Servlets -->

    <listener>
        <listener-class>guice.tomcat.MyGuiceServletConfig</listener-class>
    </listener>

    <filter>
        <filter-name>guiceFilter</filter-name>
        <filter-class>com.google.inject.servlet.GuiceFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>guiceFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <resource-ref>
        <res-ref-name>jdbc/derby</res-ref-name>
        <res-type>javax.sql.DataSource</res-type>
        <res-auth>Container</res-auth>
    </resource-ref>

</web-app>

META-INF/persistence.xml

If you don’t want to use a DataSource on Tomcat, specify the connection information in javax.persistence.jdbc.* properties instead of non-jta-data-source.

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
    <persistence-unit name="myJpaUnit" transaction-type="RESOURCE_LOCAL">
        <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
        <non-jta-data-source>java:comp/env/jdbc/derby</non-jta-data-source>
        <class>guice.tomcat.MyEntity</class>
        <properties>
            <property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
            <property name="eclipselink.logging.level" value="FINE"/>
            <property name="eclipselink.logging.level.sql" value="FINE"/>
        </properties>
    </persistence-unit>
</persistence>

MyGuiceServletConfig.java

The last two statements will take care of JPA integration.

public class MyGuiceServletConfig extends GuiceServletContextListener {
    @Override
    protected Injector getInjector() {
        return Guice.createInjector(new ServletModule() {
            @Override
            protected void configureServlets() {
                serve("/*").with(MyServlet.class);
                bind(MyService.class).to(MyServiceImpl.class);

                install(new JpaPersistModule("myJpaUnit"));
                filter("/*").through(PersistFilter.class);
            }
        });
    }
}

MyEntity.java

This is a very simple JPA entity which keeps only generated id and ts (a timestamp).

@Entity
@NamedQuery(name = "MyEntity.findAll", query = "SELECT e FROM MyEntity e ORDER BY e.id DESC")
public class MyEntity implements Serializable {

    @Id
    @GeneratedValue
    private Long id;
    @Column(nullable = false)
    @Temporal(TemporalType.TIMESTAMP)
    private Date ts;

... accessors are omitted

Service class

MyService.java

public interface MyService {
    void save(MyEntity e);
    List<MyEntity> findAll();
}

MyServiceImpl.java

Note that you need to annotate a method with @com.google.inject.persist.Transactional if you need a transaction on it.

public class MyServiceImpl implements MyService {

    // Transactions doesn't start if EntityManager is directly injected via @Inject.
    // I have no idea why...

    // According to https://github.com/google/guice/wiki/JPA,
    // "Note that if you make MyService a @Singleton, then you should inject Provider<EntityManager> instead."
    @Inject
    private Provider<EntityManager> emp;

    @Override
    // @javax.transaction.Transactional is not supported yet. https://github.com/google/guice/issues/797
    @com.google.inject.persist.Transactional
    public void save(MyEntity e) {
        EntityManager em = emp.get();
        em.persist(e);
    }

    @Override
    public List<MyEntity> findAll() {
        return emp.get().createNamedQuery("MyEntity.findAll", MyEntity.class).getResultList();
    }
}

MyServlet.java

This servlet saves a MyEntity with current timestamp, then fetches all of rows and returns them to the client on every request.

@javax.inject.Singleton
public class MyServlet extends HttpServlet {

    @javax.inject.Inject
    private MyService myService;

    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        MyEntity myEntity = new MyEntity();
        myEntity.setTs(new Date());
        myService.save(myEntity);

        PrintWriter writer = resp.getWriter();
        writer.write("<html><body><ul>");
        for(MyEntity e : myService.findAll()){
            writer.write("<li>");
            writer.write(e.toString());
            writer.write("</li>");
        }
        writer.write("</ul></body></html>");

    }
}

Test run

A row will be saved on every request as follows:

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