JEST in Action

Description of a Simple JEST-enabled Web Application

A sample is available to demonstrate JEST usage. You will find the example in OpenJPA code repository under openjpa-examples/jest tree with its source code and deployment descriptors WEB-INF/web.xml and META-INF/persistence.xml. The example is a simple servlet demo.SimpleApp that instantiates a persistent unit named estdemo and populates a database with few sample records of two persistent entities demo.Actor and demo.Movie.

Persistence Unit Initialization

The following code from demo.SimpleApp shows the initialization process at the servlet's init() method.

Simple Servlet initializes a pooled Persistent Unit

public class SimpleApp extends HttpServlet {
    EntityManagerFactory _emf;
    private static String UNIT_NAME = "jestdemo";

    @Override 
    public void init(ServletConfig config) throws ServletException {
       super.init(config);
        config.getServletContext().log("Initializing persistence unit [" + UNIT_NAME + "]");
        try {
            Map<String,Object> props = new HashMap<String, Object>();
            props.put("openjpa.EntityManagerFactoryPool", "true");
            _emf = Persistence.createEntityManagerFactory(UNIT_NAME,props);
            new DataLoader().populate(_emf.createEntityManager());
        } catch (Exception e) {
            throw new ServletException(e);
        }
        config.getServletContext().log("Initialized with persistence unit [" + UNIT_NAME + "]");
    }

It is important to notice that the persistent unit is instantiated so that the EntityManagerFactory is pooled by OpenJPA. The way to achieve that is to set configuration property openjpa.EntityManagerFactoryPool to true. It is also important to note that this property is recognized only when passed via runtime and not if it were specified in META-INF/persistence.xml unlike other OpenJPA configuration properties.

Besides instantiating the persistent unit, the servlet does not do anything else other than serving a welcome index.html web page as its doGet() method shows:

Simple Servlet that only serves a single index.html page

/**
 * The only response by this application is an <code>index.html</code> file.
 */
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    resp.setContentType("text/html");
    OutputStream out = resp.getOutputStream();
    InputStream in = getClass().getResourceAsStream("index.html");
    for (int c = 0; (c = in.read()) != -1;) {
        out.write((char)c);
    }
}

Deployment Descriptor to enable JEST

The sample web application deploys demo.SimpleApp servlet and JEST servlet. The essential aspect of the deployment descriptor WEB-INF/web.xml is shown below

web.xml deployment descriptor (edited version)

<web-app version="2.4" 
      xmlns="http://java.sun.com/xml/ns/j2ee" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
      http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
    <display-name>Demo Application with JEST Servlet</display-name>  
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>
    <servlet>
        <servlet-name>demo</servlet-name>
        <servlet-class>demo.SimpleApp</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>demo</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>jest</servlet-name>   
        <servlet-class>org.apache.openjpa.persistence.jest.JESTServlet</servlet-class>
        <init-param>
            <param-name>persistence.unit</param-name>
            <param-value>jestdemo</param-value>
        </init-param>
    </servlet>
    <servlet-mapping>
        <servlet-name>jest</servlet-name>
        <url-pattern>/jest/*</url-pattern>
    </servlet-mapping>

</web-app>

Discovery of Persistent Unit

As can be seen in WEB-INF/web.xml above, the JEST servlet org.apache.openjpa.persistence.jest.JESTServlet needs to know the persistent unit name jestdemo to discover the actual EntityManagerFactory instantiated by its sibling demo.SimpleApp servlet. This discovery of an EntityManagerFactory instantiated by another component is possible because the said EntityManagerFactory is being pooled by OpenJPA. A more general discovery mechanics where JEST can discover a EntityManagerFactory instantiated and injected by the container would be available in future.

Building the sample application

An Ant build script build.xml is provided and a build.properties to adjust for local settings. To suit your local environment, please edit the build.properties for OpenJPA library and META-INF/persistence.xml for database specifics. The supplied build file compiles two persistent domain classes demo.Actor and demo.Movie, enhanced them and packages them in an web archive demo.war. The script assumes that the OpenJPA library and the JDBC driver (the sample persistence descriptor uses MySQL, by deafult) are available in shared library of the Servlet Container and hence does not package these libraries in the web archive demo.war.

Deploying the sample application

The next step is to deploy this simple web archive demo.war in a Servlet Container such as Tomcat or a a JEE container. We do not describe these steps because they are fairly standard.

JEST in action

Once the sample web application is deployed, say in Tomcat running on localhost:8080, open the web browser http://localhost:8080/demo and you should see the web page served by demo.SimpleApp. This step initializes demo.SimpleApp and hence the persistence unit jestdemo.

Now, if you go to URL http://localhost:8080/demo/jest/, the JEST welcome page will be displayed -- which is JavaScript enabled web page that demonstrates currently available JEST facilities such as finding or querying for instances.

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