5. サーブレットからデーターベースへ問い合わせ¶
In Java EE 6, Servlet can be easily defined using a POJO, with a single annotation, and optional web.xml in most of the common cases.
ノート
This section will create a Servlet and invoke the @NamedQuery to query the database.
1. Right-click on the project, select “New”, “Servlet...”. Enter the class name as “TestServlet”, package as org.glassfish.samples, and click on “Finish”.
ノート
Expand “Web Pages”, “WEB-INF” and notice that no web.xml is generated for describing this Servlet as all the information is captured in the @WebServlet annotation.
@WebServlet(name = "TestServlet", urlPatterns = {"/TestServlet"})
public class TestServlet extends HttpServlet {
2. Inject EntityManagerFactory in Servlet by adding the following code right after the class declaration:
@PersistenceUnit EntityManagerFactory emf;
Resolve the imports by clicking on the yellow bulb
3. Add the code to use the injected EntityManagerFactory to query the database using a pre-generated @NamedQuery to retrieve all the customers. The updated try block looks like:
try {
out.println("<html>");
out.println("<head>");
out.println("<title>Servlet TestServlet</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Servlet TestServlet at " +
request.getContextPath () + "</h1>");
List<Customer> result = (List<Customer>)emf.
createEntityManager().
createNamedQuery("Customer.findAll").
getResultList();
for (Customer c : result)
out.println(c + "<br>");
out.println("</body>");
out.println("</html>");
} finally {
Since the Servlets are re-entrant and EntityManager is not thread safe, it needs to be obtained from an EntityManagerFactory during each method invocation. This is resolved when the database specific code is moved to an EJB later. Optionally, based upon the NetBeans version, you may have to un-comment the code in the try block of the processRequest method by removing the first and the last line in the try block.
ノート
Notice that no transactions are started or committed before/after the database operation as this is a read-only operation and does not have the demands of a real-life application. An explicit transaction has to be created and committed (or rollbacked) if either create, update, or delete operations are performed. In addition, JPA2 also provides support for optimistic and pessimistic locking by means of specified locking modes.
4. Right-click in the editor pane, select “Run File”, choose the defaults, and click on OK. This displays “http://localhost:8080/JavaEE6SampleApp/TestServlet” page in the browser and looks like as shown.
