GAE Datastore using Native Low Level DataStore API (in Java)

// Get the Datastore Service
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();

// The Query interface assembles a query
Query q = new Query("Person");
q.addFilter("lastName", Query.FilterOperator.EQUAL, lastNameParam);
q.addFilter("height", Query.FilterOperator.LESS_THAN, maxHeightParam);

// PreparedQuery contains the methods for fetching query results
// from the datastore
PreparedQuery pq = datastore.prepare(q);

//cycle through results
for (Entity result : pq.asIterable()) {
  String firstName = (String) result.getProperty("firstName");
  String lastName = (String) result.getProperty("lastName");
  Long height = (Long) result.getProperty("height");
  System.out.println(firstName + " " + lastName + ", " + height + " inches tall");
}

<?xml version="1.0" encoding="utf-8"?>
<datastore-indexes
  autoGenerate="true">
    <datastore-index kind="Greeting" ancestor="true">
        <property name="date" direction="desc" />
    </datastore-index>
</datastore-indexes>

Example 1 -- Book database -- ONLY storage of book Entity

(remember you can also access with higher level API like JPA and JDO)         

package booklowlevel;
         import java.io.IOException;
         import java.io.PrintWriter;
         import java.text.SimpleDateFormat;
         import java.util.Calendar;
         import java.util.Date;
         import java.util.GregorianCalendar;
         import java.util.SimpleTimeZone;
         import javax.servlet.http.*;
         import com.google.appengine.api.datastore.DatastoreService;
         import com.google.appengine.api.datastore.DatastoreServiceFactory;
         import com.google.appengine.api.datastore.Entity;
         import com.google.appengine.api.datastore.KeyFactory;


         @SuppressWarnings("serial")
         public class GAE_booklowlevelServlet extends HttpServlet {
                public void doGet(HttpServletRequest req, HttpServletResponse resp)throws IOException {
                      resp.setContentType("text/html");
                      PrintWriter out = resp.getWriter();
                      SimpleDateFormat fmt = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss.SSSSSS");


                      //STEP 1:   Create Datastoreservice
                      DatastoreService ds = DatastoreServiceFactory.getDatastoreService();
                     

                      //STEP 2: Create Entity
                      Entity book = new Entity("Book");


                      //STEP 3: setup properties of Entity
                      book.setProperty("title", "The Grapes of Wrath");
                      book.setProperty("author", "John Steinbeck");
                      book.setProperty("copyrightYear", 1939);
                      Date authorBirthdate =    new GregorianCalendar(1902, Calendar.FEBRUARY, 27).getTime();
                      book.setProperty("authorBirthdate", authorBirthdate);


                      //STEP 4: put Entity in DataStore
                      ds.put(book);
                      out.println("<p>Added a Book entity to the datastore via the low-level API, key: " + KeyFactory.keyToString(book.getKey()) + "</p>");
                      fmt.setTimeZone(new SimpleTimeZone(0, ""));
                      out.println("<p>The time is: " + fmt.format(new Date()) + "</p>");
                }
         }

result of running above code (locally)

Example 2 --- Guestbook DataStore storage(user,date,content) AND retrieval

package guestbook;

import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.KeyFactory;
import com.google.appengine.api.users.User;
import com.google.appengine.api.users.UserService;
import com.google.appengine.api.users.UserServiceFactory;

import java.io.IOException;
import java.util.Date;
import java.util.logging.Logger;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class SignGuestbookServlet extends HttpServlet {
    private static final Logger log =
            Logger.getLogger(SignGuestbookServlet.class.getName());

    public void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws IOException {
        UserService userService = UserServiceFactory.getUserService();
        User user = userService.getCurrentUser();

        // We have one entity group per Guestbook with all Greetings residing
        // in the same entity group as the Guestbook to which they belong.
        // This lets us run an ancestor query to retrieve all Greetings for a
        // given Guestbook. However, the write rate to each Guestbook should be
        // limited to ~1/second.
        String guestbookName = req.getParameter("guestbookName");
        Key guestbookKey = KeyFactory.createKey("Guestbook", guestbookName);
        String content = req.getParameter("content");
        Date date = new Date();

        //STEP 2&3: create Entity and set properties
        Entity greeting = new Entity("Greeting", guestbookKey);
        greeting.setProperty("user", user);     //storing for Greeting Entity - user,date,content --user passed by authentication, 
        greeting.setProperty("date", date);     //date is generated above
        greeting.setProperty("content", content); //content is from request data


        //STEP 1: get DataStore
        DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();

        //STEP 4: put Entity in DataStore
        datastore.put(greeting);

        resp.sendRedirect("/guestbook.jsp?guestbookName=" + guestbookName);
    }
}

guestbook.jsp contains how to get stored data      ALSO contains form that will call servlet passing date and content info

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page import="java.util.List" %>
<%@ page import="com.google.appengine.api.users.User" %>
<%@ page import="com.google.appengine.api.users.UserService" %>
<%@ page import="com.google.appengine.api.users.UserServiceFactory" %>
<%@ page import="com.google.appengine.api.datastore.DatastoreServiceFactory" %>
<%@ page import="com.google.appengine.api.datastore.DatastoreService" %>
<%@ page import="com.google.appengine.api.datastore.Query" %>
<%@ page import="com.google.appengine.api.datastore.Entity" %>
<%@ page import="com.google.appengine.api.datastore.FetchOptions" %>
<%@ page import="com.google.appengine.api.datastore.Key" %>
<%@ page import="com.google.appengine.api.datastore.KeyFactory" %>

<html>
  <head>
    <link type="text/css" rel="stylesheet" href="/stylesheets/main.css" />
  </head>

  <body>

<%
    String guestbookName = request.getParameter("guestbookName");
    if (guestbookName == null) {
        guestbookName = "default";
    }
    UserService userService = UserServiceFactory.getUserService();
    User user = userService.getCurrentUser();
    if (user != null) {
%>
<p>Hello, <%= user.getNickname() %>! (You can
<a href="<%= userService.createLogoutURL(request.getRequestURI()) %>">sign out</a>.)</p>
<%
    } else {
%>
<p>Hello!
<a href="<%= userService.createLoginURL(request.getRequestURI()) %>">Sign in</a>
to include your name with greetings you post.</p>
<%
    }
%>

<%
    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    Key guestbookKey = KeyFactory.createKey("Guestbook", guestbookName);
    // Run an ancestor query to ensure we see the most up-to-date
    // view of the Greetings belonging to the selected Guestbook.
    Query query = new Query("Greeting", guestbookKey).addSort("date", Query.SortDirection.DESCENDING);
    List<Entity> greetings = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(5));
    if (greetings.isEmpty()) {
        %>
        <p>Guestbook '<%= guestbookName %>' has no messages.</p>
        <%
    } else {
        %>
        <p>Messages in Guestbook '<%= guestbookName %>'.</p>
        <%
        for (Entity greeting : greetings) {
            if (greeting.getProperty("user") == null) {
                %>
                <p>An anonymous person wrote:</p>
                <%
            } else {
                %>
                <p><b><%= ((User) greeting.getProperty("user")).getNickname() %></b> wrote:</p>
                <%
            }
            %>
            <blockquote><%= greeting.getProperty("content") %></blockquote>
            <%
        }
    }
%>

    <form action="/sign" method="post">
      <div><textarea name="content" rows="3" cols="60"></textarea></div>
      <div><input type="submit" value="Post Greeting" /></div>
      <input type="hidden" name="guestbookName" value="<%= guestbookName %>"/>
    </form>

  </body>
</html>