Installing the java sdk

Installing the Java SDK
You develop and upload Java applications for Google App Engine using the App Engine Java software development
kit (SDK).
The SDK includes software for a web server that you can run on your own computer to test yourJava applications.
The server simulates all of the App Engine services, including a local version of the datastore,Google Accounts,and
the ability to fetch URLs and send email from yourcomputer using the App Engine APIs.
Getting Java
When App Engine runs your Java application, it uses the Java 7 virtual machine (JVM) and standard libraries. App
Engine also supports Java 6. Ideally, you should use Java 7 for compiling and testing your application to ensure that
the local server behaves similarly to App Engine. For developers that don't have easy access to Java 7, the App
Engine SDK is compatible with Java 6. You can upload compiled classes and JARs made with Java 6 to App
Engine.
If necessary,download and install the Java SE Development Kit (JDK) for your platform. Mac users,see Apple's
Java developer site to download and install the latest version of the Java Developer Kit available for Mac OS X.
Once the JDK is installed, run the following commands from a command prompt (for Windows, Command Prompt;
for Mac OS X, Terminal) to verify that you can run the commands, and to determine which version is installed. If
you have Java 7 installed, these commands will report a version number similar to 1.7.0. If you have Java 5
installed, the version number will be similar to 1.6.0.
java -version
javac -version
Using Eclipse and the Google Plugin for Eclipse
If you are using the Eclipse development environment, the easiest way to develop, test and upload App Engine apps
is to use the Google Plugin for Eclipse. The plugin includes everything you need to build, test and deploy your app,
entirely within Eclipse.
The plugin is available for Eclipse versions 3.3, 3.4, 3.5, 3.6, and 3.7. You can install the plugin using the Software
Update feature of Eclipse. The installation locations are as follows:
 The Google Plugin for Eclipse, for Eclipse 3.3 (Europa):

https://dl.google.com/eclipse/plugin/3.3
 The Google Plugin for Eclipse, for Eclipse 3.4 (Ganymede):
 The Google Plugin for Eclipse, for Eclipse 3.5 (Galileo):
 The Google Plugin for Eclipse, for Eclipse 3.6 (Helios):
 The Google Plugin for Eclipse, for Eclipse 3.7 (Indigo):
 The Google Plugin for Eclipse, for Eclipse 3.8/4.2 (Juno):
For details on how to use Software Update to install the plugin, and how to create a new project, see Using the
Google Eclipse Plugin.
Using Apache Ant
If you want to compile and run yourapplication using the command line, or an IDE other than Eclipse, you will
need to install something to manage this process. Apache Antis one such solution.Full directions on installing and
setting up Apache Ant to work with App Engine can be found at Using Apache Ant.
Getting the SDK
If you are using Eclipse and the Google Plugin, you can install the App Engine SDK from Eclipse using Software
Update. If you haven't already, install the "Google App Engine Java SDK" component using the locations above.

If you are not using Eclipse or the Google Plugin, you can download the App Engine Java SDK as a Zip archive.
Download the App Engine Java SDK. Unpack the archive in a convenient location on your hard drive.
Note: Unpacking the archive creates a directory whose name is something like appengine-java-sdk-X.X.X,
where X.X.X is the SDK version number. Throughout this documentation, this directory will be referred to
as appengine-java-sdk/. You may want to rename the directory after unpacking.
Trying a Demo Application
The App Engine Java SDK includes several demo applications in the demos/ directory. The final version of the
guest book application you will create in this tutorial is included under the directory guestbook/. This demo has
been precompiled for you so you can try it right away.
If you are using Eclipse, the SDK is located in your Eclipse installation directory,
under plugins/com.google.appengine.eclipse.sdkbundle_VERSION/, whereVERSION is a
version identifier for the SDK. From the command line, change the current working directory to this directory to run
the following command. If you're using Mac OS X or Linux, you may need to give the command files executable
permissions before you can run them (such as with the command chmod u+x dev_appserver.sh).
If you are using Windows, start the guest book demo in the development server by running the following command
at a command prompt:
appengine-java-sdkbindev_appserver.cmd appengine-java-
sdkdemosguestbookwar
If you are using Mac OS X or Linux, run the following command:
./appengine-java-sdk/bin/dev_appserver.sh appengine-java-
sdk/demos/guestbook/war
The development server starts,and listens for requests on port 8080. Visit the following URL in your browser:
 http://localhost:8080/
Note: When you start the development server from within Eclipse using the Google Plugin for Eclipse (discussed
later), the server uses the port 8888 by default:http://localhost:8888/
For more information about running the development web server from the command line, including how to change
which port it uses,see the Dev Web Server reference.
To stop the server, make sure the command prompt window is active, then press Control-C.
Next...

The development environment lets you develop and test complete App Engine applications on your computer. Let's
start with a simple project.
Continue to Creating a Project.
Using the Google Plugin for Eclipse
It's easy to use the Eclipse development environment to develop your Java App Engine application, just as you can
to develop any other servlet-based web application. With the Google Plugin for Eclipse, it's even easier. The plugin
lets you create, test and upload App Engine applications from within Eclipse.
The Google Plugin for Eclipse also makes it easy to develop applications using Google Web Toolkit (GWT), to run
on App Engine or in any other environment.
This article describes how to install the Google Plugin for Eclipse, create a new App Engine project, and debug it
using the development server running within Eclipse. The article also describes how to use the plugin to upload your
project to App Engine.
For more information about the plugin, including how to use it for Google Web Toolkit projects,see the Google
Plugin for Eclipse documentation.
 Getting Eclipse
 Installing the Google Plugin for Eclipse
 Creating a Project
 Running the Project
 Uploading to Google App Engine
 Running the Command Line Tools
Getting Eclipse
The Google Plugin for Eclipse is available for Eclipse 3.3 (Europa), Eclipse 3.4 (Ganymede), Eclipse 3.5 (Galileo),
Eclipse 3.6 (Helios), Eclipse 3.7 (Indigo), and Eclipse 4.2 (Juno). The "Eclipse IDE for Java EE Developers"
includes all of the components you will need for web application development.
In addition to the Google Plugin for Eclipse, we recommend the Web Tools Platform (WTP) plugins for web
development. See the Web Tools Platform website. Among other things,WTP provides editing modes for JSP and
HTML files.
Installing the Google Plugin for Eclipse
You can install the Google Plugin for Eclipse using the Software Update feature of Eclipse.

To install the plugin, using Eclipse 4.2 (Juno):
1. Select the Help menu > Install New Software....
2. In the Work with text box, enter:
Click the Add... button.In the dialog that shows,click OK (keep the name blank, it will be retrieved from the
update site.)
3. Click the plus icon next to "Google Plugin for Eclipse" and "SDKs". Check the boxes next to "Google Plugin
for Eclipse 4.2" and "Google App Engine Java SDK". You can also select the "Google Web Toolkit SDK" if
you'd like to use Google Web Toolkit with your apps.Make sure you check the checkbox labeled "Contact all
update sites during install to find required software." Click the Next button.Follow the prompts to accept the
terms of service and install the plugin.
4. When the installation is complete, Eclipse prompts you to restart. Click Yes. Eclipse restarts.The plugin is
installed.
To install the plugin, using Eclipse 3.7 (Indigo):
update site.)
3. Click the plus icon next to "Google Plugin for Eclipse" and "SDKs". Check the boxes next to "Google Plugin
for Eclipse 3.7" and "Google App Engine Java SDK". You can also select the "Google Web Toolkit SDK" if
you'd like to use Google Web Toolkit with your apps.Make sure you check the checkbox labeled "Contact all
update sites during install to find required software." Click the Next button.Follow the prompts to accept the
terms of service and install the plugin.
installed.
To install the plugin, using Eclipse 3.6 (Helios):

update site.)
3. Click the triangle next to "Plugin" and "SDKs". Check the boxes next to "Google Plugin for Eclipse 3.6" and
"Google App Engine Java SDK". You can also select the "Google Web Toolkit SDK" if you'd like to
use Google Web Toolkit with your apps.Make sure you check the checkbox labeled "Contact all update sites
during install to find required software." Click the Next button.Follow the prompts to accept the terms of
service and install the plugin.
installed.
The process for installing the plugin for Eclipse 3.5 (Galileo) is almost exactly the same, with only a different install
location. The location for the Google Plugin for Eclipse 3.5 is as follows:
 https://dl.google.com/eclipse/plugin/3.5
To install the plugin, using Eclipse 3.4 (Ganymede):
1. Select the Help menu > Software Updates... The "Software Updates and Add-ons" window opens.
2. Select the Available Software tab. Click the Add Site... button.The "Add Site" window opens.For "Location,"
enter the install location for the Eclipse 3.4 version of the plugin:
Click OK. The dialog closes, and the new location is added to the list of available software.
3. Click the triangle next to the new install location, then click the triangle next to "Google" to display installation
options.Check the boxes next to "Google Plugin for Eclipse 3.4" and "Google App Engine Java SDK". You can
also select the "Google Web Toolkit SDK" if you'd like to use Google Web Toolkit with yourapps. Click
the Install... button.Follow the prompts to accept the terms of service and install the plugin.
Note: The plugin install process may take several minutes to complete as Eclipse updates dependencies.See the
FAQ for more information.
installed.
The process for installing the plugin for Eclipse 3.3 (Europa) is similar, but with different prompts and a different
install location. The location for the Google Plugin for Eclipse 3.3 is as follows:
 https://dl.google.com/eclipse/plugin/3.3
See the Eclipse documentation for more information about Software Update.

Creating a Project
To create a new App Engine project:
1. Select the File menu > New > Web Application Project (If you do not see this menu option,select
the Window menu > Reset Perspective..., click OK, then try the File menu again.) Alternatively, click the
Google button in the Eclipse toolbar and select "New Web Application Project."
2. The "Create a Web Application Project" wizard opens.For "Project name," enter a name for yourproject, such
as Guestbook for the project described in theGetting Started Guide. For "Package," enter an appropriate
package name, such as guestbook.
3. If you're not using Google Web Toolkit, uncheck "Use Google Web Toolkit." Verify that "Use Google App
Engine" is checked.
4. If you installed the App Engine SDK using Software Update, the plugin is already configured to use the SDKs
that were installed. If you would like to use a separate installation of the App Engine SDK, click Configure
SDKs..., and follow the prompts to add a configuration with yourSDK's appengine-java-sdk/ directory.
5. Click Finish to create the project.
The wizard creates a directory structure for the project, including a src/ directory for Java source files, and
a war/ directory for compiled classes and other files for the application, libraries, configuration files, static files
such as images and CSS, and other data files. The wizard also creates a servlet source file and two configuration
files. The complete directory structure looks like this:
Guestbook/
src/
guestbook/
server/
GuestbookServlet.java
META-INF/
jdoconfig.xml
log4j.properties
logging.properties

war/
WEB-INF/
lib/
...App Engine JARs...
appengine-web.xml
web.xml
index.html
The war/ directory uses the WAR standard layout for bundling web applications. (WAR archive files are not yet
supported by the SDK.) The Eclipse plugin uses this directory for running the development server, and for deploying
the app to App Engine.
When Eclipse builds your project, it creates a directory named classes/ in war/WEB-INF/, and puts compiled
class files here. Eclipse also copies non-source files found in src/ to war/WEB-INF/classes/,
including META-INF/ and the log4j.properties and logging.properties files. The final contents of
the war/ directory make up your application for testing and deployment.
For details about the new project that the plugin creates, see the Getting Started Guide.
Running the Project
The App Engine SDK includes a web server for testing your application in a simulated environment. The Google
Plugin for Eclipse adds new items to the Run menu for starting this server.
To run your application in the web serverinside the Eclipse debugger,select the Run menu, Debug As > Web
Application. Eclipse builds the project, switches to the Debug perspective,and the server starts.If the server starts
successfully,the serverdisplays several messages,including a message similar to the following, in the Console:
The server is running at http://localhost:8888/
If you want to customize how the server is started,you can create a new Run/Debug configuration of the type "Web
Application."
To test the new application that the plugin created, start the server as above,then visit the following URL in your
browser (using a URL path appropriate to your application):

 http://localhost:8888/guestbook
With Eclipse, you can leave the server running in the debuggerwhile you make changes to source code, JSPs, static
files and appengine-web.xml. When you save changes to source code, Eclipse compiles the class
automatically, then attempts to insert it into the running web server dynamically. In most cases,you can simply
reload the page in yourbrowser to test the new version of the code. Changes to JSPs, static files and appengine-
web.xml are recognized by the development server automatically, and also take effect without restarting the
server. If you change web.xml or otherconfiguration files, you must stop and start the server for the changes to
take effect.
To stop the server, make sure the Debug panel is selected, then click the Terminate button:
Uploading to Google App Engine
Before you upload yourapp for the first time, you must register an application ID with App Engine using the Admin
Console. Register an application ID, then edit theappengine-web.xml file and change
the <application>...</application> element to contain the new ID.
To begin deployment of your application, click the Google button in the Eclipse toolbar, and select "Deploy to
App Engine." Eclipse may prompt you for your administrator account username (your email address)and password.
Enter your account information and click the Upload button to complete the upload. Eclipse gets the application ID
and version information from the appengine-web.xml file, and uploads the contents of the war/ directory.
Test your application on App Engine by visiting its URL:
 http://your_app_id.appspot.com/guestbook
Running the Command Line Tools
Some features of the App Engine Java SDK tools are only available by running the tools directly from the command
line. If you have installed the SDK using Eclipse, you can run these tools from the Eclipse plugin installation
directory.
The SDK is located in your Eclipse installation directory,
under plugins/com.google.appengine.eclipse.sdkbundle_VERSION/, where VERSION is a
version identifier for the SDK. In this directory is the appengine-java-sdk/bin/ subdirectory containing the
tools.
Note: If you are using Mac OS X or Linux, you must give the command files executable permission before you can
run them. For example, to give appcfg.shexecutable permission, enter the following command from
the appengine-java-sdk/bin/ directory: chmod u+x appcfg.sh
For more information on the features available exclusively from the command line, see Uploading and Managing.
Using the Users Service

Google App Engine provides several useful services based on Google infrastructure, accessible by applications
using libraries included with the SDK. One such service is the Users service, which lets your application integrate
with Google useraccounts.With the Users service, your users can use the Google accounts they already have to sign
in to your application.
Let's use the Users service to personalize this application's greeting.
Using Users
Edit src/guestbook/GuestbookServlet.java as indicated to resemble the following:
package guestbook;
import java.io.IOException;
import javax.servlet.http.*;
import com.google.appengine.api.users.User;
import com.google.appengine.api.users.UserService;
import com.google.appengine.api.users.UserServiceFactory;
public class GuestbookServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws IOException {
UserService userService = UserServiceFactory.getUserService();
User user = userService.getCurrentUser();
if (user != null) {
resp.setContentType("text/plain");
resp.getWriter().println("Hello, " + user.getNickname());
} else {
resp.sendRedirect(userService.createLoginURL(req.getRequestURI()));
}
}
}
If you are using Eclipse and yourdevelopment server is running in the debugger,when you save your changes to
this file, Eclipse compiles the new code automatically, then attempts to insert the new code into the already-running
server. Changes to classes,JSPs, static files and appengine-web.xml are reflected immediately in the running
server without needing to restart. If you change web.xml or other configuration files, you must stop and start the
server to see the changes.
If you are using Ant, you must stop the server and rebuild the project to see changes made to source code. Changes
to JSPs and static files do not require restarting the server.
Rebuild yourproject and restart the server, if necessary.Test the application by visiting the servlet URL in your
browser:
 http://localhost:8888/guestbook

Instead of displaying the message, the server prompts you for an email address.Enter any email address (such
as alfred@example.com, then click "Log In." The app displays a message, this time containing the email
address you entered.
The new code for the GuestbookServlet class uses the Users API to check if the user is signed in with a
Google Account.If not,the user is redirected to the Google Accounts sign-in
screen. userService.createLoginURL(...) returns the URL of the sign-in screen.The sign-in facility
knows to redirect the userback to the app by the URL passed to createLoginURL(...), which in this case is
the URL of the current page.
The development server knows how to simulate the Google Accounts sign-in facility. When run on your local
machine, the redirect goes to the page where you can enter any email address to simulate an account sign-in. When
run on App Engine, the redirect goes to the actual Google Accounts screen.
You are now signed in to your test application. If you reload the page, the message will display again.
To allow the userto sign out,provide a link to the sign-out screen, generated by the
method createLogoutURL(). Note that a sign-out link will sign the user out of all Google services.
Next...
Now that we know how to identify the user, we can invite the userto post messages to the guest book. Let's design a
user interface for this application using JavaServer Pages (JSPs).
Continue to Using JSPs.
Using JSPs
While we could output the HTML for our user interface directly from the Java servlet code, this would be difficult to
maintain as the HTML gets complicated. It's better to use a template system, with the user interface designed and
implemented in separate files with placeholders and logic to insert data provided by the application. There are many
template systems available for Java, any of which would work with App Engine.
For this tutorial, we'll use JavaServer Pages (JSPs) to implement the user interface for the guest book. JSPs are part
of the servlet standard.App Engine compiles JSP files in the application's WAR automatically as one large JAR file,
then maps the URL paths accordingly.
Hello, JSP!
Our guest book app writes strings to an output stream, but this could also be written as a JSP. Let's begin by porting
the latest version of the example to a JSP.
In the directory war/, create a file named guestbook.jsp with the following contents:

<%@ 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" %>
<%@ taglib prefix="fn" uri="http://www.oracle.com/technetwork/java/javaee/jsp/index.html" %>
<html>
<body>
<%
User user= userService.getCurrentUser();
if (user != null) {
pageContext.setAttribute("user",user);
%>
<p>Hello, ${fn:escapeXml(user.nickname)}! (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>
<%
}
%>
</body>
</html>
By default, any file in war/ or a subdirectory (other than WEB-INF/) whose name ends in .jsp is automatically
mapped to a URL path.The URL path is the path to the.jsp file, including the filename. This JSP will be mapped
automatically to the URL /guestbook.jsp.
For the guest book app, we want this to be the application's home page, displayed when someone accesses the
URL /. An easy way to do this is to declare in web.xmlthat guestbook.jsp is the "welcome" servlet for that
path.
Edit war/WEB-INF/web.xml and replace the current <welcome-file> element in the <welcome-file-
list>. Be sure to remove index.html from the list, as static files take precedence over JSP and servlets.
<welcome-file-list>
<welcome-file>guestbook.jsp</welcome-file>
</welcome-file-list>
Tip: If you are using Eclipse, the editor may open this file in "Design" mode. To edit this file as XML, select the
"Source" tab at the bottom of the frame.
Stop then start the development server. Visit the following URL:
 http://localhost:8888/

The app displays the contents of guestbook.jsp, including the usernickname if the useris signed in. We want
to HTML-escape any text which users provide in case that text contains HTML. To do this for the user nickname,
we use the JSP's pageContext so that java code can "see" the string, then call the escapeXML function we imported
via the taglib element.
When you upload your application to App Engine, the SDK compiles all JSPs into one JAR file, and that is what
gets uploaded.
The Guestbook Form
Our guest book application will need a web form so the usercan post a new greeting, and a way to process that
form. The HTML of the form will go into the JSP. The destination of the form will be a new URL, /sign, to be
handled by a new servlet class, SignGuestbookServlet. SignGuestbookServlet will process the form,
then redirect the user's browser back to /guestbook.jsp. For now, the new servlet will just write the posted
message to the log.
Edit guestbook.jsp, and put the following lines just above the closing </body> tag:
...
<form action="/sign" method="post">
<div><textarea name="content" rows="3" cols="60"></textarea></div>
<div><input type="submit" value="Post Greeting" /></div>
</form>
</body>
</html>
Create a new class named SignGuestbookServlet in the package guestbook. (Non-Eclipse users,create the
file SignGuestbookServlet.java in the directorysrc/guestbook/.) Give the source file the following
contents:
package guestbook;
import java.util.logging.Logger;
import javax.servlet.http.*;

public class SignGuestbookServlet extends HttpServlet {
private static final Logger log = Logger.getLogger(SignGuestbookServlet.class.getName());
public void doPost(HttpServletRequest req, HttpServletResponse resp)
String content = req.getParameter("content");
if (content == null) {
content = "(No greeting)";
}
if (user != null) {
log.info("Greeting posted by user" + user.getNickname() + ": " + content);
} else {
log.info("Greeting posted anonymously:" + content);
}
resp.sendRedirect("/guestbook.jsp");
}
}
Edit war/WEB-INF/web.xml and add the following lines to declare the SignGuestbookServlet servlet
and map it to the /sign URL:
<web-app xmlns="http://www.oracle.com/webfolder/technetwork/jsc/xml/ns/javaee/index.html" version="2.5">
...
<servlet>
<servlet-name>sign</servlet-name>
<servlet-class>guestbook.SignGuestbookServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>sign</servlet-name>
<url-pattern>/sign</url-pattern>
</servlet-mapping>
...
</web-app>
This new servlet uses the java.util.logging.Logger class to write messages to the log. You can control the
behavior of this class using a logging.propertiesfile, and a systemproperty set in the app's appengine-
web.xml file. If you are using Eclipse, your app was created with a default version of this file in your
app's src/and the appropriate systemproperty.
If you are not using Eclipse, you must set up the Logger configuration file manually. Copy the example file from the
SDK appengine-java-sdk/config/user/logging.properties to your app's war/WEB-
INF/ directory. Then edit the app's war/WEB-INF/appengine-web.xml file as indicated:
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
...

<system-properties>
<property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
</system-properties>
</appengine-web-app>
The servlet logs messages using the INFO log level (using log.info()). The default log level is WARNING,
which suppresses INFO messages from the output.To change the log level for all classes in
the guestbook package, edit the logging.properties file and add an entry for guestbook.level, as
follows:
.level = WARNING
guestbook.level = INFO
...
Tip: When your app logs messages using the java.util.logging.Logger API while running on App Engine, App
Engine records the messages and makes them available for browsing in the Admin Console, and available for
downloading using the AppCfg tool. The Admin Console lets you browse messages by log level.
Rebuild and restart, then test http://localhost:8888/. The form displays. Enter some text in the form, and submit. The
browser sends the form to the app,then redirects back to the empty form. The greeting data you entered is logged to
the console by the server.
Next...
We have a userinterface for prompting the userto enter a greeting. Now we need a way to remember the greetings
users post,and show them to other visitors. For that, we'll use the App Engine datastore.
Storing data in a scalable web application can be tricky. A user could be interacting with any of dozens of
web servers at a given time, and the user's next request could go to a different web server than the
previous request. All web servers need to be interacting with data that is also spread out across dozens
of machines, possibly in different locations around the world.
With Google App Engine, you don't have to worry about any of that. App Engine's infrastructure takes
care of all of the distribution, replication, and load balancing of data behind a simple API—and you get a
powerful query engine and transactions as well.
App Engine's data repository, the High Replication Datastore (HRD), uses the Paxos algorithm to replicate
data across multiple datacenters. Data is written to the Datastore in objects known as entities. Each entity

has a key that uniquely identifies it. An entity can optionally designate another entity as its parent; the first
entity is a child of the parent entity. The entities in the Datastore thus form a hierarchically structured
space similar to the directory structure of a file system. An entity's parent, parent's parent, and so on
recursively, are its ancestors; its children, children's children, and so on, are its descendants. An entity
without a parent is a root entity.
The Datastore is extremely resilient in the face of catastrophic failure, but its consistency guarantees may
differ from what you're familiar with. Entities descended from a common ancestor are said to belong to the
same entity group; the common ancestor's key is the group's parent key, which serves to identify the entire
group. Queries over a single entity group, called ancestor queries, refer to the parent key instead of a
specific entity's key. Entity groups are a unit of both consistency and transactionality: whereas queries
over multiple entity groups may return stale, eventually consistent results, those limited to a single entity
group always return up-to-date, strongly consistentresults.
The code samples in this guide organize related entities into entity groups, and use ancestor queries on
those entity groups to return strongly consistent results. In the example code comments, we highlight
some ways this might affect the design of your application. For more detailed information, see Structuring
Data for Strong Consistency.
Note: If you built your application using an earlier version of this Getting Started Guide, please note that
the sample application has changed. You can still find the sample code for the original Guestbook
application, which does not use ancestor queries, in the demos directory of the SDK.
The Datastore is one of several App Engine services offering a choice of standards-based or low-level
APIs. The standards-based APIs decouple your application from the underlying App Engine services,
making it easier to port your application to other hosting environments and other database technologies, if
you ever need to. The low-level APIs expose the service's capabilities directly; you can use them as a
base on which to implement new adapter interfaces, or just use them directly in your application.
App Engine includes support for two different API standards for the Datastore: Java Data Objects (JDO) and
the Java Persistence API (JPA). These interfaces are provided byDataNucleus Access Platform, an open-
source implementation of several Java persistence standards, with an adapter for the App Engine
Datastore.
For clarity getting started, we'll use the low-level API to retrieve and post messages left by users.
Updating Our Servlet to Store Data
Here is an updated version of src/SignGuestbookServlet.java that stores greetings in the
Datastore. We will discuss the changes made here below.
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 java.util.Date;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class SignGuestbookServlet extends HttpServlet {
public void doPost(HttpServletRequest req, HttpServletResponse resp)
// 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 a transactionalancestorquery 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();
Entity greeting = new Entity("Greeting", guestbookKey);
greeting.setProperty("user",user);
greeting.setProperty("date",date);
greeting.setProperty("content",content);
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
datastore.put(greeting);
resp.sendRedirect("/guestbook.jsp?guestbookName=" + guestbookName);
}
}
Storing the Submitted Greetings
The low-level Datastore API for Java provides a schemaless interface for creating and storing entities.
The low-level API does not require entities of the same kind to have the same properties, nor for a given
property to have the same type for different entities. The following code snippet constructs
the Greeting entity in the same entity group as the guestbook to which it belongs:
Entity greeting = new Entity("Greeting", guestbookKey);
greeting.setProperty("user",user);
greeting.setProperty("date",date);
greeting.setProperty("content",content);

In our example, each Greeting has the posted content, and also stores the user information about who
posted, and the date on which the post was submitted. When initializing the entity, we supply the entity
name, Greeting, as well as a guestbookKey argument that sets the parent of the entity we are storing.
Objects in the Datastore that share a common ancestor belong to the same entity group.
After we construct the entity, we instantiate the Datastore service, and put the entity in the Datastore:
DatastoreService datastore =
DatastoreServiceFactory.getDatastoreService();
datastore.put(greeting);
Because querying is strongly consistent only within entity groups, we assign all Greetings to the same
entity group by setting the same parent for each Greeting. This means a user will always see a Greeting
immediately after it was written. However, the rate at which you can write to the same entity group is
limited to 1 write to the entity group per second. When you design a real application, you'll need to keep
this fact in mind. Note that by using services such as Memcache, you can mitigate the chance that a user
won't see fresh results when querying across entity groups immediately after a write.
Updating the JSP
We also need to modify the JSP we wrote earlier to display Greetings from the Datastore, and also
include a form for submitting Greetings. Here is our updated guestbook.jsp:
<%@ 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" %>
<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>
<html>
<head>
<link type="text/css" rel="stylesheet" href="/stylesheets/main.css" />
</head>
<body>
<%
String guestbookName = request.getParameter("guestbookName");
if (guestbookName == null) {
guestbookName = "default";
}

pageContext.setAttribute("guestbookName", guestbookName);
User user= userService.getCurrentUser();
if (user != null) {
pageContext.setAttribute("user",user);
%>
<p>Hello, ${fn:escapeXml(user.nickname)}! (You can
<a href="<%= userService.createLogoutURL(request.getRequestURI()) %>">sign out</a>.)</p>
<%
} else {
%>
<p>Hello!
<a href="<%= userService.createLoginURL(request.getRequestURI()) %>">Sign in</a>
to include your name with greetings you post.</p>
<%
}
%>
<%
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key guestbookKey = KeyFactory.createKey("Guestbook", guestbookName);
// Run an ancestorquery to ensure we see the most up-to-date
// view of the Greetings belonging to the selected Guestbook.
Query query = new Query("Greeting", guestbookKey).addSort("date",Query.SortDirection.DESCENDING);
List<Entity> greetings = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(5));
if (greetings.isEmpty()) {
%>
<p>Guestbook '${fn:escapeXml(guestbookName)}' has no messages.</p>
<%
} else {
%>
<p>Messages in Guestbook '${fn:escapeXml(guestbookName)}'.</p>
<%
for (Entity greeting : greetings) {
pageContext.setAttribute("greeting_content",
greeting.getProperty("content"));
if (greeting.getProperty("user")== null) {
%>
<p>An anonymous person wrote:</p>
<%
} else {
pageContext.setAttribute("greeting_user",
greeting.getProperty("user"));
%>
<p><b>${fn:escapeXml(greeting_user.nickname)}</b> wrote:</p>
<%
}
%>
<blockquote>${fn:escapeXml(greeting_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="${fn:escapeXml(guestbookName)}"/>
</form>
</body>
</html>
Warning! Whenever you display user-supplied text in HTML, you must escape the string using
the fn:escapeXml JSTL function, or a similar escaping mechanism. If you do not correctly and
consistently escape user-supplied data, the user could supply a malicious script as text input, causing
harm to later visitors.
Retrieving the Stored Greetings
The low-level Java API provides a Query class for constructing queries and a PreparedQuery class for
fetching and returning the entities that match the query from the Datastore. The code that fetches the
data is here:
Query query = new Query("Greeting", guestbookKey).addSort("date",Query.SortDirection.DESCENDING);
List<Entity> greetings = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(5));
This code creates a new query on the Greeting entity, and sets the guestbookKey as the required
parent entity for all entities that will be returned. We also sort on thedate property, returning the newest
Greeting first.
After you construct the query, it is prepared and returned as a list of Entity objects. For a description of
the Query and PreparedQuery interfaces, see the Datastore reference.
A Word About Datastore Indexes
Every query in the App Engine Datastore is computed from one or more indexes. Indexes are tables that
map ordered property values to entity keys. This is how App Engine is able to serve results quickly
regardless of the size of your application's Datastore. Many queries can be computed from the builtin
indexes, but the Datastore requires you to specify a custom index for some, more complex, queries.
Without a custom index, the Datastore can't execute the query efficiently.
Our guest book example above, which filters results by ancestor and orders by date, uses an ancestor
query and a sort order. This query requires a custom index to be specified in your
application's datastore-indexes.xml file. When you run your application in the SDK, it will
automatically add an entry to this file. When you upload your application, the custom index definition will
be automatically uploaded, too. The entry for this query will look like:
<?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>
You can read all about Datastore indexes on the Datastore Indexes page.
Clearing the Datastore
The development web server uses a local version of the Datastore for testing your application, using local
files. The data persists as long as the temporary files exist, and the web server does not reset these files
unless you ask it to do so.
The file is named local_db.bin, and it is created in your application's WAR directory, in the WEB-
INF/appengine-generated/ directory. To clear the Datastore, delete this file.
Next...
Every web application returns dynamically generated HTML from the application code, via templates or
some other mechanism. Most web applications also need to serve static content, such as images, CSS
stylesheets, or JavaScript files. For efficiency, App Engine treats static files differently from application
source and data files. Let's create a CSS stylesheet for this application, as a static file.
Continue to Using Static Files.
Using Static Files
There are many cases where you want to serve static files directly to the web browser. Images, CSS stylesheets,
JavaScript code, movies and Flash animations are all typically served directly to the browser. For efficiency , App
Engine serves static files from separate servers than those that invoke servlets.
By default, App Engine makes all files in the WAR available as static files except JSPs and files in WEB-INF/.
Any request for a URL whose path matches a static file serves the file directly to the browser—even if the path also
matches a servlet or filter mapping. You can configure which files App Engine treats as static files using
theappengine-web.xml file.
Let's spruce up our guest book's appearance with a CSS stylesheet.For this example, we will not change the
configuration for static files. See App Configuration for more information on configuring static files and
resource files.
A Simple Stylesheet
In the directory war/, create a directory named stylesheets/. In this directory, create a file
named main.css with the following contents:

body {
font-family: Verdana, Helvetica, sans-serif;
background-color: #FFFFCC;
}
Edit war/guestbook.jsp and insert the following lines just after the <html> line at the top:
<html>
<head>
<link type="text/css" rel="stylesheet" href="/stylesheets/main.css" />
</head>
<body>
...
</body>
</html>
Visit http://localhost:8888/. The new version uses the stylesheet.
Next...
The time has come to reveal your finished application to the world.
Uploading Your Application
You create and manage applications in App Engine using the Administration Console. Once you have registered an
application ID for your application, you upload it to App Engine using either the Eclipse plugin, or a command -line
tool in the SDK.

Note: Once you register an application ID, you can delete it, but you can't re-register that same application ID after
it has been deleted. You can skip these next steps if you don't want to register an ID at this time.
Note : If you have an App Engine Premier account,you can specify that your new application should reside in the
European Union rather than the United States. For developers that do not have a Premier account, you will need
to enable billing for applications that should reside in the European Union.
Hosting applications in the European Union is especially useful if your users are closer to Europe than to the United
States. There is less network latency and the End User Content will be stored at rest in the European Union. You
must specify this location by clicking the "Edit" link in the "Location Options" section when you register the
application; you cannot change it later.
Registering the Application
You create and manage App Engine web applications from the App Engine Administration Console, at the
following URL:
https://appengine.google.com/
Sign in to App Engine using your Google account.If you do not have a Google account, you can create a Google
account with an email address and password.
To create a new application, click the "Create an Application" button.Follow the instructions to register an
application ID, a name unique to this application.
Edit the appengine-web.xml file, then change the value of the <application> element to be your
registered application ID.
For this tutorial, you should probably elect to use the free appspot.comdomain name, and so the full URL for the
application will behttp://your_app_id.appspot.com/. You can also purchase a top-level domain name
for your app, or use one that you have already registered.
For Authentication Options (Advanced), the default option,"Open to all Google Accounts users",is the simplest
choice for this tutorial. If you choose "Restricted to the following Google Apps domain", then your domain
administrator must add your new app as a service on that domain. If you choose the Google Apps domain
authentication option, then failure to add your app to yourGoogle Apps domain will result in an HTTP 500 where
the stacktrace shows the error "Unexpected exception from servlet: java.lang.IllegalArgumentException: The
requested URL was not allowed: /guestbook.jsp".If you see this error, add the app to your domain. See Configuring
Google Apps to Authenticate on Appspot forinstructions.
If you have an App Engine Premier account,you can specify that yournew application should reside in the
European Union rather than the United States. This is especially useful if your application's users are closer to
Europe than to the United States. There is less network latency and the End User Content will be stored at rest in the
European Union. You must specify this location when you register the application; you cannot change it later. Click
the Edit link in the Location Options section; select a location option, either United States or European Union.

Uploading the Application
You can upload your application using Eclipse, or using a command at the command prompt.
Currently, you can only upload applications with a maximum size of 32 megabytes.
Uploading From Eclipse
You can upload your application code and files from within Eclipse using the Google Plugin.
To upload your application from Eclipse, click on the Google button in the Eclipse toolbar, then select
"Deploy to App Engine."
If prompted, follow the instructions to provide the Application ID from the App Engine console that you would like
to use for this app, your Google account username (your email address),and your password.Then click
the Deploy button.Eclipse will then automatically upload the contents ofthe war/ directory.
Uploading Using the Command Prompt
You can upload your application code and files using a command included in the SDK
named appcfg.cmd (Windows) or appcfg.sh (Mac OS X, Linux).
AppCfg is a multi-purpose tool for interacting with your app on App Engine. The command takes the name of an
action, the path to your app's war/ directory, and otheroptions.To upload the app code and files to App Engine,
you use the update action.
To upload the app, using Windows:
..appengine-java-sdkbinappcfg.cmd update war
To upload the app, using Mac OS X or Linux:
../appengine-java-sdk/bin/appcfg.sh update war
Enter your Google username and password at the prompts.
Checking Your Application State
After your application is uploaded, its Datastore Indexes will be automatically generated.This operation may take
some time, and any visitors to your site will receive aDatastoreNeedIndexException until the indexes have

been built. You can monitor the progress of the operation by visiting the App Engine console, selecting your
application, and then selecting the Datastore Indexes link.
Accessing Your Application
You can now see your application running on App Engine. If you set up a free appspot.comdomain name, the URL
for your website begins with your application ID:
http://your_app_id.appspot.com/

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