Google App Engine tutorial

Google App Engine
tutorial

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Google App Engine tutorial - 2013

Table of contents
• Open an account
• Installing GAE plugin for eclipse
• Installing JRE and JDK
• Register & Login example
• GAE Junit
• More features of GAE
• Example source code
Note: As for now, it is possible to write your server side code in Python, Java
or PHP. This slideshow demonstrates an example using Java.

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Open an account
Suppose we have a Gmail account: nameforthetutorial@gmail.com.
Go to the address: https://developers.google.com/appengine/ and click Sign Up.

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Open an account
Fill in the project name.
The Project ID is your unique server name.
You can use the one that was generate for you or set one as you like as long
as no one else is already using it.
Your server address will be http://<your_project_id>.appspot.com.
Click Get Started.

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Open an account
In the next window click your project name.

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Open an account
Here you can view all of your projects.
You can open up to 10 projects per Gmail account.
Click your project name.

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Open an account
Now click App Engine.

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Open an account
You made it. That’s your main dashboard.
Here you manage your app.

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Installing GAE plugin for eclipse
Depends on your eclipse version, follow the instructions in the relevant link:
https://developers.google.com/eclipse/docs/install-eclipse-4.3
When finished you’ll see a blue Google symbol (
Now sign in to your account:
In the lower right corner click Sign in to Google:

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) in your upper toolbar.

Installing JRE and JDK
You may have problems with your JDK or JRE, so download the latest versions
from the java site:
http://www.oracle.com/technetwork/java/javase/downloads/jre7-downloads1880261.html
http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads1880260.html

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Register & Login example
A simple example only to understand how to communicate between your
Android app and your GAE server.
We will use POST request.
Usually Get request is for getting back data from the sever, while POST request
is for storing data in the server.
In Get request the parameters are passed in the url, while in POST request a
package is being sent to the server with the parameters encoded inside.
The following example uses POST request both for getting back data from the
server and for storing data in the server. The reason is that in order to get back
the data from the server, our password is being sent on the web and we don’t
want it to appear in the url.
Note: Using POST request is not a sufficient protection for sending password on
the web. An SSL protection is needed as will be shown next.

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Register & Login example – Server side
Click the Google symbol (
Project…

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) in your eclipse. Then choose New Web Application


Fill in your server side app name and also a
package name.
Click Finish.

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Your server side project should look like in the image.
Predictable problems:
- If you are missing the App Engine SDK or the JRE
System Library then:
Right click your project name > properties > Java Build
Path > choose Libraries tab > in the menu on the right
choose Add Library… > choose Google App Engine >
Next > Finish.
In the same way for the JRE but instead of choosing
Google App Engine choose JRE System Library.
- If the warWEB-INFlib is empty then try:
Right click your project name > properties > expand Google > App Engine > uncheck
Use Google App Engine box > OK.
Then do it again but now check the Use Google App Engine box and your libraries
should be added.
Note: In general Project > Clean do magic.
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Now Go to appengineweb.xml and set your
project id.
That’s the connection
between your code and
the Google App Engine
project you opened
earlier.

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Create two classes: Register & Login.
We need them in order to define two servlets:
https://clean-abacus-384.appspot.com/login
https://clean-abacus-384.appspot.com/register

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Define the two classes we’ve just created as
servlets, and map them to the corresponding urls.
For example, according to the written on the right
the url
https://clean-abacus-384.appspot.com/login
is mapped to the servlet Login which is referred to
the com.serversideexample.Login class, which we
defined in the previous slide.

In lines 34-42 you can see the definition of the
security constraint for using SSL.
For further reading about applying SSL protection
in GAE:
https://developers.google.com/appengine/docs/ja
va/config/webxml#Secure_URLs

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Register class extends
HttpServlet in order to
define the Register
servlet.
The datastore may
contain many kinds of
entities all together in
the same table (there
is only one table). Here
we create a Person
kind entity by
specifying it in the
entity’s constructor.

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As before Login class
extends HttpServlet in
order to define the Login
servlet.
We should specify which
entity we’re looking for (line
47).
Note: in CompositeFilter
also “or” operator may be
used.
In FilterOperator also “gt”,
”gte”, ”lt”, ”lte”, ”ne”, ”in”
may be used.

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Deploy your project:
Right click your project name > Google > Deploy to App Engine and then click Deploy.

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Register & Login example – Client side
Suppose you have a little experience in Android.
Create a new app with the following permissions:

Now set main_activity.xml to look like that:

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A simple activity which
listens the two buttons
and calls the execute
function in the
ClientAsync class in order
to perform the POST
request to the GAE server.

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ClientAsync extends
AsyncTask in order to work in
background, sets a POST
request with the encoded
parameters which the server
is expecting for.
ClientAsync uses
HttpsURLConnection for
supporting GAE’s SSL security
constraint.
Finally displays the server’s
answer with the TextView
object.

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Register & Login example – Testing
Starting point:
Datastore is empty. Fill in some details and click Register.

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After clicking Register the Person will be shown in the datastore.
Note: The App engine gives every entity an unique id (key).
For further reading about keys:
https://developers.google.com/appengine/docs/java/datastore/entities#Java_Genera
ting_keys

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Further Login Testing:

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GAE Junit
Test your code in a convenient way.
Setting up the framework:
• Right click your project name > properties > Java Build Path > choose Libraries tab > in
the menu on the right choose Add External Jars… > go to
eclipsepluginscom.google.appengine.eclipse.sdkbundle_1.8.6appengine-java-sdk1.8.6libimpl and select appengine-api.jar, appengine-api-labs.jar, appengine-apistubs.jar > Open > OK.

the menu on the right choose Add External Jars… > go to
eclipsepluginscom.google.appengine.eclipse.sdkbundle_1.8.6appengine-java-sdk1.8.6libtesting and select appengine-testing.jar > Open > OK.
the menu on the right choose Add Library… > choose Juint > Next > choose Junit 4 >
Finish.
For further reading about Junit:
https://developers.google.com/appengine/docs/java/tools/localunittesting
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GAE Junit
Junit:
To run the tests
do Run > Run
As… > Junit Test
Note: The
changes are local
and have no
effect on your
GAE datastore.

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More features of GAE
Jackson/Gson:
An easy way to transfer objects threw the web. User class should be at the server side and
at the client side. In the server side we convert user to json string into the response and in
the client side we read the json string and convert it back to user.
Server side:

Client side:

For further reading about Jackson:
http://wiki.fasterxml.com/JacksonInFiveMinutes
http://wiki.fasterxml.com/JacksonDownload
For further reading about Gson:
https://code.google.com/p/google-gson/
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Objectify/JDO/JPA:
It’s possible to define an entity as a class.
More comfortable, less dirty code.

For further reading about Objectify:
https://code.google.com/p/objectify-appengine/
For further reading about JDO:
https://developers.google.com/appengine/docs/java/datastore/jdo/overview-dn2
For further reading about JPA:
https://developers.google.com/appengine/docs/java/datastore/jpa/overview
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Sending mail from
your server:
You can send and
receive emails on
behalf of your project
ID.
Great for password
recovery, confirm
registration etc.

For further reading about mailing options:
https://developers.google.com/appengine/docs/java/mail/

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Logging messages to Logs
console:
You can log out notes of
some categories: severe,
warning and info.
You can view your logs in
the Logs section in your
app’s console.
Great for debugging 

…

…

For further reading about logging:
https://developers.google.com/appengine/docs/java/#Java_Logging

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Running on local host:
You can run your app locally. Do Run As > Web Application. Your server’s url now is
http://localhost:8888/.
You can get to your app’s console in: http://localhost:8888/_ah/admin.
Note: you may need to remove your SSL protection (use http://...) when testing locally.

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More relevant topics:
• Transactions: atomic operations in the datastore. Obligatory for concurrent writes to
datastore.
https://developers.google.com/appengine/docs/java/datastore/transactions
• ThreadSafe: allowing your server to run your server side in parallel.
https://developers.google.com/appengine/docs/java/config/appconfig#Java_appengine_w
eb_xml_Using_concurrent_requests
• Backends: background services for heavy computations.
https://developers.google.com/appengine/docs/java/backends/
•Blobstore: storing large files like images, videos etc.
https://developers.google.com/appengine/docs/java/blobstore/
• Quotas: limits for free datastore usage.
https://developers.google.com/appengine/docs/quotas#Datastore
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Example source code
https://drive.google.com/folderview?id=0B0ZJh5uX_P5iLXNQdXVZc2stSDQ&usp=sharing

Daniel Abitbul – abitbul6@gmail.com
Omer Shpigelman – omer.shpigelman@gmail.com

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