MCS 270 Spring 2014 Object-Oriented Software Development

MCS 270 Spring 2014

Object-Oriented Software Development

GUSTAVUS ADOLPHUS COLLEGE gustavus.edu

Today’s schedule

G

AE – Google App Engine

Modules

User Service

Blob Service

MCS 270 Object-Oriented Software Development


Overview

Google App Engine is:

- runtime platform that provides web application

hosting, data storage, and high-speed networking

- infrastructure (servers + storage ) at Google



Overview

GAE Advantages:

Does one thing well: running web appsApp Engine handles HTTP(S) requests, nothing

else

Simple (relatively) app configuration

Automatic Scaling to Application Needs

Secure

Easy on budget: Pay only for what you use (small apps

are free)



GAE Runtime Environment

Java: Java 7 Virtual Machine (JVM)

Java Servlet interface for client-server

Python: Python 2.7

Go

Java or Python?

Python: powerful python syntax, library, shorter code

Java: can use Java Data Objects (JDO) and Java

Persistance API (JPA)





GAE Sandbox

Applications run in a secure environment - limited access

to underlying OS

Plus - App will NEVER affect other applications on the same server

Minuses - Cannot spawn additional processes or threads

Cannot make arbitrary network connections

Only read its own code and resource files and cannot create or

modify files



GAE Sandbox Restrictions

- App can only access other computers on the Internet

through URL fetch and email services.

- App interaction to server only by HTTP (or HTTPS)

requests on the standard ports

- Applications cannot write to the file system



GAE Services

URLFetch – fetch web resources/services (servlets)

Images – manipulate images: resize, rotate, flip, crop

User Services: Google Accounts

Mail

XMPP – instant messages

Task Queue – message queue; allow integration with non-GAPPs

Datastore – managing data objects

Blobstore – large files, much larger than objects in datastore



GAE User Services

Google App Engine service based on Google infrastructure

Accessible by applications using libraries included with GAE SDK.

Provides integration with Google user accounts.

Users use existing Google accounts to sign in to Application



GAE User Services

Demo



Program Flow – GusList.java

public void onModuleLoad() {

glView.setController(GusList.this);

homeURL = Window.Location.getHref();

clientModelService.setAppBaseURL(homeURL,

new AsyncCallback<String>() {

public void onFailure(Throwable caught) {return;}

public void onSuccess(String result) {}});

clientModelService.isUserLoggedIn(

new AsyncCallback<Boolean>() {

public void onFailure(Throwable caught) {return;}

public void onSuccess(Boolean result) {

if(result) glView.viewWelcomePage();

else glView.setWindow("../GusListWelcome.html");

}});}



Program Flow – GusListView.java

public void setWindow(String url) {

Window.Location.replace(url);

}



Program Flow – GusListWelcome.html

<p style="text-align: center;">

Welcome to GusList - the Gustavus on-line classified ad system. To start

checking out the latest ads from the coolest students, please

<a href="guslist/loginservice">login</a>.

If you are new to the system, you will need to

<a href="https://accounts.google.com/SignUp">create

a Google account</a> to login. </p>



Program Flow – web.xml

<servlet>

<servlet-name>LoginService</servlet-name>

<servlet-class>edu.gac.mcs270.hvidsten.guslistgae.server.LoginService</

servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>LoginService</servlet-name>

<url-pattern>/guslist/loginservice</url-pattern>

</servlet-mapping>



Program Flow - LoginService

public class LoginService extends HttpServlet {

public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException

{

UserService userService = UserServiceFactory.getUserService();

User user = userService.getCurrentUser();

if (user != null) {

resp.setStatus(HttpServletResponse.SC_SEE_OTHER);

resp.setHeader("Location", "../GusList.html”);

} else {

String logInLink = userService.createLoginURL("../GusList.html”);


resp.setHeader("Location", logInLink);

}}}



Login by GAE


Note:

Login and logout pages are handle by GAE automatically, but the workflow is different :

Run on local – It will simulate Google Accounts sign-in page (no password authentication).

Run on GAE – It will redirect to actual Google Account login screen.


Program Flow – After Login returns to GusList.html


<body> Basically Empty – filled in by GusListView

</body>


RPC vs HTTP Servlets

HTTP – Simplest way to retrieve data from server. Data

can be anything (text, HTML, XML, binary data, etc).

Programmer must make sense of data.

RPC – Easiest way to tranfer Java objects from server<-

>client. Data can only contain Serializable objects.

Programmer knows exactly what form data is in.



RPC

“A remote procedure call (RPC) is an inter-process

communication that allows a program to cause a

procedure to execute in another address space

(commonly on another computer on a shared network)

without the programmer explicitly coding the details for

this remote interaction. When the software in question

uses object-oriented principles, RPC is called remote

invocation or remote method invocation.” (Wikipedia)



HTTP Request / Response Cycle

http://www.oreilly.com/openbook/cgi/ch04_02.html

Browser

Web Server

HTTPRequest

HTTPResponse

IE, FireFox, Chrome,Safari



HTTP Request / Response Cycle

http://www.oreilly.com/openbook/cgi/ch04_02.html

Browser

Web Server

HTTPRequest

HTTPResponse

IE, FireFox, Chrome,Safari


GET /index.html

<head> .. </head><body><h1>Welcome to my application</h1> ....</body>


RPC vs HTTP Servlets

LoginService - Transfers actual URL’s, comes from GET

request. Must be HTTP servlet.



Program Flow - LoginService

public class LoginService extends HttpServlet {

public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException

{

UserService userService = UserServiceFactory.getUserService();

User user = userService.getCurrentUser();

if (user != null) {


resp.setHeader("Location", "../GusList.html”);

} else {

String logInLink = userService.createLoginURL("../GusList.html”);


resp.setHeader("Location", logInLink);

}}}



GAE Blob Service

Blobstore API - mechanism to store “blobs” of information

Blob can be up to 50MB in size (typically large files –video, images)

Datastore vs Blob Service

Datastore – data size limited to 1mb, Access only through a GAE

app.

Blob Service – Size up to 50mb. Cheaper than Datastore. Data can

be served directly as url access. Designed for images – can do

image transformations in place.



GAE Blob Service

Demo



Program Flow - GusListView

FormPanel submitFormPanel

// The submitFormPanel, when submitted, will trigger an HTTP call to the

// servlet. The following parameters must be set

submitFormPanel.setEncoding(FormPanel.ENCODING_MULTIPART);

submitFormPanel.setMethod(FormPanel.METHOD_POST);

// Set Names for the text boxes so that they can be retrieved from the

// HTTP call as parameters

nameTextbox.setName("name");

titleTextbox.setName("title");

descrText.setName("description");

priceTextbox.setName("price");

upload.setName("upload");



Program Flow - GusListView

submitButton.addClickHandler(new ClickHandler() {

public void onClick(ClickEvent event) {

control.handlePostFromSubmitForm(submitFormPanel);

}});



Program Flow - GusList

public void handlePostFromSubmitForm(final FormPanel submitFormPanel) {

blobService.getBlobStoreUploadUrl( new AsyncCallback<String>() {

public void onSuccess(String result) {

// Set the form action to the newly created blobstore upload URL

submitFormPanel.setAction(result.toString());

// Submit the form to complete the upload

// This causes activation of an upload HTTP servlet

submitFormPanel.submit();

}

public void onFailure(Throwable caught) {

glView.sendErrorMessage("Upload Failed");

}});

}



Program Flow - BlobService

public class BlobServiceImpl extends RemoteServiceServlet implements BlobService {

//Start a GAE BlobstoreService session

BlobstoreService blobstoreService = BlobstoreServiceFactory.getBlobstoreService();

//Generate a Blobstore Upload URL

public String getBlobStoreUploadUrl() {

// Map the BlobService UploadURL to the

// HTTP servlet which will be called when

// submitting the FormPanel (see web.xml for servlet def)

return blobstoreService.createUploadUrl("/guslist/uploadservice");

}



Program Flow - SubmitPostHTTPService



Program Flow - SubmitPostHTTPService

//Get the parameters from the request to post the ad

String name = req.getParameter("name");

String title = req.getParameter("title");

String descr = req.getParameter("description");

double price = Double.valueOf(req.getParameter("price"));

//Map the ImageURL to the blobservice servlet, which will serve the image

String url = "/guslist/blobservice?blob-key=" + blobKey.getKeyString();

PostData post = new PostData(title,descr,price, url, new Seller(name), null);

GusListModel.storePost(post);

}}


Documents

MCS 270 Spring 2014 Object-Oriented Software Development