TweetFollow Us on Twitter

Sep 01 Adv WebObjects

Volume Number: 17 (2001)
Issue Number: 09
Column Tag: Advvanced WebObjects 5

Deep into the request/Response Loop

by Emmanuel Proulx

Customizing Web Component Processing

Preface

This new column covers various advanced topics of programming Web applications with WebObjects 5. It is targeted towards knowledgeable WebObjects developers who are looking for that extra wisdom to help them go further.

In this first article, I introduce the Request/Reply loop, and how to customize it and control how Components are processed. I hope this will be valuable to you.

Overview

From the time a user clicks on a hyperlink to the time the page is displayed in the browser, lots of things happen. Of course, the browser and the Web server first initiate the communication and ask for a page. If you installed WebObjects with the CGI adaptor (as opposed to a native one), the following type of URL is used:

http://server.domain.subdomain/cgibin/WebObjects/ApplicationName

Here the WebObjects adaptor program is being called, and if there is an application called "ApplicationName" registered, it is being executed. WebObjects builds a page and returns it, calling your code to fill in the blanks. This part is very transparent. But the building of a page is not. How does it work? What happens? When does it happen?

Knowing the answer to these questions is important to expand, tweak and adapt your WebObjects program to the specific requirements of your target system. Figure 1 illustrates how it works:


Figure 1. Objects of the Request/Response Loop.

The WebObjects adaptor program builds a Request object, which encapsulates the original "Get/Post" message received by the Web Server. This object is passed around the framework as WebObjects works on the Component (page template). Then WebObjects builds the Response object, which encapsulates the returned (pure HTML) page.

During this time, many classes of the framework call each other. You know about the Application and Session classes. You just learned about the Request, Response and Component classes. When they interact, they call different functions at different times.

Knowing when the functions are called and overriding the correct function is the key to customizing the request/response loop. Figure 2 illustrates the different objects and methods and their interactions. The next sections describe them further, and prescribe when to overload them.


Figure 2. Interactions Between Methods.

Application Object

The Application class is a subclass of the virtual WOApplication class. Your Application class will only have a single instance per server. This class contains the main() function. By default, the generated code for this function calls WOApplication.main(), which creates a Singleton instance of your Application class. You have to use the following definition so your subclass will be recognized:

public class Application extends WOApplication

I have also stated that the Application object is accessible from all Sessions on the server, and its main use is to share data among them, and to hold global business logic. The Application's underling goal is to manage the Sessions and the request/response loop. Typically, you overload at the Application level if your code is general to all clients. That said, here are the main functions that you can overload to customize the framework to your needs:

  • main(String []) : Overload to initialize variables before the system is started. Note that the default behavior is to call WOApplication.main(), which creates an instance of Application.
  • Application() (Constructor): Ideal to put the system-wide initialization code.
  • handleRequest(WORequest): Takes care of a single iteration of the request/response loop. The base implementation calls awake(), takeValuesFromRequest(), invokeAction(), appendToResponse() and finally sleep().
  • awake():Called by the framework when there's a new request, before WebObjects begins processing it. Overload for example when you want to keep statistics of the frequency at which the system is being used.
  • takeValuesFromRequest(WORequest, WOContext): The base implementation calls Session.takeValuesFromRequest(). As its name stipulates, this function should be overridden when you want to process the request object from the application object.
  • WOResponse infokeAction(WORequest, WOContext): The base implementation executes the action associated to a Request, by calling Session.invokeAction(). This function should be overridden if you intend to act upon the action itself (for example, to stop one from being executed).
  • appendToResponse(WOResponse, WOContext): The base implementation calls Session.appendToResponse(). As its name says, this function should be overridden to add to the Response, or for any post-action operation.
  • sleep(): Called by the framework after a request/response is done with, before WebObjects waits for the next one. Overload for example when you want to keep statistics of the requests duration.
  • finalize(): The Application's destructor function is ideal to put the system-wide cleanup code.

Session Object

The Session object is a subclass of the virtual WOSession class. Your Session class has one instance per HTTP connection. WebObjects creates all instances for you, but you have to use the following definition:

public class Session extends WOSession

The main use of the Session class is to hold connection-specific information and business logic. Under the hood, it's also in charge of terminating itself and holding the Editing Context (database objects). You will notice that most of the functions you can overload are similar to the Application's. This is so you can decide the scope of your overloaded code. Typically, you overload at the Session level if the code is connection- or client-specific. The interesting functions are:

  • Session() (Constructor): The constructor is called a single time shortly after WOApplication.awake() is called, but won't be called again for the current client. This function is ideal to put the connection-specific initialization code.
  • awake(): Called by the framework when there's a new request, during WOApplication.awake(), before WebObjects begins processing the request. The base implementation calls WOComponent.awake(). Overload to call request-specific initialization code.
  • takeValuesFromRequest(WORequest, WOContext): Called during Application.takeValuesFromRequest(). The base implementation calls takeValuesFromRequest().in the page that was requested. This function should be overridden when you want to process the Request object from the Session object.
  • WOResponse infokeAction(WORequest, WOContext): Called during Application.invokeAction(). The base implementation executes the action associated to a Request by calling invokeAction() in the page that was requested. This function should be overridden if you intend to act upon the action itself (for example, to stop one from being executed), depending on a client-specific state.
  • appendToResponse(WOResponse, WOContext): Called during WOApplication.appendToResponse(). The base implementation calls appendToResponse() in the page that was requested. This function should be overridden to add to the Response, or for any post-action operation.
  • sleep(): Called by WOApplication.sleep() after a request/response is done with, before WebObjects waits for the next one.
  • finalize(): The Session's destructor function is ideal to put client-specific cleanup code.

Web Component

The Component objects are subclasses of the virtual WOComponent class. There can be multiple instances of a Component, and they are usually represented by local references.

WebObjects creates an instance of the Component with the name "Main" for you, returning it to the client browser, but you have to use the following definition:

public class Main extends WOComponent

You are responsible for creating instances of other Components. Usually, a Component is returned when an action is invoked. In that case, you create an instance of a Component by using this syntax:

WOComponent anAction() {
  return pageWithName("ComponentName");
}

The main use of the WOComponent subclass is to hold page-specific data and logic. On top of that, Components have the ability to generate the result page (using method generateResponse()). Again, most of the functions you can overload are similar to the Application's and the Session's. Typically, you overload at the Component level if the code is page-specific. The functions you can overload are:

  • Web Component Constructor: The constructor is called when pageWithName() is called. This function is ideal to put the page-specific one-time initialization code. Use awake() to put page-specific per-request initialization code.
  • awake(): Called by the framework when there's a new request, during WOSession.awake(), before WebObjects begins processing the request. The base implementation does nothing. Overload to call page-specific per-request initialization code.
  • takeValuesFromRequest(WORequest, WOContext): Called during WOSession.takeValuesFromRequest(). The base implementation calls WOElement.takeValuesFromRequest(), where WOElement is the root element () of the hierarchy of the page. The function is then called recursively on each element of the hierarchy. This function should be overridden when you want to process the Request object from the current page.
  • WOResponse infokeAction(WORequest, WOContext): Called during WOSession.invokeAction(). The base implementation executes the action associated to a Request directly. This function should be overridden if you intend to act upon the action itself (for example, to stop one from being executed), depending on a page-specific state.
  • appendToResponse(WOResponse, WOContext): Called during WOSession.appendToResponse(). The base implementation calls WOElement. appendToResponse(), where WOElement is the root element () of the hierarchy of the page. The function is then called recursively on each element of the hierarchy. This function should be overridden to add to the Response, or for any post-action operation.
  • sleep(): Called by WOSession.sleep() after a request/response is done with, before WebObjects waits for the next one. Ideal for per-request page-specific cleanup code.
  • finalize(): The Component's destructor function is ideal to put one-time page-specific cleanup code. Use sleep() to call per-request page-specific cleanup code.

Element Object

There's an object that I skipped on purpose here, the WOElement object. You usually don't subclass this class, unless you want to write your own customized elements. You can avoid this trouble by using custom components instead, which is easier to write. The interesting member functions are the constructor, takeValuesFromRequest(), invokeAction(), appendToResponse() and finalize(). The usage of these functions should be trivial.

Request Object

As stated before, the Request class encapsulates a Get or a Post HTTP request. The base class for WORequest is WOMessage, so some of the characteristics explained here are inherited from that base class.

The framework uses an instance of WORequest to pass it around during the pre-generation phase (awake(), takeValuesFromRequest() and invokeAction() functions). Here's a partial list of the information held in this class:

  • A list of "cookies" (cookies are key/value pairs stored in the client's Browser). Keys are user-defined only. Cookies will be discussed in their own section.
  • A list of all submitted form fields when applicable (Post request only). Form fields will be discussed later on.
  • A list of "headers" (headers are key/value pairs containing the context of the request).
  • The bits that make up the requested URL.
  • Other request information (encoding, languages, protocol, request type).

I will skip cookies and form fields; they are covered later on. As for headers and the other information, there's no prescribed way of using these. It's up to you to figure out how to use them in your advantage. I can only show you how to access them. Following is an overview of the available functions.

Headers

Accessing headers is done with these functions of the class WORequest:

  • NSArray headerKeys (): Returns a list of all available header names as Strings. Use these as keys in functions headerForKey() and headersForKey().
  • String headerForKey(String) and NSArray headersForKey(String): These functions return the value (or values) of a header, when you pass the header's name (key). Use the first for single-value headers, and the second for multiple values.

But what are the fundamental request headers and what are they used for? Here's a link that lists some of the basic request headers:

http://www.w3.org/Protocols/HTTP/HTRQ_Headers.html

But there may be other headers, because some are specific to the Web browser. To find out about those extra headers, the first thing that comes to mind is to overload takeValuesFromRequest() in one of Application, Session or Main, and print them out.The following piece of code does exactly that:

public void takeValuesFromRequest(WORequest r, WOContext c) 
{
  super.takeValuesFromRequest(r,c);

  if(r.headerKeys() == null) return;
  for(int i=0; i< r.headerKeys().count(); i++) {
    String key = (String)r.headerKeys().objectAtIndex(i);
    NSArray values = r.headersForKey(key);
    System.out.println("Found header " + key + 
      ": " + value.toString());
  }
}

Here's what the output might look like:

Found header: accept-charset = ("iso-8859-1,*,utf-8")
Found header: accept-language = (bg)
Found header: accept-encoding = (gzip)
Found header: connection = ("keep-alive")
Found header: user-agent = ("Mozilla/4.6 [en] (WinNT; I)")
Found header: host = ("localhost:3878")
Found header: accept = ("image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*")
Found header: referer = ("http://www.imaginarypenda.com/gotosite.html")

Dismantling URLs

Next, let's have a look at the functions returning the parts of the requested URL. Let's use an example for each function. Imagine the user wrote the address:
http://www.imaginarypenda.com/cgi-bin/WebObjects/App

Here are the methods of WORequest to take a URL apart:

  • String uri(): Returns the last part of the whole URL, from the first slash to the end. For example, if the user typed the URL above, this method should return "/cgi-bin/WebObjects/App".
  • String adaptorPrefix(): Provides the adaptor's name. For example it could be "/cgi-bin/WebObjects/" on Mac OS X or Unix, or "/cgi-bin/WebObjects.exe/" on Windows. This example shows the CGI adaptor prefix; it may be different if using a native Web server adaptor.
  • String applicatioNumber(): Returns the user-requested application number, when provided. This information is usually not provided by the user in the URL (as not shown above). In these cases, this functions returns -1 and meaning any one instance of the application was called.
  • String applicationName(): Returns the application's name. In our example, it is "App".

Let's dissect some URLs and see which function return which part. Here are three examples.

  • The first example below shows a MacOS X-based server, pointing to any instance of Find-A-Luv (no instance number). In this case, applicationNumber() will return -1.
  • The second is a Windows-based server, pointing to any instance (we could have skipped the -1).
  • The third is a Unix-based server, with native adaptor (no CGI involved), and pointing to the third instance.
http://www.aUrl.com   /cgi-bin/   WebObjects/          /   Find-A-Luv
http://www.aUrl.com   /cgi-bin/   WebObjects.exe/        -1   /   Find-A-Luv
http://www.aUrl.com   /WebObjects/      3      Find-A-Luv
   adaptorPrefix()      applicationNumber()      applicationName()
   
   uri()

Other Request Info

Following is an brief overview the "other information" functions:

  • method() String Any valid HTTP method, like "GET", "PUT, "POST", "HEAD", etc. See http://www.w3.org/Protocols/HTTP/ Methods.html for a complete list of methods and their usage.
  • browserLanguages() NSArray of String The list of languages (see your browser's language preferences).
  • content() NSData Always null unless there was raw data posted with the request.
  • httpVersion() String For example, "HTTP/1.0".
  • userInfo() NSDictionary A set of key/value pairs, passed around during the processing of the request. Empty by default, but you can use it to convey data around the framework.

I don't want to spend too much space on these functions since their usefulness is limited.

Response Object

The Request's counterpart is the Response object, which encapsulates the page returned to the requesting browser. The WOResponse class is also a subclass of WOMessage, so some of the characteristics we'll cover come from that base class.

Typically, the Response object is created by the framework and passed around the elements hierarchy during the generation phase. Then it is passed to the Component, Session and Application objects, during the post-generation phase. All of this is accomplished by function appendToResponse().

The information in this object is very similar to the Request object. The difference is that you can change this information, and write to the buffer holding the returned page. Here's an overview of the kind of information you can get and set:

  • A list of cookies. Cookies will be discussed in their own section.
  • A list of resulting headers.
  • The actual contents of the returned page.
  • Other request information (status, encoding, protocol).

Response Headers

We saw how to get headers in previous sections. The same getter functions exist in the WOResponse object (headerKeys(), headerForKey(), headersForKey()). On top of that, you can set the headers returned to the browser:

setHeader(String value, String key) and setHeaders(NSArray values, String key): These functions fix the value (or values) of a header, given its name (key). Use the first for single-value headers; use the second for multiple values.

A list of basic response headers can be seen at:
http://www.w3.org/Protocols/HTTP/Object_Headers.html

Response Text

For getting and setting the contents of the returned page, WOResponse has many methods. Here are a few of them:

  • NSData contents(): The raw bytes that constitute the returned HTML. An NSData object encapsulates an array of bytes (byte []), and the interesting functions are bytes() and length().
  • setContent(NSData): Replaces the whole returned page with the one you provide.
  • appendContentString(String): Adds the specified string to the end of the resulting page without changing anything.
  • appendContentHTMLString(String): Adds the specified string to the end of the page, but escapes the HTML-specific characters. For example, the character '<' will be changed to '&lt'.

WARNING: you usually don't want to use setContent() because it wipes out the previously computed HTML. Use the append...() methods instead.

Other Response Info

The "other information" methods of WOResponse include:

  • defaultEncoding()
    setDefaultEncoding(int)
    Gets and sets the default encoding, specifying the character set that should be used by the returned contents.
  • contentEncoding()
    setContentEncoding(int)
    Gets and sets the encoding, specifying the character set that is being used by the returned contents.
  • httpVersion() Gets and sets the a string indicating the protocol format
  • status()
    setStatus(int)
    Gets and sets the status, indicating if the generation was successful (status() returns 200) or if an error occurred. See this page for a list of status codes: http://www.w3.org/Protocols/HTTP/HTRESP.html
  • userInfo()
    setUserInfo(NSDictionary)
    A set of key/value pairs, passed around during the processing of the response. Empty by default, but you can use it to convey data around the framework.

Conclusion

As you've seen here, there is more to a request/response loop than simply asking for a page and getting the HTML back. Many objects and methods are called along the way. As we've seen, most of the methods can be overloaded to intervene in the loop at a specific moment. We've also looked at the ways to manipulate the request and response themselves. Again, what to do with this knowledge is up to you, but I can almost hear your brain pondering.


Emmanuel Proulx is a Course Writer, Author and Web Developer, working in the domain of Java Application Servers. He can be reached at emmanuelp@theglobe.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Latest Forum Discussions

See All

Blyss (Games)
Blyss 2.0 Device: iOS Universal Category: Games Price: $1.99, Version: 2.0 (iTunes) Description: Travel through Beautiful mountains, serene valleys and harsh deserts solving Blyss' unique and self-evolving puzzles. The endless... | Read more »
Road Not Taken (Games)
Road Not Taken 1.0 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: "It looks like a cute fairy tale, but this is a turn-based game that's thorny with challenge and packed with an incredible number... | Read more »
What is PokéVision -- and why doesn...
The biggest thing that Pokemon GO players want to know is where to find the Pokemon they don't already have. The crux of the game is catching 'em all, so tracking down elusive pocket monsters is generally task number one any time someone fires up... | Read more »
Win every gym battle in Pokemon GO
Let me know if this scenario sounds familiar. You've got a Pokemon GO gym battle within easy walking distance, and you've visited it many times. Maybe you've even dropped one of your Pokemon there to help defend the place and reap the benefits. [... | Read more »
Can PokeMatch help you find love with Po...
The unofficial Pokemon GO companion app space has exploded almost as fast as the game itself over the last few weeks. Aspiring app developers, many of them working solo, have given us apps that locate Pokemon, keep track of the server status, and... | Read more »
How to get started with Prisma
If there's one thing people like to do more than taking pictures with their smartphones, it's tinkering with those photos in some way. Numerous apps have sprung up over the last several years that allow you to use filters and special effects to... | Read more »
6 Pokemon GO updates you can expect, acc...
Pokemon GO had a scheduled appearance at this year's San Diego Comic-Con for a while, but it was only relatively close to the show that it was upgraded to a spot in Hall H. That's the biggest venue at SDCC, one usually reserved for the largest... | Read more »
How to evolve Eevee in Pokemon GO
By now, almost everyone should be hip to how to evolve Pokemon in Pokemon GO (and if not, there's a guide for that). Just gather enough candy of the appropriate type, feed them all to the Pokemon, and evolution happens. It's a miracle that would... | Read more »
CSR Racing 2: Guide to all game modes
It might not seem like there are all that many ways to go fast in a straight line, but CSR Racing 2 begs to differ. [Read more] | Read more »
Bulb Boy (Games)
Bulb Boy 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Multi-award winning 2D point & click horror adventure about a boy with a glowing head. | Read more »

Price Scanner via MacPrices.net

Bare Bones Software Releases Free TextWrangle...
Bare Bones Software has announced the release and immediate availability of TextWrangler 5.5, a significant update to its powerful, free, general purpose text editor for Mac OS X. TextWrangler is a... Read more
Apple’s 2016 Back to School promotion: Free B...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB Wi-Fi iPad Air 2: $499 - 64GB Wi-Fi iPad... Read more
13-inch 2.5GHz MacBook Pro available for $964...
Overstock has the 13″ 2.5GHz MacBook Pro available for $964.21 including free shipping. Their price is $135 off MSRP. Read more
External Keyboard Innovations For iPad Pro (1...
I’m an input device aficionado. With non-touchscreen computers, which includes all Macs, the keyboard and mouse or trackpad are the tactile points of interface between user and machine, and the... Read more
GSK Rheumatoid Arthritis Study Leverages iPho...
Global healthcare products company GlaxoSmithKline (GSK) says that since 2014 they have begun transforming the way they conduct research, by leveraging state-of-the-art digital technologies — a... Read more
Clearance 12-inch Retina MacBooks, Apple refu...
Apple has Certified Refurbished 2015 12″ Retina MacBooks available starting at $929. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1199 $100 off MSRP - 13″ 2.7GHz/... Read more
13-inch 1.6GHz/128GB MacBook Air on sale for...
Amazon has the 13″ 1.6GHz/128GB MacBook Air on sale for $200 off MSRP for a limited time. Shipping is free: - 13″ 1.6GHz/128GB MacBook Air (sku MMGF2LL/A): $799.99 $200 off MSRP Their price is the... Read more
13-inch 1.6GHz/256GB MacBook Air on sale for...
Amazon has the 13″ 1.6GHz/256GB MacBook Air on sale for $200 off MSRP for a limited time. Shipping is free: - 13″ 1.6GHz/256GB MacBook Air (sku MMGG2LL/A): $999.99 $200 off MSRP Their price is the... Read more

Jobs Board

*Apple* Retail - Multiple Positions, Willow...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Evangelist - JAMF Software (United S...
The Apple Evangelist is responsible for building and cultivating strategic relationships with Apple 's small and mid-market business development field teams. This Read more
*Apple* Solutions Consultant - APPLE (United...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
*Apple* Professional Learning Specialist - A...
Job Summary The Apple Professional Learning Specialist is a full-time position for one year with Apple in the Phoenix, AZ area. This position requires a high Read more
*Apple* Picker - Apple Hill Orchard (United...
Apple Hill Orchard, Co. Rte. 21,Whitehall, NY 9/7/16-10/228/16. Pick fresh market or processing apples Productivity of 60 boxes and 80 boxes processing fruit per Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.