Follow the Chain of Responsibility

Run through server-side and client-side CoR implementations

By David Geary, JavaWorld.com, 08/29/03

I recently switched to Mac OS X from Windows and I'm thrilled with the results. But then again, I only spent a short five-year stint on Windows NT and XP; before that I was strictly a Unix developer for 15 years, mostly on Sun Microsystems machines. I also was lucky enough to develop software under Nextstep, the lush Unix-based predecessor to Mac OS X, so I'm a little biased.

Aside from its beautiful Aqua user interface, Mac OS X is Unix, arguably the best operating system in existence. Unix has many cool features; one of the most well known is the pipe, which lets you create combinations of commands by piping one command's output to another's input. For example, suppose you want to list source files from the Struts source distribution that invoke or define a method named execute(). Here's one way to do that with a pipe:

   grep "execute(" `find $STRUTS_SRC_DIR -name "*.java"` | awk -F: '{print }'

The grep command searches files for regular expressions; here, I use it to find occurrences of the string execute( in files unearthed by the find command. grep's output is piped into awk, which prints the first token—delimited by a colon—in each line of grep's output (a vertical bar signifies a pipe). That token is a filename, so I end up with a list of filenames that contain the string execute(.

Now that I have a list of filenames, I can use another pipe to sort the list:

  grep "execute(" `find $STRUTS_SRC_DIR -name "*.java"` | awk -F: '{print }' | sort

This time, I've piped the list of filenames to sort. What if you want to know how many files contain the string execute(? It's easy with another pipe:

  grep "execute(" `find $STRUTS_SRC_DIR -name "*.java"` | awk -F: '{print }' | sort -u | wc -l

The wc command counts words, lines, and bytes. In this case, I specified the -l option to count lines, one line for each file. I also added a -u option to sort to ensure uniqueness for each filename (the -u option filters out duplicates).

Pipes are powerful because they let you dynamically compose a chain of operations. Software systems often employ the equivalent of pipes (e.g., email filters or a set of filters for a servlet). At the heart of pipes and filters lies a design pattern: Chain of Responsibility (CoR).

Note: You can download this article's source code from Resources.

CoR introduction

The Chain of Responsibility pattern uses a chain of objects to handle a request, which is typically an event. Objects in the chain forward the request along the chain until one of the objects handles the event. Processing stops after an event is handled.

Figure 1 illustrates how the CoR pattern processes requests.

In Design Patterns, the authors describe the Chain of Responsibility pattern like this:

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

The Chain of Responsibility pattern is applicable if:

• You want to decouple a request's sender and receiver
• Multiple objects, determined at runtime, are candidates to handle a request
• You don't want to specify handlers explicitly in your code

If you use the CoR pattern, remember:

• Only one object in the chain handles a request
• Some requests might not get handled

Those restrictions, of course, are for a classic CoR implementation. In practice, those rules are bent; for example, servlet filters are a CoR implementation that allows multiple filters to process an HTTP request.

Figure 2 shows a CoR pattern class diagram.

Typically, request handlers are extensions of a base class that maintains a reference to the next handler in the chain, known as the successor. The base class might implement handleRequest() like this:

   public abstract class HandlerBase {
         ...
         public void handleRequest(SomeRequestObject sro) {
            if(successor != null)
                  successor.handleRequest(sro);
         }
   }

So by default, handlers pass the request to the next handler in the chain. A concrete extension of HandlerBase might look like this:

   public class SpamFilter extends HandlerBase {
      public void handleRequest(SomeRequestObject mailMessage) {
         if(isSpam(mailMessage))   { // If the message is spam
            // take spam-related action. Do not forward message.
         }
         else { // Message is not spam.
            super.handleRequest(mailMessage); // Pass message to next filter in the chain.
         }
      }
   }

The SpamFilter handles the request (presumably receipt of new email) if the message is spam, and therefore, the request goes no further; otherwise, trustworthy messages are passed to the next handler, presumably another email filter looking to weed them out. Eventually, the last filter in the chain might store the message after it passes muster by moving through several filters.

Note the hypothetical email filters discussed above are mutually exclusive: Ultimately, only one filter handles a request. You might opt to turn that inside out by letting multiple filters handle a single request, which is a better analogy to Unix pipes. Either way, the underlying engine is the CoR pattern.

In this article, I discuss two Chain of Responsibility pattern implementations: servlet filters, a popular CoR implementation that allows multiple filters to handle a request, and the original Abstract Window Toolkit (AWT) event model, an unpopular classic CoR implementation that was ultimately deprecated.

Servlet filters

In the Java 2 Platform, Enterprise Edition (J2EE)'s early days, some servlet containers provided a handy feature known as servlet chaining, whereby one could essentially apply a list of filters to a servlet. Servlet filters are popular because they're useful for security, compression, logging, and more. And, of course, you can compose a chain of filters to do some or all of those things depending on runtime conditions.

With the advent of the Java Servlet Specification version 2.3, filters became standard components. Unlike classic CoR, servlet filters allow multiple objects (filters) in a chain to handle a request.

Discuss

Subject	Replies	Last post
CoR is admittedly nice in some ways By Jeff Varszegi	3	11/25/06 03:57 AM by Anonymous
pipeline isn't CoR By Quirn	3	10/29/06 04:29 AM by Anonymous

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