Pay As You Go MVC in JSPs and Servlets Larry Karnowski OpenNMS.org OReilly Open Source Convention Ju

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Pay As You Go MVC in JSPs and ServletsLarry
Karnowski OpenNMS.orgOReilly Open Source
ConventionJuly 22-26, 2002
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What is OpenNMS?

• The Open Network Management System
• A Java-based system for discovering, monitoring,
  and managing computer networks
• OpenNMS Web UI has almost 150 JSPs, almost 100
  servlets, and over 200 helper classes

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What should you already have?

• Basic knowledge of Model-View-Controller design
  pattern
• Basic knowledge of Java Server Pages
• Basic knowledge of Java Servlets
• A desire to do more with less in your web
  applications

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Pay As You Go MVC in JSPs and Servlets

• MVC is important in large user interfaces
• However, for small or quickly developed user
  interfaces, MVC can take too much time
• Many developers opt to not use any MVC designs at
  all, which greatly increases cost of maintenance
• Instead, use my strategies for minimal MVC that
  can be scaled up to true MVC when the cost is
  warranted

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What is MVC?

• Model-View-Controller, a trusted design pattern
  to ease user interface development
• Separates concerns to ease maintenance and
  parallel development
• Model - business logic, completely nonvisual
• View - handles human interaction, completely
  visual, no concept of business logic
• Controller - ferries events and data between the
  model and view, decoupling them
• Rule of thumb - Your model should be reusable
  without change regardless of UI medium (web,
  command line, GUI, virtual reality, etc)

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Why not always use MVC?

• In theory, you always should!
• However, not all projects seem worth the cost of
  MVC
• Requires more developer expertise than simple
  JSPs (more Java code, Servlet APIs, etc)
• Requires more up-front development time
• Quicker to prototype without MVC
• Particularly in Open Source projects - developers
  familiar with PHP or ASPs can contribute easier
  if less MVC

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How do I pay as I go?

• Use simpler UI-Delegate version of MVC
• More JSP-centric, less Java code, less separation
  of concerns
• Gives many benefits of pure MVC at less cost in
  development time and developer expertise
• Uses coding strategies to avoid poorly written
  JSPs that tightly couple model, view, and
  controller code
• Given a good template JSP, easy to copy style by
  beginner developers code reviews essential
• Easier to decouple and scale to pure MVC later

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UI-Delegate Adaptation of MVC

• Popular in Swing (JFC)
• View and controller are combined to create the
  UI
• A nonvisual model is still required, now called
  the delegate
• Same rules apply to the model from pure MVC
• Easily implemented in JSPs by just adding the
  controller logic

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UI-Delegate in JSPs

• Have controller Java code and view HTML in the
  same JSP, but not intermixed
• Avoid Java in the HTML section of your JSP, keep
  all controller code in a block at the top
• Use only expressions, for-loops, and
  if-conditionals in your HTML body (or use a tag
  library if available)
• Have any convenience methods in a declaration
  block at the far bottom
• Avoid convenience methods that generate HTML,
  keep all HTML in JSP body

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Benefits of Pay as You Go

• Open Source concept - Pages start out as
  bazaars and only necessary pages become
  cathedrals
• Extreme programming concept - Code for today
  if you are not sure yet which pages will need to
  be complex, there is no need to make them all
  complex
• Adds another tool in your toolkit - like
  Perl/Python for a Java/C developer, make sure you
  use the right tool for the job
• Ability to cheat when deadlines loom without
  hating yourself too much

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Moving to a Purer MVC Later

• When JSP gets too complex or when more parallel
  development is necessary
• Move to JSP Model 2 - move controller code to a
  servlet, have servlet forward request to JSP
• Move to Struts - move controller code to a Struts
  action object and use JSP for display
• Move to Velocity - move controller code to a
  servlet, move HTML to a Velocity template, have
  servlet fill out a Velocity context and evaluate
  the template

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Summary

• Keep all business logic in the model!!
• Loose decoupling gives you some benefits from
  both JSP syntax and MVC design pattern
• Only adapt the pages that truly need it to pure
  MVC UI-Delegate is better for most pages in most
  projects
• know when to break your own rules

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