实施UML九项注意

In the last quarter of 1997, I taught object-oriented analysis and design with UML to a wide range of developers working on various projects. The projects spanned application domains including retail shelf space management, pharmaceutical clinical trials, cellular telephony, and more. Implementation languages included Visual Basic, C++, Java, DB2, and others. This article discusses several aspects of how well the UML notation, along with my company's use case-driven Unified Object Modeling process (also based on the methods of Booch, Jacobson, and Rumbaugh), met the demands of a diverse set of projects. It provides practical guidance for tailoring your use of UML for various projects.

The UML notation is big (maybe too big) and is flexible enough to accommodate the needs of a very wide range of projects. To succeed with UML, you must streamline how you use it. Different projects will need different pieces. Which specific elements of the UML notation you need for your project will depend on its nature (client/server with a mainframe RDBMS vs. real-time embedded, for example) and on the implementation language you will be using. Some detailed C++ design constructs are not needed if you're building in Java or Smalltalk, and you may want to avoid too much use of generalization or inheritance if you're building in Visual Basic.

Sometimes, the sheer volume of modeling constructs can be overwhelming, especially to those students who are new to object-oriented analysis and design. However, the good news is that you can model almost anything using UML. There are plenty of modeling constructs to go around. You Still Need a Process

UML itself is only a notation; to build effective models, you still need a modeling process. There are many modeling approaches you can use with UML; the modeling language itself does not prescribe the process. My company has been successful teaching a slightly modified version of the Objectory process (derived from Ivar Jacobson's Object-Oriented Software Engineering: A Use Case Driven Approach, Addison-Wesley, 1992) that we developed as a synthesis of Booch/Rumbaugh/Jacobson several years prior to the advent of UML. Our approach places slightly more emphasis on identifying high-level static models (domain models) up front, in parallel with use cases. We then refine the static model iteratively and incrementally refine as we explore the use cases and dynamic models.

Whether you prefer using Objectory, Object Modeling Technique, our ICONIX Unified Object Modeling process, or some other approach, it's important to understand that UML is not a process, and that it is critically important to get your project team on the same page process-wise before undertaking a modeling effort. Once again, the size and bulk of UML (and the overwhelming weight of documentation on notation as opposed to process) can make it easy to slip into "analysis paralysis." Focusing the team on an understandable process that is supported by the UML notation can generally get the modeling effort underway.

Legacy Methods Are Important

Many of my students ask whether developing an understanding of Booch, Jacobson, and Rumbaugh's legacy methods is still important. My answer is an emphatic yes. Just as knowing the symbols used for electronic circuit design doesn't eliminate the need to know circuit theory, understanding the notational aspects of UML doesn't eliminate the need to understand object modeling theory. Since UML represents the synthesis of the works of Jacobson, Booch, and Rumbaugh, the original legacy methods are a rich source of this information.

Keep It Simple

Getting a project team to make effective use of UML is tricky. Team members will have varied experience with object-oriented analysis and design and UML. A training workshop is a good way to begin. The workshop needs to be tailored to the specific needs of each project, taking the backgrounds of the various team members into careful consideration. The most critical tailoring decisions will ultimately involve judicious choices of what gets left out of the course agenda, as well as the instructor's ability to adjust on-the-fly during the workshop.

One of the most important things to remember when learning UML is that you don't need to use every construct just because it's there. Keep the process as simple as possible. Streamlining a project's documentation set and modeling approach does wonders for generating forward momentum.

Modeling with UML is similar to sitting down to an huge plate of food-sometimes, the thought that you can't possibly eat everything on the plate just kills your appetite before you get started. A similar phenomenon can occur with UML modeling. The thought of having to produce a complete set of sequence, collaboration, state, deployment, use case, and class diagrams that comprehensively covers each and every use case of the system with a fully detailed dynamic model can intimidate a team right out of object-oriented analysis and design.

The same thought process holds true for determining which constructs are employed within a given modeling technique. For example: is it really necessary to employ both USES and EXTENDS on a use case diagram, or can we live with just USES? My experience has been that the more streamlining that gets done, the better the chances of the modeling effort being carried through.

Write the User Manual Before You Design the Classes

One of the old saws of programming is to write the user manual before you write the code. This was good advice when I learned it, and it's still good advice today. In the days of structured methods, and in the early days of object-oriented methods, it was seldom followed. In his use case-driven modeling approach, Jacobson codified this maxim into a series of steps that work for object orientation and can be described using UML. Each step builds upon the previous step in a traceable manner, so that ultimately, management can enforce this approach as a design paradigm and verify that it has been followed at each step in the analysis and design process.

The key to understanding the essence of Objectory and use case-driven object modeling at the fundamental level is simple: write the user manual before you design the classes. Keeping this idea in the front of your mind will help guide you as you travel through the maze of UML static and dynamic model notations. Each use case represents a portion of the user manual, and should be written that way if the goal of your use case analysis is to produce an object model.

Organize Use Cases into Packages

As you begin to write the user manual for your system one use case at a time, you will immediately run into the need for a high-level organization of use cases. UML lets you organize your use cases into packages. These are represented as tabbed-folder icons. Each package should consist of at least one use case diagram, which will serve as a context diagram under which you can link all the use case descriptions along with the design-level dynamic model views for each use case. Some projects start with a top-level breakdown of one package per top-level menu. While this breakdown does not always represent the final breakdown, it's sometimes a helpful starting place.

Use the Objectory Stereotypes

Since we're driving the entire design process from the use cases, it makes sense to focus strongly on describing them in the "right" way. While it's becoming increasingly popular to employ use cases as an alternative to requirements specifications and for business process modeling, and while these styles of use case modeling have somewhat different guidelines, most projects I've run across still view use cases as a way to get to an object model.

Jacobson's original OOSE/O