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Using Drools in Your Enterprise Java Application

by Paul Browne

These days enterprise Java could almost put you to sleep. How many hundreds of J2EE-EJB web applications have been written that capture information from a web page and store it in a database? What really keeps developers awake at night is trying to write and maintain the complex business logic in their applications. This is a problem not only for new applications, but increasingly, for long-lived, business-critical apps whose internal logic needs to change frequently, often at very short notice.

In an earlier article, "Give Your Business Logic a Framework with Drools," I introduced the Drools framework and showed how it could be used to organize complicated business logic. Drools replaced many tangled if ... then statements with a simple set of things known to be true. If you are ever in a meeting with business customers, and your head hurts with the complexity of what they want you to implement, then maybe you should consider a rule engine such as Drools. This article will show you how you can do this in an enterprise Java application.

Frameworks All the Way Down

Most enterprise Java developers already have their favorite frameworks. In no particular order, these include presentation frameworks (Struts, JSF, Cocoon, and Spring), persistence frameworks (JDO, Hibernate, Cayenne, and Entity Beans) and structural frameworks (EJB, Spring again, Pico, and Excalibur), as well as many others. Each framework does one very useful thing (or more), and gives developers a lot of instant "out of the box" functionality. Deploying an application using frameworks means you avoid a lot of the boring bits and concentrate on what is really needed.

Until now, there was a gap in what the frameworks were able to do, in that business logic had no framework. Tools like EJB and Spring are good, but have little to say about how to organize your if ... then statements! Adding Drools to your developer toolbox means that it is now possible to build an application with "frameworks all the way down." Figure 1 shows a diagram of such an application.

Figure 1
Figure 1. Frameworks for Java applications

This article will build on what we already know of the Drools framework and allow us to build such an application.

When Should I Use a Rule Engine?

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It's almost a cliche in software engineering to say that "if you have a hammer, everything looks like a nail." While rule engines can solve a lot of problems for us, it is worth considering whether a rule engine is really appropriate for our enterprise Java application. Some questions to ask are:

  • How complex is my application? For applications that shuffle data to and from a database, but not much more, it is probably best not to use a rule engine. However, where there is even a moderate amount of processing implemented in Java, it is worthwhile to consider the use of Drools. This is because most applications develop complexity over time, and Drools will let you cope easily with this.
  • What is the lifetime of my application? The answer to this is often "surprisingly long"--remember the mainframe programmers who thought their applications wouldn't be around for the year 2000? Using a rule engine pays off, especially in the medium to long term. As this article demonstrates, even prototypes can benefit from the combination of Drools and agile methods to take the "prototype" into production.
  • Will my application need to change? The only sure thing about your requirements is that they will change, either during or just after development. Drools helps you cope with this by specifying the business rule in one or more easy-to-configure XML files.

What About Performance?

If you're writing an enterprise application, chances are that it will need to scale to hundreds, if not thousands, of users. You know that existing Java and J2EE applications can do this, but how will a application using Drools cope with this pressure? The answer is "surprisingly well." While most developers hate to "lose control" and rely on other people's code (i.e., a framework), consider the points below--not only should your application be as fast as "traditional" coding methods, but Drools may even make your application run faster:

  • Avoids badly written code: Drools guides developers to do "the right thing." You may be sure the code you are writing is good, but would you say the same for the code of your co-developers? Using a framework makes it easier to write good, fast code.
  • Optimized framework: How often have you seen business logic that repeatedly accesses a database for the same information, slowing down the entire application? Used correctly, Drools can remember not only the information, but also the results of previous tests using this information, giving the entire application a speed boost.
  • Rete algorithm: Many times we apply "if" conditions that we didn't really need. The Rete algorithm, as implemented by Drools, replaces all of the if ... then statements with an optimized network. It is important to note that the Rete algorithm involves a tradeoff between using more memory to reduce delays at run time. While this isn't a factor in most modern servers, we wouldn't yet recommend deploying Drools on your mobile phone!

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