Simple Drools Example

Implementing an expert system using Drools is quiet easy. First of all create ordinary java project in Eclipse. The following image shows the project structure in Eclipse:

There are two source folders, first one is src/main/java to save java classes and second one is src/main/rules to save Drools rule files. Don't forget to include all required libraries in Drools distribution archive to this project. Two important files that need attention are Main.java and validation.drl. Let's see each of them.

The Main.java contains one method main(). In the main method we load the validation.drl file, create a Customer object, and then fire the validation rule. We also add a Map object that is used to save error messages during validation process. Next we will see the validation.drl.

The .drl file has its own format structure. First we state package name, this is used for grouping only. No same Drools file name exists in the same package. Then we import all classes just like we do in java file. Next we create a series of rules.

In the validation.drl above we have two rules, addressMandatory to validate address, and emailMandatory to validate email. In the when clause we get objects that are inserted through session in KnowledgeBase before; the HashMap object which we put inside a variable named errors for later use and also Customer object that we put inside a variable named cust. While addressMandatory rule checks whether the customer's address is null or not, emailMandatory rule checks whether the customer's email is either null or empty.

If the address or email is null we put an error message to the errors variable so that we can write it to console in the main() method. We can safely write any Java code inside the then clause as we do in ordinary java file.

When we run the main() method, the following output is displayed on console:

Java Compiler API

Java Compiler API has been around for a while since Java 6. We can use it to compile java files at runtime. Let's now have fun with it:

Inside main method above we have two java files to compile. Next we obtain the JavaCompiler object from ToolProvider. The JavaCompiler object then does its job to compile those files using verbose parameter supplied. Verbose parameter is useful to display compilation log. When we run the code, two .class files are created at the same location of the original files. Here is the output on console:

Let's now see another approach to compile java files.

First we state the files to compile. Then we obtain the JavaCompiler object from ToolProvider as we did before. After that we create an object of type DiagnosticCollector. This object is used to collect information during compilation process. Next, we get a StandardJavaFileManager by passing the DiagnosticCollector to the JavaCompiler's method getStandardFileManager. From the StandardJavaFileManager we create an Iterable object that contains the files being compiled. Then the actual compilation process is done by CompilationTask object by calling call() method. If an error occurs during compilation, we print the error information that has been collected by our DiagnosticCollector.

Instead of using DiagnosticCollector, we can use DiagnosticListener to collect error information. To do that we have to implement DiagnosticListener interface as shown below:

Then we can use it to compile java files as shown below:

We should be familiar with the code above since in fact we have seen it before. The only difference is that we replace the DiagnosticCollector with our own MyDiagnosticListener. Both DiagnosticCollector and MyDiagnosticListener implements DiagnosticListener interface.

So far we have seen how to compile java files reside on disk. Next we'll see how to compile java file from a string.

 

Again, the code above looks familiar but instead of getting the java content from files we get the java content from a string. The string itself is wrapped inside an object of type JavaObjectFromString which extends SimpleJavaFileObject which implements JavaFileObject as expected by our Iterable object.

To load the compiled class, we can use URLClassLoader by supplying it with a path where the compiled classes are put. Those classes are most likely compiled in the working directory.