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3.2. JBoss Rules Theory
3.2.1. Methods in JBoss Rules
Methods are different to rules. They are called directly and are used to pass specific instances. A single call results in a single execution.
3.2.2. Method Example
A method looks like this:
public void helloWorld(Person person) {
if ( person.getName().equals( "Chuck" ) ) {
System.out.println( "Hello Chuck" );
}
}
3.2.3. Rule Example
A rule looks like this:
rule "Hello World"
when
Person( name == "Chuck" )
then
System.out.println( "Hello Chuck" );
end3.2.4. Cross-Products
When two or more sets of data are combined, the result is called a cross-product.
3.2.5. Cross-Product Constraining
Procedure 3.6. Task
- To prevent a rule from outputting a huge amount of cross-products, you should constrain the cross-products themselves. Do this using the variable constraint seen below:
rule when $room : Room() $sprinkler : Sprinkler( room == $room ) then System.out.println( "room:" + $room.getName() + " sprinkler:" + $sprinkler.getRoom().getName() ); endThe following output will be displayed:room:office sprinkler:office room:kitchen sprinkler:kitchen room:livingroom sprinkler:livingroom room:bedroom sprinkler:bedroom
Result
Only four rows are outputted with the correct sprinkler for each room. Without this variable, every row in the Room table would have been joined with every row in the Sprinkler table resulting in many lines of output.
3.2.6. The Inference Engine
The inference engine is the part of the JBoss Rules engine which matches production facts and data to rules. It will then perform actions based on what it infers from the information. A production rules system's inference engine is stateful and is responsible for truth maintenance.
3.2.7. Inference Example
In this example, a Person fact with an age field and a rule that provides age policy control is used. Inference is used to determine if a Person is an adult or a minor, then act on the result:
rule "Infer Adult" when $p : Person( age >= 18 ) then insert( new IsAdult( $p ) ) end
In the above snippet, every Person who is 18 or over will have an instance of IsAdult inserted for them. This fact is special in that it is known as a relation. We can use this inferred relation in any rule:
$p : Person() IsAdult( person == $p )
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