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3.2. JBoss Rules Theory

3.2.1. Methods in JBoss Rules
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Methods are different to rules. They are called directly and are used to pass specific instances. A single call results in a single execution.
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3.2.2. Method Example
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A method looks like this: 
public void helloWorld(Person person) {
    if ( person.getName().equals( "Chuck" ) ) {
        System.out.println( "Hello Chuck" );
    }
}
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3.2.3. Rule Example
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A rule looks like this: 
rule "Hello World"
    when
        Person( name == "Chuck" )
    then
        System.out.println( "Hello Chuck" );
end
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3.2.4. Cross-Products
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When two or more sets of data are combined, the result is called a cross-product.
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3.2.5. Cross-Product Constraining
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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() );
end
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    The following output will be displayed: 
    room:office sprinkler:office
room:kitchen sprinkler:kitchen
room:livingroom sprinkler:livingroom
room:bedroom sprinkler:bedroom
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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.

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3.2.6. The Inference Engine
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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.
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3.2.7. Inference Example
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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
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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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