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HTML clipboardDefine a Rule Resource
Drools is based on rule resources. Most of the time, the rules are mapped in files with the .drl extension or in a spreadsheet format as decision tables (supported formats are Excel and CSV). A basic .drl file is a list of rules written in a file like this: #package name #list any import classes here. #declare any global variables here rule "Your First Rule" when #conditions then #actions end rule "Your Second Rule" #include attributes such as "salience" here... when #conditions then #actions end
As you can see, every rule is encapsulated between the rule and end keywords. A rule has a name and a when-then pair that represents the condition and the action. The optional parts are marked with pound signs (#). For a better understanding of what goes on in the code, consider the following code sample written as a .drl file (vote.drl):
Listing vote.drl package com.sample import com.sample.DroolsTest.Vote; rule "WrongValue – less than 0" when m : Vote( average < 0.0, vote : vote ) then m.setAverage(0.0f); update( m ); end rule "WrongValue – bigger than 10" when m : Vote( average > 10.0, vote : vote ) then m.setAverage(10.0f); update( m ); end rule "BadValue – between 0-3" when m : Vote( average >= 0.0 && average <=3.0, vote : vote ) then m.setVote("Bad!"); System.out.println( m.getVote() ); end rule "GoodValue – between 3-6" when m : Vote( average >3.0 && average <=6.0, vote : vote ) then m.setVote("Good!"); System.out.println( m.getVote() ); end rule "VeryGoodValue – between 6-9" when m : Vote( average >6.0 && average <=9.0, vote : vote ) then m.setVote("Very Good!"); System.out.println( m.getVote() ); end rule "ExcellentValue – between 9-10" when m : Vote( average >9.0 && average <=10.0, vote : vote ) then m.setVote("Excellent!"); System.out.println( m.getVote() ); end The getVote, setVote, getAverage, and setAverage functions are provided by a simple POJO like this (Vote.java):
Vote.java public static class Vote { private String vote; private float average; public String getVote() { return this.vote; } public void setVote(String vote) { this.vote = vote; } public float getAverage() { return this.average; } public void setAverage(float average) { this.average = average; } } As to decision tables, consider them if you have rules that can be expressed as rule templates plus data. In each row of a decision table, you collect data and combine it with the templates to generate a rule.
Author's Note: Keep in mind that in a decision table, each row is a rule and each column in that row is either a condition or an action for that rule.
In practice, you build decision tables on a set of keywords that you place in the right rows and columns. For example, the RuleTable keyword indicates the start of a rule table (both the starting row and column). Usually under the starting row, you will have a row that contains the keywords CONDITION and/or ACTION. Be sure to indicate that the data in the columns below are for either the LHS (Left Hand Side) or the RHS (Right Hand Side) parts of a rule. The next row will contain declarations of ObjectTypes. When you use this row, the values in the cells below become constraints on that object type. Further down, you have the rule templates themselves. Here you can use the $param placeholder to indicate where data from the cells below will be populated (or $1, $2, $3, etc. to indicate parameters from a comma-separated list in a cell below). Finally, the last two rows will contain data, which are interpolated with templates, to generate rules.
Table 1 lists more keywords (this table is available in the Drools Reference Manual).
Table 1. Drools Keywords
| Keyword | Description | Required |
| RuleSet | The cell to the right of RuleSet contains the ruleset name. | One only (or default) |
| Sequential | The cell to the right of Sequential can be true or false. If true, then "salience" is used to ensure that rules fire from the top down. | Optional |
| Import | The cell to the right of Import contains a comma-separated list of Java classes to import. | Optional |
| RuleTable | A cell starting with RuleTable indicates the start of a definition of a rule table. The actual rule table starts the next row down. The rule table is read left to right and top down until there is one blank row. | At least one. If there are more, then they are all added to the one ruleset. |
| CONDITION | This keyword indicates that this column will be for rule conditions. | At least one per rule table |
| ACTION | This keyword indicates that this column will be for rule consequences. | At least one per rule table |
| PRIORITY | This keyword indicates that this column's values will set the "salience" values for the rule row. It overrides the 'Sequential' flag. | Optional |
| DURATION | This keyword indicates that this column's values will set the duration values for the rule row. | Optional |
| NAME | This keyword indicates that this column's values will set the name for the rule generated from that row. | Optional |
| Functions | The cell immediately to the right of Functions can contain functions, which can be used in the rule snippets. Drools supports functions defined in the DRL, allowing logic to be embedded in the rule, and changed without hard coding. Use with care and with the same syntax as regular DRL. | Optional |
| Variables | The cell immediately to the right of Variables can contain global declarations, which Drools supports. This is a type, followed by a variable name. (If multiple variables are required, comma-separate them). | Optional |
| UNLOOP | This keyword indicates that if there are cell values in this column, the no-loop attribute should be set. | Optional |
| XOR-GROUP | Cell values in this column mean that the rule-row belongs to the given XOR/activation group. An activation group means that only one rule in the named group will fire (i.e., the first one to fire cancels the other rule activations). | Optional |
| Worksheet | By default, only the first worksheet is looked at for decision tables. | N/A |
