Error Messages for DRL Troubleshooting

Aletyx Enterprise Build of Drools provides standardized messages for DRL errors to help you troubleshoot and resolve problems in your DRL files. The error messages provided with some of the examples included in this section are designed to help you locate and resolve your issue.

There are four main types of error codes that will be found within this section and multiple may apply in a single file at any given time.

The Error Codes

Drools Parsing Errors: Understanding Common Issues

When working with Drools, parsing errors can be tricky. Below is a table of common errors, what they mean, and how to resolve them.

Error Code	Error Name	What It Means	Common Causes	How to Fix It
101	No Viable Alternative	The parser reached a decision point but couldn’t determine a valid option.	- Misspelled keywords (e.g., rul instead of rule) - Incorrect syntax order	- Double-check spelling and syntax - Use a Drools LSP plugin to highlight syntax issues
102	Mismatched Input	Drools expected a specific character or token, but found something else instead.	- Missing or extra symbols like ], } or ; - Unexpected data type	- Ensure proper closing brackets - Verify the expected input at that location
103	Failed Predicate	A condition within Drools' internal validation rules was not met.	- Using declare, rule, exists, or not incorrectly - Rule constraints failing validation	- Review how predicates are used in your rule - Check Drools documentation for correct syntax
105	Did Not Match Anything	The parser reached a required section in the grammar, but no valid match was found.	- An empty or incorrect rule block - Misuse of conditional constructs	- Ensure every rule has a valid structure - Use logging/debugging to track where parsing fails

DRL Error Structure Visualization Methods

rule "error test rule"
when
    WorkerPerformanceContext(
        department == "Engineering",
        rating > 3,
        yearsOfService > 5)
    exists(Employee(department == $department))
then
    System.out.println("High performer identified");
end

Let's breakdown the structure of a Drools Rules Language (DRL) error as understanding how they are presented so that we can solve the problem.

Let's first look at this line in a log from a rule we wrote

[ERR 101] Line 6:35 no viable alternative at input ')' in rule
'error test rule' in pattern WorkerPerformanceContext

+-------------+  +------------+  +-----------------------------+  +---------------------+  +--------------------------------+
| [ERR 101]   |  | Line 6:35  |  | no viable alternative       |  | in rule             |  | in pattern                     |
| Error code  |  | Location   |  | at input ')'                |  | 'test rule'         |  | WorkerPerformanceContext       |
| 1st Block   |  | 2nd Block  |  | Description - 3rd Block     |  | Component-4th Block |  | Pattern - 5th Block            |
+-------------+  +------------+  +-----------------------------+  +---------------------+  +--------------------------------+

Block	Example Value	Component	Description
1st Block	[ERR101]	Error Code	Identifies the specific type of error, in this case Error 101
2nd Block	Line 6:35	Line Number and Column	Exact location in the DRL source where the error occurred begins
3rd Block	no viable alternative at input ')'	Description	Explains what caused the error
4th Block	in rule 'test rule'	Component	Identifies which rule, function, or query contains the error
5th Block	in pattern WorkerPerformanceContext	Pattern	Specifies the pattern where the error occurred (if applicable)

• 1^st Block: Error code

• 2^nd Block: Line and column in the DRL source where the error occurred

• 3^rd Block: Description of the problem

• 4^th Block: Component in the DRL source (rule, function, query) where the error occurred

• 5^th Block: Pattern in the DRL source where the error occurred (if applicable)

Spelling Issue - No Viable Alternative

Example rule with incorrect spelling.

1: rule "simple rule"
2:   when
3:     exists /persons
4:     exits /students  // (1)
5:   then
6: end

1. Exists is spelt wrong and is exits so the rule talks Must be exists.

 [ERR 101] Line 4:4 no viable alternative at input 'exits' in rule "simple rule"

Error message.

    [ERR 101] Line 4:4 no viable alternative at input 'exits' in rule "simple rule"

Rule without a Name

Example rule without a rule name.

1: package org.drools.examples;
2: rule    // (1)
3:   when
4:     Object()
5:   then
6:     System.out.println("A RHS");
7: end

1. Must be rule "rule name" (or rule rule_name if no spacing). There is a requirement of a rule name to be present.

Error message.

    [ERR 101] Line 3:2 no viable alternative at input 'when'

In this example, the parser encountered the keyword when but expected the rule name, so it flags when as the incorrect expected token.

Incorrect Syntax Usage

Example rule with incorrect syntax

1. This is missing a closing quotation on the Andy parameter.

1: rule "simple rule"
2:    when // (1)
3:       /students[ name == "Andy ]
4:    then
5: end

1. This is missing a closing quotation on the Andy parameter.

Error message.

[ERR 102] Line 0:-1 no viable alternative at input '<eof>' in rule "simple rule" in pattern student

A line and column value of 0:-1 means the parser reached the end of the source file (<eof>) but encountered incomplete constructs, usually due to missing quotation marks "…​", apostrophes '…​', or parentheses (…​).

102: mismatched input Indicates that the parser expected a particular symbol that is missing at the current input position.

Incomplete Rule Statement

Example rule with an incomplete rule statement.

1: rule "simple rule"
2:   when
3:     $p : /persons[
        // Must be a complete rule statement

Error message.

[ERR 102] Line 0:-1 mismatched input '<eof>' expecting ']' in rule "simple rule" in pattern person

A line and column value of 0:-1 means the parser reached the end of the source file (<eof>) but encountered incomplete constructs, usually due to missing quotation marks "…​", apostrophes '…​', or parentheses (…​).

Example rule with incorrect syntax.

 1: package org.drools.examples;
 2:
 3: rule "Product Pricing Check"
 4: when
 5:     not /products[ (category == "electronics", price < 500) //(1)
 6:                      || (category == "furniture", price < 200) ]
 7: then
 8: System.out.println("All products properly priced according to minimum pricing policy");
 9: end

1. Must use && operators instead of commas ,

Error messages.

[ERR 102] Line 5:36 mismatched input ',' expecting ')' in rule "Wrong syntax" in pattern car
[ERR 101] Line 5:57 no viable alternative at input 'type' in rule "Wrong syntax"
[ERR 102] Line 5:106 mismatched input ']' expecting 'then' in rule "Wrong syntax"

In this example, the syntactic problem results in multiple error messages related to each other. The single solution of replacing the commas , with && operators resolves all errors. If you encounter multiple errors, resolve one at a time in case errors are consequences of previous errors.

103: failed predicate Indicates that a validating semantic predicate evaluated to false. These semantic predicates are typically used to identify component keywords in DRL files, such as declare, rule, exists, not, and others.

Invalid Keyword

Example rule with an invalid keyword.

1: package nesting;
2:
3: import org.drools.compiler.Person
4: import org.drools.compiler.Address
5:
6: Some text
7: // (1)
8: rule "test something"
9:   when
10:     $p: /persons[ name=="Michael" ]
11:   then
12:     $p.name = "other";
13:     System.out.println(p.name);
14: end

1. Must be a valid DRL keyword

Error message.

[ERR 103] Line 6:0 rule 'rule_key' failed predicate: (validateIdentifierKey(DroolsSoftKeywords.RULE))? in rule

The Some text line is invalid because it does not begin with or is not a part of a DRL keyword construct, so the parser fails to validate the rest of the DRL file.

This error is similar to 102: mismatched input, but usually involves DRL keywords.

105: did not match anything Indicates that the parser reached a sub-rule in the grammar that must match an alternative at least once, but the sub-rule did not match anything. The parser has entered a branch with no way out.

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