apache/skywalking · error · IllegalArgumentException

MAL filter expression parsing failed: {} in expression: {}

Error message

MAL filter expression parsing failed: {} in expression: {}

What it means

Parse failure specific to filter closure expressions: MALScriptParser parses a standalone filterExpression (the filter: block text like "{ tags -> tags.job_name == 'mysql-monitoring' }") into a ClosureArgument, and any ANTLR syntax error inside the closure aborts with IllegalArgumentException. Filter closures have their own grammar entry point, stricter than general expressions.

Source

Thrown at oap-server/analyzer/meter-analyzer/src/main/java/org/apache/skywalking/oap/meter/analyzer/v2/compiler/MALScriptParser.java:269

        final MALParser parser = new MALParser(tokens);

        final List<String> errors = new ArrayList<>();
        parser.removeErrorListeners();
        parser.addErrorListener(new BaseErrorListener() {
            @Override
            public void syntaxError(final Recognizer<?, ?> recognizer,
                                    final Object offendingSymbol,
                                    final int line,
                                    final int charPositionInLine,
                                    final String msg,
                                    final RecognitionException e) {
                errors.add(line + ":" + charPositionInLine + " " + msg);
            }
        });

        final MALParser.FilterExpressionContext tree = parser.filterExpression();
        if (!errors.isEmpty()) {
            throw new IllegalArgumentException(
                "MAL filter expression parsing failed: " + String.join("; ", errors)
                    + " in expression: " + filterExpression);
        }

        return new ClosureVisitor().visitClosure(tree.closureExpression());
    }

    /**
     * Visitor transforming ANTLR4 parse tree into MAL expression AST.
     */
    private static final class MALExprVisitor extends MALParserBaseVisitor<Expr> {

        @Override
        public Expr visitAdditiveExpression(final MALParser.AdditiveExpressionContext ctx) {
            Expr result = visit(ctx.multiplicativeExpression(0));
            for (int i = 1; i < ctx.multiplicativeExpression().size(); i++) {
                final ArithmeticOp op = ctx.getChild(2 * i - 1).getText().equals("+")
                    ? ArithmeticOp.ADD : ArithmeticOp.SUB;

View on GitHub (pinned to 102af09b4a)

Solutions

  1. Check the closure has the exact shape { tags -> <boolean expr> } with single-quoted string literals
  2. Verify boolean operators are supported ones (==, !=, and, or / && , ||) per the MAL filter grammar
  3. Look at line:column in the message relative to the start of the filter string
  4. Test with MALScriptParser/MALClassGenerator.compileFilter in a unit test to iterate fast

Example fix

# before
filter: { tags -> tags.job_name == "mysql-monitoring" }

# after
filter: { tags -> tags.job_name == 'mysql-monitoring' }
Defensive patterns

Strategy: try-catch

Validate before calling

try {
    MALClassGenerator.compileFilter(filterText, ...);
} catch (Exception e) {
    throw new AssertionError("Bad filter: " + e.getMessage());
}

Try / catch

catch IllegalArgumentException around filter compilation; include the filter text and file in the error

Prevention

When it happens

Trigger: A filter: block whose closure is malformed: missing '{' '}' braces, missing 'tags ->' parameter arrow, using double quotes where the closure grammar expects single-quoted strings, or an unsupported operator inside the closure body.

Common situations: Editing the file-level filter: in otel-rules YAMLs (e.g. filtering by job_name); quoting mistakes carried over from the rule expression style — inside closures string comparisons use single quotes ('value'), opposite of extension argument double quotes; copy-paste from docs that reformat the arrow.

Related errors

AI-assisted analysis of apache/skywalking@102af09b4a (2026-08-14). Data as JSON: /api/errors/785c2a3006d8f206. Report an issue: GitHub.