apache/skywalking · error · IllegalArgumentException

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

Error message

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

What it means

Generic MAL expression parse failure: MALScriptParser.parse ran the ANTLR parser over an expression (the rule's combined expPrefix+exp+expSuffix text in production), the error listener collected one or more syntax errors, and parsing aborted with IllegalArgumentException carrying the accumulated line:column messages. This is the top-level 'your MAL expression is not grammatically valid' error.

Source

Thrown at oap-server/analyzer/meter-analyzer/src/main/java/org/apache/skywalking/oap/meter/analyzer/v2/compiler/MALScriptParser.java:235

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

        return new MALExprVisitor().visit(tree.additiveExpression());
    }

    /**
     * Parse a standalone filter closure expression into a {@link ClosureArgument}.
     *
     * @param filterExpression e.g. {@code "{ tags -> tags.job_name == 'mysql-monitoring' }"}
     */
    public static ClosureArgument parseFilter(final String filterExpression) {
        final MALLexer lexer = new MALLexer(CharStreams.fromString(filterExpression));
        final CommonTokenStream tokens = new CommonTokenStream(lexer);
        final MALParser parser = new MALParser(tokens);

        final List<String> errors = new ArrayList<>();

View on GitHub (pinned to 102af09b4a)

Solutions

  1. Use the line:column prefix in the message to locate the offending token in the expression printed after 'in expression:'
  2. Compare with a known-good rule in the same directory (e.g. otel-rules vm.yaml) for quoting and structure
  3. Rebuild the full combined expression via MetricConvert.formatExp logic (expPrefix + exp + expSuffix) when testing locally so you see exactly what the parser sees
  4. Wrap a unit test around DSL.parse / MALScriptParser.parse to iterate quickly without restarting OAP

Example fix

# before
exp: cpu_usage.sum(['mode']) .avg(['host']

# after
expr: cpu_usage.sum(['mode']).avg(['host'])
Defensive patterns

Strategy: try-catch

Validate before calling

try {
    MALScriptParser.parse(expression);
} catch (IllegalArgumentException | IllegalStateException e) {
    throw new AssertionError("MAL syntax error: " + e.getMessage());
}

Try / catch

catch IllegalArgumentException/IllegalStateException from DSL.parse; log metricName + full expression + cause, and stop OAP so the bad rule cannot be half-loaded

Prevention

When it happens

Trigger: Any grammar violation in the final expression string: unbalanced parentheses, wrong quoting (single vs double quotes outside closures), unsupported operators, missing commas in argument lists, malformed Layer enum references, or an expSuffix scope function typed incorrectly.

Common situations: Authoring or editing otel-rules/*.yaml, meter-analyzer-config, telegraf/zabbix rule files; version upgrades that tightened the grammar (v1 Groovy accepted expressions the v2 ANTLR grammar rejects); YAML escaping issues where '>' or quotes get mangled.

Related errors

AI-assisted analysis of apache/skywalking@102af09b4a (2026-08-14). Data as JSON: /api/errors/4101ee9d992a4a7e. Report an issue: GitHub.