apache/skywalking · error · IllegalStateException

Failed to compile MAL expression for metric: {}, expression:

Error message

Failed to compile MAL expression for metric: {}, expression: {}

What it means

Wrapper exception from DSL.parse(metricName, expression, ...): it compiles the expression through the MALClassGenerator (Javassist bytecode generation), and any Exception in the pipeline — ANTLR parsing, metadata validation, extension lookup, argument type checks, or Javassist compile — is rethrown as IllegalStateException naming the metric and the full expression. This is the primary user-facing failure when a MAL rule cannot be compiled at OAP startup.

Source

Thrown at oap-server/analyzer/meter-analyzer/src/main/java/org/apache/skywalking/oap/meter/analyzer/v2/dsl/DSL.java:119

            if (pool != null && targetClassLoader != null) {
                // Per-file generator: one instance per compile is fine — it's just a thin
                // orchestrator over ClassPool. Prevents cross-contamination of classNameHint /
                // sourceRef state that the shared GENERATOR carries between calls.
                final MALClassGenerator perFile = new MALClassGenerator(pool, targetClassLoader);
                perFile.setSourceRef(sourceRef);
                // The verbatim MAL expression text is the rule's "content" — threaded
                // into the GateHolder so dsl-debugging records carry the rule source
                // inline. Same string ANTLR parsed; no extra plumbing.
                perFile.setContent(expression);
                malExpr = perFile.compile(metricName, expression);
            } else {
                GENERATOR.setSourceRef(sourceRef);
                GENERATOR.setContent(expression);
                malExpr = GENERATOR.compile(metricName, expression);
            }
            return new Expression(metricName, expression, malExpr);
        } catch (Exception e) {
            throw new IllegalStateException(
                "Failed to compile MAL expression for metric: " + metricName
                    + ", expression: " + expression, e);
        }
    }
}

View on GitHub (pinned to 102af09b4a)

Solutions

Inspect e.getCause() — the real error and its message (one of the specific compiler errors) determines the fix
Fix the underlying issue in the rule YAML (syntax, quoting, scope, extension name/arity) and restart OAP
For Javassist-level failures, set SW_DYNAMIC_CLASS_ENGINE_DEBUG=true to dump the generated .java/.class and compare with what the compiler produced
Validate rules in a unit test using DSL.parse before deploying, so errors surface in CI rather than at OAP boot

Example fix

// before
Expression e = DSL.parse(metricName, expression, sourceRef, pool, loader);

// after
Expression e;
try {
    e = DSL.parse(metricName, expression, sourceRef, pool, loader);
} catch (IllegalStateException ex) {
    log.error("MAL rule '{}' failed to compile: {}", metricName, ex.getCause().getMessage(), ex);
    throw ex;
}

Defensive patterns

Strategy: try-catch

Validate before calling

try {
    DSL.parse(metricName, expression, sourceRef, pool, targetClassLoader);
} catch (IllegalStateException e) {
    throw new AssertionError("Rule '" + metricName + "' invalid: " + e.getMessage(), e);
}

Try / catch

catch IllegalStateException around each DSL.parse during rule loading; log metricName + expression + e.getCause(), and abort startup so a half-registered rule set cannot run

Prevention

Compile all custom MAL rules in CI with DSL.parse before deploying OAP
Set SW_DYNAMIC_CLASS_ENGINE_DEBUG=true when diagnosing codegen-stage failures to inspect generated .java
After OAP upgrades, run the shipped rule comparison tests to catch grammar/behavior drift

When it happens

Trigger: DSL.parse invoked per rule during analyzer init; throws when the combined expression (expPrefix+exp+expSuffix) trips any earlier error in this catalog (49-51 parse, 42-48 extension, 40-41 decorate, 52-57 already thrown at registry init) or when Javassist rejects the generated source.

Common situations: Any malformed custom MAL rule file dropped into meter-analyzer-config / otel-rules / telegraf / zabbix / envoy rule directories; upgrading OAP to the v2 engine where previously-tolerated v1 expressions now fail; classpath issues with extension jars.

Related errors

AI-assisted analysis of apache/skywalking@102af09b4a (2026-08-14). Data as JSON: /api/errors/ab496a105f9a775e. Report an issue: GitHub.