apache/maven · error · IllegalArgumentException

Invalid threads value: '{}'. Supported are int and float val

Error message

Invalid threads value: '{}'. Supported are int and float values ending with C.

What it means

This is the parse-failure branch of MavenCli.calculateDegreeOfConcurrency(): the -T value neither parsed as a float+C multiplier nor as an integer, i.e. Integer.parseInt / Float.parseFloat threw NumberFormatException, which is rethrown as IllegalArgumentException with the supported syntax. Valid forms are integers (-T4) or floats suffixed with C (-T1.5C).

Source

Thrown at compat/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java:1649

                if (coreMultiplier <= 0.0f) {
                    throw new IllegalArgumentException("Invalid threads core multiplier value: '" + threadConfiguration
                            + "'. Value must be positive.");
                }

                int procs = Runtime.getRuntime().availableProcessors();
                int threads = (int) (coreMultiplier * procs);
                return threads == 0 ? 1 : threads;
            } else {
                int threads = Integer.parseInt(threadConfiguration);
                if (threads <= 0) {
                    throw new IllegalArgumentException(
                            "Invalid threads value: '" + threadConfiguration + "'. Value must be positive.");
                }
                return threads;
            }
        } catch (NumberFormatException e) {
            throw new IllegalArgumentException("Invalid threads value: '" + threadConfiguration
                    + "'. Supported are int and float values ending with C.");
        }
    }

    // ----------------------------------------------------------------------
    // Properties handling
    // ----------------------------------------------------------------------

    void populateProperties(
            CommandLine commandLine, Properties paths, Properties systemProperties, Properties userProperties)
            throws Exception {

        // ----------------------------------------------------------------------
        // Load environment and system properties
        // ----------------------------------------------------------------------

        EnvironmentUtils.addEnvVars(systemProperties);
        SystemProperties.addSystemProperties(systemProperties);

View on GitHub (pinned to e4093d4e12)

Solutions

1. Use one of the two supported forms: an integer count (-T8) or a float multiplier with a trailing C (-T1.5C).
2. Print the exact command line before running it in CI to spot stray characters or empty interpolations in the -T value.
3. Default empty variables: -T${JOBS:-1}C rather than -T${JOBS}C.
4. Use a dot as decimal separator in multipliers (-T0.5C, never -T0,5C).

Example fix

# before
mvn -T2.5 package        # float without C -> IllegalArgumentException

# after
mvn -T2.5C package

private static final Pattern THREADS = Pattern.compile("^(?:[1-9]\\d*|\\d+(?:\\.\\d+)?[Cc])$");
String v = argAfter(args, "-T", "--threads");
if (v != null && !THREADS.matcher(v).matches()) {
    throw new IllegalArgumentException("Bad -T value '" + v + "': use an int (-T4) or float+C (-T1.5C)");
}

try {
    mavenCli.doMain(args, ...);
} catch (IllegalArgumentException e) {
    if (e.getMessage() != null && e.getMessage().contains("Supported are int and float values ending with C")) {
        // re-invoke with sanitized value, e.g. strip whitespace / add C / fall back to -T1
    }
    throw e;
}

When it happens

Trigger: mvn -Tabc, mvn -T1.5 (float without the trailing C), mvn -T'4 ' (embedded whitespace), mvn -T4x, or an interpolated value that is empty/non-numeric (e.g. -T${JOBS}C where JOBS is empty yields 'C', which fails the float parse). Note that a value like -T1,5C with a locale-style comma also fails.

Common situations: Copy-pasting '2.5' style thread counts from tutorials without the C suffix. Quoting/escaping bugs in CI YAML leaving stray characters in the option value. Empty variables producing -TC or -T. Locale confusion where a decimal comma is used.

Related errors

• Invalid threads core multiplier value: '{}'. Value must be p
• Invalid threads value: '{}'. Value must be positive.
• Unbounded range: {}
• Ranges overlap: {}
• Range defies version ordering: {}

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/b5622f8195617e1e. Report an issue: GitHub.