quarkusio/quarkus · error · IllegalStateException

Unsupported serialization format:

Error message

Unsupported serialization format: 

What it means

ApplicationModelSerializer's constructor accepts only two serialization formats: "jos" (Java object serialization) and "json" (case-insensitive). Any other value results in this IllegalStateException at construction time, failing fast rather than failing later during serialize/deserialize.

Source

Thrown at independent-projects/bootstrap/core/src/main/java/io/quarkus/bootstrap/app/ApplicationModelSerializer.java:45

 */
public class ApplicationModelSerializer {

    private static final MappableCollectionFactory JSON_CONTAINER_FACTORY = new JsonCollectionFactory();

    private static final String QUARKUS_APPLICATION_MODEL_SERIALIZATION_FORMAT_PROP = "quarkus.bootstrap.application-model.serialization.format";
    // whether to use Java Object Serialization as a format
    private static final boolean JOS;
    static {
        final String serializationFormat = System.getProperty(QUARKUS_APPLICATION_MODEL_SERIALIZATION_FORMAT_PROP);
        if (serializationFormat == null) {
            JOS = false;
        } else {
            if (serializationFormat.equalsIgnoreCase("jos")) {
                JOS = true;
            } else {
                JOS = false;
                if (!serializationFormat.equalsIgnoreCase("json")) {
                    throw new IllegalStateException("Unsupported serialization format: " + serializationFormat);
                }
            }
        }
    }

    /**
     * Creates a temporary file to serialize an application model.
     *
     * @param test whether it's for a test model
     * @return temporary file
     * @throws IOException in case of a failure
     */
    private static Path getTempFile(boolean test) throws IOException {
        return Files.createTempFile(
                "quarkus-" + (test ? "test-" : "") + "app-model",
                ".dat");
    }

View on GitHub (pinned to e1c734241f)

Solutions

1. Pass "json" (default) or "jos" as the serialization format
2. Check the system property/env value feeding the format string for typos or stray whitespace
3. Omit the format argument to use the default format

Example fix

// before
new ApplicationModelSerializer("xml");
// after
new ApplicationModelSerializer("json");

String fmt = System.getProperty("appmodel.format", "json");
if (!fmt.equalsIgnoreCase("json") && !fmt.equalsIgnoreCase("jos")) {
    throw new IllegalArgumentException("Format must be json or jos: " + fmt);
}

try {
    new ApplicationModelSerializer(fmt);
} catch (IllegalStateException e) {
    if (e.getMessage().startsWith("Unsupported serialization format")) { /* default to json */ }
}

When it happens

Trigger: Constructing `new ApplicationModelSerializer(format)` with a string other than "jos" or "json" — typically a typo like "java", "JSONS", or an environment/config-driven value with unexpected content.

Common situations: Setting a system property or config value controlling the app-model serialization format to an unsupported string, IDE/tooling passing a custom format name, or copy-pasted config from another tool.

Related errors

• Unknown JAR package type '${value}'
• Unable to resolve locale: ${value}
• Unsupported compressor '
• proxyHost must not be null
• Invalid port number

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/753c155251fef7eb. Report an issue: GitHub.