quarkusio/quarkus · error · IllegalStateException

Unsupported value type: ${value}

Error message

Unsupported value type: ${value}

What it means

Json.toJsonString (appendValue path) only knows how to serialize String, Boolean, Integer, and Long values. When a JsonValue holds any other type (e.g. Double, Float, BigDecimal, byte[]), the writer cannot map it to JSON and throws IllegalStateException. It is a programming error indicating an unsupported value was stored in the JSON model.

Source

Thrown at independent-projects/bootstrap/json/src/main/java/io/quarkus/bootstrap/json/Json.java:541

                    if (!objectBuilder.isEmpty()) {
                        put(attribute, objectBuilder);
                    }
                }
            }
        }
    }

    static void appendValue(Appendable appendable, Object value) throws IOException {
        if (value instanceof JsonObjectBuilder jsonObj) {
            jsonObj.appendTo(appendable);
        } else if (value instanceof JsonArrayBuilder jsonArr) {
            jsonArr.appendTo(appendable);
        } else if (value instanceof String str) {
            appendStringValue(appendable, str);
        } else if (value instanceof Boolean || value instanceof Integer || value instanceof Long) {
            appendable.append(value.toString());
        } else {
            throw new IllegalStateException("Unsupported value type: " + value);
        }
    }

    static void appendStringValue(Appendable appendable, String value) throws IOException {
        appendable.append(CHAR_QUOTATION_MARK);
        appendEscaped(appendable, value);
        appendable.append(CHAR_QUOTATION_MARK);
    }

    /**
     * Escape quotation mark, reverse solidus and control characters (U+0000 through U+001F).
     *
     * @param value value to escape
     * @see <a href="https://www.ietf.org/rfc/rfc4627.txt">https://www.ietf.org/rfc/rfc4627.txt</a>
     */
    static void appendEscaped(Appendable appendable, String value) throws IOException {
        for (int i = 0; i < value.length(); i++) {
            final char c = value.charAt(i);

View on GitHub (pinned to e1c734241f)

Solutions

1. Convert the value to a supported type before storing, e.g. use value.toString() for decimals or store as String
2. Extend/patch the writer to handle the type you need, or pre-serialize custom objects into a JSON string
3. Inspect the value reported in the message and find where it was inserted into the Json model

Example fix

// before
json.set("price", 19.99d);
// after
json.set("price", "19.99");

static boolean isJsonSerializable(Object v) {
    return v instanceof String || v instanceof Boolean || v instanceof Integer || v instanceof Long;
}
// assert isJsonSerializable(value) before json.set(...)

static boolean isSupportedJsonValue(Object v) {
    return v instanceof String || v instanceof Boolean || v instanceof Integer || v instanceof Long;
}

try {
    String json = Json.toJsonString(value);
} catch (IllegalStateException e) {
    if (e.getMessage().startsWith("Unsupported value type")) {
        // fall back to String.valueOf(value) or a custom serializer
    }
}

When it happens

Trigger: Building a Json object/array programmatically and putting a value type outside {String, Boolean, Integer, Long} (e.g. Double, Float, BigDecimal), then serializing it with Json's public write method.

Common situations: Storing parsed numeric values like doubles or decimals into a Json model built by hand; mixing java.util types with the model; upgrading code that previously only stored strings/ints.

Related errors

• Unknown JSON Value
• Failed to encode as JSON:
• Json array ended without ]
• Control characters not allowed in json string
• String not closed

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/35dafa9d1856571a. Report an issue: GitHub.