theonedev/onedev · error · org.apache.wicket.util.string.StringValueConversionException

Expected single character, not

Error message

Expected single character, not "{s}"

What it means

Strings.toChar(CharSequence) expects the input string to contain exactly one character. If the string is non-null but longer than one character, it throws StringValueConversionException('Expected single character, not "..."'). This guards against silently taking the first char of a longer value that is likely a mistake.

Solutions

1. Fix the source value so it contains exactly one character (trim surrounding whitespace)
2. If the value may be longer, take the intended character explicitly: s.charAt(0) after your own validation
3. Use a different parser (toChar/ValueModel of String) if the config genuinely holds multi-char separators
4. Handle null separately — null input throws the 'Character value was null' variant

Example fix

// before
char sep = Strings.toChar(";"); // fine
char bad = Strings.toChar("; "); // throws
// after
String v = rawValue.trim();
char sep = (v.length() == 1) ? v.charAt(0) : fallbackSeparator;

// Java
String v = s == null ? null : s.trim();
if (v != null && v.length() != 1) {
    throw new IllegalArgumentException("Expected single char, got: " + s);
}
char c = Strings.toChar(v);

try {
    c = Strings.toChar(value);
} catch (StringValueConversionException e) {
    log.warn("Not a single character: {}", value);
    c = DEFAULT_SEPARATOR;
}

When it happens

Trigger: Calling Strings.toChar(s) with a multi-character string such as "ab", "null", " ," or a full word instead of a single character.

Common situations: Parsing settings files where the value was meant to be a delimiter but a multi-char string was supplied; passing 'null' or empty-ish strings from property files; locale/keyboard variants of separators (e.g. ", " with a trailing space).

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• Boolean value " " not recognized
• Cannot convert ' ' to enum constant of type ' '.
• Character value was null
• An error occurred while detaching component:
• An error occurred while generating an Url for handler

AI-assisted analysis of theonedev/onedev@d44925c47c (2026-09-06). Data as JSON: /api/errors/769eb538a5297aae. Report an issue: GitHub.

Appendix: source

Thrown at server-core/src/main/java/org/apache/wicket/util/string/Strings.java:1007

	 * Converts the 1 character string s to a character.
	 * 
	 * @param s
	 *            The 1 character string to convert to a char.
	 * @return Character value to convert
	 * @throws StringValueConversionException
	 *             when the string is longer or shorter than 1 character, or <code>null</code>.
	 */
	public static char toChar(final String s) throws StringValueConversionException
	{
		if (s != null)
		{
			if (s.length() == 1)
			{
				return s.charAt(0);
			}
			else
			{
				throw new StringValueConversionException("Expected single character, not \"" + s +
					"\"");
			}
		}

		throw new StringValueConversionException("Character value was null");
	}

	/**
	 * Converts unicodes to encoded &#92;uxxxx.
	 * 
	 * @param unicodeString
	 *            The unicode string
	 * @return The escaped unicode string, like '\u4F60\u597D'.
	 */
	public static String toEscapedUnicode(final String unicodeString)
	{
		if ((unicodeString == null) || (unicodeString.length() == 0))
		{

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