languagetool-org/languagetool · error · IllegalArgumentException

Could not parse provided serverURL: ''

Error message

Could not parse provided serverURL: ''

What it means

setServerURL accepts an optional URL used for relative request paths (e.g. behind a proxy). The input is parsed as a URI twice; if either parse fails with URISyntaxException, it is rethrown as an IllegalArgumentException wrapping the original exception.

Source

Thrown at languagetool-server/src/main/java/org/languagetool/server/HTTPServerConfig.java:635

   * @since 4.8
   * @return prefix / base URL for API requests
   */
  @Nullable
  public URI getServerURL() {
    return serverURL;
  }

  /**
   * @since 4.8
   * @param url prefix / base URL for API requests
   */
  public void setServerURL(@Nullable String url) {
    if (url != null) {
      try {
        // ignore different protocols, ports,... just use path for relative requests
        serverURL = new URI(new URI(url).getPath());
      } catch (URISyntaxException e) {
        throw new IllegalArgumentException("Could not parse provided serverURL: '" + url + "'", e);
      }
    } else {
      serverURL = null;
    }
  }

  /**
   * @param len the maximum text length allowed (in number of characters), texts that are longer
   *            will cause an exception when being checked, unless the user can provide
   *            an API key
   */
  public void setMaxTextLengthAnonymous(int len) {
    this.maxTextLengthAnonymous = len;
  }

  public void setMaxTextLengthLoggedIn(int len) {
    this.maxTextLengthLoggedIn = len;
  }

View on GitHub (pinned to 2e990059ce)

Solutions

1. Percent-encode special characters (spaces as %20, etc.) in the serverURL value
2. Fix the URL syntax — ensure a valid form like `https://host/basepath`
3. Remove the serverURL option entirely if the default (no relative path prefix) is fine; null is accepted

Example fix

// before
--serverURL http://my server/lt
// after
--serverURL http://myserver/lt

try {
  new URI(new URI(url).getPath());
} catch (URISyntaxException e) {
  throw new IllegalStateException("serverURL is not a valid URI: " + url, e);
}

try {
  config.setServerURL(url);
} catch (IllegalArgumentException e) {
  LOG.error("serverURL rejected: {}", e.getMessage());
}

When it happens

Trigger: Calling setServerURL with a syntactically invalid URI string — e.g. containing illegal characters, unescaped spaces, or a malformed path — so `new URI(url)` or `new URI(new URI(url).getPath())` throws.

Common situations: Reverse-proxy configuration values with spaces or unencoded special characters (`--serverURL http://my server/lt`), trailing config garbage, or missing scheme/typos like `http:/host`.

Understand the failure class

Background: "Invalid URL" errors: why new URL(), URI.parse, and reqwest::Url reject your string — missing scheme, whitespace, and bad path format — this error's family across 39 libraries.

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AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/cbb66ad0e9fd649b. Report an issue: GitHub.