languagetool-org/languagetool · error · IllegalConfigurationException

Property '' must be set in

Error message

Property '' must be set in 

What it means

getProperty is the config-loading helper for mandatory server properties. If a required property is absent (null) or present but empty/whitespace-only, it throws IllegalConfigurationException naming the property and the config file it was expected in.

Source

Thrown at languagetool-server/src/main/java/org/languagetool/server/HTTPServerConfig.java:1395

  /** @since 5.2 */
  public void setNgramLangIdentData(File ngramLangIdentData) {
    this.ngramLangIdentData = ngramLangIdentData;
  }

  /** @since 5.2 */
  @Nullable
  public File getNgramLangIdentData() {
    return ngramLangIdentData;
  }

  /**
   * @throws IllegalConfigurationException if property is not set
   */
  protected String getProperty(Properties props, String propertyName, File config) {
    String propertyValue = (String)props.get(propertyName);
    if (propertyValue == null || propertyValue.trim().isEmpty()) {
      throw new IllegalConfigurationException("Property '" + propertyName + "' must be set in " + config);
    }
    return propertyValue;
  }

  protected String getOptionalProperty(Properties props, String propertyName, String defaultValue) {
    String propertyValue = (String)props.get(propertyName);
    if (propertyValue == null) {
      return defaultValue;
    }
    return propertyValue;
  }

  /** @since 5.1 */
  boolean isPremiumAlways() {
    return premiumAlways;
  }

  /** @since 5.1 */

View on GitHub (pinned to 2e990059ce)

Solutions

1. Add the named property with a non-empty value to the config file mentioned in the message
2. If the value should be blank-checked, ensure it is trimmed non-empty — whitespace-only counts as missing
3. Upgrade documentation: check which properties your LanguageTool version requires

Example fix

// before (properties)
# dbPassword missing
// after (properties)
dbPassword=s3cret

for (String required : List.of("dbUsername", "dbPassword")) {
  String v = props.getProperty(required);
  if (v == null || v.trim().isEmpty())
    throw new IllegalStateException(required + " must be set in " + configFile);
}

try {
  String val = config.getProperty(props, "password", configFile);
} catch (IllegalConfigurationException e) {
  LOG.error("Missing config property: {}", e.getMessage());
  System.exit(1);
}

When it happens

Trigger: Server startup with a --config properties file that omits a required property (e.g. for the database-based pipeline: password, username, etc.), or defines it as an empty string (`prop=`).

Common situations: Missing keys in the properties file after upgrades that introduced new required options, copy-pasted config templates with placeholder values deleted, or whitespace-only values.

Understand the failure class

Background: "X is required", "must be set", "cannot be empty": the missing-required-config error family, from Vertex AI project/location to WeChat keys — this error's family across 18 libraries.

Related errors

• must be set
• code is supposed to be a 2 (or rarely 3) character code (unl
• dictionary file does not exist or is not a file: ''
• Common words path not found: ''
• No rules are active. Please make sure your rule ids (<option

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/90a04703736b124c. Report an issue: GitHub.