pentaho/pentaho-kettle · error · KettleException

JsonReader.Error.ReadUrl.Null

JsonReader.Error.ReadUrl.Null

Error message

JsonReader.Error.ReadUrl.Null

What it means

KettleException with message key JsonReader.Error.ReadUrl.Null thrown by FastJsonReader.readInput() when the Json parse of the input stream returns a null JsonReadContext. It indicates the fastjson-based reader parsed the stream but produced no usable context, so the step cannot extract any JSON data.

Solutions

  1. Check the input source: confirm the file/URL actually returns non-empty JSON content (curl the URL / cat the file).
  2. Validate the upstream service or file producer is not emitting empty payloads; add a size check before the step.
  3. Verify the step's source field (filename or URL field from a previous step) is populated and points to the intended resource.
  4. If the source legitimately can be empty, guard upstream with a filter step that skips empty files/rows.

Example fix

// before: URL may return empty body
String body = httpClient.get(url); // ""
// after: guard before feeding the step
if (body == null || body.trim().isEmpty()) { throw new IllegalStateException("Empty JSON from " + url); }
new JSONObject(body); // also validates it parses
Defensive patterns

Strategy: validation

Validate before calling

// ensure the source yields non-empty content before the JSON Input step
String body = fetch(urlOrFile);
if (body == null || body.trim().isEmpty()) throw new IllegalStateException("Empty JSON payload from source");

Try / catch

try { runTransformation(); }
catch (KettleException e) {
  if (e.getMessage() != null && e.getMessage().contains("ReadUrl.Null")) { alert("Empty JSON source"); }
  else throw e;
}

Prevention

When it happens

Trigger: parse -> readInput: getParseContext().parse(is, JSON_CHARSET) returns null — empty or whitespace-only input stream, or a stream whose content fastjson cannot build a context from (e.g. empty body fetched from a URL).

Common situations: URL source returns 200 with empty body; source file exists but is zero bytes; upstream service returns blank response during maintenance windows; wrong field chosen so the fetched resource has no content.

Related errors

AI-assisted analysis of pentaho/pentaho-kettle@f3058517a1 (2026-09-13). Data as JSON: /api/errors/19c7371ef20c5ac3. Report an issue: GitHub.

Appendix: source

Thrown at plugins/json/core/src/main/java/org/pentaho/di/trans/steps/jsoninput/reader/FastJsonReader.java:122

    return defaultPathLeafToNull;
  }

  Configuration getJsonConfiguration() {
    return jsonConfiguration;
  }

  private ParseContext getParseContext() {
    return JsonPath.using( jsonConfiguration );
  }

  private ReadContext getReadContext() {
    return jsonReadContext;
  }

  protected void readInput( InputStream is ) throws KettleException {
    jsonReadContext = getParseContext().parse( is, JSON_CHARSET );
    if ( jsonReadContext == null ) {
      throw new KettleException( BaseMessages.getString( PKG, "JsonReader.Error.ReadUrl.Null" ) );
    }
  }

  public boolean isIgnoreMissingPath() {
    return this.ignoreMissingPath;
  }

  public void setInputFields( JsonInputField[] inputFields ) {
    if ( null != inputFields ) {
      this.inputFields = inputFields;

      compiledJsonPaths = new JsonPath[ inputFields.length ];
      int i = 0;
      for ( JsonInputField inputField : inputFields ) {
        if ( System.getProperty( Const.KETTLE_COMPATIBILITY_JSON_INPUT_LEGACY_MODE, "N" ).equals( "Y" ) ) {
          compiledJsonPaths[ i++ ] = JsonPath.compile( step.environmentSubstitute( inputField.getPath(), false ).trim() );
        } else {
          compiledJsonPaths[ i++ ] = JsonPath.compile( step.environmentSubstitute( inputField.getPath(), true ) );

View on GitHub (pinned to f3058517a1)