antlr/antlr4 · error · NullPointerException

listener cannot be null.

Error message

listener cannot be null.

What it means

addErrorListener(listener) throws NullPointerException when passed null, as documented on the method. The recognizer keeps a list of error listeners and a null entry would break notification loops in reportError. It is a fail-fast argument check, not a recoverable runtime condition.

Source

Thrown at runtime/Java/src/org/antlr/v4/runtime/Recognizer.java:215

			if ( t.getType()==Token.EOF ) {
				s = "<EOF>";
			}
			else {
				s = "<"+t.getType()+">";
			}
		}
		s = s.replace("\n","\\n");
		s = s.replace("\r","\\r");
		s = s.replace("\t","\\t");
		return "'"+s+"'";
	}

	/**
	 * @exception NullPointerException if {@code listener} is {@code null}.
	 */
	public void addErrorListener(ANTLRErrorListener listener) {
		if (listener == null) {
			throw new NullPointerException("listener cannot be null.");
		}

		_listeners.add(listener);
	}

	public void removeErrorListener(ANTLRErrorListener listener) {
		_listeners.remove(listener);
	}

	public void removeErrorListeners() {
		_listeners.clear();
	}


	public List<? extends ANTLRErrorListener> getErrorListeners() {
		return _listeners;
	}

View on GitHub (pinned to 7d5770395b)

Solutions

1. Remove the null listener before it reaches the call: only add when non-null
2. If the intent was to silence errors, call removeErrorListeners() instead of adding a null dummy
3. Add a no-op listener (e.g. ANTLRErrorListener with empty methods) when you need a placeholder

Example fix

// before
parser.addErrorListener(config.getListener()); // NPE when config returns null

// after
ANTLRErrorListener l = config.getListener();
if (l != null) parser.addErrorListener(l);

Objects.requireNonNull(listener, "listener");
if (listener != null) {
    recognizer.addErrorListener(listener);
}

When it happens

Trigger: Calling recognizer.addErrorListener(null); wiring listeners from configuration or DI where the value may be unset; copy-pasted setup code that passes a null field before initialization.

Common situations: Optional listener configuration ("add listener only if configured") written without a null branch; listener fields initialized after the recognizer is built; refactors that remove the listener creation but keep the add call.

Related errors

• delegates
• tokenSource cannot be null
• tokens cannot be null
• listener
• This ATN simulator does not support clearing the DFA.

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/69e73336d4b9b34c. Report an issue: GitHub.