What causes "Class must not be null" in junit-team/junit5?

Pass the concrete class whose logger you want to inspect.

junit-team/junit5 · error · JUnitException

Class must not be null

Error message

Class must not be null

What it means

LogRecordListener.stream(Class<?>) throws JUnitException when clazz is null, because it calls clazz.getName() to match logger names. This is an @INTERNAL API used by JUnit's own tests.

Source

Thrown at junit-platform-commons/src/main/java/org/junit/platform/commons/logging/LogRecordListener.java:109

	 * {@linkplain #logRecordSubmitted submitted} to this listener by the current
	 * thread for the logger name equal to the name of the given class.
	 *
	 * <p>As stated in the Javadoc for {@code LogRecord}, a submitted
	 * {@code LogRecord} should not be updated by the client application. Thus,
	 * the {@code LogRecords} in the returned stream should only be inspected for
	 * testing purposes and not modified in any way.
	 *
	 * @param clazz the class for which to get the log records; never {@code null}
	 * @see #stream()
	 * @see #stream(Level)
	 * @see #stream(Class, Level)
	 */
	@SuppressWarnings("ConstantValue")
	public Stream<LogRecord> stream(Class<?> clazz) {
		// NOTE: we cannot use org.junit.platform.commons.util.Preconditions here
		// since that would introduce a package cycle.
		if (clazz == null) {
			throw new JUnitException("Class must not be null");
		}

		return stream().filter(logRecord -> logRecord.getLoggerName().equals(clazz.getName()));
	}

	/**
	 * Get a stream of {@link LogRecord log records} that have been
	 * {@linkplain #logRecordSubmitted submitted} to this listener by the current
	 * thread for the logger name equal to the name of the given class at the given
	 * log level.
	 *
	 * <p>As stated in the Javadoc for {@code LogRecord}, a submitted
	 * {@code LogRecord} should not be updated by the client application. Thus,
	 * the {@code LogRecords} in the returned stream should only be inspected for
	 * testing purposes and not modified in any way.
	 *
	 * @param clazz the class for which to get the log records; never {@code null}
	 * @param level the log level for which to get the log records; never {@code null}

View on GitHub (pinned to 956246301e)

Solutions

Pass the concrete class whose logger you want to inspect.
Use stream() and filter records manually if the class is unknown.
Add a nullness check in test utilities that wrap the listener.

Example fix

// before
listener.stream(clazz);   // clazz == null

// after
listener.stream(MyService.class);

Defensive patterns

Strategy: validation

Validate before calling

Class<?> clazz = null; // some computed value
if (clazz == null) {
    throw new IllegalArgumentException("Class must not be null");
}
listener.stream(clazz);

Type guard

static <T> Class<T> requireClass(Class<T> c) {
    return java.util.Objects.requireNonNull(c, "Class must not be null");
}

Try / catch

try {
    listener.stream(clazz);
} catch (JUnitException e) {
    if (e.getMessage().equals("Class must not be null")) {
        // pass a concrete Class instead
    } else throw e;
}

Prevention

Pass the concrete class literal rather than a possibly-null reference.
Fall back to stream() and filter by logger name manually when the class is unknown.
Keep the internal logging API out of non-JUnit code.

When it happens

Trigger: Calling listener.stream((Class<?>) null).

Common situations: Test refactor that passes a null Class reference; parameterization that lost the type token.

Related errors

AI-assisted analysis of junit-team/junit5@956246301e (2026-08-04). Data as JSON: /data/errors/4da2e860a96acd4d.json. Report an issue: GitHub.