spring-projects/spring-framework · error · IntrospectionException

Bad write method arg count

Error message

Bad write method arg count: {writeMethod}

What it means

Thrown as java.beans.IntrospectionException by PropertyDescriptorUtils.findPropertyType when the supplied write method does not have exactly one parameter. JavaBeans simple setters must take exactly one argument.

Solutions

1. Make the setter take exactly one parameter of the property type.
2. Rename non-conforming set-prefixed methods to avoid being treated as setters.
3. Use indexed setter convention (setXxx(int, value)) only if the property is truly indexed - but note this method only validates simple setters.

Example fix

// before
public void setAge(int age, String unit) { ... }

// after
public void setAge(int age) { ... }

if (writeMethod != null && writeMethod.getParameterCount() != 1) {
    throw new IllegalArgumentException("setter must take 1 arg: " + writeMethod);
}

static boolean validSimpleSetter(Method m) {
    return m != null && m.getParameterCount() == 1;
}

When it happens

Trigger: findPropertyType(readMethod, writeMethod) is called and writeMethod.getParameterTypes().length != 1 - e.g. a setter with 0, 2, or 3+ parameters.

Common situations: A setXxx method with no args or with extra args (overloaded setters, fluent setters), a method wrongly named with the 'set' prefix, or generated classes producing non-conformant setters.

Related errors

• Bad indexed read method arg count
• Bad indexed write method arg count
• Bad read method arg count
• Indexed read method returns void
• Non int index to indexed read method

AI-assisted analysis of spring-projects/spring-framework@69bf83ad71 (2026-08-09). Data as JSON: /api/errors/1bf6def57f666a3d. Report an issue: GitHub.

Appendix: source

Thrown at spring-beans/src/main/java/org/springframework/beans/PropertyDescriptorUtils.java:166

	public static @Nullable Class<?> findPropertyType(@Nullable Method readMethod, @Nullable Method writeMethod)
			throws IntrospectionException {

		Class<?> propertyType = null;

		if (readMethod != null) {
			if (readMethod.getParameterCount() != 0) {
				throw new IntrospectionException("Bad read method arg count: " + readMethod);
			}
			propertyType = readMethod.getReturnType();
			if (propertyType == void.class) {
				throw new IntrospectionException("Read method returns void: " + readMethod);
			}
		}

		if (writeMethod != null) {
			Class<?>[] params = writeMethod.getParameterTypes();
			if (params.length != 1) {
				throw new IntrospectionException("Bad write method arg count: " + writeMethod);
			}
			if (propertyType != null) {
				if (propertyType.isAssignableFrom(params[0])) {
					// Write method's property type potentially more specific
					propertyType = params[0];
				}
				else if (params[0].isAssignableFrom(propertyType)) {
					// Proceed with read method's property type
				}
				else {
					throw new IntrospectionException(
							"Type mismatch between read and write methods: " + readMethod + " - " + writeMethod);
				}
			}
			else {
				propertyType = params[0];
			}
		}

View on GitHub (pinned to 69bf83ad71)