spring-projects/spring-framework · error · IntrospectionException

Bad read method arg count

Error message

Bad read method arg count: {readMethod}

What it means

Thrown as java.beans.IntrospectionException by PropertyDescriptorUtils.findPropertyType when a read method passed in has a non-zero parameter count. JavaBeans getters must take no arguments, so this signals a malformed PropertyDescriptor read method.

Solutions

1. Ensure the getter method takes no parameters (e.g. getName(), not getName(int)).
2. If the method is genuinely a different accessor (indexed/computed), name it according to indexed conventions (getXxx(int)).
3. Regenerate or correct the offending class so its bean accessors are valid.

Example fix

// before
public String getName(int locale);  // misread as getter

// after
public String getName();
public String getName(int locale); // renamed usage: use indexed/getter-of-locale pattern explicitly

// before registering a read method
if (readMethod.getParameterCount() != 0) {
    throw new IllegalArgumentException("read method takes args: " + readMethod);
}

static boolean validReadMethod(Method m) {
    return m != null && m.getParameterCount() == 0 && m.getReturnType() != void.class;
}

When it happens

Trigger: findPropertyType(readMethod, writeMethod) is invoked (typically while building a GenericTypeAwarePropertyDescriptor) and readMethod.getParameterCount() != 0. Means a method with arguments was registered as a read method.

Common situations: A getter-like method that actually takes arguments, a misconfigured custom PropertyDescriptor, or a generated/bytecode-altered class exposing a method as a getter that has parameters. This is usually an internal Spring/Introspector state, surfaced as a wrapped fatal exception by callers.

Related errors

• Bad indexed read method arg count
• Bad indexed write method arg count
• Bad write 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/44db71662583080c. Report an issue: GitHub.

Appendix: source

Thrown at spring-beans/src/main/java/org/springframework/beans/PropertyDescriptorUtils.java:155

		}

		// See java.beans.PropertyDescriptor#PropertyDescriptor(PropertyDescriptor)
		target.setPropertyEditorClass(source.getPropertyEditorClass());
		target.setBound(source.isBound());
		target.setConstrained(source.isConstrained());
	}

	/**
	 * See {@link java.beans.PropertyDescriptor#findPropertyType}.
	 */
	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)) {

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