spring-projects/spring-framework · error · IntrospectionException

Bad read method arg count: ${readMethod}

Error message

Bad read method arg count: ${readMethod}

What it means

IntrospectionException thrown by PropertyDescriptorUtils.findPropertyType when a candidate read method takes any parameters. The JavaBean contract requires read methods (getters) to have zero parameters; a getter with parameters is not a valid property accessor and PropertyDescriptor construction cannot proceed.

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)) {

View on GitHub (pinned to e8729d0438)

Solutions

1. If the method is an indexed accessor, register it as indexedReadMethod on an IndexedPropertyDescriptor (it then takes one int param).
2. Remove the parameter from the getter, or rename the parameterized method so it is not mistaken for the property's read method.
3. When building descriptors programmatically, pass a zero-arg method as readMethod.

Example fix

// before
new PropertyDescriptor("items", MyClass.class.getDeclaredMethod("getItems", int.class), null);

// after
new IndexedPropertyDescriptor("items", null, null,
    MyClass.class.getDeclaredMethod("getItems", int.class), null);

// Validate a read method before constructing a descriptor
static void assertValidReadMethod(Method readMethod) {
    if (readMethod.getParameterCount() != 0) {
        throw new IllegalArgumentException(
            "Read method must take 0 params: " + readMethod);
    }
}

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

try {
    return new PropertyDescriptor(name, readMethod, writeMethod);
} catch (java.beans.IntrospectionException ex) {
    if (ex.getMessage().startsWith("Bad read method arg count")) {
        // re-resolve to the zero-arg variant
        readMethod = Arrays.stream(clazz.getMethods())
            .filter(x -> x.getName().equals(readMethod.getName()) && x.getParameterCount() == 0)
            .findFirst().orElseThrow();
        return new PropertyDescriptor(name, readMethod, writeMethod);
    }
    throw ex;
}

When it happens

Trigger: Building a PropertyDescriptor where the supplied readMethod has parameterCount != 0; happens during introspection when a getFoo(...) method is mistakenly treated as the read method for property 'foo', or when hand-constructing PropertyDescriptors.

Common situations: A class with a getFoo(int index) method intended as an indexed getter but registered as a non-indexed read method; overloaded getFoo() vs getFoo(arg); tooling that auto-derives descriptors from method names without arity checks.

Related errors

• Read method returns void: ${readMethod}
• Bean property '{propertyName}' is not readable or has an inv
• Bad write method arg count: ${writeMethod}
• Type mismatch between read and write methods: ${readMethod}
• Bad indexed read method arg count: ${indexedReadMethod}

AI-assisted analysis of spring-projects/spring-framework@e8729d0438 (2026-08-04). Data as JSON: /data/errors/17359472ba3edecd.json. Report an issue: GitHub.