spring-projects/spring-ai · error · java.lang.IllegalArgumentException

Method must not be null

Error message

Method must not be null

What it means

AbstractMcpPromptMethodCallback.validateMethod() checks that the Method resolved for an @McpPrompt-annotated callback is non-null before validating its return type and parameters. A null Method means the callback was built without a target method, which cannot be invoked reflectively, so construction fails fast with IllegalArgumentException.

Source

Thrown at mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/method/prompt/AbstractMcpPromptMethodCallback.java:81

	 * @param method The method to create a callback for
	 * @param bean The bean instance that contains the method
	 * @param prompt The prompt
	 */
	protected AbstractMcpPromptMethodCallback(Method method, Object bean, Prompt prompt) {
		this.method = method;
		this.bean = bean;
		this.prompt = prompt;
		this.validateMethod(this.method);
	}

	/**
	 * Validates that the method signature is compatible with the prompt callback.
	 * @param method The method to validate
	 * @throws IllegalArgumentException if the method signature is not compatible
	 */
	protected void validateMethod(Method method) {
		if (method == null) {
			throw new IllegalArgumentException("Method must not be null");
		}

		this.validateReturnType(method);
		this.validateParameters(method);
	}

	/**
	 * Validates that the method return type is compatible with the prompt callback.
	 * @param method The method to validate
	 * @throws IllegalArgumentException if the return type is not compatible
	 */
	protected abstract void validateReturnType(Method method);

	/**
	 * Checks if a parameter type is compatible with the exchange type.
	 * @param paramType The parameter type to check
	 * @return true if the parameter type is compatible with the exchange type, false
	 * otherwise

View on GitHub (pinned to 98a7beda4f)

Solutions

1. Ensure the Builder's method(...) is called with a non-null java.lang.reflect.Method before building the callback.
2. Fix the method-lookup code to throw when the annotated method cannot be found rather than returning null.
3. Validate the resolved Method for null before passing it into the callback constructor.

Example fix

// before
Method m = clazz.getDeclaredMethod("promptHandler"); // throws if absent, or lookup returns null
callback = MyPromptCallback.builder().method(maybeNull).bean(bean).build();
// after
Objects.requireNonNull(m, "Resolved @McpPrompt method must not be null");
callback = MyPromptCallback.builder().method(m).bean(bean).build();

Method m = lookupPromptMethod(bean);
Objects.requireNonNull(m, "@McpPrompt method must not be null");

Optional<Method> findMethod(Class<?> c, String name) { return Arrays.stream(c.getDeclaredMethods()).filter(m -> m.getName().equals(name)).findFirst(); }

try { callback = builder().method(m).bean(bean).build(); } catch (IllegalArgumentException e) { log.error("Callback construction failed: {}", e.getMessage()); }

When it happens

Trigger: Calling AbstractMcpPromptMethodCallback's constructor (directly or via a Builder.build()) with method = null, e.g. a builder whose method field was never set or was resolved to null by a method-lookup helper.

Common situations: Reflection-based lookup that fails silently and returns null instead of throwing; custom builder usage forgetting the .method(...) call; annotation scanning code passing through a null from getDeclaredMethod on a missing method name.

Related errors

• Method must not be null
• Method must not be null
• Method must not be null
• Error invoking resource list changed consumer method: " + th
• Error invoking tool list changed consumer method:

AI-assisted analysis of spring-projects/spring-ai@98a7beda4f (2026-09-11). Data as JSON: /api/errors/2463f91f635a4383. Report an issue: GitHub.