spring-projects/spring-ai · error · IllegalArgumentException

clients must not be empty

Error message

clients must not be empty

What it means

The AsyncLoggingSpecification record is a compact-constructor validation for an async MCP logging registration. The clients array must be non-null, non-empty, and contain no empty strings; otherwise an IllegalArgumentException is thrown at construction time. This fails fast so invalid logging registrations are caught before they are wired into an MCP server.

Solutions

1. Pass at least one non-empty client name in the clients array.
2. If logging should apply to all clients, check the library API for a wildcard or null/absent-clients overload instead of passing an empty array.
3. Filter out blank entries before constructing: Arrays.stream(raw).filter(s -> s != null && !s.isBlank()).toArray(String[]::new).
4. Validate the configuration source (properties/env) actually populated the client list before calling the constructor.

Example fix

// before
var spec = new AsyncLoggingSpecification(config.getLoggingClients().split(","), handler);
// after
String[] clients = config.getLoggingClients() == null || config.getLoggingClients().isBlank()
    ? new String[]{"default-client"}
    : Arrays.stream(config.getLoggingClients().split(","))
        .map(String::trim).filter(s -> !s.isEmpty()).toArray(String[]::new);
var spec = new AsyncLoggingSpecification(clients, handler);

String[] clients = /* from config */;
if (clients == null || clients.length == 0 || Arrays.stream(clients).anyMatch(c -> c == null || c.isEmpty())) {
    throw new IllegalStateException("AsyncLoggingSpecification requires at least one non-empty client name");
}

try {
    new AsyncLoggingSpecification(clients, handler);
} catch (IllegalArgumentException e) {
    logger.error("Invalid MCP logging clients config: {}", e.getMessage());
}

When it happens

Trigger: Calling new AsyncLoggingSpecification(new String[0], handler), passing null clients, or passing an array containing an empty string (e.g. new String[]{""}) while building an async logging specification for an MCP server.

Common situations: Building client lists dynamically from configuration or environment variables that resolve to an empty array or blank entries; a YAML/properties list that parsed to empty; string-splitting an empty config value producing [""] instead of a zero-length array.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• clients must not be empty
• Bean must not be null
• clients must not be empty
• clients must not be empty
• clients must not be empty

AI-assisted analysis of spring-projects/spring-ai@98a7beda4f (2026-09-11). Data as JSON: /api/errors/b9b81ee6e9575878. Report an issue: GitHub.

Appendix: source

Thrown at mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/method/logging/AsyncLoggingSpecification.java:32

 * limitations under the License.
 */

package org.springframework.ai.mcp.annotation.method.logging;

import java.util.Arrays;
import java.util.Objects;
import java.util.function.Function;

import io.modelcontextprotocol.spec.McpSchema.LoggingMessageNotification;
import reactor.core.publisher.Mono;

public record AsyncLoggingSpecification(String[] clients,
		Function<LoggingMessageNotification, Mono<Void>> loggingHandler) {

	public AsyncLoggingSpecification {
		Objects.requireNonNull(clients, "clients must not be null");
		if (clients.length == 0 || Arrays.stream(clients).anyMatch(String::isEmpty)) {
			throw new IllegalArgumentException("clients must not be empty");
		}
		Objects.requireNonNull(loggingHandler, "loggingHandler must not be null");
	}

}

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