feat: Introduce Slf4jLoggingConsumer

mcp/src/main/java/io/modelcontextprotocol/client/Slf4jLoggingConsumer.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2025 the original author or authors.
+ */
+package io.modelcontextprotocol.client;
+
+import java.util.function.Consumer;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.slf4j.event.Level;
+
+import io.modelcontextprotocol.client.McpClient.SyncSpec;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.McpSchema.LoggingMessageNotification;
+
+/**
+ * MCP Client-side consumer which logs received messages from MCP Servers using SLF4J.
+ *
+ * <p>
+ * Use this for {@link SyncSpec#loggingConsumer(Consumer)} to log received MCP messages.
+ *
+ * @author <a href="http://www.vorburger.ch">Michael Vorburger.ch</a>
+ */
+public class Slf4jLoggingConsumer implements Consumer<McpSchema.LoggingMessageNotification> {
+
+	// This class originated in
+	// https://github.com/enola-dev/enola/blob/ffc004666ea7f71357562ef12464d2b9fdbf9dbd/java/dev/enola/ai/mcp/McpServer.java#L29
+	// where it was used for https://docs.enola.dev/concepts/mcp/.
+	//
+	// It then found its way into Google's Agent Development Kit (ADK) in
+	// https://github.com/google/adk-java/pull/370. It's now been moved here to be useful
+	// to others.
+
+	private static final Logger LOG = LoggerFactory.getLogger(Slf4jLoggingConsumer.class);
+
+	@Override
+	public void accept(LoggingMessageNotification notif) {
+		if (notif.meta().isEmpty()) {
+			// If no meta, then just log the data as a message
+			LOG.atLevel(convert(notif.level())).log(notif.data());
+		}
+		else {
+			// If there is meta, then log it as a structured log message
+			var builder = LOG.atLevel(convert(notif.level())).setMessage(notif.data());
+			notif.meta().forEach((key, value) -> builder.addKeyValue(key, value));
+			builder.log();
+		}
+	}
+
+	private Level convert(McpSchema.LoggingLevel level) {
+		return switch (level) {
+			case DEBUG -> Level.DEBUG;
+			case INFO, NOTICE -> Level.INFO;
+			case WARNING -> Level.WARN;
+			case ERROR, CRITICAL, ALERT, EMERGENCY -> Level.ERROR;
+		};
+	}
+
+}