feat(config): Supports loading yaml agents in mvn google-adk@web ...

Usage:

```
mvn clean compile google-adk:web -Dagents=/path/to/agents
```

Where each subdirectory in agents with a `root_agent.yaml` file will be recognized an agent in ADK Web UI.

PiperOrigin-RevId: 794342536

Author: Jacksunwei

maven_plugin/README.md

@@ -12,7 +12,7 @@ mvn google-adk:web -Dagents=com.example.MyAgentLoader.INSTANCE
 
 ### Parameters
 
-- **`agents`** (required): Full class path to your AgentLoader implementation
+- **`agents`** (required): Full class path to your AgentLoader implementation OR path to agent configuration directory
 - **`port`** (optional, default: 8000): Port for the web server
 - **`host`** (optional, default: localhost): Host address to bind to
 - **`hotReloading`** (optional, default: true): Whether to enable hot reloading
@@ -195,6 +195,59 @@ Usage:
 mvn google-adk:web -Dagents=com.example.MultipleLoaders.ADVANCED
 ```
 
+## Config-Based Agents (Directory Path)
+
+For configuration-based agents using YAML files, you can provide a directory path instead of a class name. The plugin will automatically use `ConfigAgentLoader` to scan for agent directories.
+
+### Directory Structure
+
+Create a parent directory containing subdirectories, each representing an agent with a `root_agent.yaml` file:
+
+```
+my-agents/
+├── chat-assistant/
+│   └── root_agent.yaml
+├── search-agent/
+│   └── root_agent.yaml
+└── code-helper/
+    ├── root_agent.yaml
+    └── another_agent.yaml
+```
+
+### Usage with Directory Path
+
+```bash
+mvn google-adk:web -Dagents=my-agents
+```
+
+Or with absolute path:
+
+```bash
+mvn google-adk:web -Dagents=/home/user/my-agents
+```
+
+### Hot Reloading for Config Agents
+
+When using config-based agents, hot reloading is enabled by default. The plugin will automatically detect changes to any YAML files within the agent directories and reload agents without restarting the server.
+
+To disable hot reloading:
+
+```bash
+mvn google-adk:web -Dagents=my-agents -DhotReloading=false
+```
+
+### Example root_agent.yaml
+
+```yaml
+name: "chat_assistant"
+description: "A friendly chat assistant"
+model: "gemini-2.0-flash"
+instruction: |
+  You are a helpful and friendly assistant.
+  Answer questions clearly and concisely.
+  Be encouraging and positive in your responses.
+```
+
 ## Web UI
 
 Once the server starts, open your browser to:

maven_plugin/src/main/java/com/google/adk/maven/ConfigAgentLoader.java

@@ -0,0 +1,249 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.maven;
+
+import static java.util.stream.Collectors.toList;
+
+import com.google.adk.agents.BaseAgent;
+import com.google.adk.agents.ConfigAgentUtils;
+import com.google.common.base.Suppliers;
+import com.google.common.collect.ImmutableList;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+import javax.annotation.Nonnull;
+import javax.annotation.concurrent.ThreadSafe;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Configuration-based AgentLoader that loads agents from YAML configuration files.
+ *
+ * <p>This loader monitors a configured source directory for folders containing `root_agent.yaml`
+ * files and automatically reloads agents when the files change (if hot-reloading is enabled).
+ *
+ * <p>The loader treats each subdirectory with a `root_agent.yaml` file as an agent, using the
+ * folder name as the agent identifier. Agents are loaded lazily when first requested.
+ *
+ * <p>Directory structure expected:
+ *
+ * <pre>
+ * source-dir/
+ *   ├── agent1/
+ *   │   └── root_agent.yaml
+ *   ├── agent2/
+ *   │   └── root_agent.yaml
+ *   └── ...
+ * </pre>
+ *
+ * <p>Hot-reloading can be disabled by setting hotReloadingEnabled to false.
+ *
+ * <p>TODO: Config agent features are not yet ready for public use.
+ */
+@ThreadSafe
+class ConfigAgentLoader implements AgentLoader {
+  private static final Logger logger = LoggerFactory.getLogger(ConfigAgentLoader.class);
+  private static final String YAML_CONFIG_FILENAME = "root_agent.yaml";
+
+  private final boolean hotReloadingEnabled;
+  private final String sourceDir;
+  private final Map<String, Supplier<BaseAgent>> agentSuppliers = new ConcurrentHashMap<>();
+  private final ConfigAgentWatcher watcher;
+  private volatile boolean started = false;
+
+  /**
+   * Creates a new ConfigAgentLoader.
+   *
+   * @param sourceDir The directory to scan for agent configuration files
+   * @param hotReloadingEnabled Controls whether hot-reloading is enabled
+   */
+  public ConfigAgentLoader(String sourceDir, boolean hotReloadingEnabled) {
+    this.sourceDir = sourceDir;
+    this.hotReloadingEnabled = hotReloadingEnabled;
+    this.watcher = hotReloadingEnabled ? new ConfigAgentWatcher() : null;
+
+    try {
+      discoverAgents();
+      if (hotReloadingEnabled) {
+        start();
+      }
+    } catch (IOException e) {
+      logger.error("Failed to initialize ConfigAgentLoader", e);
+    }
+  }
+
+  /**
+   * Creates a new ConfigAgentLoader with hot-reloading enabled.
+   *
+   * @param sourceDir The directory to scan for agent configuration files
+   */
+  public ConfigAgentLoader(String sourceDir) {
+    this(sourceDir, true);
+  }
+
+  @Override
+  @Nonnull
+  public ImmutableList<String> listAgents() {
+    return ImmutableList.copyOf(agentSuppliers.keySet());
+  }
+
+  @Override
+  public BaseAgent loadAgent(String name) {
+    Supplier<BaseAgent> supplier = agentSuppliers.get(name);
+    if (supplier == null) {
+      throw new NoSuchElementException("Agent not found: " + name);
+    }
+    return supplier.get();
+  }
+
+  /**
+   * Discovers available agents from the configured source directory and creates suppliers for them.
+   *
+   * @throws IOException if there's an error accessing the source directory
+   */
+  private void discoverAgents() throws IOException {
+    if (sourceDir == null || sourceDir.isEmpty()) {
+      logger.info(
+          "Agent source directory not configured. ConfigAgentLoader will not discover any agents.");
+      return;
+    }
+
+    Path sourcePath = Paths.get(sourceDir);
+    if (!Files.isDirectory(sourcePath)) {
+      logger.warn(
+          "Agent source directory does not exist: {}. ConfigAgentLoader will not discover any"
+              + " agents.",
+          sourcePath);
+      return;
+    }
+
+    logger.info("Initial scan for YAML agents in: {}", sourcePath);
+
+    try (Stream<Path> entries = Files.list(sourcePath)) {
+      for (Path agentDir : entries.collect(toList())) {
+        if (Files.isDirectory(agentDir)) {
+          Path yamlConfigPath = agentDir.resolve(YAML_CONFIG_FILENAME);
+          if (Files.exists(yamlConfigPath) && Files.isRegularFile(yamlConfigPath)) {
+            // Use the folder name as the agent identifier
+            String agentName = agentDir.getFileName().toString();
+            logger.debug("Discovering YAML agent config: {}", yamlConfigPath);
+
+            if (agentSuppliers.containsKey(agentName)) {
+              logger.warn(
+                  "Duplicate agent name '{}' found in {}. Overwriting.", agentName, yamlConfigPath);
+            }
+            // Create a memoized supplier that will load the agent only when requested
+            agentSuppliers.put(
+                agentName, Suppliers.memoize(() -> loadAgentFromPath(yamlConfigPath)));
+
+            // Register with watcher if hot-reloading is enabled
+            if (hotReloadingEnabled && watcher != null) {
+              watcher.watch(agentDir, agentDirPath -> updateAgentSupplier(agentDirPath));
+            }
+
+            logger.info("Discovered YAML agent '{}' from: {}", agentName, yamlConfigPath);
+          }
+        }
+      }
+    }
+
+    logger.info("Initial YAML agent discovery complete. Found {} agents.", agentSuppliers.size());
+  }
+
+  /**
+   * Updates the agent supplier when a configuration changes.
+   *
+   * @param agentDirPath The path to the agent configuration directory
+   */
+  private void updateAgentSupplier(Path agentDirPath) {
+    String agentName = agentDirPath.getFileName().toString();
+    Path yamlConfigPath = agentDirPath.resolve(YAML_CONFIG_FILENAME);
+
+    if (Files.exists(yamlConfigPath)) {
+      // File exists - create/update supplier
+      agentSuppliers.put(agentName, Suppliers.memoize(() -> loadAgentFromPath(yamlConfigPath)));
+      logger.info("Updated YAML agent supplier '{}' from: {}", agentName, yamlConfigPath);
+    } else {
+      // File deleted - remove supplier
+      agentSuppliers.remove(agentName);
+      logger.info("Removed YAML agent '{}' due to deleted config file", agentName);
+    }
+  }
+
+  /**
+   * Loads an agent from the specified config path.
+   *
+   * @param yamlConfigPath The path to the YAML configuration file
+   * @return The loaded BaseAgent
+   * @throws RuntimeException if loading fails
+   */
+  private BaseAgent loadAgentFromPath(Path yamlConfigPath) {
+    try {
+      logger.debug("Loading YAML agent from: {}", yamlConfigPath);
+      BaseAgent agent = ConfigAgentUtils.fromConfig(yamlConfigPath.toString());
+      logger.info("Successfully loaded YAML agent '{}' from: {}", agent.name(), yamlConfigPath);
+      return agent;
+    } catch (Exception e) {
+      logger.error("Failed to load YAML agent from: {}", yamlConfigPath, e);
+      throw new RuntimeException("Failed to load agent from: " + yamlConfigPath, e);
+    }
+  }
+
+  /**
+   * Starts the hot-loading service. Sets up file watching.
+   *
+   * @throws IOException if there's an error accessing the source directory
+   */
+  private synchronized void start() throws IOException {
+    if (!hotReloadingEnabled || watcher == null) {
+      logger.info(
+          "Hot-reloading is disabled. YAML agents will be loaded once at startup and will not be"
+              + " monitored for changes.");
+      return;
+    }
+
+    if (started) {
+      logger.warn("ConfigAgentLoader is already started");
+      return;
+    }
+
+    logger.info("Starting ConfigAgentLoader with file watching");
+    watcher.start();
+    started = true;
+    logger.info("ConfigAgentLoader started successfully with {} agents.", agentSuppliers.size());
+  }
+
+  /** Stops the hot-loading service. */
+  public synchronized void stop() {
+    if (!started) {
+      return;
+    }
+
+    logger.info("Stopping ConfigAgentLoader...");
+    if (watcher != null) {
+      watcher.stop();
+    }
+    started = false;
+    logger.info("ConfigAgentLoader stopped.");
+  }
+}