feat: add webhook chat app

solutions/webhook-chat-app/README.md

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+# Google Chat App Webhook
+
+Please see related guide on how to
+[send messages to Google Chat with incoming webhooks](https://developers.google.com/workspace/chat/quickstart/webhooks).

solutions/webhook-chat-app/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+Copyright 2022 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
+
+    https://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.
+-->
+
+<!-- [START chat_webhook_build] -->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>com.google.chat.webhook</groupId>
+  <artifactId>webhook-app</artifactId>
+  <version>0.1.0</version>
+  <name>webhook-app</name>
+
+  <properties>
+    <maven.compiler.target>11</maven.compiler.target>
+    <maven.compiler.source>11</maven.compiler.source>
+  </properties>
+
+  <dependencies>
+    <dependency>
+        <groupId>com.google.code.gson</groupId>
+        <artifactId>gson</artifactId>
+        <version>2.9.1</version>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <pluginManagement>
+      <plugins>
+        <plugin>
+          <artifactId>maven-compiler-plugin</artifactId>
+          <version>3.8.0</version>
+        </plugin>
+      </plugins>
+    </pluginManagement>
+  </build>
+</project>
+<!-- [END chat_webhook_build] -->

solutions/webhook-chat-app/src/main/java/com/google/chat/webhook/App.java

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+/**
+ * Copyright 2022 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.chat.webhook;
+
+// [START chat_webhook]
+import com.google.gson.Gson;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.util.Map;
+import java.net.URI;
+
+public class App {
+  private static final String URL = "https://chat.googleapis.com/v1/spaces/SPACE_ID/messages?key=KEY&token=TOKEN";
+  private static final Gson gson = new Gson();
+  private static final HttpClient client = HttpClient.newHttpClient();
+
+  public static void main(String[] args) throws Exception {
+    String message = gson.toJson(Map.of(
+      "text", "Hello from Java!"
+    ));
+
+    HttpRequest request = HttpRequest.newBuilder(URI.create(URL))
+      .header("accept", "application/json; charset=UTF-8")
+      .POST(HttpRequest.BodyPublishers.ofString(message)).build();
+
+    HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
+
+    System.out.println(response.body());
+  }
+}
+// [END chat_webhook]

solutions/webhook-chat-app/src/main/java/com/google/chat/webhook/AppThread.java

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+/**
+ * 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.chat.webhook;
+
+// [START chat_webhook_thread]
+import com.google.gson.Gson;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.util.Map;
+import java.net.URI;
+
+public class App {
+  private static final String URL = "https://chat.googleapis.com/v1/spaces/SPACE_ID/messages?key=KEY&token=TOKEN&messageReplyOption=REPLY_MESSAGE_FALLBACK_TO_NEW_THREAD";
+  private static final Gson gson = new Gson();
+  private static final HttpClient client = HttpClient.newHttpClient();
+
+  public static void main(String[] args) throws Exception {
+    String message = gson.toJson(Map.of(
+      "text", "Hello from Java!",
+      "thread", Map.of(
+        "threadKey", "THREAD_KEY_VALUE"
+      )
+    ));
+
+    HttpRequest request = HttpRequest.newBuilder(URI.create(URL))
+      .header("accept", "application/json; charset=UTF-8")
+      .POST(HttpRequest.BodyPublishers.ofString(message)).build();
+
+    HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
+
+    System.out.println(response.body());
+  }
+}
+// [END chat_webhook_thread]