Java runtime environment

The Java runtime is the software stack responsible for installing your web service's code and its dependencies and running your service.

Declare the Java runtime for the App Engine standard environment in the app.yaml file. For example:

runtime: javaVERSION

Where VERSION is the Java MAJOR version number. For example, to use the latest Java version, Java 21, specify 21.

For other supported Java versions, and the corresponding Ubuntu version for your Java version, see the Runtime support schedule.

Before you begin

Download the latest version of the Google Cloud CLI or update your gcloud CLI to the current version:
```
gcloud components update
```
To deploy using Maven, you will need to add the App Engine Maven Plugin to your pom.xml file:
```
<plugin>
   <groupId>com.google.cloud.tools</groupId>
   <artifactId>appengine-maven-plugin</artifactId>
   <version>2.8.2</version>
</plugin>
```
Other options for deploying include using the gcloud app deploy command or the App Engine Gradle plugin.
Follow the instructions for your application framework to configure the build of an executable JAR file.

Framework compatibility

With the App Engine Java runtime, you can deploy executable JAR files. The runtimes do not include any web-serving framework, meaning you are not limited to using servlet-based frameworks or libraries. Use your native dependencies or networking stacks such as the Netty library.

You are not limited to these frameworks and are encouraged to try your preferred one, like Grails, Blade, Play!, Vaadin or jHipster.

Deploy Maven source projects to the Java runtime

You can deploy your Maven project as source code and have it built and deployed using Google Cloud's buildpacks.

To deploy a Maven project project as source code, go to your project's top level directory and type:

gcloud app deploy pom.xml

Build and deploy logs will be streamed, and you can see the detailed logs in the Cloud Build history section in the Google Cloud console.

Using GraalVM executables

The App Engine standard environment Java runtime supports GraalVM native image executables. Once you have compiled your Java app into a GraalVM native image, you can use the entrypoint setting in your app.yaml file to point to the executable.

For example, an executable with the filename myexecutable could have the followingapp.yaml configuration file:

runtime: 21 # or another supported runtime version.
entrypoint: ./myexecutable

Google Cloud client libraries can be used to compile applications as a GraalVM native image. For more information, refer to documentation on how to Compile native images.

Java version

The Java runtime uses the latest stable release of the version that is specified in your app.yaml file. App Engine automatically updates to new patch release versions, but it won't automatically update the minor version.

For example, your application might be deployed at Java 21.0.4 and automatically updated to version Java 21.0.5 at a later deployment of the managed platform, but it won't be automatically updated to Java 22.

To learn how to upgrade your version of Java, see Upgrade an existing application.

The runtime's Open JDK environment

App Engine runs Java apps in a container secured by gVisor on an up-to-date Ubuntu Linux distribution and its supported openjdk-17-jdk for Java 17 or openjdk-21-jdk for Java 21 runtime.

For supported Ubuntu versions for your Java version, see Runtime support schedule.

App Engine maintains the base image and updates the OpenJDK 17 and OpenJDK 21 package, without requiring you to redeploy your app.

Your deployed app is located in the runtime's /workspace directory. It is also accessible through a symbolic link at /srv.

App Engine Java releases

All released artifacts that start with version 2.x.x use the open source release mechanism. See the GitHub repository for more details.

Dependencies

For more information on declaring and managing dependencies, see Specifying dependencies.

Application startup

Frameworks such as Spring Boot, Micronaut, Ktor build an executable uber JAR by default. If your Maven or Gradle build file produces an executable Uber JAR, the runtime starts your application by running an Uber JAR application.

Alternatively, App Engine will use the contents of the optional entrypoint field in your app.yaml file. For example:

runtime: java21 # or another supported runtime
entrypoint: java -Xmx64m -jar YOUR-ARTIFACT.jar

Where the example YOUR-ARTIFACT.jar application jar must:

Be in the root directory with your app.yaml file.
Contain a Main-Class entry in its META-INF/MANIFEST.MF metadata file.
Optionally, contain a Class-Path entry with a list of relative paths to other dependent jars. These will upload with the application automatically.

For your app to receive HTTP requests, your entrypoint should start a web server that listens on the port specified by the PORT environment variable. The value of the PORT environment variable is dynamically set by the App Engine serving environment. This value cannot be set in the env_variables section of the app.yaml file.

With a custom entrypoint, you can construct and package your application as a thin JAR file which only contains your application code and direct dependencies. When deploying your application, the App Engine plugin will only upload the files that changed, rather than the entire uber JAR package.

Be sure to use the PORT environment variable

If you see the warnings about port 8080 and NGINX in your app log files, your app's web server is listening on the default port 8080. This prevents App Engine from using its NGINX layer to compress HTTP responses. We recommend that you configure your web server respond to HTTP requests on the port specified by the PORT environment variable, typically 8081. For example:

/*
 * Copyright 2019 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.example.appengine;

import com.sun.net.httpserver.HttpServer;
import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;

public class Main {

  public static void main(String[] args) throws IOException {
    // Create an instance of HttpServer bound to port defined by the 
    // PORT environment variable when present, otherwise on 8080.
    int port = Integer.parseInt(System.getenv().getOrDefault("PORT", "8080"));
    HttpServer server = HttpServer.create(new InetSocketAddress(port), 0);

    // Set root URI path.
    server.createContext("/", (var t) -> {
      byte[] response = "Hello World!".getBytes();
      t.sendResponseHeaders(200, response.length);
      try (OutputStream os = t.getResponseBody()) {
        os.write(response);
      }
    });

    // Create a second URI path.
    server.createContext("/foo", (var t) -> {
      byte[] response = "Foo!".getBytes();
      t.sendResponseHeaders(200, response.length);
      try (OutputStream os = t.getResponseBody()) {
        os.write(response);
      }
    });

    server.start();
  }
}

Compatibility with previous Java versions

For differences between Java 8 and the latest supported Java version, see Migrate from Java 8 to the latest Java runtime.

Environment variables

The following environment variables are set by the runtime:

Environment variable	Description
`GAE_APPLICATION`	The ID of your App Engine application. This ID is prefixed with '`region code`~' such as 'e~' for applications deployed in Europe.
`GAE_DEPLOYMENT_ID`	The ID of the current deployment.
`GAE_ENV`	The App Engine environment. Set to `standard`.
`GAE_INSTANCE`	The ID of the instance on which your service is currently running.
`GAE_MEMORY_MB`	The amount of memory available to the application process, in MB.
`GAE_RUNTIME`	The runtime specified in your `app.yaml` file.
`GAE_SERVICE`	The service name specified in your `app.yaml` file. If no service name is specified, it is set to `default`.
`GAE_VERSION`	The current version label of your service.
`GOOGLE_CLOUD_PROJECT`	The Google Cloud project ID associated with your application.
`PORT`	The port that receives HTTP requests.
`NODE_ENV` (Only available in the Node.js runtime)	Set to `production` when your service is deployed.

You can define additional environment variables in your app.yaml file, but the above values cannot be overridden, except for NODE_ENV.

HTTPS and forwarding proxies

App Engine terminates HTTPS connections at the load balancer and forwards requests to your application. Some applications need to determine the original request IP and protocol. The user's IP address is available in the standard X-Forwarded-For header. Applications that require this information should configure their web framework to trust the proxy.

Filesystem access

The runtime includes a writable /tmp directory, with all other directories having read-only access. Writing to /tmp takes up system memory.

Metadata server

Each instance of your application can use the App Engine metadata server to query information about the instance and your project.

You can access the metadata server through the following endpoints:

http://metadata
http://metadata.google.internal

Requests sent to the metadata server must include the request header Metadata-Flavor: Google. This header indicates that the request was sent with the intention of retrieving metadata values.

The following table lists the endpoints where you can make HTTP requests for specific metadata:

Metadata endpoint	Description
`/computeMetadata/v1/project/numeric-project-id`	The project number assigned to your project.
`/computeMetadata/v1/project/project-id`	The project ID assigned to your project.
`/computeMetadata/v1/instance/region`	The region the instance is running in.
`/computeMetadata/v1/instance/service-accounts/default/aliases`
`/computeMetadata/v1/instance/service-accounts/default/email`	The default service account email assigned to your project.
`/computeMetadata/v1/instance/service-accounts/default/`	Lists all the default service accounts for your project.
`/computeMetadata/v1/instance/service-accounts/default/scopes`	Lists all the supported scopes for the default service accounts.
`/computeMetadata/v1/instance/service-accounts/default/token`	Returns the auth token that can be used to authenticate your application to other Google Cloud APIs.

For example, to retrieve your project ID, send a request to http://metadata.google.internal/computeMetadata/v1/project/project-id.