The Java runtime

The Java runtime is the software stack responsible for installing your application code and dependencies, and then running that application in the flexible environment.

Java versions

Java 21 uses buildpacks. buildpacks help you build and configure containers that you can deploy to Google Cloud.

For the full list of supported Java versions, and their corresponding Ubuntu version, see the Runtime support schedule.

To use a supported Java version, you must:

Install gcloud CLI version 420.0.0 or later. You can update your CLI tooling by running the gcloud components update command. To view your installed version, you can run the gcloud version command.
To deploy using Maven, you must add the App Engine Maven plugin to your pom.xml:
```
<plugin>
   <groupId>com.google.cloud.tools</groupId>
   <artifactId>appengine-maven-plugin</artifactId>
   <version>2.8.1</version>
</plugin>
```
Other options for deploying include the gcloud app deploy command or the App Engine Gradle plugin.
Include the runtime_config and operating_system settings in your app.yaml file to specify an operating system.

Note: Supported Java runtimes only support deployment of executable JAR files. The supported Java runtimes don't include any web-serving frameworks that limit you from using servlet-based frameworks or libraries.
Optionally, you can specify a runtime version by including the runtime_version setting in your app.yaml file. By default, the latest Java version is used if the runtime_version setting is not specified.

Examples
- To specify Java 21 on Ubuntu 22:
```
runtime: java
env: flex

runtime_config:
    operating_system: "ubuntu22"
    runtime_version: "21"
```
- To specify the latest supported Java version on Ubuntu 22:
```
  runtime: java
  env: flex

  runtime_config:
      operating_system: "ubuntu22"
```

Previous runtime versions

For Java 8, specify runtime_config and jdk settings in your app.yaml file.

Java 8 supports:

The Eclipse Jetty 9 runtime and runs Jetty 9 using Servlet 3.1.
Your own server code that listens on port 8080 as part of your application. This feature lets you to use microservice frameworks like SparkJava and Spring-Boot.

Examples

Specifying Jetty version 9 (default / optional):

  runtime_config:
    jdk: openjdk8
    server: jetty9

Specifying Java version 8:
```
  runtime_config:
    jdk: openjdk8
```

Sample app.yaml for Java 8:

runtime: java
env: flex

handlers:
- url: /.*
  script: this field is required, but ignored

See pom.xml for configuring Maven and build.gradle for configuring Gradle.

Optional files

These configuration files are optional:

Place these files at the top level of MyDir. If you use any these files, you must deploy them separately with the gcloud app deploy command.

web.xml is optional and only needed if you are not using Servlet 3.x annotations.

You can place static web content, as well as your JavaServer pages, in your webapp/ directory. Jetty 9, which the App Engine flexible environment uses as its Servlet container, uses Apache Jasper as the default JSP implementation and includes JSTL taglibs.

The Dockerfile file is optional and used to customize the Java runtime.

Enabling gzip compression

The gzip handler is bundled with Jetty but not activated by default. To activate this module, set the environment variable, JETTY_MODULES_ENABLE=gzip, in the app.yaml file:

env_variables:
  JETTY_MODULES_ENABLE: 'gzip'

Using Quickstart

Jetty can speed up the start time of your application by pre-scanning its content and generating configuration files. If you are using an extended image you can activate quickstart by executing /scripts/jetty/quickstart.sh in your Dockerfile, after the application WAR is added.

Environment variables

You can set the following optional environment variables that are specific to the Java 8/Jetty 9 runtime. You can set OpenJDK 8 environment variables as well.

To set environment variables, use the env_variables key in the app.yaml file.

Env Var	Maven Prop	Value/Comment
`JETTY_PROPERTIES`		Comma separated list of `name=value` pairs appended to `$JETTY_ARGS`
`JETTY_MODULES_ENABLE`		Comma separated list of modules to enable by appending to `$JETTY_ARGS`
`JETTY_MODULES_DISABLE`		Comma separated list of modules to disable by removing from `$JETTY_BASE/start.d`
`JETTY_ARGS`		Arguments passed to jetty's `start.jar`. Any arguments used for custom jetty configuration should be passed here.
`JAVA_OPTS`		JVM runtime arguments

Enhanced Cloud Logging (Beta)

When running on the App Engine flexible environment, you can configure Java Util Logging to send logs to Cloud Logging by setting the JETTY_ARGS environment variable. For example:

 env_variables:
   JETTY_ARGS: -Djava.util.logging.config.file=WEB-INF/logging.properties

You must provide a logging.properties file that configures a LoggingHandler as follows:

handlers=com.google.cloud.logging.LoggingHandler

# Optional configuration
.level=INFO
com.google.cloud.logging.LoggingHandler.level=FINE
com.google.cloud.logging.LoggingHandler.log=gae_app.log
com.google.cloud.logging.LoggingHandler.formatter=java.util.logging.SimpleFormatter
java.util.logging.SimpleFormatter.format=%3$s: %5$s%6$s

Support for other Java runtimes

If you need to use a Java version that isn't supported, you can create a custom runtime and select a valid base image with the Java version you need.

For Google-supplied base images or Docker Java base images, see Building custom runtimes.

Organizing your files

Your source tree should look like this:

MyDir/
  [pom.xml]
  [build.gradle]
  [index.yaml]
  [cron.yaml]
  [dispatch.yaml]
  src/main/
    appengine/
      app.yaml
    docker/
      [Dockerfile]
    java/
      com.example.mycode/
        MyCode.java
    webapp/
      [index.html]
      [jsp.jsp]
      WEB-INF/
        [web.xml]

Local testing

When testing in your local environment, you may use emulated Google Cloud services, rather than remote Google Cloud services. You can use the following emulators:

Use the gcloud command to start them before you run your app:

gcloud beta emulators datastore start
gcloud beta emulators pubsub start
gcloud beta emulators bigtable start

Maven

Add these lines in your maven pom.xml file to add the Jetty 9 Maven Plugin:

<plugin>
  <groupId>org.eclipse.jetty</groupId>
  <artifactId>jetty-maven-plugin</artifactId>
  <version>9.4.2.v20170220</version>
</plugin>

To run your app locally, use the Maven command:

mvn jetty:run-exploded

Gradle

Follow the Creating a new Gradle project instructions to add the Gretty Gradle plugin to your build.gradle, and use the command:

gradle jettyRun

Deploy your app

To initialize the Google Cloud CLI, run:

gcloud init

After completing the configurations, you can use the Google Cloud CLI to deploy this directory containing the app.yaml and WAR files using:

gcloud app deploy app.yaml

If you are using any of the optional configuration files (index.yaml, cron.yaml, and dispatch.yaml) deploy them separately with the gcloud command. For example:

gcloud app deploy cron.yaml

Maven

Use Maven to deploy your app:

mvn package appengine:deploy -Dapp.deploy.projectId=PROJECT_ID

Replace PROJECT_ID with the ID of your Google Cloud project. If your pom.xml file already specifies your project ID, you don't need to include the -Dapp.deploy.projectId property in the command you run.

Gradle

Use Gradle to deploy your app:

gradle appengineDeploy