Moving a Java application to

It is common to have a Java application that you want to migrate to supports several styles of Java application, such as monolith, microservices, stateful, and stateless.

Minimum Requirement 

To run a Java application at you need:

Monolith/Single Application 

To start a Java application, you need to understand the structure. At minimum, you need to configure your application. You can also have two YAML files if you need:



name: app
type: "java:<version>" [1]
disk: 1024
    build: [2]
        start: [3]
  1. A Java version, e,g.: java:11
  2. Hooks define what happens when building the application. This build process typically generates an executable file such as a uber-jar e.g.: mvn clean package
  3. The commands key defines the command to launch the application. E.g.: java -jar file.jar
  4. In the start’s command needs to receive the port where the application will execute thought the PORT environment. That’s best when your app follows the port bind principle. E.g.: java -jar jar --port=$PORT


    type: upstream
    upstream: [1]
    type: redirect
    to: "https://{default}/"
  1. It defines the application will link in the route, e.g.: "app:http"


You have the option to use several languages in microservices. If you’re using Java there are several options to aggregate these services into a microservices: supports multiple applications and there are two options:

  • One application YAML file to each application
  • Aggregate all applications in a single file with an applications.yaml file
Article Content
Microservices in the cloud, part two Source
Microservices in the cloud, part one Source
Multiple Applications Source
Configure multi-applications with applications.yaml Source

Access to services included at has services managed by itself such as database, cache and search engine. However, you can use a database or any services such as a transition process, just be aware of the firewall.

When applications need to access a service, it is important to include the Relationships key, because. by default an application may not talk to any other container within a project it includes others projects as a microservices architecture.

To connect to a service from your deployed application, you need to pass the relationships information into your application’s configuration. The way to do so varies with the application. The most common mechanisms are listed below.


If you are using a framework that follows the Twelve-Factor App methodology, particularly the third point, you can configure the application directly from environment variables. Examples of such frameworks include Spring, Eclipse MicroProfile Config, Quarkus, and Micronauts.

The services information is available in the PLATFORM_RELATIONSHIPS environment variable. This variable is a base64-encoded JSON object with keys of the relationship name and values of arrays of relationship endpoint definitions. supports the jq tool, which allows to extract information from this JSON.

export DB_HOST=`echo $PLATFORM_RELATIONSHIPS | base64 --decode | jq -r ".database[0].host"`
Article Source
Spring Data MongoDB Source
Jakarta EE/MicroProfile Config Source
Spring Data JPA Source
Payara JPA Source

To reduce the number of lines in the application file and to make it cleaner, you have the option to move the variable environment to another file: a .environment file.


export DB_HOST=`echo $PLATFORM_RELATIONSHIPS | base64 --decode | jq -r ".database[0].host"`
export DB_PASSWORD=`echo $PLATFORM_RELATIONSHIPS | base64 --decode | jq -r ".database[0].password"`
export DB_USER=`echo $PLATFORM_RELATIONSHIPS | base64 --decode | jq -r ".database[0].username"`
export DB_DATABASE=`echo $PLATFORM_RELATIONSHIPS | base64 --decode | jq -r ".database[0].path"`
export JDBC=jdbc:postgresql://${HOST}/${DATABASE}
export JAVA_MEMORY=-Xmx$(jq .info.limits.memory /run/config.json)m
export JAVA_OPTS="$JAVA_MEMORY -XX:+ExitOnOutOfMemoryError"

This .environment can interact to each application file. E.g.:

name: app
type: "java:11"
disk: 1024
    build: ./mvnw package -DskipTests -Dquarkus.package.uber-jar=true
    database: "db:postgresql"
        start: java -jar $JAVA_OPTS $CREDENTIAL -Dquarkus.http.port=$PORT jarfile.jar

Using Java Config Reader 

If your framework doesn’t support configuration via environment variables, there is the Config Reader.This library provides a streamlined way to interact with a environment. It offers utility methods to access routes and relationships more cleanly than reading the raw environment variables yourself. See the maven dependency.

import Config;

Config config = new Config();
MySQL database = config.getCredential("database", MySQL::new);
DataSource dataSource = database.get();