Chapter 3. Creating a custom Java runtime environment for modular applications

Procedure

1. Create a simple Hello World application by using the Logger class.

   1. Check the base Red Hat build of OpenJDK 21 binary exists in the jdk-17 folder:

      $ ls jdk-17
      bin  conf  demo  include  jmods  legal  lib  man  NEWS  release
      $ ./jdk-17/bin/java -version
      openjdk version "17.0.10" 2021-01-19 LTS
      OpenJDK Runtime Environment 18.9 (build 17.0.10+9-LTS)
      OpenJDK 64-Bit Server VM 18.9 (build 17.0.10+9-LTS, mixed mode)

   2. Create a directory for your application:

      $ mkdir -p hello-example/sample

   3. Create hello-example/sample/HelloWorld.java file with the following content:

      package sample;
      
      import java.util.logging.Logger;
      
      public class HelloWorld {
          private static final Logger LOG = Logger.getLogger(HelloWorld.class.getName());
          public static void main(String[] args) {
              LOG.info("Hello World!");
          }
      }

   4. Create a file called hello-example/module-info.java and include the following code in the file:

      module sample
      {
          requires java.logging;
      }

   5. Compile your application:

      $ ./jdk-17/bin/javac -d example $(find hello-example -name \*.java)

   6. Run your application without a custom JRE:

      $ ./jdk-17/bin/java -cp example sample.HelloWorld
      Mar 09, 2021 10:48:59 AM sample.HelloWorld main
      INFO: Hello World!

      The previous example shows the base Red Hat build of OpenJDK requiring 311 MB to run a single class.

   7. (Optional) You can inspect the Red Hat build of OpenJDK and see many non-required modules for your application:

      $ du -sh jdk-17/
      313M	jdk-17/

      $ ./jdk-17/bin/java --list-modules
      java.base@17.0.1
      java.compiler@17.0.1
      java.datatransfer@17.0.1
      java.desktop@17.0.1
      java.instrument@17.0.1
      java.logging@17.0.1
      java.management@17.0.1
      java.management.rmi@17.0.1
      java.naming@17.0.1
      java.net.http@17.0.1
      java.prefs@17.0.1
      java.rmi@17.0.1
      java.scripting@17.0.1
      java.se@17.0.1
      java.security.jgss@17.0.1
      java.security.sasl@17.0.1
      java.smartcardio@17.0.1
      java.sql@17.0.1
      java.sql.rowset@17.0.1
      java.transaction.xa@17.0.1
      java.xml@17.0.1
      java.xml.crypto@17.0.1
      jdk.accessibility@17.0.1
      jdk.attach@17.0.1
      jdk.charsets@17.0.1
      jdk.compiler@17.0.1
      jdk.crypto.cryptoki@17.0.1
      jdk.crypto.ec@17.0.1
      jdk.dynalink@17.0.1
      jdk.editpad@17.0.1
      jdk.hotspot.agent@17.0.1
      jdk.httpserver@17.0.1
      jdk.incubator.foreign@17.0.1
      jdk.incubator.vector@17.0.1
      jdk.internal.ed@17.0.1
      jdk.internal.jvmstat@17.0.1
      jdk.internal.le@17.0.1
      jdk.internal.opt@17.0.1
      jdk.internal.vm.ci@17.0.1
      jdk.internal.vm.compiler@17.0.1
      jdk.internal.vm.compiler.management@17.0.1
      jdk.jartool@17.0.1
      jdk.javadoc@17.0.1
      jdk.jcmd@17.0.1
      jdk.jconsole@17.0.1
      jdk.jdeps@17.0.1
      jdk.jdi@17.0.1
      jdk.jdwp.agent@17.0.1
      jdk.jfr@17.0.1
      jdk.jlink@17.0.1
      jdk.jpackage@17.0.1
      jdk.jshell@17.0.1
      jdk.jsobject@17.0.1
      jdk.jstatd@17.0.1
      jdk.localedata@17.0.1
      jdk.management@17.0.1
      jdk.management.agent@17.0.1
      jdk.management.jfr@17.0.1
      jdk.naming.dns@17.0.1
      jdk.naming.rmi@17.0.1
      jdk.net@17.0.1
      jdk.nio.mapmode@17.0.1
      jdk.random@17.0.1
      jdk.sctp@17.0.1
      jdk.security.auth@17.0.1
      jdk.security.jgss@17.0.1
      jdk.unsupported@17.0.1
      jdk.unsupported.desktop@17.0.1
      jdk.xml.dom@17.0.1
      jdk.zipfs@17.0.1

      This sample Hello World application has very few dependencies. You can use jlink to create custom runtime images for your application. With these images you can run your application with only the required Red Hat build of OpenJDK dependencies.

2. Create your application module:

   $ mkdir sample-module
   $ ./jdk-17/bin/jmod create --class-path example/ --main-class sample.HelloWorld --module-version 1.0.0 -p example sample-module/hello.jmod

3. Create a custom JRE with the required modules and a custom application launcher for your application:

   $ ./jdk-17/bin/jlink --launcher hello=sample/sample.HelloWorld --module-path sample-module --add-modules sample --output custom-runtime

4. List the modules of the produced custom JRE.

   Observe that only a fraction of the original Red Hat build of OpenJDK remains.

   $ du -sh custom-runtime
   50M	custom-runtime/
   $ ./custom-runtime/bin/java --list-modules
   java.base@17.0.10
   java.logging@17.0.10
   sample@1.0.0

   Note

   Red Hat build of OpenJDK reduces the size of your custom Java runtime image from a 313 M runtime image to a 50 M runtime image.

5. Launch the application using the hello launcher:

   $ ./custom-runtime/bin/hello
   Jan 14, 2021 12:13:26 PM HelloWorld main
   INFO: Hello World!

   The generated JRE with your sample application does not have any other dependencies besides java.base, java.logging, and sample module.

   You can distribute your application that is bundled with the custom runtime in custom-runtime. This custom runtime includes your application.

   Note

   You must rebuild the custom Java runtime images for your application with every security update of your base Red Hat build of OpenJDK.