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

Procedure

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

   1. You have the base Red Hat build of OpenJDK 11 available in the jdk-11 folder:

      $ ls jdk-11
      bin  conf  demo  include  jmods  legal  lib  man  NEWS  release
      $ ./jdk-11/bin/java -version
      openjdk version "11.0.10" 2021-01-19 LTS
      OpenJDK Runtime Environment 18.9 (build 11.0.10+9-LTS)
      OpenJDK 64-Bit Server VM 18.9 (build 11.0.10+9-LTS, mixed mode)

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   2. Create a directory for your application:

      $ mkdir -p hello-example/sample

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   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!");
          }
      }

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   4. Create hello-example/module-info.java file with the following content:

      module sample
      {
          requires java.logging;
      }

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   5. Compile your application:

      $ ./jdk-11/bin/javac -d example $(find hello-example -name \*.java)

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   6. Run your application without a custom JRE:

      $ ./jdk-11/bin/java -cp example sample.HelloWorld
      Mar 09, 2021 10:48:59 AM sample.HelloWorld main
      INFO: Hello World!

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      In this case, the base Red Hat build of OpenJDK takes 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-11/
      313M	jdk-11/

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      $ ./jdk-11/bin/java --list-modules
      java.base@11.0.10
      java.compiler@11.0.10
      java.datatransfer@11.0.10
      java.desktop@11.0.10
      java.instrument@11.0.10
      java.logging@11.0.10
      java.management@11.0.10
      java.management.rmi@11.0.10
      java.naming@11.0.10
      java.net.http@11.0.10
      java.prefs@11.0.10
      java.rmi@11.0.10
      java.scripting@11.0.10
      java.se@11.0.10
      java.security.jgss@11.0.10
      java.security.sasl@11.0.10
      java.smartcardio@11.0.10
      java.sql@11.0.10
      java.sql.rowset@11.0.10
      java.transaction.xa@11.0.10
      java.xml@11.0.10
      java.xml.crypto@11.0.10
      jdk.accessibility@11.0.10
      jdk.aot@11.0.10
      jdk.attach@11.0.10
      jdk.charsets@11.0.10
      jdk.compiler@11.0.10
      jdk.crypto.cryptoki@11.0.10
      jdk.crypto.ec@11.0.10
      jdk.dynalink@11.0.10
      jdk.editpad@11.0.10
      jdk.hotspot.agent@11.0.10
      jdk.httpserver@11.0.10
      jdk.internal.ed@11.0.10
      jdk.internal.jvmstat@11.0.10
      jdk.internal.le@11.0.10
      jdk.internal.opt@11.0.10
      jdk.internal.vm.ci@11.0.10
      jdk.internal.vm.compiler@11.0.10
      jdk.internal.vm.compiler.management@11.0.10
      jdk.jartool@11.0.10
      jdk.javadoc@11.0.10
      jdk.jcmd@11.0.10
      jdk.jconsole@11.0.10
      jdk.jdeps@11.0.10
      jdk.jdi@11.0.10
      jdk.jdwp.agent@11.0.10
      jdk.jfr@11.0.10
      jdk.jlink@11.0.10
      jdk.jshell@11.0.10
      jdk.jsobject@11.0.10
      jdk.jstatd@11.0.10
      jdk.localedata@11.0.10
      jdk.management@11.0.10
      jdk.management.agent@11.0.10
      jdk.management.jfr@11.0.10
      jdk.naming.dns@11.0.10
      jdk.naming.ldap@11.0.10
      jdk.naming.rmi@11.0.10
      jdk.net@11.0.10
      jdk.pack@11.0.10
      jdk.rmic@11.0.10
      jdk.scripting.nashorn@11.0.10
      jdk.scripting.nashorn.shell@11.0.10
      jdk.sctp@11.0.10
      jdk.security.auth@11.0.10
      jdk.security.jgss@11.0.10
      jdk.unsupported@11.0.10
      jdk.unsupported.desktop@11.0.10
      jdk.xml.dom@11.0.10
      jdk.zipfs@11.0.10

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      Sample Hello World application has very few dependencies. You can use jlink to create custom runtime images for your application. These images help you run your application with only required Red Hat build of OpenJDK dependencies.

2. Create your application module:

   $ mkdir sample-module
   $ ./jdk-11/bin/jmod create --class-path example/ --main-class sample.HelloWorld --module-version 1.0.0 -p example sample-module/hello.jmod

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

   $ ./jdk-11/bin/jlink --launcher hello=sample/sample.HelloWorld --module-path sample-module --add-modules sample --output custom-runtime

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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@11.0.10
   java.logging@11.0.10
   sample@1.0.0

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   Note

   The size of your custom java runtime image is being reduced to 50M runtime image from 313M 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!

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

   You can distribute your application bundled with the custom runtime in custom-runtime. It includes your application.