====== Preparing minimal Java Runtime Environment ======

Standard distribution of Java Runtime Environment (JRE) unpacks to around 125MB. This can be significantly reduced by only including those JRE components that are actually used by Workcraft. For this we use Java Development Kit (JDK), preferably the one from https://adoptium.net/, and an unpacked Workcraft distribution.

<WRAP important 90% center round>
RedHat7 and CentOS 7 use GCLIB v2.17, which is incompatible with ''*.so'' files in ''openjdk-11-jdk'' package from Ubuntu 18.04 and newer -- they require GLIBC v2.27. Therefore, minimal JRE should be created either using locally installed JDK from https://adoptium.net/ (its ''*.so'' files are compatible with GLIBC v2.17), or using ''openjdk-11-jdk'' package in Ubuntu 16.04 (which is still based on "old enough" GLIBC v2.23).
</WRAP>

Print module dependencies of all Workcraft ''bin/*.jar'' files with their dependencies located in ''lib/'':
<code shell>
jdeps --recursive --print-module-deps --ignore-missing-deps --multi-release base --class-path lib/* bin/*
</code>
The reported dependencies are as follows:
<code shell>
java.base,java.compiler,java.desktop,java.management,java.naming,java.sql,jdk.xml.dom
</code>

The reported dependencies can be explained as follows:
  * ''java.base'', ''java.desktop'' - core functionality
  * ''java.compiler'' - used in Rhino library for JavaScript support
  * ''jdk.xml.dom'' - used in ''xml-apis-ext'' for SVG support
  * ''java.management'', ''java.naming'', ''java.sql'' - used in ''log4j'' which is needed by SON Plugin and Apache Batik (specifically for PDF Transcoder, and maybe something else)

Another dependency reported on one-by-one analysis of ''lib/*jar'' libraries (affected functionality is probably not used in Workcraft):
  * ''java.logging'' -- used in ''common-logging'' and ''javax.activation'' libraries

Create minimal JRE with specific modules as follows:
<code shell>
jlink --output jre --strip-debug --no-man-pages --no-header-files --compress=2 \
--add-modules java.base,java.compiler,java.desktop,java.management,java.naming,java.sql,jdk.xml.dom,java.logging
</code>

In Linux (depending on the used JDK) some of ''*.so'' files in the produced JRE are significantly larger than expected. This is a known problem and the solution is to strip dynamic libraries and binaries of debug symbols. In some cases it helped to reduce JRE size from 450M to 45M:
<code console>
strip --preserve-dates --strip-unneeded jre/lib/*.so jre/lib/server/*.so jre/bin/java jre/bin/keytool
</code>