Maven toolchains.xml Script

Sep 15, 2019

Apache Maven has introduced Maven Toolchains to ease configuring plug-ins and to avoid specifying any JDK location in the project POMs. Available JDKs are configured in ${HOME}/.m2/toolchains.xml. This article introduces a script to automate the generation of the toolchains.xml file, toolchains.xml.bash.

Theory of Operation

The toolchains.xml file contains a <toolchain/> element for each configured JDK.

<?xml version="1.0" encoding="UTF-8"?>
<toolchains>
  ...
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>VERSION</version>
      <vendor>VENDOR</vendor>
    </provides>
    <configuration>
      <jdkHome>JDKHOMEDIR</jdkHome>
    </configuration>
  </toolchain>
  ...
</toolchains>

For each JDKHOMEDIR candidate, the VERSION is specified by the JVM system property java.specification.version and a consistent vendor name is specified by java.vendor. This simple Java program (compatible with Java 1.5 and later) will provide the template. (The JDK directory must be supplied as a command line argument.)

public class ToolchainEntry {
    public static void main(String[] argv) {
        System.out
            .format("  <toolchain>\n")
            .format("    <type>jdk</type>\n")
            .format("    <provides>\n")
            .format("      <version>%s</version>\n",
                    System.getProperty("java.specification.version"))
            .format("      <vendor>%s</vendor>\n",
                    System.getProperty("java.vendor"))
            .format("    </provides>\n")
            .format("    <configuration>\n")
            .format("      <jdkHome>%s</jdkHome>\n", argv[0])
            .format("    </configuration>\n")
            .format("  </toolchain>\n");
    }
}

This program is combined with a bash script in the next section.

Script

A bash script is used to drive the execution of the Java program described in the previous section with responsibilities to:

1. Identify the candidate JVM directories:
   • Mac OS X (Darwin): /Library/Java/JavaVirtualMachines/*.jdk/Contents/Home
   • Linux: /usr/lib/jvm/*jdk*
2. Create Toolchain.java and compile to Toolchain.class¹
3. Create a new <toolchains/> document and iterate therough each candidate JDK directory executing Toolchain.class with the candidate JVM

#!/bin/bash

shopt -s nullglob
JDKS+=({/Library/Java/JavaVirtualMachines/*.jdk/Contents/Home,/usr/lib/jvm/*jdk*})

cat > ToolchainEntry.java <<EOF
public class ToolchainEntry {
    public static void main(String[] argv) {
        System.out
            .format("  <toolchain>\n")
            .format("    <type>jdk</type>\n")
            .format("    <provides>\n")
            .format("      <version>%s</version>\n",
                    System.getProperty("java.specification.version"))
            .format("      <vendor>%s</vendor>\n",
                    System.getProperty("java.vendor"))
            .format("    </provides>\n")
            .format("    <configuration>\n")
            .format("      <jdkHome>%s</jdkHome>\n", argv[0])
            .format("    </configuration>\n")
            .format("  </toolchain>\n");
    }
}
EOF

javac -Xlint:none -source 1.7 -target 1.7 ToolchainEntry.java

echo '<?xml version="1.0" encoding="UTF-8"?>' > toolchains.xml
echo '<toolchains>' >> toolchains.xml

for jdk in ${JDKS[@]}; do
    if [ -d ${jdk} ]; then
        java=${jdk}/bin/java

        if [ -x ${java} ]; then
            ${java} ToolchainEntry ${jdk} >> toolchains.xml
        fi
    fi
done

echo '</toolchains>' >> toolchains.xml

rm -rf ToolchainEntry.*

Output

Example generated toolchains.xml files for Mac OS X and Linux (Google Cloud Shell) are provided below.

<?xml version="1.0" encoding="UTF-8"?>
<toolchains>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>10</version>
      <vendor>AdoptOpenJDK</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/adoptopenjdk-10.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>11</version>
      <vendor>AdoptOpenJDK</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/adoptopenjdk-11.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>12</version>
      <vendor>AdoptOpenJDK</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/adoptopenjdk-12.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.8</version>
      <vendor>AdoptOpenJDK</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/adoptopenjdk-8.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>9</version>
      <vendor>Oracle Corporation</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/adoptopenjdk-9.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>11</version>
      <vendor>Amazon.com Inc.</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/amazon-corretto-11.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>12</version>
      <vendor>Oracle Corporation</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/jdk-12.0.2.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.7</version>
      <vendor>Oracle Corporation</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.8</version>
      <vendor>Oracle Corporation</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.8</version>
      <vendor>Oracle Corporation</vendor>
    </provides>
    <configuration>
      <jdkHome>/Library/Java/JavaVirtualMachines/jdk1.8.0_202.jdk/Contents/Home</jdkHome>
    </configuration>
  </toolchain>
</toolchains>

<?xml version="1.0" encoding="UTF-8"?>
<toolchains>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>11</version>
      <vendor>Debian</vendor>
    </provides>
    <configuration>
      <jdkHome>/usr/lib/jvm/java-1.11.0-openjdk-amd64</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>11</version>
      <vendor>Debian</vendor>
    </provides>
    <configuration>
      <jdkHome>/usr/lib/jvm/java-11-openjdk-amd64</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.8</version>
      <vendor>Oracle Corporation</vendor>
    </provides>
    <configuration>
      <jdkHome>/usr/lib/jvm/java-1.8.0-openjdk-amd64</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.8</version>
      <vendor>Oracle Corporation</vendor>
    </provides>
    <configuration>
      <jdkHome>/usr/lib/jvm/java-8-openjdk-amd64</jdkHome>
    </configuration>
  </toolchain>
</toolchains>

[1] Even though the earliest Java version supported by the java source is 1.5, the earliest code Oracle’s JDK 12 can produce is for 1.7. ↩