The project object model pom.xml -

Figure 640. «Hello, world» pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
              http://maven.apache.org/xsd/maven-4.0.0.xsd">

  <modelVersion>4.0.0</modelVersion>
  <groupId>de.hdm_stuttgart.mi</groupId>
  <artifactId>first</artifactId>
  <version>0.9</version>

</project>

Figure 641. Executing «compile» phase

mkdir -p src/main/java ❶

vim src/main/java/Hello.java ❷

first> mvn compile ...
[WARNING] File encoding has not been set, using platform encoding UTF-8, ❸
    i.e. build is platform dependent! ...
[ERROR] error: Source option 5 is no longer supported. ❹ Use 6 or later.
[ERROR] error: Target option 1.5 is no longer supported. ❺ Use 1.6 or later.

❶	Create an empty folder designated for hosting Java™ sources. See Figure 636, “Maven project layout ” for reference.
❷	Create an executable «Hello,world ...» class: `public class Hello { public static void main(String[] args) { System.out.println("Hello!"); }`
❸	Ask Maven for execution of the «compile» phase.
❹	We have not yet specified an encoding on project level. Thus two platforms using different encoding may provide different artifact outcomes.
❺	The world has moved: Java™ 1.5 compilers are outdated.

Figure 642. Examining the Java™ version culprit

>mvn help:effective-pom

<project ...>
   ...
  <plugin>
    <artifactId>maven-compiler-plugin</artifactId> ...

> find ~/.m2/repository/ -name maven-compiler-plugin\* ...
       
>jar -xf ~/.m2/repository/org/apache/maven/plugins/maven-compiler-plugin/3.7.0/maven-compiler-plugin-3.7.0.jar
>cat META-INF/maven/plugin.xml

<encoding implementation="java.lang.String" default-value="${project.build.sourceEncoding}">${encoding}</encoding>
<source implementation="java.lang.String" default-value="1.5">${maven.compiler.source}</source>
<target implementation="java.lang.String" default-value="1.5">${maven.compiler.target}</target>

Figure 643. Resolving encoding / Java™ version issues

<project ... xsd/maven-4.0.0.xsd">
...
  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>

    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
  </properties> ...

rm -rf ~/.m2/repository/
mvn compile;     find ~/.m2/repository/ -type f|wc -l
220

Figure 644. POM inheritance

‹›

Figure 645. The Super POM

jar -tf /usr/share/apache-maven-3.0.5/lib/maven-model-builder-3.0.5.jar

...
org/apache/maven/model/plugin/ReportingConverter.class
org/apache/maven/model/pom-4.0.0.xml
org/apache/maven/model/profile/activation/FileProfileActivator$1.class
...

Figure 646. pom-4.0.0.xml content

...                          <!-- Does this ring a (security related?) bell? -->
<repositories>
  <repository>
    <id>central</id>
    <name>Central Repository</name>
    <url>http://repo.maven.apache.org/maven2</url>
...
  <pluginRepositories>
    <pluginRepository>
      <id>central</id>
      <name>Central Repository</name>
      <url>http://repo.maven.apache.org/maven2</url>
...

Figure 647. Favour https in ~/.m2/settings.xml

<settings ... settings-1.0.0.xsd">

  <mirrors>
    <mirror>
      <id>central-secure</id>
      <mirrorOf>central</mirrorOf>
      <name>Maven Central: Favour https over http.</name>
      <url>https://repo.maven.apache.org/maven2</url>
    </mirror>
  </mirrors>
  ...

Figure 648. Resolving to effective pom.xml

first> mvn help:effective-pom
... Effective POMs, after inheritance, interpolation, and profiles are applied:

<project xmlns="http://maven.apache.org/POM/4.0.0" ...>
  <modelVersion>4.0.0</modelVersion>
...
      <plugin>
        <artifactId>maven-jar-plugin</artifactId>
        <version>2.3.2</version>
        <executions>
          <execution>
            <id>default-jar</id>
...

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The project object model pom.xml

The project object model `pom.xml`