Reinstate InitializeMojo

This reverts commit 13db53e.

src/main/java/org/jenkinsci/maven/plugins/hpi/InitializeMojo.java

@@ -0,0 +1,73 @@
+package org.jenkinsci.maven.plugins.hpi;
+
+import hudson.util.VersionNumber;
+import io.jenkins.lib.versionnumber.JavaSpecificationVersion;
+import org.apache.maven.plugin.MojoExecutionException;
+import org.apache.maven.plugins.annotations.LifecyclePhase;
+import org.apache.maven.plugins.annotations.Mojo;
+
+/**
+ * Configure Maven for the desired version of Java.
+ *
+ * @author Basil Crow
+ */
+@Mojo(name = "initialize", defaultPhase = LifecyclePhase.INITIALIZE)
+public class InitializeMojo extends AbstractJenkinsMojo {
+
+    @Override
+    public void execute() throws MojoExecutionException {
+        setCompilerProperties();
+    }
+
+    private void setCompilerProperties() throws MojoExecutionException {
+        if (!project.getProperties().containsKey("maven.compiler.source")
+                && !project.getProperties().containsKey("maven.compiler.release")) {
+            // On an older plugin parent POM that predates the setting of these values as Maven properties.
+            return;
+        }
+
+        JavaSpecificationVersion javaVersion = getMinimumJavaVersion();
+        if (JavaSpecificationVersion.forCurrentJVM().isOlderThan(new VersionNumber("9"))) {
+            // Should always be set already, but just in case...
+            setProperty("maven.compiler.source", javaVersion.toString());
+            setProperty("maven.compiler.target", javaVersion.toString());
+            setProperty("maven.compiler.testSource", javaVersion.toString());
+            setProperty("maven.compiler.testTarget", javaVersion.toString());
+            // Should never be set already, but just in case...
+            unsetProperty("maven.compiler.release");
+            unsetProperty("maven.compiler.testRelease");
+        } else {
+            /*
+             * When compiling with a Java 9+ compiler, we always rely on "release" in favor of "source" and "target",
+             * even when compiling to Java 8 bytecode.
+             */
+            setProperty("maven.compiler.release", Integer.toString(javaVersion.toReleaseVersion()));
+            setProperty("maven.compiler.testRelease", Integer.toString(javaVersion.toReleaseVersion()));
+
+            /*
+             * While it does not hurt to have these set to the Java specification version, it is also not needed when
+             * "release" is in use.
+             */
+            unsetProperty("maven.compiler.source");
+            unsetProperty("maven.compiler.target");
+            unsetProperty("maven.compiler.testSource");
+            unsetProperty("maven.compiler.testTarget");
+        }
+    }
+
+    private void setProperty(String key, String value) {
+        String currentValue = project.getProperties().getProperty(key);
+        if (currentValue == null || !currentValue.equals(value)) {
+            getLog().info("Setting " + key + " to " + value);
+            project.getProperties().setProperty(key, value);
+        }
+    }
+
+    private void unsetProperty(String key) {
+        String currentValue = project.getProperties().getProperty(key);
+        if (currentValue != null && !currentValue.isEmpty()) {
+            getLog().info("Unsetting " + key);
+            project.getProperties().remove(key);
+        }
+    }
+}

src/main/java/org/jenkinsci/maven/plugins/hpi/extensions/HpiLifecycleMappingProvider.java

@@ -26,6 +26,7 @@ static Map<String, LifecyclePhase> getBindings() {
                 "validate",
                 new LifecyclePhase(
                         "org.jenkins-ci.tools:maven-hpi-plugin:validate,org.jenkins-ci.tools:maven-hpi-plugin:validate-hpi"));
+        bindings.put("initialize", new LifecyclePhase("org.jenkins-ci.tools:maven-hpi-plugin:initialize"));
         bindings.put(
                 "process-resources",
                 new LifecyclePhase("org.apache.maven.plugins:maven-resources-plugin:2.6:resources"));