Add enforcer rule to require elements in the POM

Previously, we noted in the description of our base POMs that inheriting
projects needed to override a boatload of elements. But nothing enforced
it, so everyone was free to ignore that advice and instead unknowingly
inherit the configuration of the parent, which was often incorrect.

This rule makes the requirement to override all of these elements
iron-clad, avoiding this sort of confusion henceforth.

Author: ctrueden

src/main/java/org/scijava/maven/plugin/enforcer/RequireElements.java

@@ -0,0 +1,203 @@
+/*
+ * #%L
+ * A plugin for managing SciJava-based projects.
+ * %%
+ * Copyright (C) 2014 - 2015 Board of Regents of the University of
+ * Wisconsin-Madison.
+ * %%
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * #L%
+ */
+
+package org.scijava.maven.plugin.enforcer;
+
+import java.io.File;
+import java.io.IOException;
+
+import javax.xml.parsers.DocumentBuilder;
+import javax.xml.parsers.DocumentBuilderFactory;
+import javax.xml.parsers.ParserConfigurationException;
+import javax.xml.xpath.XPath;
+import javax.xml.xpath.XPathConstants;
+import javax.xml.xpath.XPathExpressionException;
+import javax.xml.xpath.XPathFactory;
+
+import org.apache.maven.enforcer.rule.api.EnforcerRule;
+import org.apache.maven.enforcer.rule.api.EnforcerRuleException;
+import org.apache.maven.enforcer.rule.api.EnforcerRuleHelper;
+import org.apache.maven.plugin.logging.Log;
+import org.apache.maven.project.MavenProject;
+import org.codehaus.plexus.component.configurator.expression.ExpressionEvaluationException;
+import org.codehaus.plexus.util.StringUtils;
+import org.w3c.dom.Document;
+import org.w3c.dom.NodeList;
+import org.xml.sax.SAXException;
+
+/**
+ * This rule checks that particular XML elements are set in the project POM
+ * (<em>not</em> inherited from an ancestor!).
+ *
+ * @author Curtis Rueden
+ */
+public class RequireElements implements EnforcerRule {
+
+	/** The required elements. Must be given. */
+	private String[] elements;
+
+	private Document doc;
+	private XPath xpath;
+
+	private final String ruleName = //
+		StringUtils.lowercaseFirstLetter(getClass().getSimpleName());
+
+	// -- EnforcerRule methods --
+
+	/**
+	 * Execute the rule.
+	 *
+	 * @param helper the helper
+	 * @throws EnforcerRuleException the enforcer rule exception
+	 */
+	@Override
+	public void execute(final EnforcerRuleHelper helper)
+		throws EnforcerRuleException
+	{
+		final Log log = helper.getLog();
+
+		final MavenProject project = getMavenProject(helper);
+
+		// Validate rule inputs.
+		if (elements == null || elements.length == 0) {
+			fail("no elements were specified");
+		}
+		for (final String element : elements) {
+			if (element.matches("[^A-Za-z0-9_]")) {
+				fail("invalid character in element name '" + element + "'");
+			}
+		}
+
+		// Locate the project's POM file.
+		final File pomFile = project.getFile();
+		if (pomFile == null) {
+			fail("cannot locate project POM");
+		}
+
+		// Parse the project POM directly. We do this, rather than leaning on the
+		// MavenProject, because we do _not_ want to interpolate the POM. Instead,
+		// we want to know if this specific POM includes the requisite element.
+		try {
+			doc = loadXML(pomFile);
+		}
+		catch (final ParserConfigurationException exc) {
+			log.warn("Cannot parse project POM", exc);
+			return;
+		}
+		catch (final SAXException exc) {
+			log.warn("Cannot parse project POM", exc);
+			return;
+		}
+		catch (final IOException exc) {
+			log.warn("Cannot parse project POM", exc);
+			return;
+		}
+
+		xpath = XPathFactory.newInstance().newXPath();
+		final StringBuilder errors = new StringBuilder();
+		for (final String element : elements) {
+			// Find matching element(s).
+			final NodeList nodes = xpath("//project/" + element);
+			if (nodes == null || nodes.getLength() <= 0) {
+				errors.append("* " + element + ": element is missing\n");
+				continue;
+			}
+
+			// Ensure first element has actual content beneath it.
+			final String text = nodes.item(0).getTextContent();
+			if (text == null || text.trim().isEmpty()) {
+				errors.append("* " + element + ": element has no content\n");
+			}
+		}
+		if (errors.length() > 0) {
+			throw new EnforcerRuleException(
+				"The following required elements have errors:\n" + errors);
+		}
+	}
+
+	@Override
+	public String getCacheId() {
+		return "0";
+	}
+
+	@Override
+	public boolean isCacheable() {
+		return false;
+	}
+
+	@Override
+	public boolean isResultValid(final EnforcerRule cachedRule) {
+		return false;
+	}
+
+	// -- Helper methods --
+
+	private void fail(final String message) throws EnforcerRuleException {
+		throw new EnforcerRuleException(ruleName + ": " + message);
+	}
+
+	private MavenProject getMavenProject(final EnforcerRuleHelper helper)
+		throws EnforcerRuleException
+	{
+		try {
+			return (MavenProject) helper.evaluate("${project}");
+		}
+		catch (final ExpressionEvaluationException exc) {
+			throw new EnforcerRuleException("Unable to get project.", exc);
+		}
+	}
+
+	private NodeList xpath(final String expression) throws EnforcerRuleException {
+		final Object o;
+		try {
+			o = xpath.evaluate(expression, doc, XPathConstants.NODESET);
+		}
+		catch (final XPathExpressionException exc) {
+			throw new EnforcerRuleException("Cannot parse xpath expression", exc);
+		}
+		if (!(o instanceof NodeList)) {
+			final String className = o == null ? "null" : o.getClass().getName();
+			fail("Unexpected xpath result type: " + className);
+		}
+		return (NodeList) o;
+	}
+
+	private static Document loadXML(final File file)
+		throws ParserConfigurationException, SAXException, IOException
+	{
+		return createBuilder().parse(file.getAbsolutePath());
+	}
+
+	private static DocumentBuilder createBuilder()
+		throws ParserConfigurationException
+	{
+		return DocumentBuilderFactory.newInstance().newDocumentBuilder();
+	}
+}