Improve validation mojo

pom.xml

@@ -163,7 +163,7 @@
     <dependency>
       <groupId>org.jenkins-ci</groupId>
       <artifactId>version-number</artifactId>
-      <version>1.9</version>
+      <version>1.10-rc142.172b_43188179</version> <!-- TODO https://github.com/jenkinsci/lib-version-number/pull/41 -->
     </dependency>
     <dependency>
       <groupId>org.kohsuke.stapler</groupId>

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

@@ -1,6 +1,13 @@
 package org.jenkinsci.maven.plugins.hpi;
 
 import hudson.util.VersionNumber;
+import io.jenkins.lib.versionnumber.JavaSpecificationVersion;
+import java.io.DataInputStream;
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.jar.JarFile;
+import java.util.zip.ZipEntry;
 import org.apache.maven.artifact.Artifact;
 import org.apache.maven.artifact.factory.ArtifactFactory;
 import org.apache.maven.execution.MavenSession;
@@ -12,7 +19,9 @@
 import org.apache.maven.project.MavenProject;
 import org.apache.maven.project.MavenProjectHelper;
 import org.apache.maven.project.ProjectBuilder;
+import org.apache.maven.shared.transfer.artifact.DefaultArtifactCoordinate;
 import org.apache.maven.shared.transfer.artifact.resolve.ArtifactResolver;
+import org.apache.maven.shared.transfer.artifact.resolve.ArtifactResolverException;
 
 /**
  * Mojos that need to figure out the Jenkins version it's working with.
@@ -89,6 +98,48 @@ protected String findJenkinsVersion() throws MojoExecutionException {
         throw new MojoExecutionException("Failed to determine Jenkins version this plugin depends on.");
     }
 
+    protected JavaSpecificationVersion getMinimumJavaVersion() throws MojoExecutionException {
+        Artifact core = resolveJenkinsCore();
+        File jar = wrap(core).getFile();
+        try (JarFile jarFile = new JarFile(jar)) {
+            ZipEntry entry = jarFile.getEntry("jenkins/model/Jenkins.class");
+            if (entry == null) {
+                throw new MojoExecutionException("Failed to find Jenkins.class in " + jar);
+            }
+            try (InputStream is = jarFile.getInputStream(entry); DataInputStream dis = new DataInputStream(is)) {
+                int magic = dis.readInt();
+                if (magic != 0xcafebabe) {
+                    throw new MojoExecutionException("Jenkins.class is not a valid class file in " + jar);
+                }
+                dis.readUnsignedShort(); // discard minor version
+                return JavaSpecificationVersion.fromClassVersion(dis.readUnsignedShort());
+            }
+        } catch (IOException e) {
+            throw new MojoExecutionException("Failed to read minimum Java version from " + jar, e);
+        }
+    }
+
+    private Artifact resolveJenkinsCore() throws MojoExecutionException {
+        DefaultArtifactCoordinate artifactCoordinate = new DefaultArtifactCoordinate();
+        if (jenkinsCoreId != null) {
+            String[] parts = jenkinsCoreId.split(":");
+            artifactCoordinate.setGroupId(parts[0]);
+            artifactCoordinate.setArtifactId(parts[1]);
+        } else {
+            artifactCoordinate.setGroupId("org.jenkins-ci.main");
+            artifactCoordinate.setArtifactId("jenkins-core");
+        }
+        artifactCoordinate.setVersion(findJenkinsVersion());
+
+        try {
+            return artifactResolver
+                    .resolveArtifact(session.getProjectBuildingRequest(), artifactCoordinate)
+                    .getArtifact();
+        } catch (ArtifactResolverException e) {
+            throw new MojoExecutionException("Couldn't download artifact: ", e);
+        }
+    }
+
     protected MavenArtifact wrap(Artifact a) {
         return new MavenArtifact(
                 a,

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

@@ -1,8 +1,8 @@
 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.plugin.MojoFailureException;
 import org.apache.maven.plugins.annotations.LifecyclePhase;
 import org.apache.maven.plugins.annotations.Mojo;
 
@@ -13,24 +13,16 @@
  */
 @Mojo(name = "validate", defaultPhase = LifecyclePhase.VALIDATE)
 public class ValidateMojo extends AbstractJenkinsMojo {
-    @Override
-    public void execute() throws MojoExecutionException, MojoFailureException {
-        if (!isMustangOrAbove())
-            throw new MojoExecutionException("JDK6 or later is necessary to build a Jenkins plugin");
 
-        if (new VersionNumber(findJenkinsVersion()).compareTo(new VersionNumber("1.419.99"))<=0)
-            throw new MojoExecutionException("This version of maven-hpi-plugin requires Jenkins 1.420 or later");
-    }
+    @Override
+    public void execute() throws MojoExecutionException {
+        JavaSpecificationVersion javaVersion = getMinimumJavaVersion();
+        if (JavaSpecificationVersion.forCurrentJVM().isOlderThan(javaVersion)) {
+            throw new MojoExecutionException("Java " + javaVersion + " or later is necessary to build this plugin.");
+        }
 
-    /**
-     * Are we running on JDK6 or above?
-     */
-    private static boolean isMustangOrAbove() {
-        try {
-            Class.forName("javax.annotation.processing.Processor");
-            return true;
-        } catch(ClassNotFoundException e) {
-            return false;
+        if (new VersionNumber(findJenkinsVersion()).compareTo(new VersionNumber("2.204")) < 0) {
+            throw new MojoExecutionException("This version of maven-hpi-plugin requires Jenkins 2.204 or later");
         }
     }
 }