Maven plugin can't load class

Question

I\'m trying to make a maven plugin that needs to use reflection. I want a project to run the plugin, and give it the full name of a class in the project, and the plugin will

Answer 1

I ran across this exact issue, today. The above suggestions didn't work for me, thought I would submit my solution to the list. I used the HibernateExporter mojo source which can be viewed at: http://grepcode.com/file/repo1.maven.org/maven2/org.codehaus.mojo/hibernate3-maven-plugin/2.2/org/codehaus/mojo/hibernate3/HibernateExporterMojo.java?av=f

/**
 * @parameter expression="${project}"
 * @required
 * @readonly
 */
private MavenProject project;

private ClassLoader getClassLoader() throws MojoExecutionException
{
  try
  {
    List<String> classpathElements = project.getCompileClasspathElements();
    classpathElements.add(project.getBuild().getOutputDirectory() );
    classpathElements.add(project.getBuild().getTestOutputDirectory() );
    URL urls[] = new URL[classpathElements.size()];

    for ( int i = 0; i < classpathElements.size(); ++i )
    {
      urls[i] = new File( (String) classpathElements.get( i ) ).toURI().toURL();
    }
    return new URLClassLoader(urls, getClass().getClassLoader() );
  }
  catch (Exception e)//gotta catch em all
  {
    throw new MojoExecutionException("Couldn't create a classloader.", e);
  }
}

public void execute() throws MojoExecutionException
{
  ClassLoader oldClassLoader = Thread.currentThread().getContextClassLoader();
  Thread.currentThread().setContextClassLoader(getClassLoader());

   //... your code here ...
}

Also make sure you are using the right MavenProject class. add this to your pom

<dependency>
  <groupId>org.apache.maven</groupId>
  <artifactId>maven-core</artifactId>
  <version>3.0.3</version>
</dependency>

<dependency>
  <groupId>org.apache.maven</groupId>
  <artifactId>maven-plugin-api</artifactId>
  <version>3.0.3</version>
</dependency>

Answer 2

You should consider using this to add the runtime class path elements to the current class realm. (You can use the PluginDescriptor to retrieve the class realm.

List<String> runtimeClasspathElements = project.getRuntimeClasspathElements();
ClassRealm realm = descriptor.getClassRealm();

for (String element : runtimeClasspathElements)
{
    File elementFile = new File(element);
    realm.addURL(elementFile.toURI().toURL());
}

This worked perfectly for me!

As Dave asked, here is the way to get the PluginDescriptor:

/**
 * The plugin descriptor
 * 
 * @parameter default-value="${descriptor}"
 */
private PluginDescriptor descriptor;

Answer 3

I'm sure there's a better way, but here's how I got it to work:

Add the following to the javadoc at the top of your mojo: @requiresDependencyResolution runtime

Add a MavenProject parameter:

/**
 * @parameter expression="${project}"
 * @required
 * @readonly
 */
private MavenProject project;

Then you can get the dependencies at runtime, and make your own classloader:

List runtimeClasspathElements = project.getRuntimeClasspathElements();
URL[] runtimeUrls = new URL[runtimeClasspathElements.size()];
for (int i = 0; i < runtimeClasspathElements.size(); i++) {
  String element = (String) runtimeClasspathElements.get(i);
  runtimeUrls[i] = new File(element).toURI().toURL();
}
URLClassLoader newLoader = new URLClassLoader(runtimeUrls,
  Thread.currentThread().getContextClassLoader());

Then you can load your class using this new classloader:

Class bundle = newLoader.loadClass("package.MyClass");

Answer 4

This worked for me and maven3 to get dependencies into the plugin classpath.

The trick is to use @Component to inject the PluginDescriptor. Otherwise it will not be set up correctly.

@Component
private MavenProject project;
@Component
private PluginDescriptor descriptor;

private void addDependenciesToClasspath(String artifactId) {
    for (Artifact artifact : project.getDependencyArtifacts()) {
        if (artifact.getArtifactId().equals(artifactId)) {
            try {
                final URL url = artifact.getFile().toURI().toURL();
                final ClassRealm realm = descriptor.getClassRealm();
                realm.addURL(url);
            }
            catch (MalformedURLException e) {
                throw new RuntimeException(e);
            }
        }
    }
}