java - How do I use JDK6 ToolProvider and JavaCompiler with the context classloader?

Question

My usage case is compiling generated source files from a java program using the ToolProvider and JavaCompiler classes provided in JDK 6. The source files contain references to classes in the context classloader (it runs in a J2EE container), but not in the system classloader. My understanding is that by default the ToolProvider will create the JavaCompiler instance with the system classloader.

Is there a way to specify a classloader for JavaCompiler to use?

I tried this approach, modified from something on IBM DeveloperWorks:

FileManagerImpl fm = 
    new FileManagerImpl(compiler.getStandardFileManager(null, null, null););

with FileManagerImpl defined as:

static final class FileManagerImpl 
    extends ForwardingJavaFileManager<JavaFileManager> {

   public FileManagerImpl(JavaFileManager fileManager) {
      super(fileManager);
   }

   @Override
   public ClassLoader getClassLoader(JavaFileManager.Location location) {
      new Exception().printStackTrace();
      return Thread.currentThread().getContextClassLoader();
   }

}

The stacktrace indicates it's only called once during annotation processing. I verified the class referenced in the source file to be compiled is not on the system classpath but is available from the context classloader.

Answer 1

If you know the classpath to the files that are known to the contextclassloader you can pass them to the compiler:

    StandardJavaFileManager fileManager = compiler.getStandardFileManager(this /* diagnosticlistener */, null, null);
// get compilationunits from somewhere, for instance via fileManager.getJavaFileObjectsFromFiles(List<file> files)
List<String> options = new ArrayList<String>();
options.add("-classpath");
StringBuilder sb = new StringBuilder();
URLClassLoader urlClassLoader = (URLClassLoader) Thread.currentThread().getContextClassLoader();
for (URL url : urlClassLoader.getURLs())
    sb.append(url.getFile()).append(File.pathSeparator);
options.add(sb.toString());
CompilationTask task = compiler.getTask(null, fileManager, this /* diagnosticlistener */, options, null, compilationUnits);
task.call();

This example assumes you're using a URLClassloader (which allows you to retrieve the classpath) but you could insert your own classpath if you wanted to.