How to set up the context class loader's classpath for runtime compilation?

Question

I would like to compile and load new classes at runtime within a weblogic 10.3 server. Class loading seems to be somewhat straightforward:

class ClassFileManager 
extends ForwardingJavaFileManager<StandardJavaFileManager> {

  Map<String, JavaClassObject> classes = new HashMap<String, JavaClassObject>();

  public ClassFileManager(StandardJavaFileManager standardManager) {
    super(standardManager);
  }

  @Override
  public ClassLoader getClassLoader(Location location) {
    return new SecureClassLoader(currentThread().getContextClassLoader()) {
      @Override
      protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] b = classes.get(name).getBytes();
        return super.defineClass(name, b, 0, b.length);
      }
    };
  }

  @Override
  public JavaFileObject getJavaFileForOutput(
      Location location, String className, Kind kind, FileObject sibling)
      throws IOException {
    JavaClassObject result = new JavaClassObject(className, kind);
    classes.put(className, result);
    return result;
  }
}

The simplest way to perform class loading seems to be to initialise a SecureClassLoader and have it use the contextClassLoader as the parent.

But when setting up the -classpath option for the JDK's runtime compiler, I cannot seem to find a "context classpath" in a string form. The following is a bit of a hack that works "well enough":

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
ClassFileManager fileManager = 
    new ClassFileManager(compiler.getStandardFileManager(null, null, null));
List<String> options = new ArrayList<String>();
options.add("-classpath");
options.add(System.getProperty("java.class.path") + ";" +
    getClass().getProtectionDomain()
              .getCodeSource().getLocation()
              .toURI().toString()
              .replace("file:/", "").replace("/", "\\"));

But it doesn't generate the complete class path of the context class loader. How can I do it, reliably? Can I?

Answer 1

WebLogic 10.3.6 has a fairly complex ClassLoader implementation. Fortunately the classloader used for web applications exposes a getClassPath method.

ClassLoader cl = Thread.currentThread().getContextClassLoader();
String classPath = ((weblogic.utils.classloaders.GenericClassLoader)cl).getClassPath();

// Once we have a classpath it's standard procedure
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager sfm = compiler.getStandardFileManager(null, null, null);
List<String> optionList = new ArrayList<String>();
optionList.addAll(Arrays.asList("-classpath", classPath));
compiler.getTask(null, sfm, null, optionList, null, sources).call();

Answer 2

Maybe this can help you. It works for my project on WebLogic.

String getClassPath() {
    final String BASE_PATH = "<your_project_folder_name>";
    String path = "";

    String classPathProperty = System.getProperty("java.class.path");
    if (classPathProperty != null) {
        path = classPathProperty + File.pathSeparator;
    }

    URL classLocation = this.getClass().getProtectionDomain().getCodeSource().getLocation();
    URL classesLocation = this.getClass().getClassLoader().getResource("/");
    if (classesLocation == null) {
        path = path + classLocation.getPath();
    }
    else {
        String classesLocationPath = classesLocation.getPath();
        String libsLocationPath = classesLocationPath + "../lib";
        File libsLocation = new File(libsLocationPath);
        if (libsLocation.exists() == false) {
            libsLocationPath = URLDecoder.decode(classesLocationPath + "../" + BASE_PATH + "/WEB-INF/lib/");
            libsLocation = new File(libsLocationPath);
        }

        File[] filesInLibraryPath = libsLocation.listFiles(new FilenameFilter() {
            @Override
            public boolean accept(File dir, String name) {
                return name.endsWith(".jar");
            }
        });
        if (filesInLibraryPath != null) {
            for (File libraryFile : filesInLibraryPath) {
                libsLocationPath += File.pathSeparator + URLDecoder.decode(libraryFile.getAbsolutePath());
            }
        }
        path =  path +
                classLocation.getPath() + File.pathSeparator + 
                classesLocationPath + File.pathSeparator + 
                libsLocationPath;
        path = URLDecoder.decode(path);
    }
    return path;
}

Answer 3

The Open Source Jasper JSP compiler used by Tomcat interrogates context URLClassLoader to generate a classpath string that is passed to the compiler.

If WebLogic does not expose getURLs method, an alternative is to use a custom implementation of JavaFileManager that uses context classloader getResource() method to fetch class files.

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
JavaFileManager customFileManager = ...
compiler.getTask(null, customFileManager, null, null, null, sources).call();

There is a complete example available here.

Answer 4

I suggest you make your custom JavaFileManager aware of the context classloader and specify it as an argument to JavaCompiler.getTask, along the lines of @anttix's idea.

For more info and a sample implementation including explanation (which is too verbose to repeat here) see blog post Using built-in JavaCompiler with a custom classloader.

Answer 5

Since "WebApplicationClassLoader" is a kind of "URLClassLoader", maybe you can use this code snippet.

ClassLoader classLoader = getClass().getClassLoader();
System.out.println("ClassLoader: " + classLoader);
URLClassLoader urlClassLoader = (URLClassLoader)classLoader;
URL[] urls = urlClassLoader.getURLs();
for (URL u : urls) {
    System.out.println("url: " + u);
}

This code lists all jars and directories in classpath.