src/java/org/apache/sqoop/util/ClassLoaderStack.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.sqoop.util;

import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Allows you to add and remove jar-files from the running JVM by
 * instantiating classloaders for them.
 */
public final class ClassLoaderStack {

  public static final Log LOG = LogFactory.getLog(
      ClassLoaderStack.class.getName());

  private ClassLoaderStack() {
  }

  /**
   * Sets the classloader for the current thread.
   */
  public static void setCurrentClassLoader(ClassLoader cl) {
    LOG.debug("Restoring classloader: " + cl.toString());
    Thread.currentThread().setContextClassLoader(cl);
  }

  /**
   * Adds a ClassLoader to the top of the stack that will load from the Jar
   * file of your choice. Returns the previous classloader so you can restore
   * it if need be, later.
   *
   * @param jarFile The filename of a jar file that you want loaded into this
   * JVM.
   * @param testClassName The name of the class to load immediately
   * (optional).
   */
  public static ClassLoader addJarFile(String jarFile, String testClassName)
      throws IOException {

    ClassLoader prevClassLoader =
        Thread.currentThread().getContextClassLoader();

    if (null != testClassName) {
      try {
        // Test to see if testClassName is already available. If so, do not
        // load this jar.
        LOG.debug("Checking for existing class: " + testClassName);
        Class.forName(testClassName, true, prevClassLoader);
        LOG.debug("Class is already available. Skipping jar " + jarFile);
        return prevClassLoader;
      } catch (ClassNotFoundException cnfe) {
        // Expected this; we need to load the jar. continue.
      }
    }

    String urlPath = "jar:" + new File(jarFile).toURI().toURL() + "!/";
    LOG.debug("Attempting to load jar through URL: " + urlPath);
    LOG.debug("Previous classloader is " + prevClassLoader);
    URL [] jarUrlArray = {new URL(urlPath)};
    URLClassLoader cl = URLClassLoader.newInstance(jarUrlArray,
        prevClassLoader);
    try {
      if (null != testClassName) {
        // try to load a class from the jar to force loading now.
        LOG.debug("Testing class in jar: " + testClassName);
        Class.forName(testClassName, true, cl);
      }
      LOG.debug("Loaded jar into current JVM: " + urlPath);
    } catch (ClassNotFoundException cnfe) {
      throw new IOException("Could not load jar " + jarFile
          + " into JVM. (Could not find class "
          + testClassName + ".)", cnfe);
    }

    LOG.debug("Added classloader for jar " + jarFile + ": " + cl);
    Thread.currentThread().setContextClassLoader(cl);
    return prevClassLoader;
  }

}