org.apache.hadoop.sqoop.util.ClassLoaderStack.java Source code

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Here is the source code for org.apache.hadoop.sqoop.util.ClassLoaderStack.java

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/**
 * 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.hadoop.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.info("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 tableClassName The name of the class to load immediately (optional)
     */
    public static ClassLoader addJarFile(String jarFile, String testClassName) throws IOException {

        // load the classes from the ORM JAR file into the current VM
        ClassLoader prevClassLoader = Thread.currentThread().getContextClassLoader();
        String urlPath = "jar:file://" + new File(jarFile).getAbsolutePath() + "!/";
        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.
                Class.forName(testClassName, true, cl);
            }
            LOG.info("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.info("Added classloader for jar " + jarFile + ": " + cl);
        Thread.currentThread().setContextClassLoader(cl);
        return prevClassLoader;
    }
}