load Class Via Context - Java Reflection

Java examples for Reflection:Class Loader

  1. HOME
  2. Java
  3. Reflection
  4. Class Loader

Description

load Class Via Context

Demo Code

/*/*  w w  w.  j  a  v a 2  s .  com*/
 * Copyright 2011-2013 Amazon Technologies, Inc.
 *
 * Licensed 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://aws.amazon.com/apache2.0
 *
 * This file 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 com.java2s;

public class Main {
    public static void main(String[] argv) throws Exception {
        String fqcn = "java2s.com";
        System.out.println(loadClassViaContext(fqcn));
    }

    private static Class<?> loadClassViaContext(String fqcn) {
        ClassLoader loader = Thread.currentThread().getContextClassLoader();
        try {
            return loader == null ? null : loader.loadClass(fqcn);
        } catch (ClassNotFoundException e) {
        }
        return null;
    }

    /**
     * Loads the class via the optionally specified classes in the order of
     * their specification, and if not found, via the context class loader of
     * the current thread, and if not found, from the caller class loader as the
     * last resort.
     * 
     * @param fqcn
     *            fully qualified class name of the target class to be loaded
     * @param classes
     *            class loader providers
     * @return the class loaded; never null
     * 
     * @throws ClassNotFoundException
     *             if failed to load the class
     */
    public static Class<?> loadClass(String fqcn, Class<?>... classes)
            throws ClassNotFoundException {
        return loadClass(fqcn, true, classes);
    }

    /**
     * If classesFirst is false, loads the class via the context class
     * loader of the current thread, and if not found, via the class loaders of
     * the optionally specified classes in the order of their specification, and
     * if not found, from the caller class loader as the
     * last resort.
     * <p>
     * If classesFirst is true, loads the class via the optionally
     * specified classes in the order of their specification, and if not found,
     * via the context class loader of the current thread, and if not found,
     * from the caller class loader as the last resort.
     * 
     * @param fqcn
     *            fully qualified class name of the target class to be loaded
     * @param classesFirst
     *            true if the class loaders of the optionally specified classes
     *            take precedence over the context class loader of the current
     *            thread; false if the opposite is true.
     * @param classes
     *            class loader providers
     * @return the class loaded; never null
     * 
     * @throws ClassNotFoundException if failed to load the class
     */
    public static Class<?> loadClass(String fqcn, boolean classesFirst,
            Class<?>... classes) throws ClassNotFoundException {
        Class<?> target = null;
        if (classesFirst) {
            target = loadClassViaClasses(fqcn, classes);
            if (target == null) {
                target = loadClassViaContext(fqcn);
            }
        } else {
            target = loadClassViaContext(fqcn);
            if (target == null) {
                target = loadClassViaClasses(fqcn, classes);
            }
        }
        return target == null ? Class.forName(fqcn) : target;
    }

    private static Class<?> loadClassViaClasses(String fqcn,
            Class<?>[] classes) {
        if (classes != null) {
            for (Class<?> c : classes) {
                ClassLoader loader = c.getClassLoader();
                if (loader != null) {
                    try {
                        return loader.loadClass(fqcn);
                    } catch (ClassNotFoundException e) {
                        // move on to try the next class loader
                    }
                }
            }
        }
        return null;
    }
}

Related Tutorials