ClassLoaderUtil.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.struts2.util;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.Set;
/**
* <p>
* This class is extremely useful for loading resources and classes in a fault tolerant manner
* that works across different applications servers.
* </p>
*
* <p>
* It has come out of many months of frustrating use of multiple application servers at Atlassian,
* please don't change things unless you're sure they're not going to break in one server or another!
* </p>
*/
public class ClassLoaderUtil {
/**
* <p>
* Load all resources with a given name, potentially aggregating all results
* from the searched classloaders. If no results are found, the resource name
* is prepended by '/' and tried again.
* </p>
*
* <p>
* This method will try to load the resources using the following methods (in order):
* </p>
*
* <ul>
* <li>From Thread.currentThread().getContextClassLoader()
* <li>From ClassLoaderUtil.class.getClassLoader()
* <li>callingClass.getClassLoader()
* </ul>
*
* @param resourceName The name of the resources to load
* @param callingClass The Class object of the calling object
* @param aggregate aggregate
*
* @return all resources found
*
* @throws IOException in case of IO errors
*/
public static Iterator<URL> getResources(String resourceName, Class callingClass, boolean aggregate) throws IOException {
AggregateIterator<URL> iterator = new AggregateIterator<>();
iterator.addEnumeration(Thread.currentThread().getContextClassLoader().getResources(resourceName));
if (!iterator.hasNext() || aggregate) {
iterator.addEnumeration(ClassLoaderUtil.class.getClassLoader().getResources(resourceName));
}
if (!iterator.hasNext() || aggregate) {
ClassLoader cl = callingClass.getClassLoader();
if (cl != null) {
iterator.addEnumeration(cl.getResources(resourceName));
}
}
if (!iterator.hasNext() && (resourceName != null) && ((resourceName.isEmpty()) || (resourceName.charAt(0) != '/'))) {
return getResources('/' + resourceName, callingClass, aggregate);
}
return iterator;
}
/**
* Load a given resource.
* <p>
* This method will try to load the resource using the following methods (in order):
* </p>
*
* <ul>
* <li>From {@link Thread#getContextClassLoader() Thread.currentThread().getContextClassLoader()}
* <li>From {@link Class#getClassLoader() ClassLoaderUtil.class.getClassLoader()}
* <li>From the {@link Class#getClassLoader() callingClass.getClassLoader() }
* </ul>
*
* @param resourceName The name of the resource to load
* @param callingClass The Class object of the calling object
*
* @return the resource
*/
public static URL getResource(String resourceName, Class callingClass) {
URL url = Thread.currentThread().getContextClassLoader().getResource(resourceName);
if (url == null) {
url = ClassLoaderUtil.class.getClassLoader().getResource(resourceName);
}
if (url == null) {
ClassLoader cl = callingClass.getClassLoader();
if (cl != null) {
url = cl.getResource(resourceName);
}
}
if ((url == null) && (resourceName != null) && ((resourceName.isEmpty()) || (resourceName.charAt(0) != '/'))) {
return getResource('/' + resourceName, callingClass);
}
return url;
}
/**
* This is a convenience method to load a resource as a stream.
*
* The algorithm used to find the resource is given in getResource()
*
* @param resourceName The name of the resource to load
* @param callingClass The Class object of the calling object
* @return resource as a stream
*/
public static InputStream getResourceAsStream(String resourceName, Class callingClass) {
URL url = getResource(resourceName, callingClass);
try {
return (url != null) ? url.openStream() : null;
} catch (IOException e) {
return null;
}
}
/**
* <p>
* Load a class with a given name.
* </p>
*
* <p>
* It will try to load the class in the following order:
* </p>
*
* <ul>
* <li>From {@link Thread#getContextClassLoader() Thread.currentThread().getContextClassLoader()}
* <li>Using the basic {@link Class#forName(java.lang.String) }
* <li>From {@link Class#getClassLoader() ClassLoaderUtil.class.getClassLoader()}
* <li>From the {@link Class#getClassLoader() callingClass.getClassLoader() }
* </ul>
*
* @param className The name of the class to load
* @param callingClass The Class object of the calling object
* @return class with for the given name
* @throws ClassNotFoundException If the class cannot be found anywhere.
*/
public static Class loadClass(String className, Class callingClass) throws ClassNotFoundException {
try {
return Thread.currentThread().getContextClassLoader().loadClass(className);
} catch (ClassNotFoundException e) {
try {
return Class.forName(className);
} catch (ClassNotFoundException ex) {
try {
return ClassLoaderUtil.class.getClassLoader().loadClass(className);
} catch (ClassNotFoundException exc) {
return callingClass.getClassLoader().loadClass(className);
}
}
}
}
/**
* Prints the current classloader hierarchy - useful for debugging.
*/
public static void printClassLoader() {
System.out.println("ClassLoaderUtils.printClassLoader");
printClassLoader(Thread.currentThread().getContextClassLoader());
}
/**
* Prints the classloader hierarchy from a given classloader - useful for debugging.
*
* @param cl the class loader
*/
public static void printClassLoader(ClassLoader cl) {
System.out.println("ClassLoaderUtils.printClassLoader(cl = " + cl + ")");
if (cl != null) {
printClassLoader(cl.getParent());
}
}
/**
* Aggregates Enumeration instances into one iterator and filters out duplicates. Always keeps one
* ahead of the enumerator to protect against returning duplicates.
*/
static class AggregateIterator<E> implements Iterator<E> {
LinkedList<Enumeration<E>> enums = new LinkedList<>();
Enumeration<E> cur = null;
E next = null;
Set<E> loaded = new HashSet<>();
public AggregateIterator<E> addEnumeration(Enumeration<E> e) {
if (e.hasMoreElements()) {
if (cur == null) {
cur = e;
next = e.nextElement();
loaded.add(next);
} else {
enums.add(e);
}
}
return this;
}
public boolean hasNext() {
return (next != null);
}
public E next() {
if (next != null) {
E prev = next;
next = loadNext();
return prev;
} else {
throw new NoSuchElementException();
}
}
private Enumeration<E> determineCurrentEnumeration() {
if (cur != null && !cur.hasMoreElements()) {
if (!enums.isEmpty()) {
cur = enums.removeLast();
} else {
cur = null;
}
}
return cur;
}
private E loadNext() {
if (determineCurrentEnumeration() != null) {
E tmp = cur.nextElement();
int loadedSize = loaded.size();
while (loaded.contains(tmp)) {
tmp = loadNext();
if (tmp == null || loaded.size() > loadedSize) {
break;
}
}
if (tmp != null) {
loaded.add(tmp);
}
return tmp;
}
return null;
}
public void remove() {
throw new UnsupportedOperationException();
}
}
}