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/*
* Copyright 2011-2018 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.amazonaws.util;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
/**
* The class loader helper class.
*/
@SuppressWarnings({"checkstyle:nowhitespacebefore", "checkstyle:emptyblock"})
public enum ClassLoaderHelper {
;
/**
* Retrieves the resource 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 class loader of {@link ClassLoaderHelper} as the last resort.
*
* @param resource resource to be loaded
* @param classes class loader providers
* @return the resource loaded as an URL or null if not found.
*/
public static URL getResource(String resource, Class... classes) {
return getResource(resource, false, classes);
}
/**
* If classesFirst is false, retrieves the resource 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 class loader of {@link ClassLoaderHelper} as the
* last resort.
*
* If classesFirst is true, retrieves the resource 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 class loader of {@link ClassLoaderHelper} as the last resort.
*
* @param resource resource 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 resource loaded as an URL or null if not found.
*/
public static URL getResource(String resource, boolean classesFirst,
Class... classes) {
URL url;
if (classesFirst) {
url = getResourceViaClasses(resource, classes);
if (url == null) {
url = getResourceViaContext(resource);
}
} else {
url = getResourceViaContext(resource);
if (url == null) {
url = getResourceViaClasses(resource, classes);
}
}
return url == null ? ClassLoaderHelper.class.getResource(resource) : url;
}
private static URL getResourceViaClasses(String resource, Class[] classes) {
if (classes != null) {
for (Class c : classes) {
URL url = c.getResource(resource);
if (url != null)
return url;
}
}
return null;
}
private static URL getResourceViaContext(String resource) {
ClassLoader loader = Thread.currentThread().getContextClassLoader();
return loader == null ? null : loader.getResource(resource);
}
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;
}
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.
*
* 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;
}
/**
* Retrieves the resource as an input stream 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 class loader of {@link ClassLoaderHelper} as the last
* resort.
*
* @param resource resource to be loaded
* @param classes class loader providers
* @return the resource loaded as an input stream or null if not found.
*/
public static InputStream getResourceAsStream(String resource,
Class... classes) {
return getResourceAsStream(resource, false, classes);
}
/**
* If classesFirst is false, retrieves the resource as an input stream 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 class loader of
* {@link ClassLoaderHelper} as the last resort.
*
* If classesFirst is true, retrieves the resource as an input stream 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 class loader of {@link ClassLoaderHelper} as the last
* resort.
*
* @param resource resource 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 resource loaded as an input stream or null if not found.
*/
public static InputStream getResourceAsStream(String resource,
boolean classesFirst, Class... classes) {
URL url = getResource(resource, classesFirst, classes);
try {
return url != null ? url.openStream() : null;
} catch (IOException e) {
return null;
}
}
}