Added SpringFactoriesLoader
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Arjen Poutsma
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/*
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* Copyright 2002-2012 the original author or authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package org.springframework.core;
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import java.io.IOException;
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import java.net.URL;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.Collections;
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import java.util.Enumeration;
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import java.util.List;
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import java.util.Properties;
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import org.apache.commons.logging.Log;
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import org.apache.commons.logging.LogFactory;
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import org.springframework.core.io.UrlResource;
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import org.springframework.core.io.support.PropertiesLoaderUtils;
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import org.springframework.util.Assert;
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import org.springframework.util.ClassUtils;
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import org.springframework.util.StringUtils;
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/**
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* General purpose factory loading mechanism.
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*
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* <p>The {@code SpringFactoriesLoader} loads and instantiates factories of a given type
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* from a given file location. If a location is not given, the {@linkplain
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* #DEFAULT_FACTORIES_LOCATION default location} is used.
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*
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* <p>The file should be in {@link Properties} format, where the keys is the fully
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* qualified interface or abstract class name, and the value is a comma separated list of
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* implementations. For instance:
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* <pre class="code">
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* example.MyService=example.MyServiceImpl1,example.MyServiceImpl2
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* </pre>
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* where {@code MyService} is the name of the interface, and {@code MyServiceImpl1} and
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* {@code MyServiceImpl2} are the two implementations.
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*
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* @author Arjen Poutsma
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* @since 3.2
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*/
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public abstract class SpringFactoriesLoader {
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private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);
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/** The location to look for the factories. Can be present in multiple JAR files. */
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public static final String DEFAULT_FACTORIES_LOCATION = "META-INF/spring.factories";
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/**
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* Loads the factory implementations of the given type from the default location, using
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* the given class loader.
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*
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* <p>The returned factories are ordered in accordance with the {@link OrderComparator}.
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*
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* @param factoryClass the interface or abstract class representing the factory
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* @param classLoader the ClassLoader to use for loading, can be {@code null} to use the
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* default
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* @throws IllegalArgumentException in case of errors
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*/
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public static <T> List<T> loadFactories(Class<T> factoryClass,
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ClassLoader classLoader) {
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return loadFactories(factoryClass, classLoader, null);
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}
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/**
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* Loads the factory implementations of the given type from the given location, using the
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* given class loader.
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*
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* <p>The returned factories are ordered in accordance with the {@link OrderComparator}.
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*
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* @param factoryClass the interface or abstract class representing the factory
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* @param classLoader the ClassLoader to use for loading, can be {@code null} to
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* use the default
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* @param factoriesLocation the factories file location, can be {@code null} to use the
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* {@linkplain #DEFAULT_FACTORIES_LOCATION default}
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* @throws IllegalArgumentException in case of errors
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*/
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public static <T> List<T> loadFactories(Class<T> factoryClass,
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ClassLoader classLoader,
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String factoriesLocation) {
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Assert.notNull(factoryClass, "'factoryClass' must not be null");
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if (classLoader == null) {
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classLoader = ClassUtils.getDefaultClassLoader();
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}
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if (factoriesLocation == null) {
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factoriesLocation = DEFAULT_FACTORIES_LOCATION;
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}
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List<String> factoryNames =
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loadFactoryNames(factoryClass, classLoader, factoriesLocation);
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if (logger.isTraceEnabled()) {
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logger.trace(
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"Loaded [" + factoryClass.getName() + "] names: " + factoryNames);
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}
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List<T> result = new ArrayList<T>(factoryNames.size());
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for (String factoryName : factoryNames) {
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result.add(instantiateFactory(factoryName, factoryClass, classLoader));
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}
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Collections.sort(result, new OrderComparator());
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return result;
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}
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private static List<String> loadFactoryNames(Class factoryClass,
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ClassLoader classLoader,
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String factoriesLocation) {
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String factoryClassName = factoryClass.getName();
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try {
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List<String> result = new ArrayList<String>();
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Enumeration urls = classLoader.getResources(factoriesLocation);
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while (urls.hasMoreElements()) {
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URL url = (URL) urls.nextElement();
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Properties properties =
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PropertiesLoaderUtils.loadProperties(new UrlResource(url));
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String factoryClassNames = properties.getProperty(factoryClassName);
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result.addAll(Arrays.asList(
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StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
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}
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return result;
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}
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catch (IOException ex) {
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throw new IllegalArgumentException(
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"Unable to load [" + factoryClass.getName() +
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"] factories from location [" +
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factoriesLocation + "]", ex);
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}
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}
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@SuppressWarnings("unchecked")
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private static <T> T instantiateFactory(String instanceClassName,
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Class<T> factoryClass,
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ClassLoader classLoader) {
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try {
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Class<?> instanceClass = ClassUtils.forName(instanceClassName, classLoader);
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if (!factoryClass.isAssignableFrom(instanceClass)) {
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throw new IllegalArgumentException(
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"Class [" + instanceClassName + "] is not assignable to [" +
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factoryClass.getName() + "]");
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}
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return (T) instanceClass.newInstance();
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}
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catch (ClassNotFoundException ex) {
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throw new IllegalArgumentException(
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factoryClass.getName() + " class [" + instanceClassName +
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"] not found", ex);
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}
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catch (LinkageError err) {
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throw new IllegalArgumentException(
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"Invalid " + factoryClass.getName() + " class [" + instanceClassName +
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"]: problem with handler class file or dependent class", err);
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}
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catch (InstantiationException ex) {
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throw new IllegalArgumentException(
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"Could not instantiate bean class [" + instanceClassName +
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"]: Is it an abstract class?", ex);
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}
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catch (IllegalAccessException ex) {
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throw new IllegalArgumentException(
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"Could not instantiate bean class [" + instanceClassName +
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"Is the constructor accessible?", ex);
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}
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}
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}
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