spring扩展机制非常重要的一个累，用来读取spring.handlers的NameSpaceHandler

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.beans.factory.xml;

import java.io.IOException;
import java.util.Map;
import java.util.Properties;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.beans.BeanUtils;
import org.springframework.beans.FatalBeanException;
import org.springframework.core.io.support.PropertiesLoaderUtils;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.CollectionUtils;

/**
 * Default implementation of the {@link NamespaceHandlerResolver} interface.
 * Resolves namespace URIs to implementation classes based on the mappings
 * contained in mapping file.
 *
 * <p>By default, this implementation looks for the mapping file at
 * {@code META-INF/spring.handlers}, but this can be changed using the
 * {@link #DefaultNamespaceHandlerResolver(ClassLoader, String)} constructor.
 *
 * @author Rob Harrop
 * @author Juergen Hoeller
 * @since 2.0
 * @see NamespaceHandler
 * @see DefaultBeanDefinitionDocumentReader
 */
public class DefaultNamespaceHandlerResolver implements NamespaceHandlerResolver {

	/**
	 * The location to look for the mapping files. Can be present in multiple JAR files.
	 */
	public static final String DEFAULT_HANDLER_MAPPINGS_LOCATION = "META-INF/spring.handlers";


	/** Logger available to subclasses */
	protected final Log logger = LogFactory.getLog(getClass());

	/** ClassLoader to use for NamespaceHandler classes */
	private final ClassLoader classLoader;

	/** Resource location to search for */
	private final String handlerMappingsLocation;

	/** Stores the mappings from namespace URI to NamespaceHandler class name / instance */
	private volatile Map<String, Object> handlerMappings;


	/**
	 * Create a new {@code DefaultNamespaceHandlerResolver} using the
	 * default mapping file location.
	 * <p>This constructor will result in the thread context ClassLoader being used
	 * to load resources.
	 * @see #DEFAULT_HANDLER_MAPPINGS_LOCATION
	 */
	public DefaultNamespaceHandlerResolver() {
		this(null, DEFAULT_HANDLER_MAPPINGS_LOCATION);
	}

	/**
	 * Create a new {@code DefaultNamespaceHandlerResolver} using the
	 * default mapping file location.
	 * @param classLoader the {@link ClassLoader} instance used to load mapping resources
	 * (may be {@code null}, in which case the thread context ClassLoader will be used)
	 * @see #DEFAULT_HANDLER_MAPPINGS_LOCATION
	 */
	public DefaultNamespaceHandlerResolver(ClassLoader classLoader) {
		this(classLoader, DEFAULT_HANDLER_MAPPINGS_LOCATION);
	}

	/**
	 * Create a new {@code DefaultNamespaceHandlerResolver} using the
	 * supplied mapping file location.
	 * @param classLoader the {@link ClassLoader} instance used to load mapping resources
	 * may be {@code null}, in which case the thread context ClassLoader will be used)
	 * @param handlerMappingsLocation the mapping file location
	 */
	public DefaultNamespaceHandlerResolver(ClassLoader classLoader, String handlerMappingsLocation) {
		Assert.notNull(handlerMappingsLocation, "Handler mappings location must not be null");
		this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
		this.handlerMappingsLocation = handlerMappingsLocation;
	}


	/**
	 * Locate the {@link NamespaceHandler} for the supplied namespace URI
	 * from the configured mappings.
	 * @param namespaceUri the relevant namespace URI
	 * @return the located {@link NamespaceHandler}, or {@code null} if none found
	 */
	@Override
	public NamespaceHandler resolve(String namespaceUri) {
		Map<String, Object> handlerMappings = getHandlerMappings();
		Object handlerOrClassName = handlerMappings.get(namespaceUri);
		if (handlerOrClassName == null) {
			return null;
		}
		else if (handlerOrClassName instanceof NamespaceHandler) {
			return (NamespaceHandler) handlerOrClassName;
		}
		else {
			String className = (String) handlerOrClassName;
			try {
				Class<?> handlerClass = ClassUtils.forName(className, this.classLoader);
				if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {
					throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +
							"] does not implement the [" + NamespaceHandler.class.getName() + "] interface");
				}
				NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
				namespaceHandler.init();
				handlerMappings.put(namespaceUri, namespaceHandler);
				return namespaceHandler;
			}
			catch (ClassNotFoundException ex) {
				throw new FatalBeanException("NamespaceHandler class [" + className + "] for namespace [" +
						namespaceUri + "] not found", ex);
			}
			catch (LinkageError err) {
				throw new FatalBeanException("Invalid NamespaceHandler class [" + className + "] for namespace [" +
						namespaceUri + "]: problem with handler class file or dependent class", err);
			}
		}
	}

	/**
	 * Load the specified NamespaceHandler mappings lazily.
	 */
	private Map<String, Object> getHandlerMappings() {
		Map<String, Object> handlerMappings = this.handlerMappings;
		if (handlerMappings == null) {
			synchronized (this) {
				handlerMappings = this.handlerMappings;
				if (handlerMappings == null) {
					if (logger.isDebugEnabled()) {
						logger.debug("Loading NamespaceHandler mappings from [" + this.handlerMappingsLocation + "]");
					}
					try {
						Properties mappings =
								PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
						if (logger.isDebugEnabled()) {
							logger.debug("Loaded NamespaceHandler mappings: " + mappings);
						}
						handlerMappings = new ConcurrentHashMap<String, Object>(mappings.size());
						CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
						this.handlerMappings = handlerMappings;
					}
					catch (IOException ex) {
						throw new IllegalStateException(
								"Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
					}
				}
			}
		}
		return handlerMappings;
	}


	@Override
	public String toString() {
		return "NamespaceHandlerResolver using mappings " + getHandlerMappings();
	}

}