Springboot下的配置文件加载接口PropertySourceLoader以及它的两个实现类

原由：

今天报了个JRebel: ERROR Class 'org.springframework.boot.env.PropertiesPropertySourceLoader' could not be processed by org.zeroturnaround.javarebel.integration的错误。再网上找错误原因的时候，自己了解的一下PropertySourceLoader这个接口，做一下总结。

先是找了一下PropertiesPropertySourceLoader文件。

/*
 * Copyright 2012-2018 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
 *
 *      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.springframework.boot.env;

import java.io.IOException;
import java.util.Collections;
import java.util.List;
import java.util.Map;

import org.springframework.core.env.PropertySource;
import org.springframework.core.io.Resource;
import org.springframework.core.io.support.PropertiesLoaderUtils;

/**
 * Strategy to load '.properties' files into a {@link PropertySource}.
 *
 * @author Dave Syer
 * @author Phillip Webb
 * @author Madhura Bhave
 */
public class PropertiesPropertySourceLoader implements PropertySourceLoader {

	private static final String XML_FILE_EXTENSION = ".xml";

	@Override
	public String[] getFileExtensions() {
		return new String[] { "properties", "xml" };
	}

	@Override
	public List> load(String name, Resource resource)
			throws IOException {
		Map properties = loadProperties(resource);
		if (properties.isEmpty()) {
			return Collections.emptyList();
		}
		return Collections
				.singletonList(new OriginTrackedMapPropertySource(name, properties));
	}

	@SuppressWarnings({ "unchecked", "rawtypes" })
	private Map loadProperties(Resource resource) throws IOException {
		String filename = resource.getFilename();
		if (filename != null && filename.endsWith(XML_FILE_EXTENSION)) {
			return (Map) PropertiesLoaderUtils.loadProperties(resource);
		}
		return new OriginTrackedPropertiesLoader(resource).load();
	}

}

这个类的作用是加载Springboot下的properties和xml类型的配置文件的，它实现了PropertySourceLoader这个接口。

/*
 * Copyright 2012-2018 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
 *
 *      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.springframework.boot.env;

import java.io.IOException;
import java.util.List;

import org.springframework.core.env.PropertySource;
import org.springframework.core.io.Resource;
import org.springframework.core.io.support.SpringFactoriesLoader;

/**
 * Strategy interface located via {@link SpringFactoriesLoader} and used to load a
 * {@link PropertySource}.
 *
 * @author Dave Syer
 * @author Phillip Webb
 */
public interface PropertySourceLoader {

	/**
	 * Returns the file extensions that the loader supports (excluding the '.').
	 * @return the file extensions
	 */
	String[] getFileExtensions();

	/**
	 * Load the resource into one or more property sources. Implementations may either
	 * return a list containing a single source, or in the case of a multi-document format
	 * such as yaml a source for each document in the resource.
	 * @param name the root name of the property source. If multiple documents are loaded
	 * an additional suffix should be added to the name for each source loaded.
	 * @param resource the resource to load
	 * @return a list property sources
	 * @throws IOException if the source cannot be loaded
	 */
	List> load(String name, Resource resource) throws IOException;

}

PropertySourceLoader这个接口是用于加载Springboot下的配置文件的，例如properties、xml、yml和yaml。它有两个实现类，分别是PropertiesPropertySourceLoader和YamlPropertySourceLoader。

1.PropertiesPropertySourceLoader用于加载properties和xml配置文件。

2.YamlPropertySourceLoader.java用于加载yml和yaml配置文件。

如下图:

总结：

因为它们都继承于PropertySourceLoader，举一反三，以后如果想引入其他类型的配置文件，可以实现PropertySourceLoader，写直接的配置文件加载器了。以后有时间自己试一试。