FileSystemResourceLoader继承自DefaultResourceLoader,并重写了getResourceByPath（String path）方法，返回FileSystemContextResource类型资源，源码如下：

/*
 * Copyright 2002-2017 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.core.io;

/**
 * {@link ResourceLoader} implementation that resolves plain paths as
 * file system resources rather than as class path resources
 * (the latter is {@link DefaultResourceLoader}'s default strategy).
 *
 * <p><b>NOTE:</b> Plain paths will always be interpreted as relative
 * to the current VM working directory, even if they start with a slash.
 * (This is consistent with the semantics in a Servlet container.)
 * <b>Use an explicit "file:" prefix to enforce an absolute file path.</b>
 *
 * <p>{@link org.springframework.context.support.FileSystemXmlApplicationContext}
 * is a full-fledged ApplicationContext implementation that provides
 * the same resource path resolution strategy.
 *
 * @author Juergen Hoeller
 * @since 1.1.3
 * @see DefaultResourceLoader
 * @see org.springframework.context.support.FileSystemXmlApplicationContext
 */
public class FileSystemResourceLoader extends DefaultResourceLoader {

	/**
	 * Resolve resource paths as file system paths.
	 * <p>Note: Even if a given path starts with a slash, it will get
	 * interpreted as relative to the current VM working directory.
	 * @param path the path to the resource
	 * @return the corresponding Resource handle
	 * @see FileSystemResource
	 * @see org.springframework.web.context.support.ServletContextResourceLoader#getResourceByPath
	 */
	@Override
	protected Resource getResourceByPath(String path) {
		if (path.startsWith("/")) {
			path = path.substring(1);
		}
		return new FileSystemContextResource(path);
	}


	/**
	 * FileSystemResource that explicitly expresses a context-relative path
	 * through implementing the ContextResource interface.
	 */
	private static class FileSystemContextResource extends FileSystemResource implements ContextResource {

		public FileSystemContextResource(String path) {
			super(path);
		}

		@Override
		public String getPathWithinContext() {
			return getPath();
		}
	}

}

FileSystemResourceLoader表示从文件系统中加载资源，所以咱们用FileSystemResourceLoader加载资源时应该给出资源的绝对路径，假设在classpath下有一个文件叫test.xml，咱们加载它该怎么写呢？ 可以这么写：

@Controller
public class LoginController  {				
	@RequestMapping(value="/index.html")
	public String loginPage() throws IOException {		
		ResourceLoader resourceLoader=new FileSystemResourceLoader();
		Resource resource=resourceLoader.getResource("D:\\ALANWANG-AIA\\Horse-workspace\\chapter3\\target\\classes\\test.xml");
		System.out.println(resource.getInputStream()); 
		return "index";
	}	  
}

启动服务器，访问index.html ，观察输出：

sun.nio.ch.ChannelInputStream@274a058c

这个测试代码为什么要加一句

	System.out.println(resource.getInputStream()); 

呢？这里要填一个坑，咱们知道，不管是DeafultResoureLoader还是FileSystemResourceLoader也好，都是ResourceLoader的子子孙孙，ResourceLoader中 getResource(String location);方法有一个现象就是该方法返回的Resource实例并不保证该Resource一定是存在的！重要的话再说一遍！该方法返回的Resource实例并不保证该Resource一定是存在的！，所以要加一个输出看看是否真的加载到了，咱们实际使用时一定要用resource.exists()判断一下！

当然，咱们已可以皮一下，非要传个相对路径进来，像这样：

@Controller
public class LoginController  {
				
	@RequestMapping(value="/index.html")
	public String loginPage() throws IOException {		
		ResourceLoader resourceLoader=new FileSystemResourceLoader();
		Resource resource=resourceLoader.getResource("test.xml");
		System.out.println(resource.getInputStream()); 
				
		return "index";
	}	  
}

启动服务器，访问index.html,观察输出：

[INFO] Restart completed at Fri Jul 06 10:48:31 CST 2018
[WARNING] /chapter3/index.html java.nio.file.NoSuchFileException: test.xml

发现挂了，实际上test.xml是放在classpath下的，假如这么写肯定是没问题的，

Resource resource=resourceLoader.getResource("classpath:test.xml");

那咱们非要用相对路径，又非要用FileSystemResourceLoader咋办？咱们看一下FileSystemResourceLoader是如何实现的：

	/**
	 * Resolve resource paths as file system paths.
	 * <p>Note: Even if a given path starts with a slash, it will get
	 * interpreted as relative to the current VM working directory.
	 * @param path the path to the resource
	 * @return the corresponding Resource handle
	 * @see FileSystemResource
	 * @see org.springframework.web.context.support.ServletContextResourceLoader#getResourceByPath
	 */
	@Override
	protected Resource getResourceByPath(String path) {
		if (path.startsWith("/")) {
			path = path.substring(1);
		}
		return new FileSystemContextResource(path);
	}

如果path以斜杠开头，会把斜杠截取掉，这段代码说明就算咱们传个斜杠打头的进来，也会被转成相对路径，只不过该方法的注释信息已经告诉咱们了，如果是个相对路径的话，相对的就是虚拟机的工作目录，也就是说该项目的根目录，咱们的这个测试项目叫chapter3,现在chapter3就是虚拟机的工作目录，因此咱们可以这么写：

@Controller
public class LoginController  {
				
	@RequestMapping(value="/index.html")
	public String loginPage() throws IOException {
		
		ResourceLoader resourceLoader=new FileSystemResourceLoader();
		Resource resource=resourceLoader.getResource("target\\classes\\test.xml");
		System.out.println(resource.getInputStream()); 
				
		return "index";
	}	  
}

启动服务器，访问index.html,观察输出：

[INFO] Restart completed at Fri Jul 06 10:59:47 CST 2018
sun.nio.ch.ChannelInputStream@783ccd29

成功了，不过是不是感觉这种写法怪怪的？反正我觉得是，所以咱们的总结就是，使用FileSystemResourceLoader加载资源时，最好给出资源的绝对路径，如果使用“＼”打头的或者是相对路径，可能会有坑。也由此得出一个结论，已斜杠打头资源路径指的是项目的根路径，或者说jvm的启动路径等等。普通的java项目就是项目路径，web项目就要看服务器的启动路径了，tomcat是bin,本例中咱们用的是jetty,咱们知道，jetty就是个jar包，用的时候扔到项目的classpath里就可以了，所以咱们测试项目的jvm启动路径就是chapter3 。是不是挺麻烦，是不是有点绕，所以除非有特殊原因必须使用FileSystemResourceLoader，否这咱们还是老实巴交的用classpath吧！

以上纯属个人理解，如有错误，还望读多指正，非常感谢！！！