Class,ClassLoader的getResource()和getResourceAsStream()区别

今天研究Spring源码中Resource的时候，用到其工具类ResourceUtils其中的getFile(String resourceLocation)。发现了Class和ClassLoader的这两个方法。先把我的工程目录贴到这儿吧，如下图：

Java代码如下：

public class ResourceTest ... {

    public static String FILE_NAME = "ResourceTest_zh_CN.properties";
    public static String ABSOLUTE_PATH =
    "E:/project/designpattern/classes/";
   
    public static void main(String[] args) throws Exception ...{
    File file = ResourceUtils.getFile("classpath:" + FILE_NAME);
    print(file);
    file = ResourceUtils.getFile("file:" + ABSOLUTE_PATH + FILE_NAME);
    print(file);
    }
   
    public static void print(File file) throws Exception ...{
    System.out.println("======================================================");
    System.out.println("class name = " + file.getClass().getName());
    System.out.println("file name = " + file.getName());
    System.out.println("file exists? = " + file.exists());
    System.out.println("file absolute path = " + file.getAbsolutePath());
    }
}

以上都是运行结果正确的，输出如下：

======================================================
class name = java.io.File
file name = ResourceTest_zh_CN.properties
file exists ? = true
file absolute path = E:/project/designpattern/classes/ResourceTest_zh_CN.properties
======================================================
class name = java.io.File
file name = ResourceTest_zh_CN.properties
file exists ? = true
file absolute path = E:/project/designpattern/classes/ResourceTest_zh_CN.properties

但是如果我把ResourceTest_zh_CN.properties剪切到resource目录下，无论怎么执行都报FileNotFoundException。看了一下ResourceUtils的源代码：

public static File getFile(String resourceLocation) throws FileNotFoundException ... {
        Assert.notNull(resourceLocation, "Resource location must not be null");
        if (resourceLocation.startsWith(CLASSPATH_URL_PREFIX)) ...{
            String path = resourceLocation.substring(CLASSPATH_URL_PREFIX.length());
            String description = "class path resource [" + path + "]";
            URL url = ClassUtils.getDefaultClassLoader().getResource(path);
            if (url == null) ...{
                throw new FileNotFoundException(
                        description + " cannot be resolved to absolute file path " +
                        "because it does not reside in the file system");
            }
            return getFile(url, description);
        }
        try ...{
            // try URL
            return getFile(new URL(resourceLocation));
        }
        catch (MalformedURLException ex) ...{
            // no URL -> treat as file path
            return new File(resourceLocation);
        }
}

从上可以看出ResourceUtils是通过ClassLoader的getResource方法来取得文件的。这儿岔开话题一下，可以注意到Class也有getResource和getResourceAsStream方法，这两个有什么区别呢？用代码来说话吧，先看一下目录图如下：

代码如下：

public class ResourceTest ... {

    public static String FILE_NAME = "ResourceTest_zh_CN.properties";
    public static String ABSOLUTE_PATH =
    "E:/project/designpattern/classes/";
   
    public static void main(String[] args) throws Exception ...{

    System.out.println(ResourceTest.class.getClassLoader().getResource("com/learn/spring/test/resource/" + FILE_NAME));
    System.out.println(ResourceTest.class.getClassLoader().getResourceAsStream("com/learn/spring/test/resource/" + FILE_NAME));   
    System.out.println(ResourceTest.class.getResource(FILE_NAME));
    System.out.println(ResourceTest.class.getResourceAsStream(FILE_NAME));
    }
}

这样输出结果正确，显然ClassLoader和Class的两个方法的区别一看便知，它们相对路径不一样，Class是把class文件所在的目录做为根目录，ClassLoader是把加载所有classpath的目录为根目录，也就是“..../classes”。

好，回到Spring的代码，如果把ResourceTest_zh_CN.properties剪切到resource目录下，而且要第一段代码执行正确的话，我们可以这样修改（上面的Class和ClassLoader的两个方法的对比已经很清楚），更改代码如下就可以了：

public class ResourceTest ... {

    public static String FILE_NAME = "ResourceTest_zh_CN.properties";
    public static String ABSOLUTE_PATH =
    "E:/project/designpattern/classes/";
   
    public static void main(String[] args) throws Exception ...{
    // TODO Auto-generated method stub
    Resource resource = null;
   
    resource = new ClassPathResource(FILE_NAME, ResourceTest.class);
    print(resource);
    resource = new FileSystemResource(ABSOLUTE_PATH + "com/learn/spring/test/resource/" + FILE_NAME);
    print(resource);
    File file = ResourceUtils.getFile("classpath:" +  "com/learn/spring/test/resource/" + FILE_NAME);
    print(file);
    file = ResourceUtils.getFile("file:" + ABSOLUTE_PATH + "com/learn/spring/test/resource/" + FILE_NAME);
    print(file);
    }
   
    public static void print(Resource resource) throws Exception ...{
    System.out.println("======================================================");
    System.out.println("class name = " + resource.getClass().getName());
    System.out.println("file name = " + resource.getFilename());
    System.out.println("file exists? = " + resource.exists());
    System.out.println("file url = " + resource.getURL());
    System.out.println("file description = " + resource.getDescription());
    }
   
    public static void print(File file) throws Exception ...{
    System.out.println("======================================================");
    System.out.println("class name = " + file.getClass().getName());
    System.out.println("file name = " + file.getName());
    System.out.println("file exists? = " + file.exists());
    System.out.println("file absolute path = " + file.getAbsolutePath());
    }
}