在Java中，将不同来源的资源抽象成URL,通过注册不同的handler来处理不同 的资源（“file,http,jar”）等，URL没有默认相对于”classpath:“或ServletContext等资源的handler，因此spring对其内部使用的资源实现了自己的抽象结构：Resource接口来封装底层资源。

Resouce接口提供了File、url、uri的转换。提供getDescription用于在错误处理中打印信息。

类继承层次图：

定义了3个成员变量：

    private final String path;
    private ClassLoader classLoader;
    private Class<?> clazz;

构造方法

    public ClassPathResource(String path) {
        this(path, (ClassLoader)null);
    }

    public ClassPathResource(String path, ClassLoader classLoader) {
        Assert.notNull(path, "Path must not be null");
        String pathToUse = StringUtils.cleanPath(path);
        if(pathToUse.startsWith("/")) {
            pathToUse = pathToUse.substring(1);
        }

        this.path = pathToUse;
        this.classLoader = classLoader != null?classLoader:ClassUtils.getDefaultClassLoader();
    }

    public ClassPathResource(String path, Class<?> clazz) {
        Assert.notNull(path, "Path must not be null");
        this.path = StringUtils.cleanPath(path);
        this.clazz = clazz;
    }
    protected ClassPathResource(String path, ClassLoader classLoader, Class<?> clazz) {
        this.path = StringUtils.cleanPath(path);
        this.classLoader = classLoader;
        this.clazz = clazz;
    }

核心方法：

//获取文件资源的输入流，路径是基于classpath的
 public InputStream getInputStream() throws IOException {
        InputStream is;
        if(this.clazz != null) {
            is = this.clazz.getResourceAsStream(this.path);
        } else if(this.classLoader != null) {
            is = this.classLoader.getResourceAsStream(this.path);
        } else {
            is = ClassLoader.getSystemResourceAsStream(this.path);
        }

        if(is == null) {
            throw new FileNotFoundException(this.getDescription() + " cannot be opened because it does not exist");
        } else {
            return is;
        }
    }
//基于当前文件所在位置，创建相关的资源
public Resource createRelative(String relativePath) {
        String pathToUse = StringUtils.applyRelativePath(this.path, relativePath);
        return this.clazz != null?new ClassPathResource(pathToUse, this.clazz):new ClassPathResource(pathToUse, this.classLoader);
    }
//获取文件位置的描述信息
 public String getDescription() {
        StringBuilder builder = new StringBuilder("class path resource [");
        String pathToUse = this.path;
        if(this.clazz != null && !pathToUse.startsWith("/")) {
            builder.append(ClassUtils.classPackageAsResourcePath(this.clazz));
            builder.append('/');
        }

        if(pathToUse.startsWith("/")) {
            pathToUse = pathToUse.substring(1);
        }

        builder.append(pathToUse);
        builder.append(']');
        return builder.toString();
    }