spring（四）：Resource

Resource

Spring的Resource接口代表底层外部资源，提供了对底层外部资源的一致性访问接口。

public interface Resource extends InputStreamSource {
    boolean exists();
    default boolean isReadable() { return this.exists(); }
    default boolean isOpen() { return false; }
    default boolean isFile() { return false; }
    URL getURL() throws IOException;
    URI getURI() throws IOException;
    File getFile() throws IOException;
    default ReadableByteChannel readableChannel() throws IOException {
        return Channels.newChannel(this.getInputStream());
    }
    long contentLength() throws IOException;
    long lastModified() throws IOException;
    Resource createRelative(String var1) throws IOException;
    @Nullable
    String getFilename();
    String getDescription();
}

Resource接口提供了足够的抽象，足够满足我们日常使用。而且提供了很多内置Resource实现：ByteArrayResource、InputStreamResource 、FileSystemResource 、UrlResource 、ClassPathResource、ServletContextResource、VfsResource等。

ByteArrayResource

	private final byte[] byteArray;
    private final String description;

	return new ByteArrayInputStream(this.byteArray);

InputStreamResource

    private final InputStream inputStream;
    private final String description;
    private boolean read;

FileSystemResource

    private final String path;
    @Nullable
    private final File file;
    private final Path filePath;
    
    return Files.newInputStream(this.filePath);

UrlResource

    @Nullable
    private final URI uri;
    private final URL url;
    private final URL cleanedUrl;
    
    URLConnection con = this.url.openConnection();
    return con.getInputStream();

ClassPathResource

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

	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);
    }

ServletContextResource

代表web应用资源

    private final ServletContext servletContext;
    private final String path;
    
    InputStream is = this.servletContext.getResourceAsStream(this.path);

VfsResource

代表虚拟文件系统资源

    private final Object resource;
    
    return VfsUtils.getInputStream(this.resource);

ResourceLoader接口

public interface ResourceLoader {
    String CLASSPATH_URL_PREFIX = "classpath:";
    Resource getResource(String var1);
    @Nullable
    ClassLoader getClassLoader();
}

对于目前所有ApplicationContext都实现了ResourceLoader，因此可以使用其来加载资源。

ClassPathXmlApplicationContext：不指定前缀将返回默认的ClassPathResource资源，否则将根据前缀来加载资源；

    // 第一类构造器是根据提供的配置文件路径使用“ResourcePatternResolver”
	// 的“getResources()”接口通过匹配获取资源；即如“classpath:config.xml”
    public ClassPathXmlApplicationContext(String configLocation)...
    public ClassPathXmlApplicationContext(String... configLocations)...
    public ClassPathXmlApplicationContext(String[] configLocations, @Nullable ApplicationContext parent)...
    public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh)...
    public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)...

	// 第二类构造器是根据提供的路径和clazz来构造ClassResource资源。
    // 即采用“public ClassPathResource(String path, Class<?> clazz)”构造器获取资源。
    public ClassPathXmlApplicationContext(String path, Class<?> clazz)...
    public ClassPathXmlApplicationContext(String[] paths, Class<?> clazz)...
    public ClassPathXmlApplicationContext(String[] paths, Class<?> clazz, @Nullable ApplicationContext parent)...

FileSystemXmlApplicationContext：不指定前缀将返回FileSystemResource，否则将根据前缀来加载资源；

	// 将加载相对于当前工作目录的“configLocation”位置的资源
	// 在linux系统上不管“configLocation”是否带“/”，都作为相对路径
	// 在window系统上如“D:/resourceInject.xml”是绝对路径。
	// 因此在除非很必要的情况下，不建议使用该ApplicationContext。
    public FileSystemXmlApplicationContext(String configLocation)...
    public FileSystemXmlApplicationContext(String... configLocations)...
    public FileSystemXmlApplicationContext(String[] configLocations, ApplicationContext parent)...
    public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh)...
    public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)...

WebApplicationContext：不指定前缀将返回ServletContextResource，否则将根据前缀来加载资源；

其他：不指定前缀根据当前上下文返回Resource实现，否则将根据前缀来加载资源。