Spring的Resource接口为资源访问提供了统一的接口，不同的实现类实现了从不同上下文获取资源。下面是该接口的方法:
public interface Resource extends InputStreamSource {
boolean exists();
default boolean isReadable() {
return true;
}
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(getInputStream());
}
long contentLength() throws IOException;
long lastModified() throws IOException;
Resource createRelative(String relativePath) throws IOException;
@Nullable
String getFilename();
String getDescription();
}
public interface InputStreamSource {
InputStream getInputStream() throws IOException;
}
下面分析几个常用的Resource实现类。
1.AbstractResource提供了Resource的默认实现。
public abstract class AbstractResource implements Resource {
@Override
public boolean exists() {
try {
return getFile().exists();
}
catch (IOException ex) {
try {
InputStream is = getInputStream();
is.close();
return true;
}
catch (Throwable isEx) {
return false;
}
}
}
@Override
public boolean isReadable() {
return true;
}
@Override
public boolean isOpen() {
return false;
}
@Override
public boolean isFile() {
return false;
}
@Override
public URL getURL() throws IOException {
throw new FileNotFoundException(getDescription() + " cannot be resolved to URL");
}
@Override
public URI getURI() throws IOException {
URL url = getURL();
try {
return ResourceUtils.toURI(url);
}
catch (URISyntaxException ex) {
throw new NestedIOException("Invalid URI [" + url + "]", ex);
}
}
@Override
public File getFile() throws IOException {
throw new FileNotFoundException(getDescription() + " cannot be resolved to absolute file path");
}
@Override
public ReadableByteChannel readableChannel() throws IOException {
return Channels.newChannel(getInputStream());
}
@Override
public long contentLength() throws IOException {
InputStream is = getInputStream();
try {
long size = 0;
byte[] buf = new byte[255];
int read;
while ((read = is.read(buf)) != -1) {
size += read;
}
return size;
}
finally {
try {
is.close();
}
catch (IOException ex) {
}
}
}
@Override
public long lastModified() throws IOException {
long lastModified = getFileForLastModifiedCheck().lastModified();
if (lastModified == 0L) {
throw new FileNotFoundException(getDescription() +
" cannot be resolved in the file system for resolving its last-modified timestamp");
}
return lastModified;
}
protected File getFileForLastModifiedCheck() throws IOException {
return getFile();
}
@Override
public Resource createRelative(String relativePath) throws IOException {
throw new FileNotFoundException("Cannot create a relative resource for " + getDescription());
}
@Override
@Nullable
public String getFilename() {
return null;
}
@Override
public String toString() {
return getDescription();
}
@Override
public boolean equals(Object obj) {
return (obj == this ||
(obj instanceof Resource && ((Resource) obj).getDescription().equals(getDescription())));
}
@Override
public int hashCode() {
return getDescription().hashCode();
}
}
2.UrlResource 封装了一个 java.net.URL 对象，用来访问 URL 可以正常访问的任意对象，比如文件。
public class UrlResource extends AbstractFileResolvingResource {
@Nullable
private final URI uri;
private final URL url;
private final URL cleanedUrl;
public UrlResource(URI uri) throws MalformedURLException {
Assert.notNull(uri, "URI must not be null");
this.uri = uri;
this.url = uri.toURL();
this.cleanedUrl = getCleanedUrl(this.url, uri.toString());
}
public UrlResource(URL url) {
Assert.notNull(url, "URL must not be null");
this.url = url;
this.cleanedUrl = getCleanedUrl(this.url, url.toString());
this.uri = null;
}
public UrlResource(String path) throws MalformedURLException {
Assert.notNull(path, "Path must not be null");
this.uri = null;
this.url = new URL(path);
this.cleanedUrl = getCleanedUrl(this.url, path);
}
public UrlResource(String protocol, String location) throws MalformedURLException {
this(protocol, location, null);
}
public UrlResource(String protocol, String location, @Nullable String fragment) throws MalformedURLException {
try {
this.uri = new URI(protocol, location, fragment);
this.url = this.uri.toURL();
this.cleanedUrl = getCleanedUrl(this.url, this.uri.toString());
}
catch (URISyntaxException ex) {
MalformedURLException exToThrow = new MalformedURLException(ex.getMessage());
exToThrow.initCause(ex);
throw exToThrow;
}
}
private URL getCleanedUrl(URL originalUrl, String originalPath) {
try {
return new URL(StringUtils.cleanPath(originalPath));
}
catch (MalformedURLException ex) {
// Cleaned URL path cannot be converted to URL
// -> take original URL.
return originalUrl;
}
}
@Override
public InputStream getInputStream() throws IOException {
URLConnection con = this.url.openConnection();
ResourceUtils.useCachesIfNecessary(con);
try {
return con.getInputStream();
}
catch (IOException ex) {
// Close the HTTP connection (if applicable).
if (con instanceof HttpURLConnection) {
((HttpURLConnection) con).disconnect();
}
throw ex;
}
}
@Override
public URL getURL() {
return this.url;
}
@Override
public URI getURI() throws IOException {
if (this.uri != null) {
return this.uri;
}
else {
return super.getURI();
}
}
@Override
public boolean isFile() {
if (this.uri != null) {
return super.isFile(this.uri);
}
else {
return super.isFile();
}
}
@Override
public File getFile() throws IOException {
if (this.uri != null) {
return super.getFile(this.uri);
}
else {
return super.getFile();
}
}
@Override
public Resource createRelative(String relativePath) throws MalformedURLException {
if (relativePath.startsWith("/")) {
relativePath = relativePath.substring(1);
}
return new UrlResource(new URL(this.url, relativePath));
}
@Override
public String getFilename() {
return StringUtils.getFilename(this.cleanedUrl.getPath());
}
@Override
public String getDescription() {
return "URL [" + this.url + "]";
}
@Override
public boolean equals(Object obj) {
return (obj == this ||
(obj instanceof UrlResource && this.cleanedUrl.equals(((UrlResource) obj).cleanedUrl)));
}
@Override
public int hashCode() {
return this.cleanedUrl.hashCode();
}
}
3.可以使用 ClassPathResource 来获取类路径上的资源。ClassPathResource 可以使用线程上下文的加载器、调用者提供的加载器或指定的类中的任意一个来加载资源。
public class ClassPathResource extends AbstractFileResolvingResource {
private final String path;
@Nullable
private ClassLoader classLoader;
@Nullable
private Class<?> clazz;
public ClassPathResource(String path) {
this(path, (ClassLoader) null);
}
public ClassPathResource(String path, @Nullable 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, @Nullable Class<?> clazz) {
Assert.notNull(path, "Path must not be null");
this.path = StringUtils.cleanPath(path);
this.clazz = clazz;
}
@Deprecated
protected ClassPathResource(String path, @Nullable ClassLoader classLoader, @Nullable Class<?> clazz) {
this.path = StringUtils.cleanPath(path);
this.classLoader = classLoader;
this.clazz = clazz;
}
public final String getPath() {
return this.path;
}
@Nullable
public final ClassLoader getClassLoader() {
return (this.clazz != null ? this.clazz.getClassLoader() : this.classLoader);
}
@Override
public boolean exists() {
return (resolveURL() != null);
}
@Nullable
protected URL resolveURL() {
if (this.clazz != null) {
return this.clazz.getResource(this.path);
}
else if (this.classLoader != null) {
return this.classLoader.getResource(this.path);
}
else {
return ClassLoader.getSystemResource(this.path);
}
}
@Override
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(getDescription() + " cannot be opened because it does not exist");
}
return is;
}
@Override
public URL getURL() throws IOException {
URL url = resolveURL();
if (url == null) {
throw new FileNotFoundException(getDescription() + " cannot be resolved to URL because it does not exist");
}
return url;
}
@Override
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));
}
@Override
@Nullable
public String getFilename() {
return StringUtils.getFilename(this.path);
}
@Override
public String getDescription() {
StringBuilder builder = new StringBuilder("class path resource [");
String pathToUse = 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();
}
@Override
public boolean equals(Object obj) {
if (obj == this) {
return true;
}
if (obj instanceof ClassPathResource) {
ClassPathResource otherRes = (ClassPathResource) obj;
return (this.path.equals(otherRes.path) &&
ObjectUtils.nullSafeEquals(this.classLoader, otherRes.classLoader) &&
ObjectUtils.nullSafeEquals(this.clazz, otherRes.clazz));
}
return false;
}
@Override
public int hashCode() {
return this.path.hashCode();
}
}
4.FileSystemResource是针对 java.io.File 提供的 Resource 实现。
public class FileSystemResource extends AbstractResource implements WritableResource {
private final File file;
private final String path;
public FileSystemResource(File file) {
Assert.notNull(file, "File must not be null");
this.file = file;
this.path = StringUtils.cleanPath(file.getPath());
}
public FileSystemResource(String path) {
Assert.notNull(path, "Path must not be null");
this.file = new File(path);
this.path = StringUtils.cleanPath(path);
}
public final String getPath() {
return this.path;
}
@Override
public boolean exists() {
return this.file.exists();
}
@Override
public boolean isReadable() {
return (this.file.canRead() && !this.file.isDirectory());
}
@Override
public InputStream getInputStream() throws IOException {
try {
return Files.newInputStream(this.file.toPath());
}
catch (NoSuchFileException ex) {
throw new FileNotFoundException(ex.getMessage());
}
}
@Override
public boolean isWritable() {
return (this.file.canWrite() && !this.file.isDirectory());
}
@Override
public OutputStream getOutputStream() throws IOException {
return Files.newOutputStream(this.file.toPath());
}
@Override
public URL getURL() throws IOException {
return this.file.toURI().toURL();
}
@Override
public URI getURI() throws IOException {
return this.file.toURI();
}
@Override
public boolean isFile() {
return true;
}
@Override
public File getFile() {
return this.file;
}
@Override
public ReadableByteChannel readableChannel() throws IOException {
try {
return FileChannel.open(this.file.toPath(), StandardOpenOption.READ);
}
catch (NoSuchFileException ex) {
throw new FileNotFoundException(ex.getMessage());
}
}
@Override
public WritableByteChannel writableChannel() throws IOException {
return FileChannel.open(this.file.toPath(), StandardOpenOption.WRITE);
}
@Override
public long contentLength() throws IOException {
return this.file.length();
}
@Override
public Resource createRelative(String relativePath) {
String pathToUse = StringUtils.applyRelativePath(this.path, relativePath);
return new FileSystemResource(pathToUse);
}
@Override
public String getFilename() {
return this.file.getName();
}
@Override
public String getDescription() {
return "file [" + this.file.getAbsolutePath() + "]";
}
@Override
public boolean equals(Object obj) {
return (obj == this ||
(obj instanceof FileSystemResource && this.path.equals(((FileSystemResource) obj).path)));
}
@Override
public int hashCode() {
return this.path.hashCode();
}
}
5.ServletContextResource为了获取 web 根路径的 ServletContext 资源而提供的 Resource 实现。
public class ServletContextResource extends AbstractFileResolvingResource implements ContextResource {
private final ServletContext servletContext;
private final String path;
public ServletContextResource(ServletContext servletContext, String path) {
// check ServletContext
Assert.notNull(servletContext, "Cannot resolve ServletContextResource without ServletContext");
this.servletContext = servletContext;
// check path
Assert.notNull(path, "Path is required");
String pathToUse = StringUtils.cleanPath(path);
if (!pathToUse.startsWith("/")) {
pathToUse = "/" + pathToUse;
}
this.path = pathToUse;
}
public final ServletContext getServletContext() {
return this.servletContext;
}
public final String getPath() {
return this.path;
}
@Override
public boolean exists() {
try {
URL url = this.servletContext.getResource(this.path);
return (url != null);
}
catch (MalformedURLException ex) {
return false;
}
}
@Override
public boolean isReadable() {
InputStream is = this.servletContext.getResourceAsStream(this.path);
if (is != null) {
try {
is.close();
}
catch (IOException ex) {
// ignore
}
return true;
}
else {
return false;
}
}
@Override
public boolean isFile() {
try {
URL url = this.servletContext.getResource(this.path);
if (url != null && ResourceUtils.isFileURL(url)) {
return true;
}
else {
return (this.servletContext.getRealPath(this.path) != null);
}
}
catch (MalformedURLException ex) {
return false;
}
}
@Override
public InputStream getInputStream() throws IOException {
InputStream is = this.servletContext.getResourceAsStream(this.path);
if (is == null) {
throw new FileNotFoundException("Could not open " + getDescription());
}
return is;
}
@Override
public URL getURL() throws IOException {
URL url = this.servletContext.getResource(this.path);
if (url == null) {
throw new FileNotFoundException(
getDescription() + " cannot be resolved to URL because it does not exist");
}
return url;
}
@Override
public File getFile() throws IOException {
URL url = this.servletContext.getResource(this.path);
if (url != null && ResourceUtils.isFileURL(url)) {
// Proceed with file system resolution...
return super.getFile();
}
else {
String realPath = WebUtils.getRealPath(this.servletContext, this.path);
return new File(realPath);
}
}
@Override
public Resource createRelative(String relativePath) {
String pathToUse = StringUtils.applyRelativePath(this.path, relativePath);
return new ServletContextResource(this.servletContext, pathToUse);
}
@Override
@Nullable
public String getFilename() {
return StringUtils.getFilename(this.path);
}
@Override
public String getDescription() {
return "ServletContext resource [" + this.path + "]";
}
@Override
public String getPathWithinContext() {
return this.path;
}
@Override
public boolean equals(Object obj) {
if (obj == this) {
return true;
}
if (obj instanceof ServletContextResource) {
ServletContextResource otherRes = (ServletContextResource) obj;
return (this.servletContext.equals(otherRes.servletContext) && this.path.equals(otherRes.path));
}
return false;
}
@Override
public int hashCode() {
return this.path.hashCode();
}
}
6.在确实没有找到其他合适的 Resource 实现时，才使用 InputSteamResource。
public class InputStreamResource extends AbstractResource {
private final InputStream inputStream;
private final String description;
private boolean read = false;
public InputStreamResource(InputStream inputStream) {
this(inputStream, "resource loaded through InputStream");
}
public InputStreamResource(InputStream inputStream, @Nullable String description) {
Assert.notNull(inputStream, "InputStream must not be null");
this.inputStream = inputStream;
this.description = (description != null ? description : "");
}
@Override
public boolean exists() {
return true;
}
@Override
public boolean isOpen() {
return true;
}
@Override
public InputStream getInputStream() throws IOException, IllegalStateException {
if (this.read) {
throw new IllegalStateException("InputStream has already been read - " +
"do not use InputStreamResource if a stream needs to be read multiple times");
}
this.read = true;
return this.inputStream;
}
@Override
public String getDescription() {
return "InputStream resource [" + this.description + "]";
}
@Override
public boolean equals(Object obj) {
return (obj == this ||
(obj instanceof InputStreamResource && ((InputStreamResource) obj).inputStream.equals(this.inputStream)));
}
@Override
public int hashCode() {
return this.inputStream.hashCode();
}
}
- 当需要从字节数组加载内容时，ByteArrayResource 是一个不错的选择，使用 ByteArrayResource 可以不用求助于 InputStreamResource。
public class ByteArrayResource extends AbstractResource {
private final byte[] byteArray;
private final String description;
public ByteArrayResource(byte[] byteArray) {
this(byteArray, "resource loaded from byte array");
}
public ByteArrayResource(byte[] byteArray, @Nullable String description) {
Assert.notNull(byteArray, "Byte array must not be null");
this.byteArray = byteArray;
this.description = (description != null ? description : "");
}
public final byte[] getByteArray() {
return this.byteArray;
}
@Override
public boolean exists() {
return true;
}
@Override
public long contentLength() {
return this.byteArray.length;
}
@Override
public InputStream getInputStream() throws IOException {
return new ByteArrayInputStream(this.byteArray);
}
@Override
public String getDescription() {
return "Byte array resource [" + this.description + "]";
}
@Override
public boolean equals(Object obj) {
return (obj == this ||
(obj instanceof ByteArrayResource && Arrays.equals(((ByteArrayResource) obj).byteArray, this.byteArray)));
}
@Override
public int hashCode() {
return (byte[].class.hashCode() * 29 * this.byteArray.length);
}
}
Java 面试宝典是大明哥全力打造的 Java 精品面试题，它是一份靠谱、强大、详细、经典的 Java 后端面试宝典。它不仅仅只是一道道面试题，而是一套完整的 Java 知识体系，一套你 Java 知识点的扫盲贴。
它的内容包括：
- 大厂真题：Java 面试宝典里面的题目都是最近几年的高频的大厂面试真题。
- 原创内容：Java 面试宝典内容全部都是大明哥原创，内容全面且通俗易懂，回答部分可以直接作为面试回答内容。
- 持续更新：一次购买，永久有效。大明哥会持续更新 3+ 年，累计更新 1000+，宝典会不断迭代更新，保证最新、最全面。
- 覆盖全面：本宝典累计更新 1000+，从 Java 入门到 Java 架构的高频面试题，实现 360° 全覆盖。
- 不止面试：内容包含面试题解析、内容详解、知识扩展，它不仅仅只是一份面试题，更是一套完整的 Java 知识体系。
- 宝典详情：https://www.yuque.com/chenssy/sike-java/xvlo920axlp7sf4k
- 宝典总览：https://www.yuque.com/chenssy/sike-java/yogsehzntzgp4ly1
- 宝典进展：https://www.yuque.com/chenssy/sike-java/en9ned7loo47z5aw
目前 Java 面试宝典累计更新 400+ 道，总字数 42w+。大明哥还在持续更新中，下图是大明哥在 2024-12 月份的更新情况：
想了解详情的小伙伴，扫描下面二维码加大明哥微信【daming091】咨询
同时，大明哥也整理一套目前市面最常见的热点面试题。微信搜[大明哥聊 Java]或扫描下方二维码关注大明哥的原创公众号[大明哥聊 Java] ，回复【面试题】 即可免费领取。
