编写下载服务器。第二部分：标头：Last-Modified，ETag和If-None-Match

客户端缓存是万维网的基础之一。服务器应告知客户端资源的有效性，客户端应尽可能快地对其进行缓存。如我们所见，如果不缓存Web，将会非常慢。只需在任何网站上Ctrl + F5并将其与普通F5进行比较-后者就会更快，因为它使用了已缓存的资源。缓存对于下载也很重要。如果我们已经获取了几兆字节的数据，并且它们没有改变，则通过网络推送它们是非常浪费的。

使用

HTTP ETag标头可用于避免重复下载客户端已有的资源。服务器与第一响应服务器一起返回ETag标头，该标头通常是文件内容的哈希值。客户端可以保留ETag并在以后请求相同资源时将其发送（在If-None-Match请求标头中）。如果在此期间未更改，则服务器可以简单地返回304 Not Modified响应。让我们从对ETag支持的集成测试开始：

def 'should send file if ETag not present'() {expect:mockMvc.perform(get('/download/' + FileExamples.TXT_FILE_UUID)).andExpect(status().isOk())}def 'should send file if ETag present but not matching'() {expect:mockMvc.perform(get('/download/' + FileExamples.TXT_FILE_UUID).header(IF_NONE_MATCH, '"WHATEVER"')).andExpect(status().isOk())
}def 'should not send file if ETag matches content'() {given:String etag = FileExamples.TXT_FILE.getEtag()expect:mockMvc.perform(get('/download/' + FileExamples.TXT_FILE_UUID).header(IF_NONE_MATCH, etag)).andExpect(status().isNotModified()).andExpect(header().string(ETAG, etag))
}

有趣的是，Spring框架中内置了ShallowEtagHeaderFilter 。安装它会使所有测试通过，包括最后一个测试：

@WebAppConfiguration
@ContextConfiguration(classes = [MainApplication])
@ActiveProfiles("test")
class DownloadControllerSpec extends Specification {private MockMvc mockMvc@Autowiredpublic void setWebApplicationContext(WebApplicationContext wac) {mockMvc = MockMvcBuilders.webAppContextSetup(wac).addFilter(new Sha512ShallowEtagHeaderFilter(), "/download/*").build()}//tests...}

我实际上插入了使用SHA-512而不是默认MD5的自己的Sha512ShallowEtagHeaderFilter 。同样由于某种原因，默认实现在哈希值前面加上0 ：

public class ShallowEtagHeaderFilter {protected String generateETagHeaderValue(byte[] bytes) {StringBuilder builder = new StringBuilder("\"0");DigestUtils.appendMd5DigestAsHex(bytes, builder);builder.append('"');return builder.toString();}//...
}

与：

public class Sha512ShallowEtagHeaderFilter extends ShallowEtagHeaderFilter {@Overrideprotected String generateETagHeaderValue(byte[] bytes) {final HashCode hash = Hashing.sha512().hashBytes(bytes);return "\"" + hash + "\"";}
}

不幸的是，我们无法使用内置过滤器，因为它们必须首先完全读取响应主体才能计算ETag 。这基本上关闭了上一篇文章中介绍的主体流传输–整个响应都存储在内存中。我们必须自己实现ETag功能。从技术上讲， If-None-Match可以包含多个ETag值。但是，谷歌浏览器和ShallowEtagHeaderFilter支持它，因此我们也将跳过它。为了控制响应头，我们现在返回ResponseEntity<Resource> ：

@RequestMapping(method = GET, value = "/{uuid}")
public ResponseEntity<Resource> download(@PathVariable UUID uuid,@RequestHeader(IF_NONE_MATCH) Optional<String> requestEtagOpt) {return storage.findFile(uuid).map(pointer -> prepareResponse(pointer, requestEtagOpt)).orElseGet(() -> new ResponseEntity<>(NOT_FOUND));
}private ResponseEntity<Resource> prepareResponse(FilePointer filePointer, Optional<String> requestEtagOpt) {return requestEtagOpt.filter(filePointer::matchesEtag).map(this::notModified).orElseGet(() -> serveDownload(filePointer));
}private ResponseEntity<Resource> notModified(String etag) {log.trace("Cached on client side {}, returning 304", etag);return ResponseEntity.status(NOT_MODIFIED).eTag(etag).body(null);
}private ResponseEntity<Resource> serveDownload(FilePointer filePointer) {log.debug("Serving '{}'", filePointer);final InputStream inputStream = filePointer.open();final InputStreamResource resource = new InputStreamResource(inputStream);return ResponseEntity.status(OK).eTag(filePointer.getEtag()).body(resource);
}

该过程由可选的requestEtagOpt控制。如果存在并且与客户端发送的内容匹配，则返回304。否则照常发送200 OK。本示例中介绍的FilePointer新方法如下所示：

import com.google.common.hash.HashCode;
import com.google.common.hash.Hashing;
import com.google.common.io.Files;public class FileSystemPointer implements FilePointer {private final File target;private final HashCode tag;public FileSystemPointer(File target) {try {this.target = target;this.tag = Files.hash(target, Hashing.sha512());} catch (IOException e) {throw new IllegalArgumentException(e);}}@Overridepublic InputStream open() {try {return new BufferedInputStream(new FileInputStream(target));} catch (FileNotFoundException e) {throw new IllegalArgumentException(e);}}@Overridepublic String getEtag() {return "\"" + tag + "\"";}@Overridepublic boolean matchesEtag(String requestEtag) {return getEtag().equals(requestEtag);}
}

在这里，您将看到FileSystemPointer实现，该实现直接从文件系统读取文件。关键部分是缓存标记，而不是在每次请求时都重新计算标记。上面的实现的行为符合预期，例如，Web浏览器不会再次下载资源。

3.使用

与ETag和If-None-Match标头类似，还有Last-Modified和If-Modified-Since 。我猜它们很容易解释：第一个服务器返回Last-Modified响应标头，指示给定资源的最后修改时间（ duh！ ）。客户端缓存此时间戳，并将其与后续请求一起传递给If-Modified-Since请求标头中的相同资源。如果同时未更改资源，则服务器将响应304，从而节省带宽。这是一个后备机制，同时实现ETag和Last-Modified是一个很好的实践。让我们从集成测试开始：

def 'should not return file if wasn\'t modified recently'() {given:Instant lastModified = FileExamples.TXT_FILE.getLastModified()String dateHeader = toDateHeader(lastModified)expect:mockMvc.perform(get('/download/' + FileExamples.TXT_FILE_UUID).header(IF_MODIFIED_SINCE, dateHeader)).andExpect(status().isNotModified())
}def 'should not return file if server has older version than the client'() {given:Instant lastModifiedLaterThanServer = FileExamples.TXT_FILE.getLastModified().plusSeconds(60)String dateHeader = toDateHeader(lastModifiedLaterThanServer)expect:mockMvc.perform(get('/download/' + FileExamples.TXT_FILE_UUID).header(IF_MODIFIED_SINCE, dateHeader)).andExpect(status().isNotModified())
}def 'should return file if was modified after last retrieval'() {given:Instant lastModifiedRecently = FileExamples.TXT_FILE.getLastModified().minusSeconds(60)String dateHeader = toDateHeader(lastModifiedRecently)expect:mockMvc.perform(get('/download/' + FileExamples.TXT_FILE_UUID).header(IF_MODIFIED_SINCE, dateHeader)).andExpect(status().isOk())
}private static String toDateHeader(Instant lastModified) {ZonedDateTime dateTime = ZonedDateTime.ofInstant(lastModified, ZoneOffset.UTC)DateTimeFormatter.RFC_1123_DATE_TIME.format(dateTime)
}

并执行：

@RequestMapping(method = GET, value = "/{uuid}")
public ResponseEntity<Resource> download(@PathVariable UUID uuid,@RequestHeader(IF_NONE_MATCH) Optional<String> requestEtagOpt,@RequestHeader(IF_MODIFIED_SINCE) Optional<Date> ifModifiedSinceOpt) {return storage.findFile(uuid).map(pointer -> prepareResponse(pointer,requestEtagOpt,ifModifiedSinceOpt.map(Date::toInstant))).orElseGet(() -> new ResponseEntity<>(NOT_FOUND));
}private ResponseEntity<Resource> prepareResponse(FilePointer filePointer, Optional<String> requestEtagOpt, Optional<Instant> ifModifiedSinceOpt) {if (requestEtagOpt.isPresent()) {final String requestEtag = requestEtagOpt.get();if (filePointer.matchesEtag(requestEtag)) {return notModified(filePointer);}}if (ifModifiedSinceOpt.isPresent()) {final Instant isModifiedSince = ifModifiedSinceOpt.get();if (filePointer.modifiedAfter(isModifiedSince)) {return notModified(filePointer);}}return serveDownload(filePointer);
}private ResponseEntity<Resource> serveDownload(FilePointer filePointer) {log.debug("Serving '{}'", filePointer);final InputStream inputStream = filePointer.open();final InputStreamResource resource = new InputStreamResource(inputStream);return response(filePointer, OK, resource);
}private ResponseEntity<Resource> notModified(FilePointer filePointer) {log.trace("Cached on client side {}, returning 304", filePointer);return response(filePointer, NOT_MODIFIED, null);
}private ResponseEntity<Resource> response(FilePointer filePointer, HttpStatus status, Resource body) {return ResponseEntity.status(status).eTag(filePointer.getEtag()).lastModified(filePointer.getLastModified().toEpochMilli()).body(body);
}

可悲的是，习惯上使用Optional不再看起来不错，所以我坚持使用isPresent() 。我们同时检查If-Modified-Since和If-None-Match 。如果两者都不匹配，我们将照常提供文件。只是为了让您了解这些标头的工作方式，让我们执行一些端到端测试。第一个要求：

> GET /download/4a8883b6-ead6-4b9e-8979-85f9846cab4b HTTP/1.1
> ...
> 
< HTTP/1.1 200 OK
< ETag: "8b97c678a7f1d2e0af...921228d8e"
< Last-Modified: Sun, 17 May 2015 15:45:26 GMT
< ...

带有ETag后续请求（已缩短）：

> GET /download/4a8883b6-ead6-4b9e-8979-85f9846cab4b HTTP/1.1
> If-None-Match: "8b97c678a7f1d2e0af...921228d8e"
> ...
> 
< HTTP/1.1 304 Not Modified
< ETag: "8b97c678a7f1d2e0af...921228d8e"
< Last-Modified: Sun, 17 May 2015 15:45:26 GMT
< ...

如果我们的客户仅支持Last-Modified ：

> GET /download/4a8883b6-ead6-4b9e-8979-85f9846cab4b HTTP/1.1
> If-Modified-Since: Tue, 19 May 2015 06:59:55 GMT
> ...
> 
< HTTP/1.1 304 Not Modified
< ETag: "8b97c678a7f1d2e0af9cda473b36c21f1b68e35b93fec2eb5c38d182c7e8f43a069885ec56e127c2588f9495011fd8ce032825b6d3136df7adbaa1f921228d8e"
< Last-Modified: Sun, 17 May 2015 15:45:26 GMT

有许多内置工具，例如过滤器，可以为您处理缓存。但是，如果您需要确保在服务器端流传输文件而不是对其进行预先缓冲，则需要格外小心。