前言
ShallowEtagHeaderFilter用于处理Etag，HTTP1.1用ETag来判断请求的文件是否被修改，如果未修改则返回304，让浏览器使用缓存的数据。
一、注册
@Configuration
public class FilterConfiguration {
@Bean
public FilterRegistrationBean filterTestRegistrationBean(){
FilterRegistrationBean filterRegistry = new FilterRegistrationBean();
filterRegistry.setOrder(Ordered.HIGHEST_PRECEDENCE + 3);
//注册过滤器
filterRegistry.setFilter(new ShallowEtagHeaderFilter());
filterRegistry.addUrlPatterns("/*");
filterRegistry.setName("eTagFilter");
return filterRegistry;
}
}
二、流程
（1）doFilterInternal( )
protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
throws ServletException, IOException {
HttpServletResponse responseToUse = response;
if (!isAsyncDispatch(request) && !(response instanceof ConditionalContentCachingResponseWrapper)) {
//包装response、request
responseToUse = new ConditionalContentCachingResponseWrapper(response, request);
}
//执行过滤器连
filterChain.doFilter(request, responseToUse);
if (!isAsyncStarted(request) && !isContentCachingDisabled(request)) {
updateResponse(request, responseToUse);
}
}
（2）updateResponse( )
private void updateResponse(HttpServletRequest request, HttpServletResponse response) throws IOException {
ConditionalContentCachingResponseWrapper wrapper =
WebUtils.getNativeResponse(response, ConditionalContentCachingResponseWrapper.class);
Assert.notNull(wrapper, "ContentCachingResponseWrapper not found");
HttpServletResponse rawResponse = (HttpServletResponse) wrapper.getResponse();
//校验Etag
if (isEligibleForEtag(request, wrapper, wrapper.getStatus(), wrapper.getContentInputStream())) {
String eTag = wrapper.getHeader(HttpHeaders.ETAG);
if (!StringUtils.hasText(eTag)) {
//不存在时，生成Etag
eTag = generateETagHeaderValue(wrapper.getContentInputStream(), this.writeWeakETag);
//设置响应header
rawResponse.setHeader(HttpHeaders.ETAG, eTag);
}
//校验Etag，资源Etag不变时，return
if (new ServletWebRequest(request, rawResponse).checkNotModified(eTag)) {
return;
}
}
//Etag改变时，将响应题写入Response 响应给浏览器
wrapper.copyBodyToResponse();
}
（3）isEligibleForEtag( )
protected boolean isEligibleForEtag(HttpServletRequest request, HttpServletResponse response,
int responseStatusCode, InputStream inputStream) {
//response没有完成，响应码在200~300之间，并且是get请求
if (!response.isCommitted() &&
responseStatusCode >= 200 && responseStatusCode < 300 &&
HttpMethod.GET.matches(request.getMethod())) {
String cacheControl = response.getHeader(HttpHeaders.CACHE_CONTROL);
//并且cacheControl为null或者cacheControl不包含no-store
return (cacheControl == null || !cacheControl.contains(DIRECTIVE_NO_STORE));
}
return false;
}
（4）generateETagHeaderValue( )
对资源的inputStream进行md5计算
protected String generateETagHeaderValue(InputStream inputStream, boolean isWeak) throws IOException {
// length of W/ + " + 0 + 32bits md5 hash + "
StringBuilder builder = new StringBuilder(37);
if (isWeak) {
builder.append("W/");
}
builder.append("\"0");
DigestUtils.appendMd5DigestAsHex(inputStream, builder);
builder.append('"');
return builder.toString();
}
（5）checkNotModified( )
定位到Etag的校验
validateIfNoneMatch(etag)
private boolean validateIfNoneMatch(@Nullable String etag) {
if (!StringUtils.hasLength(etag)) {
return false;
}
Enumeration<String> ifNoneMatch;
try {
//获取请求header中的If-None-Match
ifNoneMatch = getRequest().getHeaders(HttpHeaders.IF_NONE_MATCH);
}
catch (IllegalArgumentException ex) {
return false;
}
if (!ifNoneMatch.hasMoreElements()) {
return false;
}
// We will perform this validation...
etag = padEtagIfNecessary(etag);
if (etag.startsWith("W/")) {
etag = etag.substring(2);
}
while (ifNoneMatch.hasMoreElements()) {
String clientETags = ifNoneMatch.nextElement();
//正则出Etag
Matcher etagMatcher = ETAG_HEADER_VALUE_PATTERN.matcher(clientETags);
// Compare weak/strong ETags as per https://tools.ietf.org/html/rfc7232#section-2.3
while (etagMatcher.find()) {
//通过equals进行值的比对
if (StringUtils.hasLength(etagMatcher.group()) && etag.equals(etagMatcher.group(3))) {
//匹配到任意一个Etag
this.notModified = true;
break;
}
}
}
return true;
}
总结
通过Etag，资源内容没有发生变化，就可以让浏览器使用缓存。