原文鏈接：http://www.javaarch.net/jiagoushi/867.htm
這兩個默認servlet名稱都是defaultservlet，然後在web.xml中就可以添加下面的配置讓應用支持都靜態資源的處理，對應的這些靜態資源的目錄則是在webapp根目錄下，這裏其實可以不用配置servlet名稱，對於名稱爲default的url，tomcat和jetty都會作爲靜態資源文件處理
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.css</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.gif</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.jpg</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.js</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.swf</url-pattern>
</servlet-mapping>那麼我們來看看tomcat和jetty對靜態資源的客戶端緩存的處理邏輯：
tomcat，tomcat在default的servlet支持一些參數，如果有需要那麼就需要配置servlet了，
<servlet>
<servlet-name>default</servlet-name>
<servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
<init-param>
<param-name>debug</param-name>
<param-value>0</param-value>
</init-param>
<init-param>
<param-name>listings</param-name>
<param-value>false</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet> 在對於靜態資源處理的客戶端緩存的代碼如下
// ETag header
response.setHeader("ETag", cacheEntry.attributes.getETag());
// Last-Modified header
response.setHeader("Last-Modified", cacheEntry.attributes.getLastModifiedHttp());這裏的etag計算規則如下：
long contentLength = getContentLength();
long lastModified = getLastModified();
if ((contentLength >= 0) || (lastModified >= 0)) {
weakETag = "W/\"" + contentLength + "-" +
lastModified + "\"";
}
輸出的reponse header如下：
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"7482-1371188756000"
Last-Modified: Fri, 14 Jun 2013 05:45:56 GMT
Content-Type: text/css;charset=GBK
Content-Length: 7482
Date: Sun, 16 Jun 2013 07:05:37 GMT第二次請求時，會先對reqeust的etag和last-modified進行比對，如果沒更新的話則返回304
protected ArrayList<Range> parseRange(HttpServletRequest request,
HttpServletResponse response,
ResourceAttributes resourceAttributes) throws IOException {
// Checking If-Range
String headerValue = request.getHeader("If-Range");
if (headerValue != null) {
long headerValueTime = (-1L);
try {
headerValueTime = request.getDateHeader("If-Range");
} catch (IllegalArgumentException e) {
// Ignore
}
String eTag = resourceAttributes.getETag();
long lastModified = resourceAttributes.getLastModified();
if (headerValueTime == (-1L)) {
// If the ETag the client gave does not match the entity
// etag, then the entire entity is returned.
if (!eTag.equals(headerValue.trim()))
return FULL;
} else {
// If the timestamp of the entity the client got is older than
// the last modification date of the entity, the entire entity
// is returned.
if (lastModified > (headerValueTime + 1000))
return FULL;
}
}
第二次返回：
HTTP/1.1 304 Not Modified
Server: Apache-Coyote/1.1
ETag: W/"2640-1371187966000"
Date: Sun, 16 Jun 2013 07:23:27 GMT那麼這裏對靜態資源瀏覽器就自動能夠緩存起來了。當然tomcat和jetty服務器也會對靜態資源進行緩存。
jetty對這個處理也差不多，不過jetty對於靜態資源的緩存策略可以做更多參數設置，這些參數都是在web.xml配置servlet的時候可以進行設置的。
_cacheControl=getInitParameter("cacheControl");
String resourceCache = getInitParameter("resourceCache");
int max_cache_size=getInitInt("maxCacheSize", -2);
int max_cached_file_size=getInitInt("maxCachedFileSize", -2);
int max_cached_files=getInitInt("maxCachedFiles", -2);
if (resourceCache!=null)
{
if (max_cache_size!=-1 || max_cached_file_size!= -2 || max_cached_files!=-2)
LOG.debug("ignoring resource cache configuration, using resourceCache attribute");
if (_relativeResourceBase!=null || _resourceBase!=null)
throw new UnavailableException("resourceCache specified with resource bases");
_cache=(ResourceCache)_servletContext.getAttribute(resourceCache);
LOG.debug("Cache {}={}",resourceCache,_cache);
}
_etags = getInitBoolean("etags",_etags);處理代碼：
寫header
/* ------------------------------------------------------------ */
protected void writeHeaders(HttpServletResponse response,HttpContent content,long count)
throws IOException
{
if (content.getContentType()!=null && response.getContentType()==null)
response.setContentType(content.getContentType().toString());
if (response instanceof Response)
{
Response r=(Response)response;
HttpFields fields = r.getHttpFields();
if (content.getLastModified()!=null)
fields.put(HttpHeader.LAST_MODIFIED,content.getLastModified());
else if (content.getResource()!=null)
{
long lml=content.getResource().lastModified();
if (lml!=-1)
fields.putDateField(HttpHeader.LAST_MODIFIED,lml);
}
if (count != -1)
r.setLongContentLength(count);
writeOptionHeaders(fields);
if (_etags)
fields.put(HttpHeader.ETAG,content.getETag());
}
else
{
long lml=content.getResource().lastModified();
if (lml>=0)
response.setDateHeader(HttpHeader.LAST_MODIFIED.asString(),lml);
if (count != -1)
{
if (count<Integer.MAX_VALUE)
response.setContentLength((int)count);
else
response.setHeader(HttpHeader.CONTENT_LENGTH.asString(),Long.toString(count));
}
writeOptionHeaders(response);
if (_etags)
response.setHeader(HttpHeader.ETAG.asString(),content.getETag().toString());
}
}
判斷靜態資源是否修改過
/* ------------------------------------------------------------ */
/* Check modification date headers.
*/
protected boolean passConditionalHeaders(HttpServletRequest request,HttpServletResponse response, Resource resource, HttpContent content)
throws IOException
{
try
{
if (!HttpMethod.HEAD.is(request.getMethod()))
{
if (_etags)
{
String ifm=request.getHeader(HttpHeader.IF_MATCH.asString());
if (ifm!=null)
{
boolean match=false;
if (content!=null && content.getETag()!=null)
{
QuotedStringTokenizer quoted = new QuotedStringTokenizer(ifm,", ",false,true);
while (!match && quoted.hasMoreTokens())
{
String tag = quoted.nextToken();
if (content.getETag().toString().equals(tag))
match=true;
}
}
if (!match)
{
Response r = Response.getResponse(response);
r.reset(true);
r.setStatus(HttpServletResponse.SC_PRECONDITION_FAILED);
return false;
}
}
String ifnm=request.getHeader(HttpHeader.IF_NONE_MATCH.asString());
if (ifnm!=null && content!=null && content.getETag()!=null)
{
// Look for GzipFiltered version of etag
if (content.getETag().toString().equals(request.getAttribute("o.e.j.s.GzipFilter.ETag")))
{
Response r = Response.getResponse(response);
r.reset(true);
r.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
r.getHttpFields().put(HttpHeader.ETAG,ifnm);
return false;
}
// Handle special case of exact match.
if (content.getETag().toString().equals(ifnm))
{
Response r = Response.getResponse(response);
r.reset(true);
r.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
r.getHttpFields().put(HttpHeader.ETAG,content.getETag());
return false;
}
// Handle list of tags
QuotedStringTokenizer quoted = new QuotedStringTokenizer(ifnm,", ",false,true);
while (quoted.hasMoreTokens())
{
String tag = quoted.nextToken();
if (content.getETag().toString().equals(tag))
{
Response r = Response.getResponse(response);
r.reset(true);
r.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
r.getHttpFields().put(HttpHeader.ETAG,content.getETag());
return false;
}
}
// If etag requires content to be served, then do not check if-modified-since
return true;
}
}
// Handle if modified since
String ifms=request.getHeader(HttpHeader.IF_MODIFIED_SINCE.asString());
if (ifms!=null)
{
//Get jetty's Response impl
Response r = Response.getResponse(response);
if (content!=null)
{
String mdlm=content.getLastModified();
if (mdlm!=null)
{
if (ifms.equals(mdlm))
{
r.reset(true);
r.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
r.flushBuffer();
return false;
}
}
}
long ifmsl=request.getDateHeader(HttpHeader.IF_MODIFIED_SINCE.asString());
if (ifmsl!=-1)
{
if (resource.lastModified()/1000 <= ifmsl/1000)
{
r.reset(true);
r.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
r.flushBuffer();
return false;
}
}
}
// Parse the if[un]modified dates and compare to resource
long date=request.getDateHeader(HttpHeader.IF_UNMODIFIED_SINCE.asString());
if (date!=-1)
{
if (resource.lastModified()/1000 > date/1000)
{
response.sendError(HttpServletResponse.SC_PRECONDITION_FAILED);
return false;
}
}
}
}
catch(IllegalArgumentException iae)
{
if(!response.isCommitted())
response.sendError(400, iae.getMessage());
throw iae;
}
return true;
}
第一次訪問的response header頭：
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"7482-1371188756000"
Last-Modified: Fri, 14 Jun 2013 05:45:56 GMT
Content-Type: text/css;charset=GBK
Content-Length: 7482
Date: Sun, 16 Jun 2013 07:05:37 GMT
第二次訪問：
HTTP/1.1 304 Not Modified
Server: Jetty(6.1.26)不過處理靜態資源大型網站肯定不是tomcat或者jetty，基本都是用apache或者nginx等來處理靜態處理，性能更好。這裏只是列出tomcat和jetyy對靜態資源的處理和客戶端緩存的支持。