提升Tomcat性能方法有很多种，使用NIO Connector和启用gzip压缩是其中两种。
NIO：Java New IO，使用了多路复用的技术，无疑要比普通的IO socket要高效。
gzip：对需要传输到前台的内容首先在内存中进行gzip压缩，这样可以大大的减少网络带宽占用。前提是前台的Accept-Encoding允许gzip。
但是，当同时配置了这两个时，会发现大于48KB的文件并没有进行压缩。
经查Tomcat源码，发现org.apache.catalina.servlets.DefaultServlet中：
/**
* Check if sendfile can be used.
*/
protected boolean checkSendfile(HttpServletRequest request,
HttpServletResponse response,
CacheEntry entry,
long length, Range range) {
if ((sendfileSize > 0)
&& (entry.resource != null)
&& ((length > sendfileSize) || (entry.resource.getContent() == null))
&& (entry.attributes.getCanonicalPath() != null)
&& (Boolean.TRUE == request.getAttribute("org.apache.tomcat.sendfile.support"))
&& (request.getClass().getName().equals("org.apache.catalina.connector.RequestFacade"))
&& (response.getClass().getName().equals("org.apache.catalina.connector.ResponseFacade"))) {
request.setAttribute("org.apache.tomcat.sendfile.filename", entry.attributes.getCanonicalPath());
if (range == null) {
request.setAttribute("org.apache.tomcat.sendfile.start", new Long(0L));
request.setAttribute("org.apache.tomcat.sendfile.end", new Long(length));
} else {
request.setAttribute("org.apache.tomcat.sendfile.start", new Long(range.start));
request.setAttribute("org.apache.tomcat.sendfile.end", new Long(range.end + 1));
}
return true;
} else {
return false;
}
}
此处的sendfileSize = 48*1024，默认值为48KB，可以发现，当文件大小大于48KB时，Tomcat并未马上将内容写回到output中，而是把文件的路径记录下来。
/**
* Serve the specified resource, optionally including the data content.
*
* @param request The servlet request we are processing
* @param response The servlet response we are creating
* @param content Should the content be included?
*
* @exception IOException if an input/output error occurs
* @exception ServletException if a servlet-specified error occurs
*/
protected void serveResource(HttpServletRequest request,
HttpServletResponse response,
boolean content)
throws IOException, ServletException {
......
// Copy the input stream to our output stream (if requested)
if (content) {
try {
response.setBufferSize(output);
} catch (IllegalStateException e) {
// Silent catch
}
if (ostream != null) {
if (!checkSendfile(request, response, cacheEntry, contentLength, null))
copy(cacheEntry, renderResult, ostream);
} else {
copy(cacheEntry, renderResult, writer);
}
}
......
}