公司有个渠道系统，专门对接三方渠道使用，没有什么业务逻辑，主要是转换报文和参数校验之类的工作，起着一个承上启下的作用。
最近在优化接口的响应时间，优化了代码之后，但是时间还是达不到要求；有一个诡异的100ms左右的耗时问题，在接口中打印了请求处理时间后，和调用方的响应时间还有差了100ms左右。比如程序里记录150ms，但是调用方等待时间却为250ms左右。
下面记录下当时详细的定位&解决流程（其实解决很简单，关键在于怎么定位并找到解决问题的方法）。
渠道系统是一个常见的spring-boot web工程，使用了集成的Tomcat。分析了代码之后，发现并没有特殊的地方，没有特殊的过滤器或者拦截器，所以初步排除是业务代码问题。
出现这个问题之后，首先确认了下接口的调用流程。由于是内部测试，所以调用流程较少。
Nginx -反向代理-> 渠道系统
公司是云服务器，网络走的也是云的内网。由于不明确问题的原因，所以用排除法，首先确认服务器网络是否有问题。
先确认发送端到Nginx Host是否有问题：
[jboss@VM_0_139_centos ~]$ ping 10.0.0.139
PING 10.0.0.139 (10.0.0.139) 56(84) bytes of data.
64 bytes from 10.0.0.139: icmp_seq=1 ttl=64 time=0.029 ms
64 bytes from 10.0.0.139: icmp_seq=2 ttl=64 time=0.041 ms
64 bytes from 10.0.0.139: icmp_seq=3 ttl=64 time=0.040 ms
64 bytes from 10.0.0.139: icmp_seq=4 ttl=64 time=0.040 ms
从ping结果上看，发送端到Nginx主机的延迟是无问题的，接下来查看Nginx到渠道系统的网络。
# 由于日志是没问题的，这里直接复制上面日志了
[jboss@VM_0_139_centos ~]$ ping 10.0.0.139
PING 10.0.0.139 (10.0.0.139) 56(84) bytes of data.
64 bytes from 10.0.0.139: icmp_seq=1 ttl=64 time=0.029 ms
64 bytes from 10.0.0.139: icmp_seq=2 ttl=64 time=0.041 ms
64 bytes from 10.0.0.139: icmp_seq=3 ttl=64 time=0.040 ms
64 bytes from 10.0.0.139: icmp_seq=4 ttl=64 time=0.040 ms
从ping结果上看，Nginx到渠道系统服务器网络延迟也是没问题的。
既然网络看似没问题，那么可以继续排除法，砍掉Nginx，客户端直接再渠道系统的服务器上，通过回环地址（localhost）直连，避免经过网卡/DNS，缩小问题范围看看能否复现（这个应用和地址是我后期模拟的，测试的是一个空接口）：
[jboss@VM_10_91_centos tmp]$ curl -w "@curl-time.txt" http://127.0.0.1:7744/send
success
http: 200
dns: 0.001s
redirect: 0.000s
time_connect: 0.001s
time_Appconnect: 0.000s
time_pretransfer: 0.001s
time_starttransfer: 0.073s
size_download: 7bytes
speed_download: 95.000B/s
----------
time_total: 0.073s 请求总耗时
从curl日志上看，通过回环地址调用一个空接口耗时也有73ms。这就奇怪了，跳过了中间所有调用节点（包括过滤器&拦截器之类），直接请求应用一个空接口，都有73ms的耗时，再请求一次看看：
[jboss@VM_10_91_centos tmp]$ curl -w "@curl-time.txt" http://127.0.0.1:7744/send
success
http: 200
dns: 0.001s
redirect: 0.000s
time_connect: 0.001s
time_appconnect: 0.000s
time_pretransfer: 0.001s
time_starttransfer: 0.003s
size_download: 7bytes
speed_download: 2611.000B/s
----------
time_total: 0.003s
更奇怪的是，第二次请求耗时就正常了，变成了3ms。经查阅资料，linux curl是默认开启http keep-alive的。就算不开启keep-alive，每次重新handshake，也不至于需要70ms。
经过不断分析测试发现，连续请求的话时间就会很短，每次请求只需要几毫秒，但是如果隔一段时间再请求，就会花费70ms以上。
从这个现象猜想，可能是某些缓存机制导致的，连续请求因为有缓存，所以速度快，时间长缓存失效后导致时间长。
那么这个问题点到底在哪一层呢？tomcat层还是spring-webmvc呢？
光猜想定位不了问题，还是得实际测试一下，把渠道系统的代码放到本地ide里启动测试能否复现
但是导入本地Ide后，在Ide中启动后并不能复现问题，并没有70+ms的延迟问题。这下头疼了，本地无法复现，不能Debug，由于问题点不在业务代码，也不能通过加日志的方式来Debug
这时候可以祭出神器Arthas了。
Arthas 是Alibaba开源的JAVA诊断工具，深受开发者喜爱。当你遇到以下类似问题而束手无策时，Arthas可以帮助你解决：
上面是Arthas的官方简介，这次我只需要用他的一个小功能trace。动态计算方法调用路径和时间，这样我就可以定位时间在哪个地方被消耗了。
有了神器，那么该追踪什么方法呢？由于我对Tomcat源码不是很熟，所以只能从spring mvc下手，先来trace一下spring mvc的入口：
[arthas@24851]$ trace org.springframework.web.servlet.DispatcherServlet *
Press Q or Ctrl+C to abort.
Affect(class-cnt:1 , method-cnt:44) cost in 508 ms.
`---ts=2019-09-14 21:07:44;thread_name=http-nio-7744-exec-2;id=11;is_daemon=true;priority=5;TCCL=org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader@7c136917
`---[2.952142ms] org.springframework.web.servlet.DispatcherServlet:buildLocaleContext()
`---ts=2019-09-14 21:07:44;thread_name=http-nio-7744-exec-2;id=11;is_daemon=true;priority=5;TCCL=org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader@7c136917
`---[18.08903ms] org.springframework.web.servlet.DispatcherServlet:doService()
+---[0.041346ms] org.Apache.commons.logging.Log:isDebugEnabled() #889
+---[0.022398ms] org.springframework.web.util.WebUtils:isIncludeRequest() #898
+---[0.014904ms] org.springframework.web.servlet.DispatcherServlet:getWebApplicationContext() #910
+---[1.071879ms] javax.servlet.http.HttpServletRequest:setAttribute() #910
+---[0.020977ms] javax.servlet.http.HttpServletRequest:setAttribute() #911
+---[0.017073ms] javax.servlet.http.HttpServletRequest:setAttribute() #912
+---[0.218277ms] org.springframework.web.servlet.DispatcherServlet:getThemeSource() #913
| `---[0.137568ms] org.springframework.web.servlet.DispatcherServlet:getThemeSource()
| `---[min=0.00783ms,max=0.014251ms,total=0.022081ms,count=2] org.springframework.web.servlet.DispatcherServlet:getWebApplicationContext() #782
+---[0.019363ms] javax.servlet.http.HttpServletRequest:setAttribute() #913
+---[0.070694ms] org.springframework.web.servlet.FlashMapManager:retrieveAndUpdate() #916
+---[0.01839ms] org.springframework.web.servlet.FlashMap:<init>() #920
+---[0.016943ms] javax.servlet.http.HttpServletRequest:setAttribute() #920
+---[0.015268ms] javax.servlet.http.HttpServletRequest:setAttribute() #921
+---[15.050124ms] org.springframework.web.servlet.DispatcherServlet:doDispatch() #925
| `---[14.943477ms] org.springframework.web.servlet.DispatcherServlet:doDispatch()
| +---[0.019135ms] org.springframework.web.context.request.async.WebAsyncUtils:getAsyncManager() #953
| +---[2.108373ms] org.springframework.web.servlet.DispatcherServlet:checkMultipart() #960
| | `---[2.004436ms] org.springframework.web.servlet.DispatcherServlet:checkMultipart()
| | `---[1.890845ms] org.springframework.web.multipart.MultipartResolver:isMultipart() #1117
| +---[2.054361ms] org.springframework.web.servlet.DispatcherServlet:getHandler() #964
| | `---[1.961963ms] org.springframework.web.servlet.DispatcherServlet:getHandler()
| | +---[0.02051ms] java.util.List:iterator() #1183
| | +---[min=0.003805ms,max=0.009641ms,total=0.013446ms,count=2] java.util.Iterator:hasNext() #1183
| | +---[min=0.003181ms,max=0.009751ms,total=0.012932ms,count=2] java.util.Iterator:next() #1183
| | +---[min=0.005841ms,max=0.015308ms,total=0.021149ms,count=2] org.apache.commons.logging.Log:isTraceEnabled() #1184
| | `---[min=0.474739ms,max=1.19145ms,total=1.666189ms,count=2] org.springframework.web.servlet.HandlerMapping:getHandler() #1188
| +---[0.013071ms] org.springframework.web.servlet.HandlerExecutionChAIn:getHandler() #971
| +---[0.372236ms] org.springframework.web.servlet.DispatcherServlet:getHandlerAdapter() #971
| | `---[0.280073ms] org.springframework.web.servlet.DispatcherServlet:getHandlerAdapter()
| | +---[0.004804ms] java.util.List:iterator() #1224
| | +---[0.003668ms] java.util.Iterator:hasNext() #1224
| | +---[0.003038ms] java.util.Iterator:next() #1224
| | +---[0.006451ms] org.apache.commons.logging.Log:isTraceEnabled() #1225
| | `---[0.012683ms] org.springframework.web.servlet.HandlerAdapter:supports() #1228
| +---[0.012848ms] javax.servlet.http.HttpServletRequest:getMethod() #974
| +---[0.013132ms] java.lang.String:equals() #975
| +---[0.003025ms] org.springframework.web.servlet.HandlerExecutionChain:getHandler() #977
| +---[0.008095ms] org.springframework.web.servlet.HandlerAdapter:getLastModified() #977
| +---[0.006596ms] org.apache.commons.logging.Log:isDebugEnabled() #978
| +---[0.018024ms] org.springframework.web.context.request.ServletWebRequest:<init>() #981
| +---[0.017869ms] org.springframework.web.context.request.ServletWebRequest:checkNotModified() #981
| +---[0.038542ms] org.springframework.web.servlet.HandlerExecutionChain:applyPreHandle() #986
| +---[0.00431ms] org.springframework.web.servlet.HandlerExecutionChain:getHandler() #991
| +---[4.248493ms] org.springframework.web.servlet.HandlerAdapter:handle() #991
| +---[0.014805ms] org.springframework.web.context.request.async.WebAsyncManager:isConcurrentHandlingStarted() #993
| +---[1.444994ms] org.springframework.web.servlet.DispatcherServlet:applyDefaultViewName() #997
| | `---[0.067631ms] org.springframework.web.servlet.DispatcherServlet:applyDefaultViewName()
| +---[0.012027ms] org.springframework.web.servlet.HandlerExecutionChain:applyPostHandle() #998
| +---[0.373997ms] org.springframework.web.servlet.DispatcherServlet:processDispatchResult() #1008
| | `---[0.197004ms] org.springframework.web.servlet.DispatcherServlet:processDispatchResult()
| | +---[0.007074ms] org.apache.commons.logging.Log:isDebugEnabled() #1075
| | +---[0.005467ms] org.springframework.web.context.request.async.WebAsyncUtils:getAsyncManager() #1081
| | +---[0.004054ms] org.springframework.web.context.request.async.WebAsyncManager:isConcurrentHandlingStarted() #1081
| | `---[0.011988ms] org.springframework.web.servlet.HandlerExecutionChain:triggerAfterCompletion() #1087
| `---[0.004015ms] org.springframework.web.context.request.async.WebAsyncManager:isConcurrentHandlingStarted() #1018
+---[0.005055ms] org.springframework.web.context.request.async.WebAsyncUtils:getAsyncManager() #928
`---[0.003422ms] org.springframework.web.context.request.async.WebAsyncManager:isConcurrentHandlingStarted() #928
[jboss@VM_10_91_centos tmp]$ curl -w "@curl-time.txt" http://127.0.0.1:7744/send
success
http: 200
dns: 0.001s
redirect: 0.000s
time_connect: 0.001s
time_appconnect: 0.000s
time_pretransfer: 0.001s
time_starttransfer: 0.115s
size_download: 7bytes
speed_download: 60.000B/s
----------
time_total: 0.115s
本次调用，调用端时间花费115ms，但是从arthas trace上看，spring mvc只消耗了18ms，那么剩下的97ms去哪了呢？
本地测试后已经可以排除spring mvc的问题了，最后也是唯一可能出问题的点就是tomcat
可是本人并不熟悉tomcat中的源码，就连请求入口都不清楚，tomcat里需要trace的类都不好找。。。
不过没关系，有神器Arthas，可以通过stack命令来反向查找调用路径，以org.springframework.web.servlet.DispatcherServlet作为参数：
stack 输出当前方法被调用的调用路径很多时候我们都知道一个方法被执行，但这个方法被执行的路径非常多，或者你根本就不知道这个方法是从那里被执行了，此时你需要的是 stack 命令。
[arthas@24851]$ stack org.springframework.web.servlet.DispatcherServlet *
Press Q or Ctrl+C to abort.
Affect(class-cnt:1 , method-cnt:44) cost in 495 ms.
ts=2019-09-14 21:15:19;thread_name=http-nio-7744-exec-5;id=14;is_daemon=true;priority=5;TCCL=org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader@7c136917
@org.springframework.web.servlet.FrameworkServlet.processRequest()
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:866)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790)
at org.apache.tomcat.util.NET.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1468)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)
ts=2019-09-14 21:15:19;thread_name=http-nio-7744-exec-5;id=14;is_daemon=true;priority=5;TCCL=org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader@7c136917
@org.springframework.web.servlet.DispatcherServlet.doService()
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:974)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:866)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1468)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)
从stack日志上可以很直观的看出DispatchServlet的调用栈，那么这么长的路径，该trace哪个类呢（这里跳过spring mvc中的过滤器的trace过程，实际排查的时候也trace了一遍，但这诡异的时间消耗不是由这里过滤器产生的）？
有一定经验的老司机从名字上大概也能猜出来从哪里下手比较好，那就是org.apache.coyote.http11.Http11Processor.service，从名字上看，http1.1处理器，这可能是一个比较好的切入点。下面来trace一下：
[arthas@24851]$ trace org.apache.coyote.http11.Http11Processor service
Press Q or Ctrl+C to abort.
Affect(class-cnt:1 , method-cnt:1) cost in 269 ms.
`---ts=2019-09-14 21:22:51;thread_name=http-nio-7744-exec-8;id=17;is_daemon=true;priority=5;TCCL=org.springframework.boot.loader.LaunchedURLClassLoader@20ad9418
`---[131.650285ms] org.apache.coyote.http11.Http11Processor:service()
+---[0.036851ms] org.apache.coyote.Request:getRequestProcessor() #667
+---[0.009986ms] org.apache.coyote.RequestInfo:setStage() #668
+---[0.008928ms] org.apache.coyote.http11.Http11Processor:setSocketWrapper() #671
+---[0.013236ms] org.apache.coyote.http11.Http11InputBuffer:init() #672
+---[0.00981ms] org.apache.coyote.http11.Http11OutputBuffer:init() #673
+---[min=0.00213ms,max=0.007317ms,total=0.009447ms,count=2] org.apache.coyote.http11.Http11Processor:getErrorState() #683
+---[min=0.002098ms,max=0.008888ms,total=0.010986ms,count=2] org.apache.coyote.ErrorState:isError() #683
+---[min=0.002448ms,max=0.007149ms,total=0.009597ms,count=2] org.apache.coyote.http11.Http11Processor:isAsync() #683
+---[min=0.002399ms,max=0.00852ms,total=0.010919ms,count=2] org.apache.tomcat.util.net.AbstractEndpoint:isPaused() #683
+---[min=0.033587ms,max=0.11832ms,total=0.151907ms,count=2] org.apache.coyote.http11.Http11InputBuffer:parseRequestLine() #687
+---[0.005384ms] org.apache.tomcat.util.net.AbstractEndpoint:isPaused() #695
+---[0.007924ms] org.apache.coyote.Request:getMimeHeaders() #702
+---[0.006744ms] org.apache.tomcat.util.net.AbstractEndpoint:getMaxHeaderCount() #702
+---[0.012574ms] org.apache.tomcat.util.http.MimeHeaders:setLimit() #702
+---[0.14319ms] org.apache.coyote.http11.Http11InputBuffer:parseHeaders() #703
+---[0.003997ms] org.apache.coyote.Request:getMimeHeaders() #743
+---[0.026561ms] org.apache.tomcat.util.http.MimeHeaders:values() #743
+---[min=0.002869ms,max=0.01203ms,total=0.014899ms,count=2] java.util.Enumeration:hasMoreElements() #745
+---[0.070114ms] java.util.Enumeration:nextElement() #746
+---[0.010921ms] java.lang.String:toLowerCase() #746
+---[0.008453ms] java.lang.String:contains() #746
+---[0.002698ms] org.apache.coyote.http11.Http11Processor:getErrorState() #775
+---[0.00307ms] org.apache.coyote.ErrorState:isError() #775
+---[0.002708ms] org.apache.coyote.RequestInfo:setStage() #777
+---[0.171139ms] org.apache.coyote.http11.Http11Processor:prepareRequest() #779
+---[0.009349ms] org.apache.tomcat.util.net.SocketWrapperBase:decrementKeepAlive() #794
+---[0.002574ms] org.apache.coyote.http11.Http11Processor:getErrorState() #800
+---[0.002696ms] org.apache.coyote.ErrorState:isError() #800
+---[0.002499ms] org.apache.coyote.RequestInfo:setStage() #802
+---[0.005641ms] org.apache.coyote.http11.Http11Processor:getAdapter() #803
+---[129.868916ms] org.apache.coyote.Adapter:service() #803
+---[0.003859ms] org.apache.coyote.http11.Http11Processor:getErrorState() #809
+---[0.002365ms] org.apache.coyote.ErrorState:isError() #809
+---[0.003844ms] org.apache.coyote.http11.Http11Processor:isAsync() #809
+---[0.002382ms] org.apache.coyote.Response:getStatus() #809
+---[0.002476ms] org.apache.coyote.http11.Http11Processor:statusDropsConnection() #809
+---[0.002284ms] org.apache.coyote.RequestInfo:setStage() #838
+---[0.00222ms] org.apache.coyote.http11.Http11Processor:isAsync() #839
+---[0.037873ms] org.apache.coyote.http11.Http11Processor:endRequest() #843
+---[0.002188ms] org.apache.coyote.RequestInfo:setStage() #845
+---[0.002112ms] org.apache.coyote.http11.Http11Processor:getErrorState() #849
+---[0.002063ms] org.apache.coyote.ErrorState:isError() #849
+---[0.002504ms] org.apache.coyote.http11.Http11Processor:isAsync() #853
+---[0.009808ms] org.apache.coyote.Request:updateCounters() #854
+---[0.002008ms] org.apache.coyote.http11.Http11Processor:getErrorState() #855
+---[0.002192ms] org.apache.coyote.ErrorState:isIoAllowed() #855
+---[0.01968ms] org.apache.coyote.http11.Http11InputBuffer:nextRequest() #856
+---[0.010065ms] org.apache.coyote.http11.Http11OutputBuffer:nextRequest() #857
+---[0.002576ms] org.apache.coyote.RequestInfo:setStage() #870
+---[0.016599ms] org.apache.coyote.http11.Http11Processor:processSendfile() #872
+---[0.008182ms] org.apache.coyote.http11.Http11InputBuffer:getParsingRequestLinePhase() #688
+---[0.0075ms] org.apache.coyote.http11.Http11Processor:handleIncompleteRequestLineRead() #690
+---[0.001979ms] org.apache.coyote.RequestInfo:setStage() #875
+---[0.001981ms] org.apache.coyote.http11.Http11Processor:getErrorState() #877
+---[0.001934ms] org.apache.coyote.ErrorState:isError() #877
+---[0.001995ms] org.apache.tomcat.util.net.AbstractEndpoint:isPaused() #877
+---[0.002403ms] org.apache.coyote.http11.Http11Processor:isAsync() #879
`---[0.006176ms] org.apache.coyote.http11.Http11Processor:isUpgrade() #881
日志里有一个129ms的耗时点（时间比没开arthas的时候更长是因为arthas本身带来的性能消耗，所以生产环境小心使用），这个就是要找的问题点。
打问题点找到了，那怎么定位是什么导致的问题呢，又如何解决呢？
继续trace吧，细化到具体的代码块或者内容。trace由于性能考虑，不会展示所有的调用路径，如果调用路径过深，只有手动深入trace，原则就是trace耗时长的那个方法：
[arthas@24851]$ trace org.apache.coyote.Adapter service
Press Q or Ctrl+C to abort.
Affect(class-cnt:1 , method-cnt:1) cost in 608 ms.
`---ts=2019-09-14 21:34:33;thread_name=http-nio-7744-exec-1;id=10;is_daemon=true;priority=5;TCCL=org.springframework.boot.loader.LaunchedURLClassLoader@20ad9418
`---[81.70999ms] org.apache.catalina.connector.CoyoteAdapter:service()
+---[0.032546ms] org.apache.coyote.Request:getNote() #302
+---[0.007148ms] org.apache.coyote.Response:getNote() #303
+---[0.007475ms] org.apache.catalina.connector.Connector:getXpoweredBy() #324
+---[0.00447ms] org.apache.coyote.Request:getRequestProcessor() #331
+---[0.007902ms] java.lang.ThreadLocal:get() #331
+---[0.006522ms] org.apache.coyote.RequestInfo:setWorkerThreadName() #331
+---[73.793798ms] org.apache.catalina.connector.CoyoteAdapter:postParseRequest() #336
+---[0.001536ms] org.apache.catalina.connector.Connector:getService() #339
+---[0.004469ms] org.apache.catalina.Service:getContainer() #339
+---[0.007074ms] org.apache.catalina.Engine:getPipeline() #339
+---[0.004334ms] org.apache.catalina.Pipeline:isAsyncSupported() #339
+---[0.002466ms] org.apache.catalina.connector.Request:setAsyncSupported() #339
+---[6.01E-4ms] org.apache.catalina.connector.Connector:getService() #342
+---[0.001859ms] org.apache.catalina.Service:getContainer() #342
+---[9.65E-4ms] org.apache.catalina.Engine:getPipeline() #342
+---[0.005231ms] org.apache.catalina.Pipeline:getFirst() #342
+---[7.239154ms] org.apache.catalina.Valve:invoke() #342
+---[0.006904ms] org.apache.catalina.connector.Request:isAsync() #345
+---[0.00509ms] org.apache.catalina.connector.Request:finishRequest() #372
+---[0.051461ms] org.apache.catalina.connector.Response:finishResponse() #373
+---[0.007244ms] java.util.concurrent.atomic.AtomicBoolean:<init>() #379
+---[0.007314ms] org.apache.coyote.Response:action() #380
+---[0.004518ms] org.apache.catalina.connector.Request:isAsyncCompleting() #382
+---[0.001072ms] org.apache.catalina.connector.Request:getContext() #394
+---[0.007166ms] java.lang.System:currentTimeMillis() #401
+---[0.004367ms] org.apache.coyote.Request:getStartTime() #401
+---[0.011483ms] org.apache.catalina.Context:logAccess() #401
+---[0.0014ms] org.apache.coyote.Request:getRequestProcessor() #406
+---[min=8.0E-4ms,max=9.22E-4ms,total=0.001722ms,count=2] java.lang.Integer:<init>() #406
+---[0.001082ms] java.lang.reflect.Method:invoke() #406
+---[0.001851ms] org.apache.coyote.RequestInfo:setWorkerThreadName() #406
+---[0.035805ms] org.apache.catalina.connector.Request:recycle() #410
`---[0.007849ms] org.apache.catalina.connector.Response:recycle() #411
一段无聊的手动深入trace之后………………
[arthas@24851]$ trace org.apache.catalina.webresources.AbstractArchiveResourceSet getArchiveEntries
Press Q or Ctrl+C to abort.
Affect(class-cnt:4 , method-cnt:2) cost in 150 ms.
`---ts=2019-09-14 21:36:26;thread_name=http-nio-7744-exec-3;id=12;is_daemon=true;priority=5;TCCL=org.springframework.boot.loader.LaunchedURLClassLoader@20ad9418
`---[75.743681ms] org.apache.catalina.webresources.JarWarResourceSet:getArchiveEntries()
+---[0.025731ms] java.util.HashMap:<init>() #106
+---[0.097729ms] org.apache.catalina.webresources.JarWarResourceSet:openJarFile() #109
+---[0.091037ms] java.util.jar.JarFile:getJarEntry() #110
+---[0.096325ms] java.util.jar.JarFile:getInputStream() #111
+---[0.451916ms] org.apache.catalina.webresources.TomcatJarInputStream:<init>() #113
+---[min=0.001175ms,max=0.001176ms,total=0.002351ms,count=2] java.lang.Integer:<init>() #114
+---[0.00104ms] java.lang.reflect.Method:invoke() #114
+---[0.045105ms] org.apache.catalina.webresources.TomcatJarInputStream:getNextJarEntry() #114
+---[min=5.02E-4ms,max=0.008531ms,total=0.028864ms,count=31] java.util.jar.JarEntry:getName() #116
+---[min=5.39E-4ms,max=0.022805ms,total=0.054647ms,count=31] java.util.HashMap:put() #116
+---[min=0.004452ms,max=34.479307ms,total=74.206249ms,count=31] org.apache.catalina.webresources.TomcatJarInputStream:getNextJarEntry() #117
+---[0.018358ms] org.apache.catalina.webresources.TomcatJarInputStream:getManifest() #119
+---[0.006429ms] org.apache.catalina.webresources.JarWarResourceSet:setManifest() #120
+---[0.010904ms] org.apache.tomcat.util.compat.JreCompat:isJre9Available() #121
+---[0.003307ms] org.apache.catalina.webresources.TomcatJarInputStream:getMetaInfEntry() #133
+---[5.5E-4ms] java.util.jar.JarEntry:getName() #135
+---[6.42E-4ms] java.util.HashMap:put() #135
+---[0.001981ms] org.apache.catalina.webresources.TomcatJarInputStream:getManifestEntry() #137
+---[0.064484ms] org.apache.catalina.webresources.TomcatJarInputStream:close() #141
+---[0.007961ms] org.apache.catalina.webresources.JarWarResourceSet:closeJarFile() #151
`---[0.004643ms] java.io.InputStream:close() #155
发现了一个值得暂停思考的点：
+---[min=0.004452ms,max=34.479307ms,total=74.206249ms,count=31] org.apache.catalina.webresources.TomcatJarInputStream:getNextJarEntry() #117
这行代码加载了31次，一共耗时74ms；从名字上看，应该是tomcat加载jar包时的耗时，那么是加载了31个jar包的耗时，还是加载了jar包内的某些资源31次耗时呢？
TomcatJarInputStream这个类源码的注释写到：
The purpose of this sub-class is to obtain references to the JarEntry objects for META-INF/ and META-INF/MANIFEST.MF that are otherwise swallowed by the JarInputStream implementation.
大概意思也就是，获取jar包内META-INF/，META-INF/MANIFEST的资源，这是一个子类，更多的功能在父类JarInputStream里。
其实看到这里大概也能猜到问题了，tomcat加载jar包内META-INF/，META-INF/MANIFEST的资源导致的耗时，至于为什么连续请求不会耗时，应该是tomcat的缓存机制（下面介绍源码分析）不着急定位问题，试着通过Arthas最终定位问题细节，继续手动深入trace
[arthas@24851]$ trace org.apache.catalina.webresources.TomcatJarInputStream *
Press Q or Ctrl+C to abort.
Affect(class-cnt:1 , method-cnt:4) cost in 44 ms.
`---ts=2019-09-14 21:37:47;thread_name=http-nio-7744-exec-5;id=14;is_daemon=true;priority=5;TCCL=org.springframework.boot.loader.LaunchedURLClassLoader@20ad9418
`---[0.234952ms] org.apache.catalina.webresources.TomcatJarInputStream:createZipEntry()
+---[0.039455ms] java.util.jar.JarInputStream:createZipEntry() #43
`---[0.007827ms] java.lang.String:equals() #44
`---ts=2019-09-14 21:37:47;thread_name=http-nio-7744-exec-5;id=14;is_daemon=true;priority=5;TCCL=org.springframework.boot.loader.LaunchedURLClassLoader@20ad9418
`---[0.050222ms] org.apache.catalina.webresources.TomcatJarInputStream:createZipEntry()
+---[0.001889ms] java.util.jar.JarInputStream:createZipEntry() #43
`---[0.001643ms] java.lang.String:equals() #46
#这里一共31个trace日志，删减了剩下的
从方法名上看，还是加载资源之类的意思。都已经到jdk源码了，这时候来看一下TomcatJarInputStream这个类的源码:
/**
* Creates a new <code>JarEntry</code> (<code>ZipEntry</code>) for the
* specified JAR file entry name. The manifest attributes of
* the specified JAR file entry name will be copied to the new
* <CODE>JarEntry</CODE>.
*
* @param name the name of the JAR/ZIP file entry
* @return the <code>JarEntry</code> object just created
*/
protected ZipEntry createZipEntry(String name) {
JarEntry e = new JarEntry(name);
if (man != null) {
e.attr = man.getAttributes(name);
}
return e;
}
这个createZipEntry有个name参数，从注释上看，是jar/zip文件名，如果能得到文件名这种关键信息，就可以直接定位问题了；还是通过Arthas，使用watch命令，动态监测方法调用数据
让你能方便的观察到指定方法的调用情况。能观察到的范围为：返回值、抛出异常、入参，通过编写 OGNL 表达式进行对应变量的查看。
watch 该方法的入参：
[arthas@24851]$ watch org.apache.catalina.webresources.TomcatJarInputStream createZipEntry "{params[0]}"
Press Q or Ctrl+C to abort.
Affect(class-cnt:1 , method-cnt:1) cost in 27 ms.
ts=2019-09-14 21:51:14; [cost=0.14547ms] result=@ArrayList[
@String[META-INF/],
]
ts=2019-09-14 21:51:14; [cost=0.048028ms] result=@ArrayList[
@String[META-INF/MANIFEST.MF],
]
ts=2019-09-14 21:51:14; [cost=0.046071ms] result=@ArrayList[
@String[META-INF/resources/],
]
ts=2019-09-14 21:51:14; [cost=0.033855ms] result=@ArrayList[
@String[META-INF/resources/swagger-ui.html],
]
ts=2019-09-14 21:51:14; [cost=0.039138ms] result=@ArrayList[
@String[META-INF/resources/webjars/],
]
ts=2019-09-14 21:51:14; [cost=0.033701ms] result=@ArrayList[
@String[META-INF/resources/webjars/springfox-swagger-ui/],
]
ts=2019-09-14 21:51:14; [cost=0.033644ms] result=@ArrayList[
@String[META-INF/resources/webjars/springfox-swagger-ui/favicon-16x16.png],
]
ts=2019-09-14 21:51:14; [cost=0.033976ms] result=@ArrayList[
@String[META-INF/resources/webjars/springfox-swagger-ui/springfox.css],
]
ts=2019-09-14 21:51:14; [cost=0.032818ms] result=@ArrayList[
@String[META-INF/resources/webjars/springfox-swagger-ui/swagger-ui-standalone-preset.js.map],
]
ts=2019-09-14 21:51:14; [cost=0.04651ms] result=@ArrayList[
@String[META-INF/resources/webjars/springfox-swagger-ui/swagger-ui.css],
]
ts=2019-09-14 21:51:14; [cost=0.034793ms] result=@ArrayList[
@String[META-INF/resources/webjars/springfox-swagger-ui/swagger-ui.js.map],
这下直接看到了具体加载的资源名，这么熟悉的名字：swagger-ui，一个国外的rest接口文档工具，又有国内开发者基于swagger-ui做了一套spring mvc的集成工具，通过注解就可以自动生成swagger-ui需要的接口定义json文件，用起来还比较方便，就是侵入性较强。
删除swagger的jar包后问题，诡异的70+ms就消失了：
<!--pom 里删除这两个引用，这两个包时国内开发者封装的，swagger-ui并没有提供java spring-mvc的支持包，swagger只是一个浏览器端的ui+editor -->
<dependency>
<groupId>io.springfox</groupId>
<artifactId>springfox-swagger2</artifactId>
<version>2.9.2</version>
</dependency>
<dependency>
<groupId>io.springfox</groupId>
<artifactId>springfox-swagger-ui</artifactId>
<version>2.9.2</version>
</dependency>
那么为什么swagger会导致请求耗时呢，为什么每次请求偶读会加载swagger内部的静态资源呢？
其实这是tomcat-embed的一个bug吧，下面详细介绍一下该Bug
源码分析过程实在太漫长，而且也不是本文的重点，所以就不介绍了， 下面直接介绍下分析结果，顺便贴一张tomcat处理请求的核心类图：
(1) 为什么每次请求会加载Jar包内的静态资源
关键在于org.apache.catalina.mapper.Mapper#internalMapWrapper这个方法，该版本下处理请求的方式有问题，导致每次都校验静态资源。
(2) 为什么连续请求不会出现问题
因为Tomcat对于这种静态资源的解析是有缓存的，优先从缓存查找，缓存过期后再重新解析。具体参考org.apache.catalina.webresources.Cache，默认过期时间ttl是5000ms。
(3) 为什么本地不会复现
其实确切的说，是通过spring-boot打包插件后不能复现。由于启动方式的不同，tomcat使用了不同的类去处理静态资源，所以没问题。
(4) 如何解决
升级tomcat-embed版本即可。
当前出现Bug的版本为：
如果项目是maven是继承的springboot，即parent配置为springboot的，或者dependencyManagement中import spring boot包的：
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.0.2.RELEASE</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>
pom中直接覆盖properties即可：
<properties>
<tomcat.version>8.5.40</tomcat.version>
</properties>
升级spring boot版本：springboot 2.1.0.RELEASE中的tomcat embed版本已经大于8.5.31了，所以直接将springboot升级至该版本及以上版本就可以解决此问题。
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