前面已经分析完了Tomcat的启动和关闭过程。本篇就来接着分析一下Tomcat中请求的处理过程。
在開始本文之前，咋们首先来看看一个Http请求处理的过程。普通情况下是浏览器发送http请求->建立Socket连接->通过Socket读取数据->依据http协议解析数据->调用后台服务完毕响应,具体的流程图如上图所看到的，等读者读完本篇，应该就清楚了上图所表达的意思。Tomcat既是一个HttpServer也是一个Servlet
容器，那么这里必定也涉及到如上过程，首先依据HTTP协议规范解析请求数据，然后将请求转发给Servlet进行处理。因此顺应这种思路，本文也将从Http协议请求解析，请求怎样转发给Servlet两个方面来进行分析。首先来看Http协议请求解析。
Http协议请求解析
在Tomcat启动过程（Tomcat源代码解析三）一文中，我们已经知道Tomcat启动以后。默认情况下会通过org.apache.tomcat.util.net.JIoEndpoint.Acceptor监听Socket连接。当监听到有Socket连接的时候，就会调用org.apache.tomcat.util.net.JIoEndpoint#processSocket方法进行处理，以下我们就来看看此方法的代码，为了节省版面，仅仅保留与本文相关的代码。
protected boolean processSocket(Socket socket) {
// Process the request from this socket
try {
SocketWrapper<Socket> wrapper = new SocketWrapper<Socket>(socket);
wrapper.setKeepAliveLeft(getMaxKeepAliveRequests());
// During shutdown, executor may be null - avoid NPE
if (!running) {
return false;
}
getExecutor().execute(new SocketProcessor(wrapper));
} catch (RejectedExecutionException x) {
//exception handler ...
return false;
}
return true;
}通过上面的代码，我们能够看出首先将Socket封装为SocketWrapper，然后通过SocketProcessor来进行处理。由于Tomcat必定面对用户并发请求，因此这里Socket的处理通过新的线程池来处理。接下来我们再来看看SocketProcess的代码。相同省略了一些非核心的代码，代码例如以下：
<span style="color: rgb(102, 102, 102); font-family: 'Open Sans', HelveticaNeue-Light, 'Helvetica Neue Light', 'Helvetica Neue', Helvetica, Arial, sans-serif; font-size: 12.6000003814697px; line-height: 25.2000007629395px; text-align: justify; background-color: rgb(236, 236, 236);">org.apache.tomcat.util.net.JIoEndpoint.SocketProcessor#run</span>
public void run() {
boolean launch = false;
synchronized (socket) {
try {
SocketState state = SocketState.OPEN;
try {
// SSL handshake
serverSocketFactory.handshake(socket.getSocket());
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
if (log.isDebugEnabled()) {
log.debug(sm.getString("endpoint.err.handshake"), t);
}
// Tell to close the socket
state = SocketState.CLOSED;
}
if ((state != SocketState.CLOSED)) {
if (status == null) {
// 1
state = handler.process(socket, SocketStatus.OPEN);
} else {
state = handler.process(socket,status);
}
}
if (state == SocketState.CLOSED) {
// Close socket
if (log.isTraceEnabled()) {
log.trace("Closing socket:"+socket);
}
countDownConnection();
try {
socket.getSocket().close();
} catch (IOException e) {
// Ignore
}
} else if (state == SocketState.OPEN ||
state == SocketState.UPGRADING ||
state == SocketState.UPGRADED){
socket.setKeptAlive(true);
socket.access();
launch = true;
} else if (state == SocketState.LONG) {
socket.access();
waitingRequests.add(socket);
}
} finally {
//other code
}
}
socket = null;
// Finish up this request
}
}默认情况下。代码会执行到标注1的地方，标注1的地方又通过org.apache.tomcat.util.net.JIoEndpoint.Handler#process的方法进行处理，而通过前面Tomcat启动的文章，我们已经知道handler属性是在org.apache.coyote.http11.Http11Protocol的构造方法中初始化的，构造方法例如以下：
public Http11Protocol() {
endpoint = new JIoEndpoint();
cHandler = new Http11ConnectionHandler(this);
((JIoEndpoint) endpoint).setHandler(cHandler);
setSoLinger(Constants.DEFAULT_CONNECTION_LINGER);
setSoTimeout(Constants.DEFAULT_CONNECTION_TIMEOUT);
setTcpNoDelay(Constants.DEFAULT_TCP_NO_DELAY);
}从构造方法中，我们能够清楚的看到。事实上初始化了org.apache.coyote.http11.Http11Protocol.Http11ConnectionHandler的实例，那么接下来我们就来看看它的process方法，由于Http11ConnectionHandler继承了org.apache.coyote.AbstractProtocol.AbstractConnectionHandler，而自己没有实现process方法。因此会调用到父类的process方法，那么接下来我们就来看看AbstractConnectionHandler的process方法，代码例如以下：
public SocketState process(SocketWrapper<S> socket,
SocketStatus status) {
Processor<S> processor = connections.remove(socket.getSocket());
if (status == SocketStatus.DISCONNECT && processor == null) {
//nothing more to be done endpoint requested a close
//and there are no object associated with this connection
return SocketState.CLOSED;
}
socket.setAsync(false);
try {
if (processor == null) {
processor = recycledProcessors.poll();
}
if (processor == null) {
processor = createProcessor();
}
initSsl(socket, processor);
SocketState state = SocketState.CLOSED;
do {
if (status == SocketStatus.DISCONNECT &&
!processor.isComet()) {
// Do nothing here, just wait for it to get recycled
// Don't do this for Comet we need to generate an end
// event (see BZ 54022)
} else if (processor.isAsync() ||
state == SocketState.ASYNC_END) {
state = processor.asyncDispatch(status);
} else if (processor.isComet()) {
state = processor.event(status);
} else if (processor.isUpgrade()) {
state = processor.upgradeDispatch();
} else {
state = processor.process(socket);
}
if (state != SocketState.CLOSED && processor.isAsync()) {
state = processor.asyncPostProcess();
}
if (state == SocketState.UPGRADING) {
// Get the UpgradeInbound handler
UpgradeInbound inbound = processor.getUpgradeInbound();
// Release the Http11 processor to be re-used
release(socket, processor, false, false);
// Create the light-weight upgrade processor
processor = createUpgradeProcessor(socket, inbound);
inbound.onUpgradeComplete();
}
} while (state == SocketState.ASYNC_END ||
state == SocketState.UPGRADING);
return state;
} catch(java.net.SocketException e) {
// exception handler
}
return SocketState.CLOSED;
}通过查看上面的代码。默认一个新连接的情况下。会调用
org.apache.coyote.Processor#process方法,而Processor的实例实在org.apache.coyote.AbstractProtocol.AbstractConnectionHandler#createProcessor中创建的。通过查看createProcessor代码。我们发现是创建了一个org.apache.coyote.http11.Http11Processor的实例，那么接下来，我们就来看看它的process方法，由于Http11Processor继承了AbstractHttp11Processor，终于事实上调用的是AbstractHttp11Processor的process方法。代码例如以下：
public SocketState process(SocketWrapper<S> socketWrapper)
throws IOException {
RequestInfo rp = request.getRequestProcessor();
rp.setStage(org.apache.coyote.Constants.STAGE_PARSE);
// Setting up the I/O
// 1
setSocketWrapper(socketWrapper);
getInputBuffer().init(socketWrapper, endpoint);
getOutputBuffer().init(socketWrapper, endpoint);
// Flags
error = false;
keepAlive = true;
comet = false;
openSocket = false;
sendfileInProgress = false;
readComplete = true;
if (endpoint.getUsePolling()) {
keptAlive = false;
} else {
keptAlive = socketWrapper.isKeptAlive();
}
if (disableKeepAlive()) {
socketWrapper.setKeepAliveLeft(0);
}
while (!error && keepAlive && !comet && !isAsync() &&
upgradeInbound == null && !endpoint.isPaused()) {
// Parsing the request header
try {
setRequestLineReadTimeout();
//2
if (!getInputBuffer().parseRequestLine(keptAlive)) {
if (handleIncompleteRequestLineRead()) {
break;
}
}
if (endpoint.isPaused()) {
// 503 - Service unavailable
response.setStatus(503);
error = true;
} else {
// Make sure that connectors that are non-blocking during
// header processing (NIO) only set the start time the first
// time a request is processed.
if (request.getStartTime() < 0) {
request.setStartTime(System.currentTimeMillis());
}
keptAlive = true;
// Set this every time in case limit has been changed via JMX
request.getMimeHeaders().setLimit(endpoint.getMaxHeaderCount());
// Currently only NIO will ever return false here
// 3
if (!getInputBuffer().parseHeaders()) {
// We've read part of the request, don't recycle it
// instead associate it with the socket
openSocket = true;
readComplete = false;
break;
}
if (!disableUploadTimeout) {
setSocketTimeout(connectionUploadTimeout);
}
}
} catch (IOException e) {
if (getLog().isDebugEnabled()) {
getLog().debug(
sm.getString("http11processor.header.parse"), e);
}
error = true;
break;
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
UserDataHelper.Mode logMode = userDataHelper.getNextMode();
if (logMode != null) {
String message = sm.getString(
"http11processor.header.parse");
switch (logMode) {
case INFO_THEN_DEBUG:
message += sm.getString(
"http11processor.fallToDebug");
//$FALL-THROUGH$
case INFO:
getLog().info(message);
break;
case DEBUG:
getLog().debug(message);
}
}
// 400 - Bad Request
response.setStatus(400);
adapter.log(request, response, 0);
error = true;
}
if (!error) {
// Setting up filters, and parse some request headers
rp.setStage(org.apache.coyote.Constants.STAGE_PREPARE);
try {
prepareRequest();
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
if (getLog().isDebugEnabled()) {
getLog().debug(sm.getString(
"http11processor.request.prepare"), t);
}
// 400 - Internal Server Error
response.setStatus(400);
adapter.log(request, response, 0);
error = true;
}
}
if (maxKeepAliveRequests == 1) {
keepAlive = false;
} else if (maxKeepAliveRequests > 0 &&
socketWrapper.decrementKeepAlive() <= 0) {
keepAlive = false;
}
// Process the request in the adapter
if (!error) {
try {
// 4
rp.setStage(org.apache.coyote.Constants.STAGE_SERVICE);
adapter.service(request, response);
// Handle when the response was committed before a serious
// error occurred. Throwing a ServletException should both
// set the status to 500 and set the errorException.
// If we fail here, then the response is likely already
// committed, so we can't try and set headers.
if(keepAlive && !error) { // Avoid checking twice.
error = response.getErrorException() != null ||
(!isAsync() &&
statusDropsConnection(response.getStatus()));
}
setCometTimeouts(socketWrapper);
} catch (InterruptedIOException e) {
error = true;
} catch (HeadersTooLargeException e) {
error = true;
// The response should not have been committed but check it
// anyway to be safe
if (!response.isCommitted()) {
response.reset();
response.setStatus(500);
response.setHeader("Connection", "close");
}
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
getLog().error(sm.getString(
"http11processor.request.process"), t);
// 500 - Internal Server Error
response.setStatus(500);
adapter.log(request, response, 0);
error = true;
}
}
// Finish the handling of the request
rp.setStage(org.apache.coyote.Constants.STAGE_ENDINPUT);
if (!isAsync() && !comet) {
if (error) {
// If we know we are closing the connection, don't drain
// input. This way uploading a 100GB file doesn't tie up the
// thread if the servlet has rejected it.
getInputBuffer().setSwallowInput(false);
}
endRequest();
}
rp.setStage(org.apache.coyote.Constants.STAGE_ENDOUTPUT);
// If there was an error, make sure the request is counted as
// and error, and update the statistics counter
if (error) {
response.setStatus(500);
}
request.updateCounters();
if (!isAsync() && !comet || error) {
getInputBuffer().nextRequest();
getOutputBuffer().nextRequest();
}
if (!disableUploadTimeout) {
if(endpoint.getSoTimeout() > 0) {
setSocketTimeout(endpoint.getSoTimeout());
} else {
setSocketTimeout(0);
}
}
rp.setStage(org.apache.coyote.Constants.STAGE_KEEPALIVE);
if (breakKeepAliveLoop(socketWrapper)) {
break;
}
}
rp.setStage(org.apache.coyote.Constants.STAGE_ENDED);
if (error || endpoint.isPaused()) {
return SocketState.CLOSED;
} else if (isAsync() || comet) {
return SocketState.LONG;
} else if (isUpgrade()) {
return SocketState.UPGRADING;
} else {
if (sendfileInProgress) {
return SocketState.SENDFILE;
} else {
if (openSocket) {
if (readComplete) {
return SocketState.OPEN;
} else {
return SocketState.LONG;
}
} else {
return SocketState.CLOSED;
}
}
}
}
上面的代码有点长。可是经过分析，我们还是能够看清楚主干。我已经在代码中将主流程通过数字标注了。我们就来一一看看标注了数字的地方：
- 标注1的地方（第7行）将Socket的输入流和输出流通过InternalInputBuffer进行了包装，InternalInputBuffer是在Http11Processor的构造函数中初始化的。
- 标注2的地方（第35行）调用了InternalInputBuffer的parseRequesLine方法解析http请求的请求行。
(关于http请求行和请求头请看下文解释)
- 标注3的地方（第57行）调用了InternalInputBuffer的prarseHeaders方法解析http请求的请求头。解析完了以后，会将http header保存在
org.apache.tomcat.util.http.MimeHeaders - 标注4的地方（第128行）调用了org.apache.coyote.Adapter#service方法。次方法就会终于调用到详细的Servlet.
对于Http请求行和请求头，大家能够看以下的样例：
GET /contextpath/querystring HTTP/1.1 Host: 127.0.0.1:8080 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:23.0) Gecko/20100101 Firefox/23.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-US,en;q=0.5 Accept-Encoding: gzip, deflate Cookie: JSESSIONID=9F5897FEF3CDBCB234C050C132DCAE52; __atuvc=384%7C39; __utma=96992031.358732763.1380383869.1381468490.1381554710.38; __utmz=96992031.1380383869.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); Hm_lvt_21e144d0df165d6556d664e2836dadfe=1381330561,1381368826,1381395666,1381554711 Connection: keep-alive Cache-Control: max-age=0
在上面的Http协议get请求中，当中请求行就是第一行，GET /contextpath/querystring
HTTP/1.1,余下的都是请求头。
这里面须要注意依据Http协议的要求，请求行末尾必须是CRLF，而请求行与请求头，以及请求头之间必须用空行隔开，而空行也必须仅仅包括CRLF。
对于Http协议请求头的规范能够參考这里。
通过上面的描写叙述，我们能够整理出例如以下的一个请求解析流程：
org.apache.tomcat.util.net.JIoEndpoint.Acceptor#run ->org.apache.tomcat.util.net.JIoEndpoint.SocketProcessor#run(请求处理线程池中执行) -->org.apache.coyote.AbstractProtocol.AbstractConnectionHandler#process --->org.apache.coyote.http11.AbstractHttp11Processor#process ---->org.apache.coyote.http11.InternalInputBuffer#parseRequestLine ---->org.apache.coyote.http11.InternalInputBuffer#parseHeaders ---->org.apache.catalina.connector.CoyoteAdapter#service
怎样转发到Servlet
上面我们说了一个请求过来是怎样依据http协议解析Socket的数据。终于将生成org.apache.coyote.Request和org.apache.coyote.Response。接下来我们就来看看request,reponse是怎样一步步的进入终于的Servlet进行处理的。
这一步的入口就是CoyoteAdapter的service方法。 接下来我们就来看看它的代码：
<span style="color: rgb(102, 102, 102); font-family: 'Open Sans', HelveticaNeue-Light, 'Helvetica Neue Light', 'Helvetica Neue', Helvetica, Arial, sans-serif; font-size: 12.6000003814697px; line-height: 25.2000007629395px; text-align: justify; background-color: rgb(236, 236, 236);">org.apache.catalina.connector.CoyoteAdapter#service</span>
public void service(org.apache.coyote.Request req,
org.apache.coyote.Response res)
throws Exception {
Request request = (Request) req.getNote(ADAPTER_NOTES);
Response response = (Response) res.getNote(ADAPTER_NOTES);
//1
if (request == null) {
// Create objects
request = connector.createRequest();
request.setCoyoteRequest(req);
response = connector.createResponse();
response.setCoyoteResponse(res);
// Link objects
request.setResponse(response);
response.setRequest(request);
// Set as notes
req.setNote(ADAPTER_NOTES, request);
res.setNote(ADAPTER_NOTES, response);
// Set query string encoding
req.getParameters().setQueryStringEncoding
(connector.getURIEncoding());
}
if (connector.getXpoweredBy()) {
response.addHeader("X-Powered-By", POWERED_BY);
}
boolean comet = false;
boolean async = false;
try {
// Parse and set Catalina and configuration specific
// request parameters
req.getRequestProcessor().setWorkerThreadName(Thread.currentThread().getName());
//2
boolean postParseSuccess = postParseRequest(req, request, res, response);
if (postParseSuccess) {
//check valves if we support async
request.setAsyncSupported(connector.getService().getContainer().getPipeline().isAsyncSupported());
// Calling the container
//3
connector.getService().getContainer().getPipeline().getFirst().invoke(request, response);
// other code
}
// other code
} catch (IOException e) {
// Ignore
} finally {
req.getRequestProcessor().setWorkerThreadName(null);
// Recycle the wrapper request and response
if (!comet && !async) {
request.recycle();
response.recycle();
} else {
// Clear converters so that the minimum amount of memory
// is used by this processor
request.clearEncoders();
response.clearEncoders();
}
}
}
为了能够清楚的看到主流程，上面删除了一部分非主流程的代码。接下来我们逐一分析一下标注了数字的地方：
- 标注1的代码(第9行)将
org.apache.coyote.Request和org.apache.coyote.Response对象转变为org.apache.catalina.connector.Request,org.apache.catalina.connector.Response类型的对象。当中coyote包中的Request只不过包括了解析出来的http协议的数据，而connector包中的Request才是真正Servlet容器中的HttpServletRequest，它里面包括了完毕请求须要的host,context和wrapper信息,在这里每个wrapper事实上都相应web.xml配置的一个Servlet。 - 标注2（第44行）的代码调用了postParseRequest方法，这种方法里面做的事情许多，可是终于都是为了依据Request对象找到相应的Host,Conext和Wrapper对象，也就是说终于要清楚这个请求应该由哪个Servlet来处理。
- 标注3（第50）的代码将已经设置好了Host,Context,Wrapper对象的Request通过Pipeline机制链式传递给终于的Servlet。
上面仅仅是从总体上告诉了读者org.apache.catalina.connector.CoyoteAdapter#service方法做的事情，接下来我们进一步分解每个步骤都详细做了哪些工作。第一步比較简单，大家能够自己阅读，我们关键来看2，3步。首先我们来看看postParseRequest方法。
通过分析org.apache.catalina.connector.CoyoteAdapter#postParseRequest的代码，我们会发现它终于是通过org.apache.tomcat.util.http.mapper.Mapper#map方法来达到匹配请求到相应的Context和Wrapper(Servlet包装类)目的。详细代码例如以下：
<span style="color: rgb(102, 102, 102); font-family: 'Open Sans', HelveticaNeue-Light, 'Helvetica Neue Light', 'Helvetica Neue', Helvetica, Arial, sans-serif; font-size: 12.6000003814697px; line-height: 25.2000007629395px; text-align: justify; background-color: rgb(236, 236, 236);">org.apache.catalina.connector.CoyoteAdapter#postParseRequest</span>connector.getMapper().map(serverName, decodedURI, version, request.getMappingData()); request.setContext((Context) request.getMappingData().context); request.setWrapper((Wrapper) request.getMappingData().wrapper);那我们再来看看此方法。通过分析它的代码。我们发现终于事实上是调用了几个
internalMap**方法将找到的Context,Wrapper设置到org.apache.catalina.connector.Request对象的org.apache.tomcat.util.http.mapper.MappingData类型的属性中,map方法运行完以后。然后接下来就从MappingData中获取已经找到的Context和Wrapper。再设置到Request的context和wrapper中。接下来我们再来分析第3步。第3步通过pipeline链式调用机制终于调用了Servlet对象。而对于pipeline事实上是运用了责任链模式，它将各个阀门链接起来，然后一步步的调用，而至于有多少个阀门（Valve）对象。主要来源于两个地方，一个是conf/server.xml中配置的valve，我们知道全部的容器都是支持pipeline机制的，另外一个就是每个容器的构造当中自己初始化的阀门对象。
接下来一一看一下。对于StandardEngine来说有一个与之相应的StandardEngineValve。对于StandardHost有一个StandardHostValve与之相应。StandardContext有一个StandardContextValve与之相应。StandardWrapper与StandardWrapperValve相应,通过分析代码。我们能够得到例如以下的一个调用链。
->org.apache.catalina.core.StandardEngineValve#invoke -->org.apache.catalina.valves.AccessLogValve#invoke --->org.apache.catalina.valves.ErrorReportValve#invoke ---->org.apache.catalina.core.StandardHostValve#invoke ----->org.apache.catalina.authenticator.AuthenticatorBase#invoke ------>org.apache.catalina.core.StandardContextValve#invoke ------->org.apache.catalina.core.StandardWrapperValve#invoke
上述的调用栈中。最后会调用到StandardWrapperValve，它事实上也是终于调用Servlet的地方，接下来我们就来看看它的代码：
public final void invoke(Request request, Response response)
throws IOException, ServletException {
// Initialize local variables we may need
boolean unavailable = false;
Throwable throwable = null;
// This should be a Request attribute...
long t1=System.currentTimeMillis();
requestCount++;
StandardWrapper wrapper = (StandardWrapper) getContainer();
Servlet servlet = null;
Context context = (Context) wrapper.getParent();
// Allocate a servlet instance to process this request
try {
//1
if (!unavailable) {
servlet = wrapper.allocate();
}
} catch (UnavailableException e) {
container.getLogger().error(
sm.getString("standardWrapper.allocateException",
wrapper.getName()), e);
long available = wrapper.getAvailable();
if ((available > 0L) && (available < Long.MAX_VALUE)) {
response.setDateHeader("Retry-After", available);
response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
} else if (available == Long.MAX_VALUE) {
response.sendError(HttpServletResponse.SC_NOT_FOUND,
sm.getString("standardWrapper.notFound",
wrapper.getName()));
}
} // other code
MessageBytes requestPathMB = request.getRequestPathMB();
DispatcherType dispatcherType = DispatcherType.REQUEST;
if (request.getDispatcherType()==DispatcherType.ASYNC) dispatcherType = DispatcherType.ASYNC;
request.setAttribute(Globals.DISPATCHER_TYPE_ATTR,dispatcherType);
request.setAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR,
requestPathMB);
// Create the filter chain for this request
ApplicationFilterFactory factory =
ApplicationFilterFactory.getInstance();
ApplicationFilterChain filterChain =
factory.createFilterChain(request, wrapper, servlet);
// Reset comet flag value after creating the filter chain
request.setComet(false);
// Call the filter chain for this request
// NOTE: This also calls the servlet's service() method
// 2
try {
if ((servlet != null) && (filterChain != null)) {
// Swallow output if needed
if (context.getSwallowOutput()) {
try {
SystemLogHandler.startCapture();
if (request.isAsyncDispatching()) {
//TODO SERVLET3 - async
((AsyncContextImpl)request.getAsyncContext()).doInternalDispatch();
} else if (comet) {
filterChain.doFilterEvent(request.getEvent());
request.setComet(true);
} else {
filterChain.doFilter(request.getRequest(),
response.getResponse());
}
} finally {
String log = SystemLogHandler.stopCapture();
if (log != null && log.length() > 0) {
context.getLogger().info(log);
}
}
} else {
if (request.isAsyncDispatching()) {
//TODO SERVLET3 - async
((AsyncContextImpl)request.getAsyncContext()).doInternalDispatch();
} else if (comet) {
request.setComet(true);
filterChain.doFilterEvent(request.getEvent());
} else {
filterChain.doFilter
(request.getRequest(), response.getResponse());
}
}
}
} catch(Exception e){
// other code
}
}为了节省版面，上面的代码已经删除非主流程的代码。接下来我们逐一分析一下标注了数字的地方：
- 标注1（第17行）的代码实例化了Servlet对象，在实例化的过程中使用了Java双检查锁的机制来实例化Servlet。有兴趣的童鞋能够去看看org.apache.catalina.core.StandardWrapper#allocate的代码。这里须要注意的是在Servlet2.4规范之前，有一个singleThreadMode模型，这个机制类似与之前EJB的无状态会话Bean机制，每一个线程过来会通过实例池中取出一个实例来完毕响应。在Servlet规范2.4之后，单线程模型已经被废除了。详细细节能够參考这里 .
- 标注2（第55行）的代码事实上调用了大家熟悉的Servlet的过滤器链。过滤器链终于就会调用到Servlet.
最后，咋们再来看看过滤器滤链的处理。来看看org.apache.catalina.core.ApplicationFilterChain#doFilter。doFilter方法中会依据filterConfig中取的web.xml配置的过滤器，然后一个个调用，等每一个过滤器运行完了以后，终于就会调用到Servlet的Service方法。
通过上面的分析，事实上我们已经清楚了一个请求过来以后，Tomcat是怎样一步步处理的。我们再来做一个整体的总结：
- 用户浏览器发送请求，请求会发送到相应的Connector监听的Socketport。
- Connector从Socket流中获取数据。然后依据Http协议将其解析为Request和Reponse对象
- 找到Request对象相应的Host,Context,Wrapper
- 调用终于的Servelt的service进行处理。