背景
前面已经对Server、Service和Executor进行了分析讲解，本篇章开启分析解剖Tomcat如何处理Request请求，众所周知，客户端是可以发起多个请求的，Tomcat也是可以支持多个webapp的，有多个上下文，且一个webapp中可以有多个Servlet，本篇将详细分析Tomcat是如何设计组件支持请求处理的
Container设计
从代码结构可以看出
Engine、Host、Context、Wrapper都是Container的组件设计，Container顶层也是基于Lifecycle的组件设计的。
Engine：表示整个catalina的servlet引擎，多数情况下包含一个或多个子容器，这些子容器要么是Host，要么是Context实现，或者是其他自定义组件
Host：表示多个Context的虚拟主机
Context：表示一个servletContext，是一个webapp，它通常包含一个或多个wrapper
Wrapper：表示一个servlet
结合整体的框架图，可以看出
除了四个组件的嵌套之外，Container中还包含了Realm、cluster、Listeners,Pipleline等支持组件
Container的管道机制
责任链模式
管道机制在设计模式上属于责任链模式，通过责任链模式，可以为某个请求创建一个对象链，每个对象依序检查此请求并对其进行处理或者将它传给链中的下个对象
Piple机制
在一个比较复杂的大型系统中，如果一个对象或者数据流需要进行频繁的逻辑处理，我们可以选择在一个大的组件中直接处理这些繁杂的业务逻辑，这个方法虽然能达到目的，但是扩展性和重用性较差，更好的解解方法就是采用管道机制，用一条管道把多个对象(阀门部件)连接起来，整体看起来就像若干个阀门嵌套在管道中一样，而处理逻辑就放在阀门上。
Tomcat中就是使用了这样的Piple机制，通过阀门的方式让每个请求的数据流通过管道的方式在每个容器之间流动起来
Tomcat一次完整请求的处理流程
时序图
- 接收Socket请求，把请求转发给线程池，由线程池分配一个线程来处理
- 获取Socket数据包之后，解析HTTP协议，翻译成Tomcat内部的Request和Response对象，再映射到相应的Container
- Resqest和Response对象进入Tomcat中的Container容器的各级Piepline管道去流式执行，最后会流到StandardWrapper这个阀门
- 在StanderdWrapperValue这个阀门中，把当前请求的Filter以及Servlet封装成FilterChain，最终执行FilterChain，回调Web应用，安成响应。
具体源码如下：
Tomcat启动各组件
tomcat启动时，默认使用NIO模式
public final void start() throws Exception {
if (bindState == BindState.UNBOUND) {
bindWithCleanup();
bindState = BindState.BOUND_ON_START;
}
startInternal();
}
通过initServiceSocket方法初始化，使用 ServerSocketChannel.open() 打开一个 ServerSocket通道，默认绑定到 8080 端口，用于监听请求。
protected void initServerSocket() throws Exception {
if (getUseInheritedChannel()) {
// Retrieve the channel provided by the OS
Channel ic = System.inheritedChannel();
if (ic instanceof ServerSocketChannel) {
serverSock = (ServerSocketChannel) ic;
}
if (serverSock == null) {
throw new IllegalArgumentException(sm.getString("endpoint.init.bind.inherited"));
}
} else if (getUnixDomainSocketPath() != null) {
SocketAddress sa = JreCompat.getInstance().getUnixDomainSocketAddress(getUnixDomainSocketPath());
serverSock = JreCompat.getInstance().openUnixDomainServerSocketChannel();
serverSock.bind(sa, getAcceptCount());
if (getUnixDomainSocketPathPermissions() != null) {
Path path = Paths.get(getUnixDomainSocketPath());
Set<PosixFilePermission> permissions =
PosixFilePermissions.fromString(getUnixDomainSocketPathPermissions());
if (path.getFileSystem().supportedFileAttributeViews().contains("posix")) {
FileAttribute<Set<PosixFilePermission>> attrs = PosixFilePermissions.asFileAttribute(permissions);
Files.setAttribute(path, attrs.name(), attrs.value());
} else {
java.io.File file = path.toFile();
if (permissions.contains(PosixFilePermission.OTHERS_READ) && !file.setReadable(true, false)) {
log.warn(sm.getString("endpoint.nio.perms.readFail", file.getPath()));
}
if (permissions.contains(PosixFilePermission.OTHERS_WRITE) && !file.setWritable(true, false)) {
log.warn(sm.getString("endpoint.nio.perms.writeFail", file.getPath()));
}
}
}
} else {
serverSock = ServerSocketChannel.open();
socketProperties.setProperties(serverSock.socket());
InetSocketAddress addr = new InetSocketAddress(getAddress(), getPortWithOffset());
serverSock.bind(addr, getAcceptCount());
}
serverSock.configureBlocking(true); //mimic APR behavior
}
其中，open方法就是建立一个TCP连接
再来看下 startInternal()方法,创建一个线程任务Poller,用于检测Acceptor接受并处理已就绪的Socket
@Override
public void startInternal() throws Exception {
if (!running) {
running = true;
paused = false;
if (socketProperties.getProcessorCache() != 0) {
processorCache = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,
socketProperties.getProcessorCache());
}
if (socketProperties.getEventCache() != 0) {
eventCache = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,
socketProperties.getEventCache());
}
int actualBufferPool =
socketProperties.getActualBufferPool(isSSLEnabled() ? getSniParseLimit() * 2 : 0);
if (actualBufferPool != 0) {
nioChannels = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,
actualBufferPool);
}
// Create worker collection
if (getExecutor() == null) {
createExecutor();
}
initializeConnectionLatch();
// Start poller thread
poller = new Poller();
Thread pollerThread = new Thread(poller, getName() + "-Poller");
pollerThread.setPriority(threadPriority);
pollerThread.setDaemon(true);
pollerThread.start();
startAcceptorThread();
}
}
其中startAcceptorThread 创建一个线程Acceptor，作为一个接收者，无限循环接受客户端发送过来的连接请求。
protected void startAcceptorThread() {
acceptor = new Acceptor<>(this);
String threadName = getName() + "-Acceptor";
acceptor.setThreadName(threadName);
Thread t = new Thread(acceptor, threadName);
t.setPriority(getAcceptorThreadPriority());
t.setDaemon(getDaemon());
t.start();
}
Acceptor
accpetor的run方法，无限循环在这里接受连接请求
@Override
public void run() {
int errorDelay = 0;
try {
// Loop until we receive a shutdown command
while (!stopCalled) {
....
U socket = null;
try {
// Accept the next incoming connection from the server
// socket
socket = endpoint.serverSocketAccept();
} catch (Exception ioe) {}
}
当Acceptor接收到客户端请求时，调用addEvent()方法会将Socket添加到Poller的PollerEvent队列中，并调用NIO中selector.wakeup方法，唤醒Poller
public void add(NioSocketWrapper socketWrapper, int interestOps) {
PollerEvent r = null;
if (eventCache != null) {
r = eventCache.pop();
}
if (r == null) {
r = new PollerEvent(socketWrapper, interestOps);
} else {
r.reset(socketWrapper, interestOps);
}
addEvent(r);
if (close) {
processSocket(socketWrapper, SocketEvent.STOP, false);
}
}
private void addEvent(PollerEvent event) {
events.offer(event);
if (wakeupCounter.incrementAndGet() == 0) {
selector.wakeup();
}
}
Poller
Poller线程1秒阻塞一次，等待有请求过来被唤醒后，每次请求都会经过processSocket，从处理器缓存中获取当前要被执行的任务。放进任务进程中，然后获取Worker线程组，将任务放进去，到此Poll任务就完成了
public boolean processSocket(SocketWrapperBase<S> socketWrapper,
SocketEvent event, boolean dispatch) {
try {
if (socketWrapper == null) {
return false;
}
SocketProcessorBase<S> sc = null;
//从处理器缓存取出当前任务
if (processorCache != null) {
sc = processorCache.pop();
}
if (sc == null) {
sc = createSocketProcessor(socketWrapper, event);
} else {
sc.reset(socketWrapper, event);
}
//获取执行任务的线程池
Executor executor = getExecutor();
if (dispatch && executor != null) {
executor.execute(sc);
} else {
sc.run();
}
} catch (RejectedExecutionException ree) {
getLog().warn(sm.getString("endpoint.executor.fail", socketWrapper) , ree);
return false;
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
// This means we got an OOM or similar creating a thread, or that
// the pool and its queue are full
getLog().error(sm.getString("endpoint.process.fail"), t);
return false;
}
return true;
}
Worker
然后就到了Worker线程组，这次请求后续的所有操作都在这个线程中完成，Worker线程组是一个阻塞队列,worker线程被创建以后就执行ThreadPoolExecutor 的 runWorker() 方法，试图从 workQueue 中取待处理任务，但是一开始 workQueue 是空的，所以 worker 线程会阻塞在 workQueue.take() 方法。
当新任务添加到workQueue后，阻塞就会结束，然后worker线程调用SocketWrapperBase的 run() 方法对 Socket 进行处理。
@Override
public final void run() {
synchronized (socketWrapper) {
// It is possible that processing may be triggered for read and
// write at the same time. The sync above makes sure that processing
// does not occur in parallel. The test below ensures that if the
// first event to be processed results in the socket being closed,
// the subsequent events are not processed.
if (socketWrapper.isClosed()) {
return;
}
doRun();
}
}
protected abstract void doRun();
里面的doRun方法是一个抽象方法，Tomcat默认使用的NIO模式，会调用NioEndpoint里面的内部类doRun()
@Override
protected void doRun() {
/*
* Do not cache and re-use the value of socketWrapper.getSocket() in
* this method. If the socket closes the value will be updated to
* CLOSED_NIO_CHANNEL and the previous value potentially re-used for
* a new connection. That can result in a stale cached value which
* in turn can result in unintentionally closing currently active
* connections.
*/
Poller poller = NioEndpoint.this.poller;
if (poller == null) {
socketWrapper.close();
return;
}
try {
int handshake = -1; // 进行三次握手
try {
if (socketWrapper.getSocket().isHandshakeComplete()) {
// No TLS handshaking required. Let the handler
// process this socket / event combination.
handshake = 0;
} else if (event == SocketEvent.STOP || event == SocketEvent.DISCONNECT ||
event == SocketEvent.ERROR) {
// Unable to complete the TLS handshake. Treat it as
// if the handshake failed.
handshake = -1;
} else {
handshake = socketWrapper.getSocket().handshake(event == SocketEvent.OPEN_READ, event == SocketEvent.OPEN_WRITE);
// The handshake process reads/writes from/to the
// socket. status may therefore be OPEN_WRITE once
// the handshake completes. However, the handshake
// happens when the socket is opened so the status
// must always be OPEN_READ after it completes. It
// is OK to always set this as it is only used if
// the handshake completes.
event = SocketEvent.OPEN_READ;
}
} catch (IOException x) {
handshake = -1;
if (log.isDebugEnabled()) {
log.debug("Error during SSL handshake",x);
}
} catch (CancelledKeyException ckx) {
handshake = -1;
}
if (handshake == 0) {
SocketState state = SocketState.OPEN;
// Process the request from this socket
if (event == null) {
state = getHandler().process(socketWrapper, SocketEvent.OPEN_READ);
} else {
// 获取handler并进行请求处理
state = getHandler().process(socketWrapper, event);
}
if (state == SocketState.CLOSED) {
poller.cancelledKey(getSelectionKey(), socketWrapper);
}
} else if (handshake == -1 ) {
getHandler().process(socketWrapper, SocketEvent.CONNECT_FAIL);
poller.cancelledKey(getSelectionKey(), socketWrapper);
} else if (handshake == SelectionKey.OP_READ){
socketWrapper.registerReadInterest();
} else if (handshake == SelectionKey.OP_WRITE){
socketWrapper.registerWriteInterest();
}
} catch (CancelledKeyException cx) {
poller.cancelledKey(getSelectionKey(), socketWrapper);
} catch (VirtualMachineError vme) {
ExceptionUtils.handleThrowable(vme);
} catch (Throwable t) {
log.error(sm.getString("endpoint.processing.fail"), t);
poller.cancelledKey(getSelectionKey(), socketWrapper);
} finally {
socketWrapper = null;
event = null;
//return to cache
if (running && !paused && processorCache != null) {
processorCache.push(this);
}
}
}
这个socket处理器先做TCP的三次握手，我们看下getHandler().process(socketWrapper, event)方法，执行的是
AbstractProtocol.process，下面摘取部分代码进行解读
因为默认用的协议是http1.1，所以下面获取的是Http11Processor。
Processor processor = (Processor) wrapper.getCurrentProcessor();
if (getLog().isDebugEnabled()) {
getLog().debug(sm.getString("abstractConnectionHandler.connectionsGet",
processor, socket));
}
往下走这个处理器的process方法，最后到了Http11Processor.service方法中，service方法先从当前请求request中，解析请求、相关信息，封装成Request对象，然后获取adapter(CoyoteAdaptor)，调用services方法
public class Http11Processor extends AbstractProcessor {
@Override
public SocketState service(SocketWrapperBase<?> socketWrapper) {
throws IOException {
RequestInfo rp = request.getRequestProcessor();
rp.setStage(org.apache.coyote.Constants.STAGE_PARSE);
// Setting up the I/O
setSocketWrapper(socketWrapper);
// Flags
keepAlive = true;
openSocket = false;
readComplete = true;
boolean keptAlive = false;
SendfileState sendfileState = SendfileState.DONE;
...
try {
rp.setStage(org.apache.coyote.Constants.STAGE_SERVICE);
getAdapter().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 && !getErrorState().isError() && !isAsync() &&
statusDropsConnection(response.getStatus())) {
setErrorState(ErrorState.CLOSE_CLEAN, null);
}
}
....
}
}
CoyoteAdaptor
getAdapter() 获取的是CoyoteAdaptor，然后执行CoyoteAdaptor.service，源码如下
@Override
public void service(org.apache.coyote.Request req, org.apache.coyote.Response res)
throws Exception {
// 封装Request和Response对象
Request request = (Request) req.getNote(ADAPTER_NOTES);
Response response = (Response) res.getNote(ADAPTER_NOTES);
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().setQueryStringCharset(connector.getURICharset());
}
if (connector.getXpoweredBy()) {
response.addHeader("X-Powered-By", POWERED_BY);
}
boolean async = false;
boolean postParseSuccess = false;
req.getRequestProcessor().setWorkerThreadName(THREAD_NAME.get());
try {
// 查找URL的映射关系
postParseSuccess = postParseRequest(req, request, res, response);
if (postParseSuccess) {
//check valves if we support async
request.setAsyncSupported(
connector.getService().getContainer().getPipeline().isAsyncSupported());
//把封装好的对象传递给Engine容器，获取他的管道，执行里面绑定的阀门value
connector.getService().getContainer().getPipeline().getFirst().invoke(
request, response);
}
if (request.isAsync()) {
async = true;
ReadListener readListener = req.getReadListener();
if (readListener != null && request.isFinished()) {
// Possible the all data may have been read during service()
// method so this needs to be checked here
ClassLoader oldCL = null;
try {
oldCL = request.getContext().bind(false, null);
if (req.sendAllDataReadEvent()) {
req.getReadListener().onAllDataRead();
}
} finally {
request.getContext().unbind(false, oldCL);
}
}
Throwable throwable =
(Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION);
// If an async request was started, is not going to end once
// this container thread finishes and an error occurred, trigger
// the async error process
if (!request.isAsyncCompleting() && throwable != null) {
request.getAsyncContextInternal().setErrorState(throwable, true);
}
} else {
request.finishRequest();
response.finishResponse();
}
} catch (IOException e) {
// Ignore
} finally {
AtomicBoolean error = new AtomicBoolean(false);
res.action(ActionCode.IS_ERROR, error);
if (request.isAsyncCompleting() && error.get()) {
// Connection will be forcibly closed which will prevent
// completion happening at the usual point. Need to trigger
// call to onComplete() here.
res.action(ActionCode.ASYNC_POST_PROCESS, null);
async = false;
}
// Access log
if (!async && postParseSuccess) {
// Log only if processing was invoked.
// If postParseRequest() failed, it has already logged it.
Context context = request.getContext();
Host host = request.getHost();
// If the context is null, it is likely that the endpoint was
// shutdown, this connection closed and the request recycled in
// a different thread. That thread will have updated the access
// log so it is OK not to update the access log here in that
// case.
// The other possibility is that an error occurred early in
// processing and the request could not be mapped to a Context.
// Log via the host or engine in that case.
long time = System.nanoTime() - req.getStartTimeNanos();
if (context != null) {
context.logAccess(request, response, time, false);
} else if (response.isError()) {
if (host != null) {
host.logAccess(request, response, time, false);
} else {
connector.getService().getContainer().logAccess(
request, response, time, false);
}
}
}
req.getRequestProcessor().setWorkerThreadName(null);
// Recycle the wrapper request and response
if (!async) {
updateWrapperErrorCount(request, response);
request.recycle();
response.recycle();
}
}
}
按照StandardEngineValue、StandardHostValue、StandardContextValue、StandardWrapperValue顺序执行多个阀门
StandardWrapperValve
最后执行到StandardWrapperValve.invoke，将Servlet封装到FlinterChain过滤链中
// Create the filter chain for this request
ApplicationFilterChain filterChain =
ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);
// Call the filter chain for this request
// NOTE: This also calls the servlet's service() method
Container container = this.container;
try {
if ((servlet != null) && (filterChain != null)) {
// Swallow output if needed
if (context.getSwallowOutput()) {
try {
SystemLogHandler.startCapture();
if (request.isAsyncDispatching()) {
request.getAsyncContextInternal().doInternalDispatch();
} 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()) {
request.getAsyncContextInternal().doInternalDispatch();
} else {
filterChain.doFilter
(request.getRequest(), response.getResponse());
}
}
}
HttpServlet
最后执行servlet.service(request,responce),调用熟悉的HttpServlet的service方法，解析里面对应的doGet、doPost方法等，最后由Servlet将响应返回给客户端
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
// Invalid date header - proceed as if none was set
ifModifiedSince = -1;
}
if (ifModifiedSince < (lastModified / 1000 * 1000)) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}
