Connector容器主要负责解析socket请求，在tomcat中的源码位于org.apache.catalina.connector和org.apache.coyote包路径下；通过上两节的分析，我们知道了Connector是Service的子容器，而Service又是Server的子容器。在server.xml文件中配置，然后在Catalina类中通过Digester完成实例化。在server.xml中默认配置了两种Connector的实现，分别用来处理Http请求和AJP请求。
Connector的实现一共有以下三种：
1、Http Connector：解析HTTP请求，又分为BIO Http Connector和NIO Http Connector，即阻塞IO Connector和非阻塞IO Connector。本文主要分析NIO Http Connector的实现过程。
2、AJP：基于AJP协议，用于Tomcat与HTTP服务器通信定制的协议，能提供较高的通信速度和效率。如与Apache服务器集成时，采用这个协议。
3、APR HTTP Connector：用C实现，通过JNI调用的。主要提升对静态资源（如HTML、图片、CSS、JS等）的访问性能。
具体要使用哪种Connector可以在server.xml文件中通过protocol属性配置如下：
<Connector port="8080" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="8443" />
然后看一下Connector的构造器：
public Connector(String protocol) {
setProtocol(protocol);
// Instantiate protocol handler
ProtocolHandler p = null;
try {
Class<?> clazz = Class.forName(protocolHandlerClassName);
p = (ProtocolHandler) clazz.getConstructor().newInstance();
} catch (Exception e) {
log.error(sm.getString(
"coyoteConnector.protocolHandlerInstantiationFailed"), e);
} finally {
this.protocolHandler = p;
}
if (Globals.STRICT_SERVLET_COMPLIANCE) {
uriCharset = StandardCharsets.ISO_8859_1;
} else {
uriCharset = StandardCharsets.UTF_8;
}
}
public void setProtocol(String protocol) {
boolean aprConnector = AprLifecycleListener.isAprAvailable() &&
AprLifecycleListener.getUseAprConnector();
if ("HTTP/1.1".equals(protocol) || protocol == null) {
if (aprConnector) {
setProtocolHandlerClassName("org.apache.coyote.http11.Http11AprProtocol");
} else {
setProtocolHandlerClassName("org.apache.coyote.http11.Http11NioProtocol");
}
} else if ("AJP/1.3".equals(protocol)) {
if (aprConnector) {
setProtocolHandlerClassName("org.apache.coyote.ajp.AjpAprProtocol");
} else {
setProtocolHandlerClassName("org.apache.coyote.ajp.AjpNioProtocol");
}
} else {
setProtocolHandlerClassName(protocol);
}
}
通过分析Connector构造器的源码可以知道，每一个Connector对应了一个protocolHandler，一个protocolHandler被设计用来监听服务器某个端口的网络请求，但并不负责处理请求(处理请求由Container组件完成)。下面就以Http11NioProtocol为例分析Http请求的解析过程。
在Connector的startInterval方法中启动了protocolHandler,代码如下：
protected void startInternal() throws LifecycleException {
// Validate settings before starting
if (getPort() < 0) {
throw new LifecycleException(sm.getString(
"coyoteConnector.invalidPort", Integer.valueOf(getPort())));
}
setState(LifecycleState.STARTING);
try {
protocolHandler.start();
} catch (Exception e) {
throw new LifecycleException(
sm.getString("coyoteConnector.protocolHandlerStartFailed"), e);
}
}
Http11NioProtocol创建一个org.apache.tomcat.util.net.NioEndpoint实例,然后将监听端口并解析请求的工作全被委托给NioEndpoint实现。tomcat在使用Http11NioProtocol解析HTTP请求时一共设计了三种线程，分别为Acceptor，Poller和Worker。
1、Acceptor线程
Acceptor实现了Runnable接口，根据其命名就知道它是一个接收器，负责接收socket，其接收方法是serverSocket.accept()方式，获得SocketChannel对象，然后封装成tomcat自定义的org.apache.tomcat.util.net.NioChannel。虽然是Nio，但在接收socket时仍然使用传统的方法，使用阻塞方式实现。Acceptor以线程池的方式被创建和管理，在NioEndpoint的startInternal()方法中完成Acceptor的启动，源码如下：
public void startInternal() throws Exception {
if (!running) {
running = true;
paused = false;
processorCache = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,
socketProperties.getProcessorCache());
eventCache = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,
socketProperties.getEventCache());
nioChannels = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,
socketProperties.getBufferPool());
// Create worker collection
if ( getExecutor() == null ) {
createExecutor();
}
//设置最大连接数,默认值为maxConnections = 10000，通过同步器AQS实现。
initializeConnectionLatch();
//默认是2个，Math.min(2,Runtime.getRuntime().availableProcessors());和虚拟机处理器个数比较
// Start poller threads
pollers = new Poller[getPollerThreadCount()];
for (int i=0; i<pollers.length; i++) {
pollers[i] = new Poller();
Thread pollerThread = new Thread(pollers[i], getName() + "-ClientPoller-"+i);
pollerThread.setPriority(threadPriority);
pollerThread.setDaemon(true);
pollerThread.start();
}
startAcceptorThreads();
}
}
继续追踪startAcceptorThreads的源码
protected final void startAcceptorThreads() {
//启动Acceptor线程，默认是1个
int count = getAcceptorThreadCount();
acceptors = new Acceptor[count];
for (int i = 0; i < count; i++) {
acceptors[i] = createAcceptor();
String threadName = getName() + "-Acceptor-" + i;
acceptors[i].setThreadName(threadName);
Thread t = new Thread(acceptors[i], threadName);
t.setPriority(getAcceptorThreadPriority());
t.setDaemon(getDaemon());
t.start();
}
}
Acceptor线程的核心代码在它的run方法中：
protected class Acceptor extends AbstractEndpoint.Acceptor {
@Override
public void run() {
int errorDelay = 0;
// Loop until we receive a shutdown command
while (running) {
// Loop if endpoint is paused
while (paused && running) {
state = AcceptorState.PAUSED;
try {
Thread.sleep(50);
} catch (InterruptedException e) {
// Ignore
}
}
if (!running) {
break;
}
state = AcceptorState.RUNNING;
try {
//if we have reached max connections, wait
countUpOrAwaitConnection();
SocketChannel socket = null;
try {
// Accept the next incoming connection from the server
// socket
//接收socket请求
socket = serverSock.accept();
} catch (IOException ioe) {
// We didn't get a socket
countDownConnection();
if (running) {
// Introduce delay if necessary
errorDelay = handleExceptionWithDelay(errorDelay);
// re-throw
throw ioe;
} else {
break;
}
}
// Successful accept, reset the error delay
errorDelay = 0;
// Configure the socket
if (running && !paused) {
// setSocketOptions() will hand the socket off to
// an appropriate processor if successful
if (!setSocketOptions(socket)) {
closeSocket(socket);
}
} else {
closeSocket(socket);
}
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
log.error(sm.getString("endpoint.accept.fail"), t);
}
}
state = AcceptorState.ENDED;
}
private void closeSocket(SocketChannel socket) {
countDownConnection();
try {
socket.socket().close();
} catch (IOException ioe) {
if (log.isDebugEnabled()) {
log.debug(sm.getString("endpoint.err.close"), ioe);
}
}
try {
socket.close();
} catch (IOException ioe) {
if (log.isDebugEnabled()) {
log.debug(sm.getString("endpoint.err.close"), ioe);
}
}
}
}
Acceptor完成了socket请求的接收，然后交给NioEndpoint 进行配置，继续追踪Endpoint的setSocketOptions方法。
protected boolean setSocketOptions(SocketChannel socket) {
// Process the connection
try {
//disable blocking, APR style, we are gonna be polling it
//设置为非阻塞
socket.configureBlocking(false);
Socket sock = socket.socket();
socketProperties.setProperties(sock);
NioChannel channel = nioChannels.pop();
if (channel == null) {
SocketBufferHandler bufhandler = new SocketBufferHandler(
socketProperties.getAppReadBufSize(),
socketProperties.getAppWriteBufSize(),
socketProperties.getDirectBuffer());
if (isSSLEnabled()) {
channel = new SecureNioChannel(socket, bufhandler, selectorPool, this);
} else {
channel = new NioChannel(socket, bufhandler);
}
} else {
channel.setIOChannel(socket);
channel.reset();
}
//轮训pollers数组元素，调用Poller的register方法，完成channel的注册。
getPoller0().register(channel);
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
try {
log.error("",t);
} catch (Throwable tt) {
ExceptionUtils.handleThrowable(tt);
}
// Tell to close the socket
return false;
}
return true;
}
分析setSocketOptions的源码可以知道，该方法的主要功能是利用传入的SocketChannel参数生成SecureNioChannel或者NioChannel，然后注册到Poller线程的selector中，可以进一步了解Java nio的相关知识，对这一块内容有更深的理解。
2、Poller线程
Poller同样实现了Runnable接口，是NioEndpoint类的内部类。在Endpoint的startInterval方法中创建、配置并启动了Poller线程，见代码清单4。Poller主要职责是不断轮询其selector，检查准备就绪的socket(有数据可读或可写)，实现io的多路复用。其构造其中初始化了selector。
public Poller() throws IOException {
this.selector = Selector.open();
}
在分析Acceptor的时候，提到了Acceptor接受到一个socket请求后，调用NioEndpoint的setSocketOptions方法(代码清单6),该方法生成了NioChannel后调用Poller的register方法生成PoolorEvent后加入到Eventqueue，register方法的源码如下：
public void register(final NioChannel socket) {
socket.setPoller(this);
NioSocketWrapper ka = new NioSocketWrapper(socket, NioEndpoint.this);
socket.setSocketWrapper(ka);
ka.setPoller(this);
ka.setReadTimeout(getSocketProperties().getSoTimeout());
ka.setWriteTimeout(getSocketProperties().getSoTimeout());
ka.setKeepAliveLeft(NioEndpoint.this.getMaxKeepAliveRequests());
ka.setSecure(isSSLEnabled());
ka.setReadTimeout(getConnectionTimeout());
ka.setWriteTimeout(getConnectionTimeout());
PollerEvent r = eventCache.pop();
ka.interestOps(SelectionKey.OP_READ);//this is what OP_REGISTER turns into.
//生成PoolorEvent并加入到Eventqueue
if ( r==null) r = new PollerEvent(socket,ka,OP_REGISTER);
else r.reset(socket,ka,OP_REGISTER);
addEvent(r);
}
Poller的核心代码也在其run方法中:
public void run() {
// Loop until destroy() is called
// 调用了destroy()方法后终止此循环
while (true) {
boolean hasEvents = false;
try {
if (!close) {
hasEvents = events();
if (wakeupCounter.getAndSet(-1) > 0) {
//if we are here, means we have other stuff to do
//do a non blocking select
//非阻塞的 select
keyCount = selector.selectNow();
} else {
//阻塞selector，直到有准备就绪的socket
keyCount = selector.select(selectorTimeout);
}
wakeupCounter.set(0);
}
if (close) {
//该方法遍历了eventqueue中的所有PollerEvent，然后依次调用PollerEvent的run，将socket注册到selector中。
events();
timeout(0, false);
try {
selector.close();
} catch (IOException ioe) {
log.error(sm.getString("endpoint.nio.selectorCloseFail"), ioe);
}
break;
}
} catch (Throwable x) {
ExceptionUtils.handleThrowable(x);
log.error("",x);
continue;
}
//either we timed out or we woke up, process events first
if ( keyCount == 0 ) hasEvents = (hasEvents | events());
Iterator<SelectionKey> iterator =
keyCount > 0 ? selector.selectedKeys().iterator() : null;
// Walk through the collection of ready keys and dispatch
// any active event.
//遍历就绪的socket事件
while (iterator != null && iterator.hasNext()) {
SelectionKey sk = iterator.next();
NioSocketWrapper attachment = (NioSocketWrapper)sk.attachment();
// Attachment may be null if another thread has called
// cancelledKey()
if (attachment == null) {
iterator.remove();
} else {
iterator.remove();
//调用processKey方法对有数据读写的socket进行处理，在分析Worker线程时会分析该方法
processKey(sk, attachment);
}
}//while
//process timeouts
timeout(keyCount,hasEvents);
}//while
getStopLatch().countDown();
}
run方法中调用了events方法：
public boolean events() {
boolean result = false;
PollerEvent pe = null;
for (int i = 0, size = events.size(); i < size && (pe = events.poll()) != null; i++ ) {
result = true;
try {
//将pollerEvent中的每个socketChannel注册到selector中
pe.run();
pe.reset();
if (running && !paused) {
//将注册了的pollerEvent加到endPoint.eventCache
eventCache.push(pe);
}
} catch ( Throwable x ) {
log.error("",x);
}
}
return result;
}
继续跟进PollerEvent的run方法：
public void run() {
if (interestOps == OP_REGISTER) {
try {
//将SocketChannel注册到selector中,注册时间为SelectionKey.OP_READ读事件
socket.getIOChannel().register(
socket.getPoller().getSelector(), SelectionKey.OP_READ, socketWrapper);
} catch (Exception x) {
log.error(sm.getString("endpoint.nio.registerFail"), x);
}
} else {
final SelectionKey key = socket.getIOChannel().keyFor(socket.getPoller().getSelector());
try {
if (key == null) {
// The key was cancelled (e.g. due to socket closure)
// and removed from the selector while it was being
// processed. Count down the connections at this point
// since it won't have been counted down when the socket
// closed.
socket.socketWrapper.getEndpoint().countDownConnection();
((NioSocketWrapper) socket.socketWrapper).closed = true;
} else {
final NioSocketWrapper socketWrapper = (NioSocketWrapper) key.attachment();
if (socketWrapper != null) {
//we are registering the key to start with, reset the fairness counter.
int ops = key.interestOps() | interestOps;
socketWrapper.interestOps(ops);
key.interestOps(ops);
} else {
socket.getPoller().cancelledKey(key);
}
}
} catch (CancelledKeyException ckx) {
try {
socket.getPoller().cancelledKey(key);
} catch (Exception ignore) {}
}
}
}
3、Worker线程
Worker线程即SocketProcessor是用来处理Socket请求的。SocketProcessor也同样是Endpoint的内部类。在Poller的run方法中(代码清单8)监听到准备就绪的socket时会调用processKey方法进行处理：
protected void processKey(SelectionKey sk, NioSocketWrapper attachment) {
try {
if ( close ) {
cancelledKey(sk);
} else if ( sk.isValid() && attachment != null ) {
//有读写事件就绪时
if (sk.isReadable() || sk.isWritable() ) {
if ( attachment.getSendfileData() != null ) {
processSendfile(sk,attachment, false);
} else {
unreg(sk, attachment, sk.readyOps());
boolean closeSocket = false;
// Read goes before write
// socket可读时，先处理读事件
if (sk.isReadable()) {
//调用processSocket方法进一步处理
if (!processSocket(attachment, SocketEvent.OPEN_READ, true)) {
closeSocket = true;
}
}
//写事件
if (!closeSocket && sk.isWritable()) {
//调用processSocket方法进一步处理
if (!processSocket(attachment, SocketEvent.OPEN_WRITE, true)) {
closeSocket = true;
}
}
if (closeSocket) {
cancelledKey(sk);
}
}
}
} else {
//invalid key
cancelledKey(sk);
}
} catch ( CancelledKeyException ckx ) {
cancelledKey(sk);
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
log.error("",t);
}
}
继续跟踪processSocket方法：
public boolean processSocket(SocketWrapperBase<S> socketWrapper,
SocketEvent event, boolean dispatch) {
try {
if (socketWrapper == null) {
return false;
}
// 尝试循环利用之前回收的SocketProcessor对象，如果没有可回收利用的则创建新的SocketProcessor对象
SocketProcessorBase<S> sc = processorCache.pop();
if (sc == null) {
sc = createSocketProcessor(socketWrapper, event);
} else {
// 循环利用回收的SocketProcessor对象
sc.reset(socketWrapper, event);
}
Executor executor = getExecutor();
if (dispatch && executor != null) {
//SocketProcessor实现了Runneble接口，可以直接传入execute方法进行处理
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;
} //NioEndpoint中createSocketProcessor创建一个SocketProcessor。
protected SocketProcessorBase<NioChannel> createSocketProcessor(
SocketWrapperBase<NioChannel> socketWrapper, SocketEvent event) {
return new SocketProcessor(socketWrapper, event);
}
总结：
Http11NioProtocol是基于Java Nio实现的，创建了Acceptor、Poller和Worker线程实现多路io的复用。三类线程之间的关系如下图所示：
Acceptor和Poller之间是生产者消费者模式的关系，Acceptor不断向EventQueue中添加PollerEvent，Pollor轮询检查EventQueue中就绪的PollerEvent，然后发送给Work线程进行处理。