本文使用Spring Boot提供的嵌入式Tomcat为例来分析Tomcat的启动过程、线程池管理及请求过程。
从org.apache.tomcat.util.net.NioEndpoint#startInternal开始说起：
@Override
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();
}
initializeConnectionLatch();
// Start poller threads
pollers = new Poller[getPollerThreadCount()];
for (int i=0; i这里我们关注下创建Executor的代码，通过条件判断是否存在Executor，不存在则执行createExecutor()方法。
public void createExecutor() {
internalExecutor = true;
TaskQueue taskqueue = new TaskQueue();
TaskThreadFactory tf = new TaskThreadFactory(getName() + "-exec-", daemon, getThreadPriority());
executor = new ThreadPoolExecutor(getMinSpareThreads(), getMaxThreads(), 60, TimeUnit.SECONDS,taskqueue, tf);
taskqueue.setParent( (ThreadPoolExecutor) executor);
}
TaskThreadFactory是一个ThreadFactory的实现，里面定义了工作线程的线程名前缀和线程优先级，然后实例化ThreadPoolExecutor，这里的ThreadPoolExecutor其实是Tomcat自己的实现，全称为org.apache.tomcat.util.threads.ThreadPoolExecutor，其继承自juc的java.util.concurrent.ThreadPoolExecutor，我们可以理解为tomcat的ThreadPoolExecutor对juc提供的ThreadPoolExecutor进行了一层包装，最终返回一个Executor实例，这里创建Executor的工作就结束了。
值得注意的是在new ThreadPoolExecutor的时候就会创建核心线程，因为这个是通过TaskThreadFactory来创建线程的，所以我们可以稍微关注下它里面创建线程的方法：
@Override
public Thread newThread(Runnable r) {
TaskThread t = new TaskThread(group, r, namePrefix + threadNumber.getAndIncrement());
t.setDaemon(daemon);
t.setPriority(threadPriority);
// Set the context class loader of newly created threads to be the class
// loader that loaded this factory. This avoids retaining references to
// web application class loaders and similar.
if (Constants.IS_SECURITY_ENABLED) {
PrivilegedAction pa = new PrivilegedSetTccl(
t, getClass().getClassLoader());
AccessController.doPrivileged(pa);
} else {
t.setContextClassLoader(getClass().getClassLoader());
}
return t;
}
可以看到这里已经指定了线程的名称，形如http-nio-8080-exec-3。
回到startInternal方法，接着便是创建pollers并启动（线程），pollers的大小默认为2和处理器核数的最小值，创建Poller的代码如下：
public Poller() throws IOException {
this.selector = Selector.open();
}
这里说明下Poller实现了Runnable接口，且是NioEndpoint的一个内部类。
/**
* Poller class.
*/
public class Poller implements Runnable {
...
}
startInternal方法的介绍就告一段落，接着来看看Poller的run方法：
@Override
public void run() {
// Loop until destroy() is called
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
keyCount = selector.selectNow();
} else {
keyCount = selector.select(selectorTimeout);
}
wakeupCounter.set(0);
}
if (close) {
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 iterator =
keyCount > 0 ? selector.selectedKeys().iterator() : null;
// Walk through the collection of ready keys and dispatch
// any active event.
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();
processKey(sk, attachment);
}
}//while
//process timeouts
timeout(keyCount,hasEvents);
}//while
getStopLatch().countDown();
}
主要的逻辑是通过while循环去遍历SelectionKey，如果有请求过来，则会交给processKey方法处理。
protected void processKey(SelectionKey sk, NioSocketWrapper attachment) {
try {
if ( close ) {
cancelledKey(sk);
} else if ( sk.isValid() && attachment != null ) {
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
if (sk.isReadable()) {
if (!processSocket(attachment, SocketEvent.OPEN_READ, true)) {
closeSocket = true;
}
}
if (!closeSocket && sk.isWritable()) {
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);
}
}
这里对nio的SelectionKey实例进行判断，发现sk.isReadable()为true，最后交给processSocket方法。
public boolean processSocket(SocketWrapperBase socketWrapper,
SocketEvent event, boolean dispatch) {
try {
if (socketWrapper == null) {
return false;
}
SocketProcessorBase sc = processorCache.pop();
if (sc == null) {
sc = createSocketProcessor(socketWrapper, event);
} else {
sc.reset(socketWrapper, event);
}
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;
}
这里通过getExecutor方法获取之前创建的Executor实例，然后执行execute方法提交请求任务。至此，一个完整的请求便会经过Tomcat到Servlet，并最终进入对应的handler对请求进行业务处理。
以上。