Tomcat使用线程池处理远程并发请求的方法

sun.misc.Unsafe
java.util.concurrent.ThreadPoolExecutor
java.util.concurrent.ThreadPoolExecutor.Worker
java.util.concurrent.locks.AbstractQueuedSynchronizer
java.util.concurrent.locks.AbstractQueuedLongSynchronizer
java.util.concurrent.LinkedBlockingQueue

org.apache.tomcat.util.net.NioEndpoint
org.apache.tomcat.util.threads.ThreadPoolExecutor
org.apache.tomcat.util.threads.TaskThreadFactory
org.apache.tomcat.util.threads.TaskQueue

public ThreadPoolExecutor(
 int corePoolSize,
 int maximumPoolSize,
 long keepAliveTime,
 TimeUnit unit,
 BlockingQueue<Runnable> workQueue,
 ThreadFactory threadFactory,
 RejectedExecutionHandler handler) {
 
}

public void createExecutor() {
 internalExecutor = true;
 TaskQueue taskqueue = new TaskQueue();
 //TaskQueue无界队列，可以一直添加，因此handler 等同于无效
 TaskThreadFactory tf = new TaskThreadFactory(getName() + "-exec-", daemon, getThreadPriority());
 executor = new ThreadPoolExecutor(getMinSpareThreads(), getMaxThreads(), 60, TimeUnit.SECONDS,taskqueue, tf);
 taskqueue.setParent( (ThreadPoolExecutor) executor);
 }

public int prestartAllCoreThreads() {
 int n = 0;
 while (addWorker(null, true))
  ++n;
 return n;
 }

public void execute(Runnable command) {
 if (command == null)
  throw new NullPointerException();
 
 int c = ctl.get();
 	// worker数 是否小于 核心线程数 tomcat中初始化后，一般不满足第一个条件，不会addWorker
 if (workerCountOf(c) < corePoolSize) {
  if (addWorker(command, true))
  return;
  c = ctl.get();
 }
 	// workQueue.offer(command)，将任务添加到队列，
 if (isRunning(c) && workQueue.offer(command)) {
  int recheck = ctl.get();
  if (! isRunning(recheck) && remove(command))
  reject(command);
  else if (workerCountOf(recheck) == 0)
  addWorker(null, false);
 }
 else if (!addWorker(command, false))
  reject(command);
 }

public boolean offer(E e) {
 if (e == null) throw new NullPointerException();
 final AtomicInteger count = this.count;
 if (count.get() == capacity)
 return false;
 int c = -1;
 Node<E> node = new Node<E>(e);
 final ReentrantLock putLock = this.putLock;
 putLock.lock();
 try {
 if (count.get() < capacity) {
  enqueue(node); //此处将任务添加到队列
  c = count.getAndIncrement();
  if (c + 1 < capacity)
  notFull.signal();
 }
 } finally {
 putLock.unlock();
 }
 if (c == 0)
 signalNotEmpty();
 return c >= 0;
}

// 添加任务到队列
/**
 * Links node at end of queue.
 *
 * @param node the node
 */
private void enqueue(Node<E> node) {
 // assert putLock.isHeldByCurrentThread();
 // assert last.next == null;
 last = last.next = node; //链表结构 last.next = node; last = node
}

/** Delegates main run loop to outer runWorker */
 public void run() {
  runWorker(this);
 }

final void runWorker(Worker w) {
 Thread wt = Thread.currentThread();
 Runnable task = w.firstTask;
 w.firstTask = null;
 w.unlock(); // allow interrupts
 boolean completedAbruptly = true;
 try {
  while (task != null || (task = getTask()) != null) { //循环获取队列中的任务
  w.lock(); // 上锁
  try {
   // 运行前处理
   beforeExecute(wt, task);
   // 队列中的任务开始执行
   task.run();
   // 运行后处理
   afterExecute(task, thrown);
  } finally {
   task = null;
   w.completedTasks++;
   w.unlock(); // 释放锁
  }
  }
  completedAbruptly = false;
 } finally {
  processWorkerExit(w, completedAbruptly);
 }
 }

public boolean offer(E e) {
 if (e == null) throw new NullPointerException();
 final AtomicInteger count = this.count;
 if (count.get() == capacity)
  return false;
 int c = -1;
 Node<E> node = new Node<E>(e);
 final ReentrantLock putLock = this.putLock;
 putLock.lock(); //上锁
 try {
  if (count.get() < capacity) {
  enqueue(node);
  c = count.getAndIncrement();
  if (c + 1 < capacity)
   notFull.signal();
  }
 } finally {
  putLock.unlock(); //释放锁
 }
 if (c == 0)
  signalNotEmpty();
 return c >= 0;
 }

private Runnable getTask() {
 boolean timedOut = false; // Did the last poll() time out"htmlcode">

protected final boolean compareAndSetState(int expect, int update) {
 // See below for intrinsics setup to support this
 return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
 }


对应Unsafe类的代码:


//对应的java底层，实际是native方法，对应C++代码
/**
* Atomically update Java variable to <tt>x</tt> if it is currently
* holding <tt>expected</tt>.
* @return <tt>true</tt> if successful
*/
public final native boolean compareAndSwapInt(Object o, long offset,
      int expected,
      int x);


方法的作用简单来说就是 更新一个值，保证原子性操作

当你要操作一个对象o的一个成员变量offset时,修改o.offset，

高并发下为保证准确性，你在操作o.offset的时候，读应该是正确的值，并且中间不能被别的线程修改来保证高并发的环境数据操作有效。
即 expected 期望值与内存中的值比较是一样的expected == 内存中的值 ，则更新值为 x，返回true代表修改成功
否则，期望值与内存值不同，说明值被其他线程修改过，不能更新值为x，并返回false，告诉操作者此次原子性修改失败。
阻塞和唤醒线程
public native void park(boolean isAbsolute, long time); //阻塞当前线程
线程池的worker角色循环获取队列任务，如果队列中没有任务，worker.run 还是在等待的，不会退出线程，代码中用了notEmpty.await() 中断此worker线程，放入一个等待线程队列（区别去任务队列）；当有新任务需要时，再notEmpty.signal()唤醒此线程
底层分别是

unsafe.park() 阻塞当前线程

public native void park(boolean isAbsolute, long time);
unsafe.unpark() 唤醒线程

public native void unpark(Object thread);
这个操作是对应的，阻塞时，先将thread放入队列，唤醒时，从队列拿出被阻塞的线程，unsafe.unpark(thread)唤醒指定线程。
java.util.concurrent.locks.AbstractQueuedLongSynchronizer.ConditionObject 类中
通过链表存放线程信息


// 添加一个阻塞线程
private Node addConditionWaiter() {
  Node t = lastWaiter;
  // If lastWaiter is cancelled, clean out.
  if (t != null && t.waitStatus != Node.CONDITION) {
  unlinkCancelledWaiters();
  t = lastWaiter;
  }
  Node node = new Node(Thread.currentThread(), Node.CONDITION);
  if (t == null)
  firstWaiter = node;
  else
  t.nextWaiter = node;
  lastWaiter = node; //将新阻塞的线程放到链表尾部
  return node;
 }

// 拿出一个被阻塞的线程
 public final void signal() {
  if (!isHeldExclusively())
  throw new IllegalMonitorStateException();
  Node first = firstWaiter; //链表中第一个阻塞的线程
  if (first != null)
  doSignal(first);
 }

// 拿到后，唤醒此线程
final boolean transferForSignal(Node node) {
  LockSupport.unpark(node.thread);
 return true;
 }
public static void unpark(Thread thread) {
 if (thread != null)
  UNSAFE.unpark(thread);
 }