ThreadPoolExecutor 源码解读
ThreadPoolExecutor 是最普通的 ExecutorService 的实现。
参数最全的构造函数如下：
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
。。省略参数检查
this.corePoolSize = corePoolSize; //核心池大小
this.maximumPoolSize = maximumPoolSize; // 最大池大小
this.workQueue = workQueue; //任务队列
this.keepAliveTime = unit.toNanos(keepAliveTime); //线程无任务时的存活时间
this.threadFactory = threadFactory; //线程的构造工厂
this.handler = handler; //RejectedExecutionHandler，任务队列满时的处理方式
}
还有几个很重要的成员变量是：
private final ReentrantLock mainLock = new ReentrantLock(); //访问线程池时，内部状态的锁 //Set containing all worker threads in pool. Accessed only when //holding mainLock. 所有的worker，每个持有一个线程，并继承自AbstractQueuedSynchronizer private final HashSet<Worker> workers = new HashSet<Worker>(); //终止时候的等待条件。 private final Condition termination = mainLock.newCondition();
下面看看worker类，是一个私有内部类：
private final class Worker extends AbstractQueuedSynchronizer implements Runnable //实现了Runnable
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) { //这个getTask方法就是检查了状态，并且从ThreadPoolExecutor的workQueue中，拿到一个Runnable的任务。
//这个方法还有另外的作用，就是如果当前池大于core则，清理当前线程。如果小于等于，则保持。
w.lock(); //这个lock很重要，说明了这个线程当前是工作的，所以不会被interruptIdleWorkers中断，因为interruptIdleWorkers调用了worker的tryLock方法，判断是否是idle线程。
//shutdownNow方法则调用interruptWorkers，中断所有worker.
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt(); //这几句话很重要，确保中断能够发送到当前线程。
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run(); //运行这个任务
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount(); // 如果是意外终止，减少线程池worker数目
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w); //移除当前的worker
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false); //如果是非正常 或则在任务队列不为空的情况下，workerCountOf(c) < min 则。。，重新启动一个worker。
}
}
下面看看ThreadPoolExecutor 的execute方法：
submit方法，就是实例化了一个FutureTask对象返回，然后调用execute方法执行这个FutureTask。
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) { //如果小于核心池，新建一个worker
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) { //如果当前线程数量超过核心池，则加入到任务队列
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false)) //如果满了，就是addWorker(command, false)表示加入非核心
reject(command); //如果addWorker失败了，就执行rejectHandle策略.
}
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary. 检查线程池没有被关闭，任务队列不为空
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY || //如果线程池超出限制，则退出
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c)) //如果应该add，添加工作线程数目，退出循环，
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
final ReentrantLock mainLock = this.mainLock;
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int c = ctl.get();
int rs = runStateOf(c);
if (rs < SHUTDOWN || //检查线程池没有关闭，或则关闭了firstTask为null
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start(); //开始新的工作线程
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
最后看看Shutdown和ShutdowNow, 这两个方法：
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess(); //检查是否有这个权限
advanceRunState(SHUTDOWN); //设置状态为SHUTDOWN
interruptIdleWorkers(); //给idle的work发送中断
onShutdown(); // hook for ScheduledThreadPoolExecutor 关闭钩子调用
} finally {
mainLock.unlock();
}
tryTerminate();
}
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(STOP); //设置状态为STOP
interruptWorkers(); //给所有线程发送中断，正在工作的线程也会收到这个中断。
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
之后看看这篇博客，写的很好。
http://my.oschina.net/xionghui/blog/494698