java Executors源码简析
package a160229;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ExecutorsTest {
public static void main(String[] args) {
ExecutorService threadPool = Executors.newFixedThreadPool(5);
for(int i=1; i<=10; i++) {
threadPool.execute(new Task(i));
}
threadPool.shutdown();
while(!threadPool.isTerminated());
System.out.println("all over!");
}
}
class Task implements Runnable {
int id;
Task(int id) {
this.id = id;
}
@Override
public void run() {
try {
Thread.sleep(new Random().nextInt(10)*1000);
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
//e1.printStackTrace();
System.out.println("task" + id + "is interrupted");
}
System.out.println("task" + id + " is running ");
try {
Thread.sleep(new Random().nextInt(10)*1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
//e.printStackTrace();
System.out.println("task" + id + "is interrupted");
让我们来以这段代码为例简单分析Executor源码。
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue());
}
newFixedThreadPool 是创建一个固定大小的线程池，固定线程数为nThreads，如果当前线程数不足以满足当前任务数，未被线程受理的任务会进入一个队列（这里是LinkedBlokingQueue，下一篇文章分析这个队列）。
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
构造函数中需要说明的形参，corePoolSize是在未设置allowCoreThreadTimeOut的情况下，池中所维持的线程数，即便其中线程是空闲状态；maximumPoolSize是池所允许的最大线程数。
如此，一个线程池就建好了，当然此时还没有线程。
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
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);
}
让我们来看ctl是什么玩意
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
private static int More ...runStateOf(int c) { return c & ~CAPACITY; }
private static int More ...workerCountOf(int c) { return c & CAPACITY; }
private static int More ...ctlOf(int rs, int wc) { return rs | wc; }
Ctl就是control，是一个32bit二进制数（一个int），高3位代表了线程池的状态（RUNNING等），低29位表示最大线程数（非用户设置数，当用户设置数大于此数时，用户设置数并不会被重置，而是以此数为标准）。
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
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 {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(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();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
addWorker就是要向池中添加线程了，其中会有池状态检查（主要包括线程池是否处于要关闭状态、池中线程数是否到达阈值等）。
workers.add(w)就是向池（workers是一个hashset）中放入工作线程，而后t.start()便启动了工作线程，这里t（thread）和w（worker）之间的数据结构需要关注一下。
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
private static final long serialVersionUID = 6138294804551838833L;
final Thread thread;
Runnable firstTask;
volatile long completedTasks;
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
。。。。
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) {
w.lock();
// 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();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} 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);
}
}
}
可以看出runWorker才是工作线程真正的执行代码，其中task.run就是我们定义的任务的执行，执行过程中同样会检查线程池状态，如下。
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
当线程池处于STOP状态时，线程会被标记打断，但当前线程会继续执行，也即当前任务会继续执行完。while (task != null || (task = getTask()) != null)中，getTask时如果状态为STOP，即使队列中有任务，其也会返回null，这就会使得runWorker 方法中的for循环被终止，从而执行后面的processWorkerExit
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} 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);
}
}
这里的关键方法就是workers.remove，此后相应worker已被移除workers（hashset）。
final void tryTerminate() {
for (;;) {
int c = ctl.get();
if (isRunning(c) ||
runStateAtLeast(c, TIDYING) ||
(runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
return;
if (workerCountOf(c) != 0) { // Eligible to terminate
interruptIdleWorkers(ONLY_ONE);
return;
}
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {//设置TIDYING
try {
terminated();
} finally {
ctl.set(ctlOf(TERMINATED, 0));//执行完terminated后，池变为TERMINATED状态
termination.signalAll();//Wakes up all waiting threads.
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
}
tryTerminate();会试图STOP-> TERMINATED
最后看一下shutdown方法
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(SHUTDOWN);//池状态变为SHUTDOWN
interruptIdleWorkers();//将所有worker标记打断
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
}
while(!threadPool.isTerminated())，isTerminated就是检查线程池状态（ctl的值）是否为TERMINATED