线程池源码之execute
execute：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
//workerCountOf 获取线程数量，isRunning获取线程池状态
int c = ctl.get();
//如果小于核心线程池数量直接添加到线程池里
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//大于核心线程数量，加入队列，会做一个dobbule check,
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//如果当前线程池状态不是RUNNING则从队列删除任务，并执行拒绝策略
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//再次尝试添加线程，失败的话执行拒绝策略
else if (!addWorker(command, false))
reject(command);
}
总体来说就是：
- 如果线程的数量小于线程池的核心线程数直接创建线程执行；
- 如果加入线程池失败，说明超过核心线程数量尝试加入阻塞队列；
- 如果加入阻塞队列失败了，尝试重新加入线程池如果也失败了就说明超过了最大线程数量，执行拒绝策略。
addWorker：
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// check 线程池的状态和阻塞队列是否为空
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//这段for循环的代码含义是：往线程池里面添加一个线程
for (;;) {
int wc = workerCountOf(c);
// check 线程数量是否超出
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// 添加成功直接退出循环，添加失败的话再次尝试
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
}
}
//这段代码的含义是：把当前线程加入到hashSet里面，加入成功后启动线程，加入失败的话，从set里面删除，并且尝试停止线程池
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//把线程封装成Worker，worker是继承了Aqs的
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
int rs = runStateOf(ctl.get());
// check 线程池状态
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
//加入到set里
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
//加入成功启动线程
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
//加入失败，删除线程，尝试停止线程池
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
总体来说分为两步：
- 第一步：尝试给线程池的数量加一，失败继续尝试，成功退出。
- 第二步：线程池数量成功加一之后，采用加锁的方式把线程加入到set里，然后启动线程，如果启动失败删除线程，尝试停止线程池。
来看一下work对象：
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable{
final Thread thread;
Runnable firstTask;
volatile long completedTasks;
//构造方法
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}
protected boolean isHeldExclusively() {
return getState() != 0;
}
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
- 首先，这个work对象实现了runnable接口，就有了线程的属性。
- 其次，这个work继承了AbstractQueuedSynchronizer就有了锁的功能
- 最后，我们来看一下runWorker这个方法。
runWorker:
//运行当前线程
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 ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
//执行前操作
beforeExecute(wt, task);
Throwable thrown = null;
try {
//运行当前线程
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 {
//执行后操作
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
getTask:
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
// 主要功能就是从队列里面获取线程
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
// 获取任务
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
线程池源码之shutdown
先来搞清楚几个概念：
interrupt():为当前线程打上停止的标记
interrupted():测试当前是否中断。此方法具有清除功能
isInterrupted(): 测试线程是否中断。此方法不会清除
shutdown:停止线程池，优雅停掉，等队列里面的线程执行完再停掉。
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
//将线程池的状态设置为SHUTDOWN
advanceRunState(SHUTDOWN);
//为所有线程打上停止标记
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
}
//为所有线程打上停止标记
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
Thread t = w.thread;
if (!t.isInterrupted() && w.tryLock()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
// 尝试tryTerminate
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))) {
try {
terminated();
} finally {
ctl.set(ctlOf(TERMINATED, 0));
termination.signalAll();
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
}
线程池源码之shutdownNow
shutdownNow:比较粗暴的停掉，不会等阻塞队列执行完再停掉。
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
//把线程池状态改为STOP
advanceRunState(STOP);
interruptWorkers();
// 把队列的任务取出来
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
//把队列的任务放入到list里面返回
private List<Runnable> drainQueue() {
BlockingQueue<Runnable> q = workQueue;
ArrayList<Runnable> taskList = new ArrayList<Runnable>();
q.drainTo(taskList);
if (!q.isEmpty()) {
for (Runnable r : q.toArray(new Runnable[0])) {
if (q.remove(r))
taskList.add(r);
}
}
return taskList;
}
// 打上停止的标记
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
