1.ThreadPoolExecutor.execute
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);
}
這里主要做兩件事：
- 小于corePoolSize的時(shí)候創(chuàng)建核心線程
- 當(dāng)前核心線程都正在執(zhí)行則入隊(duì)，判斷是否需要?jiǎng)?chuàng)建工作線程
2.ThreadPoolExecutor.addWorker
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
// 判斷是否可以創(chuàng)建worker
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
}
}
// 創(chuàng)建Worker并啟動(dòng)
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;
}
這里采用顯示鎖的方式實(shí)現(xiàn)同步創(chuàng)建Worker
3.Worker的run()
Worker其實(shí)也是一個(gè)Runnable的子類，那么從執(zhí)行到添加Worker的過程都沒有線程復(fù)用的邏輯，那么該邏輯就應(yīng)該是在Worker的run方法中。
在這里可以看到，Worker的runWorker會(huì)調(diào)用ThreadPoolExecutor的runWorker方法。
而在runWorker方法中，會(huì)采用一個(gè)while循環(huán)從task任務(wù)隊(duì)列中取出待執(zhí)行的任務(wù)，如果任務(wù)是沒有被中斷的，則會(huì)調(diào)用task.run()方法執(zhí)行任務(wù)。
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);
}
}
getTask()的內(nèi)部是采用一個(gè)無限for循環(huán)的方式進(jìn)行循環(huán)遍歷
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
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);
// 判斷是否允許核心線程超時(shí)或者當(dāng)前工作線程是否大于核心線程數(shù)
// 如果為true
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
// 如果存在的線程數(shù)大于核心線程數(shù)，則從隊(duì)列中取任務(wù)時(shí)沒有任務(wù)返回null
// 如果存在的線程數(shù)小于核心線程數(shù)，則從隊(duì)列中取任務(wù)時(shí)沒有任務(wù)將阻塞隊(duì)列
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
而工作線程的關(guān)閉，其實(shí)可以從上面兩種情況知道，在線程通過while無限循環(huán)的時(shí)候，如果是非核心線程（即工作線程）調(diào)用BlockingQueue的poll方法，則會(huì)在隊(duì)列為空的時(shí)候返回null而不會(huì)阻塞隊(duì)列，當(dāng)返回的task是null的時(shí)候，那么runWorker中的while循環(huán)就會(huì)退出，最終就會(huì)執(zhí)行processWorkerExit(w, completedAbruptly)而completedAbruptly=false進(jìn)行線程的刪除；如果是核心線程的話，則會(huì)調(diào)用BlockingQueue的take()方法，那么在隊(duì)列為空的時(shí)候就會(huì)通過take()方法調(diào)用而阻塞等待隊(duì)列結(jié)果的返回，而不會(huì)返回一個(gè)task=null，這樣runWorker中的while循環(huán)就不會(huì)退出，就會(huì)一直等待結(jié)果。
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
所以工作線程會(huì)被關(guān)閉，而核心線程不會(huì)被關(guān)閉，就是因?yàn)楣ぷ骶€程的run()方法會(huì)通過task=null退出循環(huán)而執(zhí)行完成run()的方法體內(nèi)容，從而自動(dòng)關(guān)閉；而核心線程因?yàn)樽枞却?duì)列返回的原因而不會(huì)導(dǎo)致while循環(huán)的結(jié)束，所以不會(huì)關(guān)閉。
BlockingQueue的方法都是成對(duì)出現(xiàn)：
add和remove：這兩個(gè)是非阻塞的，當(dāng)隊(duì)列滿的時(shí)候，add會(huì)拋出異常，當(dāng)隊(duì)列為空的時(shí)候，remove會(huì)拋出異常。
offer和poll：使用offer往滿的隊(duì)列里放入元素，會(huì)返回false；poll方法往空的隊(duì)列里拿元素，會(huì)返回一個(gè)null
put和take：這是真正的阻塞方法，使用put往滿的隊(duì)列里放元素，會(huì)被阻塞；使用take往空的隊(duì)列里拿方法，會(huì)被阻塞。
