幾句閒扯：首先，我想說java的線程池真的是很繞，以前一直都感覺新建幾個線程一直不退出到底是怎麼實現的，也就有了後來學習ThreadPoolExecutor源碼。學習源碼的過程中，最噁心的其實就是幾種狀態的轉換了，這也是ThreadPoolExecutor的核心。花了將近小一週才大致的弄明白ThreadPoolExecutor的機制，遂記錄下來。
線程池有多重要
線程是一個程序員一定會涉及到的一個概念，但是線程的創建和切換都是代價比較大的。所以，我們有沒有一個好的方案能做到線程的複用呢？這就涉及到一個概念——線程池。合理的使用線程池能夠帶來3個很明顯的好處：
- 降低資源消耗：通過重用已經創建的線程來降低線程創建和銷燬的消耗
- 提高響應速度：任務到達時不需要等待線程創建就可以立即執行。
- 提高線程的可管理性：線程池可以統一管理、分配、調優和監控。
java多線程池的支持——ThreadPoolExecutor
java的線程池支持主要通過ThreadPoolExecutor來實現，我們使用的ExecutorService的各種線程池策略都是基於ThreadPoolExecutor實現的，所以ThreadPoolExecutor十分重要。要弄明白各種線程池策略，必須先弄明白ThreadPoolExecutor。
1、實現原理
首先看一個線程池的流程圖：
- step1.調用ThreadPoolExecutor的execute提交線程，首先檢查CorePool，如果CorePool內的線程小於CorePoolSize，新創建線程執行任務。
- step2.如果當前CorePool內的線程大於等於CorePoolSize，那麼將線程加入到BlockingQueue。
- step3.如果不能加入BlockingQueue，在小於MaxPoolSize的情況下創建線程執行任務。
- step4.如果線程數大於等於MaxPoolSize，那麼執行拒絕策略。
2、線程池的創建
線程池的創建可以通過ThreadPoolExecutor的構造方法實現：
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> 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;
}具體解釋一下上述參數：
- corePoolSize 核心線程池大小
- maximumPoolSize 線程池最大容量大小
- keepAliveTime 線程池空閒時，線程存活的時間
- TimeUnit 時間單位
- ThreadFactory 線程工廠
- BlockingQueue任務隊列
- RejectedExecutionHandler 線程拒絕策略
3、線程的提交
ThreadPoolExecutor的構造方法如上所示，但是隻是做一些參數的初始化，ThreadPoolExecutor被初始化好之後便可以提交線程任務，線程的提交方法主要是execute和submit。這裏主要說execute，submit會在後續的博文中分析。
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
* {@code RejectedExecutionHandler}, if the task
* cannot be accepted for execution
* @throws NullPointerException if {@code command} is null
*/
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.
* 如果當前的線程數小於核心線程池的大小，根據現有的線程作爲第一個Worker運行的線程，
* 新建一個Worker，addWorker自動的檢查當前線程池的狀態和Worker的數量，
* 防止線程池在不能添加線程的狀態下添加線程
*
* 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.
* 如果線程入隊成功，然後還是要進行double-check的，因爲線程池在入隊之後狀態是可能會發生變化的
*
* 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.
*
* 如果task不能入隊(隊列滿了)，這時候嘗試增加一個新線程，如果增加失敗那麼當前的線程池狀態變化了或者線程池已經滿了
* 然後拒絕task
*/
int c = ctl.get();
//當前的Worker的數量小於核心線程池大小時，新建一個Worker。
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))//recheck防止線程池狀態的突變，如果突變，那麼將reject線程，防止workQueue中增加新線程
reject(command);
else if (workerCountOf(recheck) == 0)//上下兩個操作都有addWorker的操作，但是如果在workQueue.offer的時候Worker變爲0，
//那麼將沒有Worker執行新的task，所以增加一個Worker.
addWorker(null, false);
}
//如果workQueue滿了，那麼這時候可能還沒到線程池的maxnum，所以嘗試增加一個Worker
else if (!addWorker(command, false))
reject(command);//如果Worker數量到達上限，那麼就拒絕此線程
}這裏需要明確幾個概念：
- Worker和Task的區別，Worker是當前線程池中的線程，而task雖然是runnable，但是並沒有真正執行，只是被Worker調用了run方法，後面會看到這部分的實現。
- maximumPoolSize和corePoolSize的區別：這個概念很重要，maximumPoolSize爲線程池最大容量，也就是說線程池最多能起多少Worker。corePoolSize是核心線程池的大小，當corePoolSize滿了時，同時workQueue full（ArrayBolckQueue是可能滿的） 那麼此時允許新建Worker去處理workQueue中的Task，但是不能超過maximumPoolSize。超過corePoolSize之外的線程會在空閒超時後終止。
核心方法：addWorker
Worker的增加和Task的獲取以及終止都是在此方法中實現的，也就是這一個方法裏面包含了很多東西。在addWorker方法中提到了Status的概念，Status是線程池的核心概念，這裏我們先看一段關於status的註釋：
/**
* 首先ctl是一個原子量，同時它裏面包含了兩個field，一個是workerCount，另一個是runState
* workerCount表示當前有效的線程數，也就是Worker的數量
* runState表示當前線程池的狀態
* The main pool control state, ctl, is an atomic integer packing
* two conceptual fields
* workerCount, indicating the effective number of threads
* runState, indicating whether running, shutting down etc
*
* 兩者是怎麼結合的呢？首先workerCount是佔據着一個atomic integer的後29位的，而狀態佔據了前3位
* 所以，workerCount上限是(2^29)-1。
* In order to pack them into one int, we limit workerCount to
* (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
* billion) otherwise representable. If this is ever an issue in
* the future, the variable can be changed to be an AtomicLong,
* and the shift/mask constants below adjusted. But until the need
* arises, this code is a bit faster and simpler using an int.
*
* The workerCount is the number of workers that have been
* permitted to start and not permitted to stop. The value may be
* transiently different from the actual number of live threads,
* for example when a ThreadFactory fails to create a thread when
* asked, and when exiting threads are still performing
* bookkeeping before terminating. The user-visible pool size is
* reported as the current size of the workers set.
*
* runState是整個線程池的運行生命週期，有如下取值：
* 1. RUNNING：可以新加線程，同時可以處理queue中的線程。
* 2. SHUTDOWN：不增加新線程，但是處理queue中的線程。
* 3.STOP 不增加新線程，同時不處理queue中的線程。
* 4.TIDYING 所有的線程都終止了（queue中），同時workerCount爲0，那麼此時進入TIDYING
* 5.terminated()方法結束，變爲TERMINATED
* The runState provides the main lifecyle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
* 狀態的轉化主要是：
* RUNNING -> SHUTDOWN（調用shutdown()）
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP(調用shutdownNow())
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING（queue和pool均empty）
* When both queue and pool are empty
* STOP -> TIDYING（pool empty，此時queue已經爲empty）
* When pool is empty
* TIDYING -> TERMINATED(調用terminated())
* When the terminated() hook method has completed
*
* Threads waiting in awaitTermination() will return when the
* state reaches TERMINATED.
*
* Detecting the transition from SHUTDOWN to TIDYING is less
* straightforward than you'd like because the queue may become
* empty after non-empty and vice versa during SHUTDOWN state, but
* we can only terminate if, after seeing that it is empty, we see
* that workerCount is 0 (which sometimes entails a recheck -- see
* below).
*/下面是狀態的代碼：
//利用ctl來保證當前線程池的狀態和當前的線程的數量。ps：低29位爲線程池容量，高3位爲線程狀態。
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
//設定偏移量
private static final int COUNT_BITS = Integer.SIZE - 3;
//確定最大的容量2^29-1
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
//幾個狀態，用Integer的高三位表示
// runState is stored in the high-order bits
//111
private static final int RUNNING = -1 << COUNT_BITS;
//000
private static final int SHUTDOWN = 0 << COUNT_BITS;
//001
private static final int STOP = 1 << COUNT_BITS;
//010
private static final int TIDYING = 2 << COUNT_BITS;
//011
private static final int TERMINATED = 3 << COUNT_BITS;
//獲取線程池狀態，取前三位
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
//獲取當前正在工作的worker,主要是取後面29位
private static int workerCountOf(int c) { return c & CAPACITY; }
//獲取ctl
private static int ctlOf(int rs, int wc) { return rs | wc; }接下來貼上addWorker方法看看：
/**
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked, which requires a
* backout of workerCount, and a recheck for termination, in case
* the existence of this worker was holding up termination.
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
/**
* rs!=Shutdown || fistTask！=null || workCount.isEmpty
* 如果當前的線程池的狀態>SHUTDOWN 那麼拒絕Worker的add 如果=SHUTDOWN
* 那麼此時不能新加入不爲null的Task，如果在WorkCount爲empty的時候不能加入任何類型的Worker，
* 如果不爲empty可以加入task爲null的Worker,增加消費的Worker
*/
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
}
}
Worker w = new Worker(firstTask);
Thread t = w.thread;
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 c = ctl.get();
int rs = runStateOf(c);
/**
* rs!=SHUTDOWN ||firstTask!=null
*
* 同樣檢測當rs>SHUTDOWN時直接拒絕減小Wc，同時Terminate，如果爲SHUTDOWN同時firstTask不爲null的時候也要Terminate
*/
if (t == null ||
(rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null))) {
decrementWorkerCount();
tryTerminate();
return false;
}
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
} finally {
mainLock.unlock();
}
t.start();
// It is possible (but unlikely) for a thread to have been
// added to workers, but not yet started, during transition to
// STOP, which could result in a rare missed interrupt,
// because Thread.interrupt is not guaranteed to have any effect
// on a non-yet-started Thread (see Thread#interrupt).
//Stop或線程Interrupt的時候要中止所有的運行的Worker
if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
t.interrupt();
return true;
}addWorker中首先進行了一次線程池狀態的檢測：
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//判斷當前線程池的狀態是不是已經shutdown，如果shutdown了拒絕線程加入
//(rs!=SHUTDOWN || first!=null || workQueue.isEmpty())
//如果rs不爲SHUTDOWN，此時狀態是STOP、TIDYING或TERMINATED，所以此時要拒絕請求
//如果此時狀態爲SHUTDOWN，而傳入一個不爲null的線程，那麼需要拒絕
//如果狀態爲SHUTDOWN，同時隊列中已經沒任務了，那麼拒絕掉
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;其實是比較難懂的，主要在線程池狀態判斷條件這裏：
- 如果是runing，那麼跳過if。
- 如果rs>=SHUTDOWN,同時不等於SHUTDOWN，即爲SHUTDOWN以上的狀態，那麼不接受新線程。
- 如果rs>=SHUTDOWN，同時等於SHUTDOWN，同時first！=null，那麼拒絕新線程，如果first==null，那麼可能是新增加線程消耗Queue中的線程。但是同時還要檢測workQueue是否isEmpty()，如果爲Empty，那麼隊列已空，不需要增加消耗線程，如果隊列沒有空那麼運行增加first=null的Worker。
從這裏是可以看出一些策略的
- 首先，在rs>SHUTDOWN時，拒絕一切線程的增加，因爲STOP是會終止所有的線程，同時移除Queue中所有的待執行的線程的，所以也不需要增加first=null的Worker了
- 其次，在SHUTDOWN狀態時，是不能增加first！=null的Worker的，同時即使first=null，但是此時Queue爲Empty也是不允許增加Worker的，SHUTDOWN下增加的Worker主要用於消耗Queue中的任務。
SHUTDOWN狀態時，是不允許向workQueue中增加線程的，isRunning(c) && workQueue.offer(command) 每次在offer之前都要做狀態檢測，也就是線程池狀態變爲>=SHUTDOWN時不允許新線程進入線程池了。
for (;;) {
int wc = workerCountOf(c);
//如果當前的數量超過了CAPACITY，或者超過了corePoolSize和maximumPoolSize（試core而定）
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//CAS嘗試增加線程數，如果失敗，證明有競爭，那麼重新到retry。
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
//判斷當前線程池的運行狀態
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}這段代碼做了一個兼容，主要是沒有到corePoolSize 或maximumPoolSize上限時，那麼允許添加線程，CAS增加Worker的數量後，跳出循環。
接下來實例化Worker,實例化Worker其實是很關鍵的，後面會說。
因爲workers是HashSet線程不安全的，那麼此時需要加鎖，所以mainLock.lock(); 之後重新檢查線程池的狀態，如果狀態不正確，那麼減小Worker的數量，爲什麼tryTerminate（）目前不大清楚。如果狀態正常，那麼添加Worker到workers。最後：
if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
t.interrupt();註釋說的很清楚，爲了能及時的中斷此Worker，因爲線程存在未Start的情況，此時是不能響應中斷的，如果此時status變爲STOP，則不能中斷線程。此處用作中斷線程之用。
接下來我們看Worker的方法：
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}這裏可以看出Worker是對firstTask的包裝，並且Worker本身就是Runnable的，看上去真心很煩。
通過ThreadFactory爲Worker自己構建一個線程。
因爲Worker是Runnable類型的，所以是有run方法的,上面也看到了會調用t.start() 其實就是執行了run方法：
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}調用了runWorker:
/**
* Main worker run loop. Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
* 1 Worker可能還是執行一個初始化的task——firstTask。
* 但是有時也不需要這個初始化的task（可以爲null）,只要pool在運行，就會
* 通過getTask從隊列中獲取Task，如果返回null，那麼worker退出。
* 另一種就是external拋出異常導致worker退出。
* 1. We may start out with an initial task, in which case we
* don't need to get the first one. Otherwise, as long as pool is
* running, we get tasks from getTask. If it returns null then the
* worker exits due to changed pool state or configuration
* parameters. Other exits result from exception throws in
* external code, in which case completedAbruptly holds, which
* usually leads processWorkerExit to replace this thread.
*
*
* 2 在運行任何task之前，都需要對worker加鎖來防止other pool中斷worker。
* clearInterruptsForTaskRun保證除了線程池stop，那麼現場都沒有中斷標誌
* 2. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and
* clearInterruptsForTaskRun called to ensure that unless pool is
* stopping, this thread does not have its interrupt set.
*
* 3. Each task run is preceded by a call to beforeExecute, which
* might throw an exception, in which case we cause thread to die
* (breaking loop with completedAbruptly true) without processing
* the task.
*
* 4. Assuming beforeExecute completes normally, we run the task,
* gathering any of its thrown exceptions to send to
* afterExecute. We separately handle RuntimeException, Error
* (both of which the specs guarantee that we trap) and arbitrary
* Throwables. Because we cannot rethrow Throwables within
* Runnable.run, we wrap them within Errors on the way out (to the
* thread's UncaughtExceptionHandler). Any thrown exception also
* conservatively causes thread to die.
*
* 5. After task.run completes, we call afterExecute, which may
* also throw an exception, which will also cause thread to
* die. According to JLS Sec 14.20, this exception is the one that
* will be in effect even if task.run throws.
*
* The net effect of the exception mechanics is that afterExecute
* and the thread's UncaughtExceptionHandler have as accurate
* information as we can provide about any problems encountered by
* user code.
*
* @param w the worker
*/
final void runWorker(Worker w) {
Runnable task = w.firstTask;
w.firstTask = null;
//標識線程是不是異常終止的
boolean completedAbruptly = true;
try {
//task不爲null情況是初始化worker時，如果task爲null，則去隊列中取線程--->getTask()
while (task != null || (task = getTask()) != null) {
w.lock();
//獲取woker的鎖，防止線程被其他線程中斷
clearInterruptsForTaskRun();//清楚所有中斷標記
try {
beforeExecute(w.thread, task);//線程開始執行之前執行此方法，可以實現Worker未執行退出，本類中未實現
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);//線程執行後執行，可以實現標識Worker異常中斷的功能，本類中未實現
}
} finally {
task = null;//運行過的task標null
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
//處理worker退出的邏輯
processWorkerExit(w, completedAbruptly);
}
}從上面代碼可以看出，execute的Task是被“包裝 ”了一層，線程啓動時是內部調用了Task的run方法。
接下來所有的核心集中在getTask()方法上：
/**
* Performs blocking or timed wait for a task, depending on
* current configuration settings, or returns null if this worker
* must exit because of any of:
* 1. There are more than maximumPoolSize workers (due to
* a call to setMaximumPoolSize).
* 2. The pool is stopped.
* 3. The pool is shutdown and the queue is empty.
* 4. This worker timed out waiting for a task, and timed-out
* workers are subject to termination (that is,
* {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
* both before and after the timed wait.
*
* @return task, or null if the worker must exit, in which case
* workerCount is decremented
*
*
* 隊列中獲取線程
*/
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//當前狀態爲>stop時，不處理workQueue中的任務，同時減小worker的數量所以返回null，如果爲shutdown 同時workQueue已經empty了，同樣減小worker數量並返回null
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
boolean timed; // Are workers subject to culling?
for (;;) {
int wc = workerCountOf(c);
timed = allowCoreThreadTimeOut || wc > corePoolSize;
if (wc <= maximumPoolSize && ! (timedOut && timed))
break;
if (compareAndDecrementWorkerCount(c))
return null;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}這段代碼十分關鍵，首先看幾個局部變量：
boolean timedOut = false;
主要是判斷後面的poll是否要超時
boolean timed;
主要是標識着當前Worker超時是否要退出。wc > corePoolSize時需要減小空閒的Worker數，那麼timed爲true，但是wc <= corePoolSize時，不能減小核心線程數timed爲false。
timedOut初始爲false，如果timed爲true那麼使用poll取線程。如果正常返回，那麼返回取到的task。如果超時，證明worker空閒，同時worker超過了corePoolSize，需要刪除。返回r=null。則 timedOut = true。此時循環到wc <= maximumPoolSize && ! (timedOut && timed)時，減小worker數，並返回null，導致worker退出。如果線程數<= corePoolSize，那麼此時調用 workQueue.take()，沒有線程獲取到時將一直阻塞，知道獲取到線程或者中斷，關於中斷後面Shutdown的時候會說。
至此線程執行過程就分析完了
關於終止線程池
我個人認爲，如果想了解明白線程池，那麼就一定要理解好各個狀態之間的轉換，想理解轉換，線程池的終止機制是很好的一個途徑。對於關閉線程池主要有兩個方法shutdown()和shutdownNow():
首先從shutdown()方法開始：
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
*
* <p>This method does not wait for previously submitted tasks to
* complete execution. Use {@link #awaitTermination awaitTermination}
* to do that.
*
* @throws SecurityException {@inheritDoc}
*/
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
//判斷是否可以操作目標線程
checkShutdownAccess();
//設置線程池狀態爲SHUTDOWN,此處之後，線程池中不會增加新Task
advanceRunState(SHUTDOWN);
//中斷所有的空閒線程
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
//轉到Terminate
tryTerminate();
}shutdown做了幾件事：
1. 檢查是否能操作目標線程
2. 將線程池狀態轉爲SHUTDOWN
3. 中斷所有空閒線程
這裏就引發了一個問題，什麼是空閒線程？
這需要接着看看interruptIdleWorkers是怎麼回事。
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
//這裏的意圖很簡單，遍歷workers 對所有worker做中斷處理。
// w.tryLock()對Worker加鎖，這保證了正在運行執行Task的Worker不會被中斷，那麼能中斷哪些線程呢？
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();
}
}這裏主要是爲了中斷worker，但是中斷之前需要先獲取鎖，這就意味着正在運行的Worker不能中斷。但是上面的代碼有w.tryLock()，那麼獲取不到鎖就不會中斷，shutdown的Interrupt只是對所有的空閒Worker（正在從workQueue中取Task，此時Worker沒有加鎖）發送中斷信號。
while (task != null || (task = getTask()) != null) {
w.lock();
//獲取woker的鎖，防止線程被其他線程中斷
clearInterruptsForTaskRun();//清楚所有中斷標記
try {
beforeExecute(w.thread, task);//線程開始執行之前執行此方法，可以實現Worker未執行退出，本類中未實現
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);//線程執行後執行，可以實現標識Worker異常中斷的功能，本類中未實現
}
} finally {
task = null;//運行過的task標null
w.completedTasks++;
w.unlock();
}
}在runWorker中，每一個Worker getTask成功之後都要獲取Worker的鎖之後運行，也就是說運行中的Worker不會中斷。因爲核心線程一般在空閒的時候會一直阻塞在獲取Task上，也只有中斷纔可能導致其退出。這些阻塞着的Worker就是空閒的線程（當然，非核心線程，並且阻塞的也是空閒線程）。在getTask方法中：
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//當前狀態爲>stop時，不處理workQueue中的任務，同時減小worker的數量所以返回null，如果爲shutdown 同時workQueue已經empty了，同樣減小worker數量並返回null
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
boolean timed; // Are workers subject to culling?
for (;;) {
//allowCoreThreadTimeOu是判斷CoreThread是否會超時的，true爲會超時，false不會超時。默認爲false
int wc = workerCountOf(c);
timed = allowCoreThreadTimeOut || wc > corePoolSize;
if (wc <= maximumPoolSize && ! (timedOut && timed))
break;
if (compareAndDecrementWorkerCount(c))
return null;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}會有兩階段的Worker：
- 剛進入getTask()，還沒進行狀態判斷。
- block在poll或者take上的Worker。
當調用ShutDown方法時，首先設置了線程池的狀態爲ShutDown，此時1階段的worker進入到狀態判斷時會返回null，此時Worker退出。
因爲getTask的時候是不加鎖的，所以在shutdown時可以調用worker.Interrupt.此時會中斷退出，Loop到狀態判斷時，同時workQueue爲empty。那麼拋出中斷異常，導致重新Loop，在檢測線程池狀態時，Worker退出。如果workQueue不爲null就不會退出，此處有些疑問，因爲沒有看見中斷標誌位清除的邏輯，那麼這裏就會不停的循環直到workQueue爲Empty退出。
這裏也能看出來SHUTDOWN只是清除一些空閒Worker，並且拒絕新Task加入，對於workQueue中的線程還是繼續處理的。
對於shutdown中獲取mainLock而addWorker中也做了mainLock的獲取，這麼做主要是因爲Works是HashSet類型的，是線程不安全的，我們也看到在addWorker後面也是對線程池狀態做了判斷，將Worker添加和中斷邏輯分離開。
接下來做了tryTerminate()操作，這操作是進行了後面狀態的轉換，在shutdownNow後面說。
接下來看看shutdownNow：
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks
* that were awaiting execution. These tasks are drained (removed)
* from the task queue upon return from this method.
*
* <p>This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
*
* <p>There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks. This implementation
* cancels tasks via {@link Thread#interrupt}, so any task that
* fails to respond to interrupts may never terminate.
*
* @throws SecurityException {@inheritDoc}
*/
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(STOP);
interruptWorkers();
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}shutdownNow和shutdown代碼類似，但是實現卻很不相同。首先是設置線程池狀態爲STOP，前面的代碼我們可以看到，是對SHUTDOWN有一些額外的判斷邏輯，但是對於>=STOP,基本都是reject，STOP也是比SHUTDOWN更加嚴格的一種狀態。此時不會有新Worker加入，所有剛執行完一個線程後去GetTask的Worker都會退出。
之後調用interruptWorkers：
/**
* Interrupts all threads, even if active. Ignores SecurityExceptions
* (in which case some threads may remain uninterrupted).
*/
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
try {
w.thread.interrupt();
} catch (SecurityException ignore) {
}
}
} finally {
mainLock.unlock();
}
}這裏可以看出來，此方法目的是中斷所有的Worker，而不是像shutdown中那樣只中斷空閒線程。這樣體現了STOP的特點，中斷所有線程，同時workQueue中的Task也不會執行了。所以接下來drainQueue：
/**
* Drains the task queue into a new list, normally using
* drainTo. But if the queue is a DelayQueue or any other kind of
* queue for which poll or drainTo may fail to remove some
* elements, it deletes them one by one.
*/
private List<Runnable> drainQueue() {
BlockingQueue<Runnable> q = workQueue;
List<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;
}獲取所有沒有執行的Task，並且返回。
這也體現了STOP的特點：
拒絕所有新Task的加入，同時中斷所有線程，WorkerQueue中沒有執行的線程全部拋棄。所以此時Pool是空的，WorkerQueue也是空的。
這之後就是進行到TIDYING和TERMINATED的轉化了：
/**
* Transitions to TERMINATED state if either (SHUTDOWN and pool
* and queue empty) or (STOP and pool empty). If otherwise
* eligible to terminate but workerCount is nonzero, interrupts an
* idle worker to ensure that shutdown signals propagate. This
* method must be called following any action that might make
* termination possible -- reducing worker count or removing tasks
* from the queue during shutdown. The method is non-private to
* allow access from ScheduledThreadPoolExecutor.
*/
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
}
}上面的代碼其實很有意思有幾種狀態是不能轉化到TIDYING的:
- RUNNING狀態
- TIDYING或TERMINATED
- SHUTDOWN狀態，但是workQueue不爲空
也說明了兩點：
1. SHUTDOWN想轉化爲TIDYING，需要workQueue爲空，同時workerCount爲0。
2. STOP轉化爲TIDYING，需要workerCount爲0
如果滿足上面的條件（一般一定時間後都會滿足的），那麼CAS成TIDYING，TIDYING也只是個過度狀態，最終會轉化爲TERMINATED。
至此，ThreadPoolExecutor一些核心思想就介紹完了，想分析清楚實在是不容易，對於ThreadPoolExecutor我還是有些不懂地方，以上只是我對源碼的片面的見解，如果有不正確之處,希望各位大佬指出
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