ThreadPoolExecutor重載了多個構造方法,不過最終都是調用的同一個:git
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.acc = System.getSecurityManager() == null ? null : AccessController.getContext(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }
其中涉及了7個參數:github
allowCoreThreadTimeOut
(默認未設置)BlockingQueue<Runnable> workQueue
:當任務沒法當即被執行時,會被存儲在隊列中。不一樣類型的隊列會致使線程池不一樣的特性,這裏不深刻討論(有興趣能夠查看: 隊列爲 直接提交隊列SynchronousQueue
,無界隊列LinkedBlockingQueue
,有界隊列ArrayBlockingQueue
時不一樣的特性,參考)new ThreadFactoryBuilder().setNameFormat("Thread-pool-%d").build();
RejectedExecutionHandler handler
:拒絕策略,當線程數達到maximumPoolSize,且workQueue已經沒法存儲更多任務時,採用拒絕策略。ThreadPoolExecutor爲咱們提供了4種拒絕策略:shell
AbortPolicy
,默認策略,拋出異常RejectedExecutionException
,告訴調用方已經來不及處理了,調用方須要處理異常和線程線程池來不及執行的任務DiscardPolicy
,靜默的忽略掉,無一致性要求的能夠這麼幹DiscardOldestPolicy
,從隊列裏拋棄掉最老的任務,無一致性要求的能夠這麼幹CallerRunsPolicy
,當任務添加到線程池中被拒絕時,會在線程池當前正在運行的Thread線程中處理被拒絕的任務。能夠必定程度緩解當前線程不夠的狀況,可是若是當前任務執行所需時間不定,有卡住主線程的風險再看看CallerRunsPolicy的實現:安全
public static class CallerRunsPolicy implements RejectedExecutionHandler { /** * Creates a {@code CallerRunsPolicy}. */ public CallerRunsPolicy() { } /** * Executes task r in the caller's thread, unless the executor * has been shut down, in which case the task is discarded. * * @param r the runnable task requested to be executed * @param e the executor attempting to execute this task */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) { r.run(); } } }
可見是經過執行r.run()
來佔用主線程執行的。less
全部的拒絕策略都是繼承RejectedExecutionHandler,因此咱們也能夠自定義拒絕策略。oop
ctl變量是ThreadPoolExecutor的一個屬性,ctl能夠理解爲control的簡寫,源碼中定義以下:源碼分析
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
源碼中ctl變量的註釋中解釋了該變量的含義,該變量包含了兩個含義,線程池的運行狀態 (runState) 和線程池內有效線程的數量 (workerCount)。 ctl用高3位來表示線程池的運行狀態, 用低29位來表示線程池內有效線程的數量。在源碼中,rs一般表示線程池運行狀態 , wc一般表示線程池中有效線程數量, 另外, ctl 也一般會簡寫做 c。ui
再看與ctl相關的幾個變量和方法:this
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 runStateOf(int c) { return c & ~CAPACITY; } private static int workerCountOf(int c) { return c & CAPACITY; } private static int ctlOf(int rs, int wc) { return rs | wc; }
111
,後29位爲0000
,後29位爲0。調用shutdown()
方法會置爲該狀態。001
,後29位爲0。調用shutdownNow()
方法會置爲該狀態010
,後29位爲0。此時表示全部的任務都執行完了,當前線程池已經沒有有效的線程,而且將要調用terminated方法011
,後29位爲0c
爲ctl變量,CAPACITY
取反結果是前三位爲1,後29位爲0,與ctl與操做便可獲得狀態對於狀態能夠簡單理解爲:RUNNING爲-1,SHUTDOWN爲0,STOP爲1,TIDYING爲2,TERMINATED爲3。RUNNING變爲SHUTDOWN或者STOP後,再變爲TIDYING,再變爲TERMINATED。
ThreadPoolExecutor繼承於AbstractExecutorService:
public class ThreadPoolExecutor extends AbstractExecutorService
AbstractExecutorService提供了最經常使用的三個添加任務到線程成的方法:
public Future<?> submit(Runnable task) { if (task == null) throw new NullPointerException(); RunnableFuture<Void> ftask = newTaskFor(task, null); execute(ftask); return ftask; } public <T> Future<T> submit(Runnable task, T result) { if (task == null) throw new NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task, result); execute(ftask); return ftask; } public <T> Future<T> submit(Callable<T> task) { if (task == null) throw new NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task); execute(ftask); return ftask; }
能夠看到最終它們都是調用了execute
方法,ThreadPoolExecutor中execute的實現以下:
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) { 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); }
源碼中的這段註釋詳細的介紹了這段代碼的做用,該方法考慮三種狀況:
若是步驟1中建立失敗
或者thread數量>=corePoolSize
,那會進入該步驟。該步驟判斷線程池處於運行狀態,則嘗試將新任務加入隊列。
workQueue.offer(command)
的時候線程池狀態改變)。若是線程池狀態改變則remove剛剛入隊的任務,並執行拒絕操做。若是在運行態,可是線程數爲0,則添加一個worker。線程池不處於運行狀態
或加入隊列失敗
則進入下一步驟若是線程池不處於運行狀態
或者處於運行狀態,可是thread數量>=corePoolSize且workQueue已滿
,則會進入該步驟。該步驟會嘗試建立一個新的線程來執行任務。若是線程池線程總數達到maximumPoolSize 或者 建立線程時線程池狀態變化再也不處於運行狀態,則會建立失敗。
在上面的代碼中主要是經過addWorker方法添加新任務的,下面咱們就來分析下這個方法的實現
源碼以下:
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,狀態不爲RUNNING //而且 //rs != SHUTDOWN || firstTask != null || workQueue.isEmpty() //一下幾種狀況 //1. 狀態不爲RUNNING和SHUTDOWN, //2. 或者 狀態爲SHUTDOWN且task不爲null, //3. 或者 狀態爲SHUTDOWN, task爲null, workQueue 爲空, //則返回false,添加失敗 if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false; //判斷是否超過線程數量的限制, for (;;) { int wc = workerCountOf(c); if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) return false; //未超過限制則嘗試把線程數加1,成功跳出retry循環 if (compareAndIncrementWorkerCount(c)) break retry; //線程數加1失敗則說明ctl有變化(狀態或數量), 從新獲取 c = ctl.get(); // Re-read ctl //若是是狀態變化則循環外層,反之循環內層 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); //從Worker構造方法能夠看到 //this.firstTask = firstTask; //this.thread = getThreadFactory().newThread(this); //故此firstTask爲null的時候, w.thread不爲null 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(); //成功添加worker後,啓動線程 workerStarted = true; } } //end of if (t != null) } finally { //worker啓動失敗則移除worker, 數量減一 if (! workerStarted) addWorkerFailed(w); } return workerStarted; }
在execute
方法中在三個地方用不用的參數調用了addWorker方法:
addWorker有兩個參數:Runnable firstTask
和 boolean core
,前者表示要執行的任務,後者表示線程數量限制的類型(基於corePoolSize
仍是maximumPoolSize
)。1和3 是相似的,惟一的不一樣就是線程數的限制不一樣,因此這裏主要分析firstTask爲null 和 不爲null 的區別。
方法中retry: for (;;) {...}
的內容主要是用於判斷是否線程池已經關閉,以及線程數量是否超過限制。若未關閉,未超過限制則把線程數加1。firstTask爲null的時候, w.thread不爲null,因此firstTask是否在addWorker中仍是沒有區別,那隻能更進一步看看worker裏對firstTask是如何處理的。
線程池中的任務都是經過worker來代理的。
private final class Worker extends AbstractQueuedSynchronizer implements Runnable{ /** Thread this worker is running in. Null if factory fails. */ final Thread thread; /** Initial task to run. Possibly null. */ Runnable firstTask; /** Per-thread task counter */ volatile long completedTasks; //後續代碼此處省略........................... }
Worker繼承與AQS,實現Runable接口,自己是線程類,且具備AQS的特性。
看worker構造方法:
Worker(Runnable firstTask) { setState(-1); // inhibit interrupts until runWorker this.firstTask = firstTask; //Worker實現了Runnable因此, //因此this.thread.start(),就是用線程執行worker的run方法 this.thread = getThreadFactory().newThread(this); }
setState(-1)
爲AQS的方法,把狀態位設置成-1,這樣任何線程都不能獲得Worker的鎖,除非調用了unlock方法。這個unlock方法會在runWorker方法中一開始就調用,這是爲了確保Worker構造出來以後,沒有任何線程可以獲得它的鎖,除非調用了runWorker以後,其餘線程才能得到Worker的鎖。
再看其run方法:
/** Delegates main run loop to outer runWorker */ public void run() { runWorker(this); }
runWorker(this)不是worker的方法,是ThreadPoolExecutor的方法,也是執行任務的方法。
又回到了ThreadPoolExecutor中,runWorker實現以下:
final void runWorker(Worker w) { Thread wt = Thread.currentThread(); Runnable task = w.firstTask; w.firstTask = null; w.unlock(); // allow interrupts,建立worker時狀態設置爲-1了,此時設置爲1 boolean completedAbruptly = true; //task是否意外終止,意外終止爲true,反之false try { //優先運行初始化時的firstTask, 若是firstTask已經執行了則從隊列取 while (task != null || (task = getTask()) != null) { w.lock(); //獲取到task後鎖定,獨佔worker,保證線程安全 // 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); //空方法,用於子類擴展 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);//空方法,用於子類擴展 } } finally { task = null; w.completedTasks++; w.unlock(); } } completedAbruptly = false; } finally { //移除執行完成的worker processWorkerExit(w, completedAbruptly); } }
到此咱們終於能回答前面的問題了,addWorker(Runnable firstTask, boolean core) 中firstTask爲null不不爲null的區別:
- 爲null,
addWorker(null, core)
表示建立一個worker,執行隊列中的task- 不爲null,
addWorker(firstTask, core)
表示建立一個worker,先執行firstTask,再執行隊列中的task他們都新增了一個線程,一個是直接執行隊列裏的任務,一個先執行當前任務,再執行隊列任務。
下面繼續分析runWorker。
線程池在runWorker方法中,經過while (task != null || (task = getTask()) != null)
不斷從隊列中取出任務執行,等待隊列中任務執行完成後,調用processWorkerExit(w, completedAbruptly)
,移除當前worker。問題來了,這麼看起來線程池中的線程只有在隊列不爲空的時候才得以複用,這不科學啊,那問題在哪兒?反覆看代碼,惟一忽略的掉的地方就是getTask()
了,看到這個方法的時候,想固然的認爲是簡單的獲取隊列中的任務,那麼咱們來看一下它的具體實現:
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. 線程池是否已經關閉 if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } int wc = workerCountOf(c); // Are workers subject to culling? //表示worker是否須要回收 //allowCoreThreadTimeOut=true時core線程超時也回收, 默認爲false //因此默認狀況下timed表示 wc > corePoolSize boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } try { Runnable r = timed ? //線程須要回收;嘗試取隊列中的任務,超過keepAliveTime還未取到返回null workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : //線程無需回收;取隊列中的任務, 隊列中沒有任務則一直等到有任務 workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
上面代碼能夠看出getTask()
確實是取任務,不過也兼任了 線程池在運行態取不到數據時 park線程
或 等待線程直到超時(parkNanos)
的工做,咱們查看線程無需回收時park在取隊列任務的線程堆棧以下:
"pool-1-thread-1@731" prio=5 tid=0xd nid=NA waiting java.lang.Thread.State: WAITING at sun.misc.Unsafe.park(Unsafe.java:-1) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039) at java.util.concurrent.ArrayBlockingQueue.take(ArrayBlockingQueue.java:403) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748)
線程處於waiting狀態,從堆棧中能夠看到at java.util.concurrent.ArrayBlockingQueue.take(ArrayBlockingQueue.java:403)
,正是被workQueue.take()
park住了。如此一來worker執行完當前線程以後,若是取不到新的任務就會一直處在park狀態,直到隊列中有新的任務進入。以ArrayBlockingQueue爲例看,看其take
和 enqueue
實現:
/** Condition for waiting takes */ private final Condition notEmpty; public E take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == 0) notEmpty.await(); //park 線程 return dequeue(); } finally { lock.unlock(); } } private void enqueue(E x) { // assert lock.getHoldCount() == 1; // assert items[putIndex] == null; final Object[] items = this.items; items[putIndex] = x; if (++putIndex == items.length) putIndex = 0; count++; notEmpty.signal(); //喚起線程 }
ThreadPoolExecutor提供了兩個關閉的方法:
shutdown()
,關閉線程池,再也不接受新的任務,可是會處理完當前線程和隊列中的線程shutdownNow()
,關閉線程池,再也不接受新的任務,且試圖中止全部正在執行的線程,並再也不處理還在池隊列中等待的任務。可是它試圖終止線程的方法是經過調用Thread.interrupt()
方法來實現的,可是interrupt的做用有限,運行中的線程不必定能成功退出(具體緣由參考)。下面看下實現:
public void shutdown() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(SHUTDOWN); //狀態設置爲SHUTDOWN interruptIdleWorkers(); //中斷空閒線程 onShutdown(); // hook for ScheduledThreadPoolExecutor,這裏爲空方法 } finally { mainLock.unlock(); } tryTerminate(); } public List<Runnable> shutdownNow() { List<Runnable> tasks; final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(STOP); //狀態設置爲STOP interruptWorkers(); //中斷所有線程 tasks = drainQueue(); //返回隊列中未執行的任務 } finally { mainLock.unlock(); } tryTerminate(); return tasks; }
能夠看到shutdown和shutdownNow的實現大體相同,不一樣的地方有兩個,
SHUTDOWN
,後者爲STOP
interruptIdleWorkers()
,只中斷空閒線程;後者interruptWorkers()
,中斷所有 線程,返回隊列中未執行的任務設置狀態的源碼:
private void advanceRunState(int targetState) { for (;;) { int c = ctl.get(); if (runStateAtLeast(c, targetState) || ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c)))) break; } }
interruptIdleWorkers():
private void interruptIdleWorkers() { interruptIdleWorkers(false); } private void interruptIdleWorkers(boolean onlyOne) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { for (Worker w : workers) { Thread t = w.thread; //若是線程未被中斷,且獲取work的鎖成功(說明空閒),則中斷線程 if (!t.isInterrupted() && w.tryLock()) { try { t.interrupt(); } catch (SecurityException ignore) { } finally { w.unlock(); } } if (onlyOne) break; } } finally { mainLock.unlock(); } }
interruptWorkers():
//ThreadPoolExecutor private void interruptWorkers() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { //中斷所有worker線程 for (Worker w : workers) w.interruptIfStarted(); } finally { mainLock.unlock(); } } //worker void interruptIfStarted() { Thread t; //若worker已經啓動(未啓動時爲-1),且thread不爲null,且未被中斷 //也就是說線程還存活着,那就發送中斷信號 if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) { try { t.interrupt(); } catch (SecurityException ignore) { } } }
tryTerminate()
除了在關閉鏈接池時調用,還在其它地方調用了,這裏只分析在關閉鏈接池時它都作了什麼:
final void tryTerminate() { for (;;) { int c = ctl.get(); //關閉鏈接池調用該方法第一次調用時: //狀態爲SHUTDOWN或STOP,都小於TIDYING,故前兩條件都不知足 //第三個條件,隊列不爲空的時候直接返回了, //若是爲shutdown()則可能隊列不爲空,可能知足條件直接返回,也可能不知足 //若是爲shutdownNow()則隊列被清空,不知足 if (isRunning(c) || runStateAtLeast(c, TIDYING) || (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty())) return; //若是worker數量不爲0則執行interruptIdleWorkers(true) //而後直接返回,完成該方法 if (workerCountOf(c) != 0) { // Eligible to terminate interruptIdleWorkers(ONLY_ONE); return; } final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { //嘗試設置狀態爲TIDYING,worker數量爲0, //期間ctl若未變更,則成功 if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) { try { terminated(); //空方法用於子類擴展 } finally { //設置狀態爲TERMINATED ctl.set(ctlOf(TERMINATED, 0)); //喚醒調用了awaitTermination(long timeout, TimeUnit unit)的線程 //awaitTermination中調用了 termination.signalAll(); } return; } } finally { mainLock.unlock(); } // else retry on failed CAS } }
tryTerminate()
在關閉鏈接池時的作的判斷能夠簡單理解爲
因此不管是shutdown仍是shutdownNow都不會阻塞線程,且不保證worker已經所有關閉。
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