前言
在熟練掌握如何使用執行緒池之後，我們來對ThreadPoolExecutor進行原始碼分析。希望大家保持對原始碼的閱讀熱情，不僅要知其然，也要知其所以然。閱讀原始碼比較苦澀，請養成反覆研究琢磨為什麼這麼寫的精神，多推敲。衝鴨！
其實有時候想不通的時候可以看一下英文註釋，還是作者解釋的精準
1 ThreadPoolExecutor類圖
2 ThreadPoolExecutor重要變數
2.1 ctl
這個變數是整個類的核心，AtomicInteger保證了原子性，這個變數儲存了2個內容
- 執行緒池的狀態
- 所有工作執行緒的數量
// int是4個位元組，有32位，這裡的ctl前3位表示執行緒池的狀態，後29位標識工作執行緒的數量 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); // Integer.SIZE - 3 = 29 private static final int COUNT_BITS = Integer.SIZE - 3; // 容量 000 11111111111111111111111111111 private static final int CAPACITY = (1 << COUNT_BITS) - 1; // runState is stored in the high-order bits // 執行中狀態 111 00000000000000000000000000000 （-536870912） 括號內為十進位制的 private static final int RUNNING = -1 << COUNT_BITS; // 關閉狀態 000 00000000000000000000000000000 （0） private static final int SHUTDOWN = 0 << COUNT_BITS; // 停止狀態 001 00000000000000000000000000000 （536870912） private static final int STOP = 1 << COUNT_BITS; // 整理狀態 010 00000000000000000000000000000 （1073741824） private static final int TIDYING = 2 << COUNT_BITS; // 終結狀態 011 00000000000000000000000000000 （1610612736） private static final int TERMINATED = 3 << COUNT_BITS; // Packing and unpacking ctl // 先非然後位與運算子獲取執行緒池執行的狀態，也就是前3位 private static int runStateOf(int c) { return c & ~CAPACITY; } // 位與運算子獲取工作執行緒數量，也就是後29位 private static int workerCountOf(int c) { return c & CAPACITY; } private static int ctlOf(int rs, int wc) { return rs | wc; }
執行緒池的狀態
- RUNNING：接收任務，處理workQueue佇列裡的任務
- SHUTDOWN：不再接收新的任務，但是處理workQueue佇列裡的任務
- STOP：拒絕新任務並且拋棄佇列裡的任務
- TIDYING：將要呼叫terminated方法
- TERMINATED：終結狀態
2.2 Woker靜態內部類
Worker實現了Runnable介面，說明可以當做一個可執行的任務。Woker也繼承了AbstractQueuedSynchronizer，說明可以實現鎖的功能，他是一個簡單的不可重入的互斥鎖，工作執行緒執行任務的時候，會先加鎖，如果想要中斷工作執行緒，需要先獲取鎖，否則無法中斷，工作執行緒執行完任務才會釋放鎖，然後接著從workQueue獲取任務繼續執行。Worker的主要作用是執行佇列的任務，並管理工作執行緒和統計一些東西。
/** * Class Worker mainly maintains interrupt control state for * threads running tasks, along with other minor bookkeeping. * This class opportunistically extends AbstractQueuedSynchronizer * to simplify acquiring and releasing a lock surrounding each * task execution. This protects against interrupts that are * intended to wake up a worker thread waiting for a task from * instead interrupting a task being run. We implement a simple * non-reentrant mutual exclusion lock rather than use * ReentrantLock because we do not want worker tasks to be able to * reacquire the lock when they invoke pool control methods like * setCorePoolSize. Additionally, to suppress interrupts until * the thread actually starts running tasks, we initialize lock * state to a negative value, and clear it upon start (in * runWorker). */ private final class Worker extends AbstractQueuedSynchronizer implements Runnable { /** * This class will never be serialized, but we provide a * serialVersionUID to suppress a javac warning. */ private static final long serialVersionUID = 6138294804551838833L; /** Thread this worker is running in. Null if factory fails. */ // 工作執行緒 final Thread thread; /** Initial task to run. Possibly null. */ // 第一個任務 Runnable firstTask; /** Per-thread task counter */ // 該工作執行緒已經完成任務的數量 volatile long completedTasks; /** * Creates with given first task and thread from ThreadFactory. * @param firstTask the first task (null if none) */ Worker(Runnable firstTask) { // 直到runWorker方法禁止被中斷 setState(-1); this.firstTask = firstTask; // 從執行緒工廠獲取執行緒，並把第一個任務給worker this.thread = getThreadFactory().newThread(this); } /** Delegates main run loop to outer runWorker */ public void run() { runWorker(this); } // Lock methods // // The value 0 represents the unlocked state. // The value 1 represents the locked state. 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) { } } } }
3 ThreadPoolExecutor重要函式
3.1 execute(Runnable command)
/** * 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. * * 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. */ // 獲取執行緒池狀態和執行緒數 int c = ctl.get(); // 工作執行緒數小於核心執行緒數 if (workerCountOf(c) < corePoolSize) { // 新增工作執行緒 if (addWorker(command, true)) return; // 新增工作執行緒失敗則重新獲取執行緒池狀態和執行緒數 c = ctl.get(); } // 執行緒池正在執行且任務可以將任務放進佇列裡 if (isRunning(c) && workQueue.offer(command)) { // 重新檢查 - 獲取執行緒池狀態和執行緒數 int recheck = ctl.get(); // 這裡重新檢查是為了以下2種情況 // 1.當offer方法執行之後，執行緒池關閉了，回滾之前放入佇列的操作並拒絕任務 if (! isRunning(recheck) && remove(command)) reject(command); // 2.執行緒池裡沒有可用的消費執行緒，比如現在核心執行緒數就1個，前一個任務拋異常了 // 那麼現在就沒有可用的消費執行緒了，所以要判斷還有沒有Worker，這步很關鍵 else if (workerCountOf(recheck) == 0) addWorker(null, false); } // 新增執行緒失敗則拒絕任務 else if (!addWorker(command, false)) reject(command); }
3.2 addWorker(Runnable firstTask, boolean core)
/* * Methods for creating, running and cleaning up after workers */ /** * 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. If the thread * creation fails, either due to the thread factory returning * null, or due to an exception (typically OutOfMemoryError in * Thread.start()), we roll back cleanly. * * @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: // 外迴圈 for (;;) { // 獲取執行緒池狀態和執行緒數 int c = ctl.get(); // 執行緒池狀態 int rs = runStateOf(c); // Check if queue empty only if necessary. // 這裡我做了一個小調整，看著舒服點，以下幾種情況會返回false // 1.執行緒池狀態為STOP，TIDYING，TERMINATED // 2.執行緒池狀態為SHUTDOWN且工作執行緒的firstTask不為空 // 3.執行緒池狀態為SHUTDOWN且佇列為空 if (rs >= SHUTDOWN && (rs != SHUTDOWN || firstTask != null || workQueue.isEmpty())) return false; // 內迴圈 for (;;) { // 獲取工作執行緒數 int wc = workerCountOf(c); // 如果工作執行緒大於容量或者工作執行緒大於核心執行緒數（或者最大執行緒數）返回false if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) return false; // 新增工作執行緒+1 if (compareAndIncrementWorkerCount(c)) break retry; // 重新獲取執行緒池狀態和執行緒數 c = ctl.get(); // Re-read ctl // 如果執行緒池狀態變了，那麼重新走外迴圈 if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop // 如果CAS操作失敗，那麼重新走內迴圈 } } // 執行緒是否開始工作 boolean workerStarted = false; // 執行緒是否新增到工作執行緒集合 boolean workerAdded = false; Worker w = null; try { w = new Worker(firstTask); final Thread t = w.thread; if (t != null) { // 利用顯式鎖加鎖新增Worker 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()); // 如果執行緒池狀態是RUNNING或者是SHUTDOWN&&第一個任務為空 if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) { // 檢查這個執行緒是否處於活動狀態 - RUNNABLE或者RUNNING if (t.isAlive()) // precheck that t is startable throw new IllegalThreadStateException(); //新增到工作執行緒集合 workers.add(w); int s = workers.size(); if (s > largestPoolSize) largestPoolSize = s; workerAdded = true; } } finally { mainLock.unlock(); } // 如果新增到工作執行緒集合則開始工作 if (workerAdded) { t.start(); workerStarted = true; } } } finally { // 如果執行緒沒有開始工作，那麼工作執行緒數量-1 if (! workerStarted) addWorkerFailed(w); } return workerStarted; }
3.3 runWorker(Worker w)
/** * Main worker run loop. Repeatedly gets tasks from queue and * executes them, while coping with a number of issues: * * 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. Before running any task, the lock is acquired to prevent * other pool interrupts while the task is executing, and then we * 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) { // 此處獲取的wt就是Worker裡的thread Thread wt = Thread.currentThread(); Runnable task = w.firstTask; w.firstTask = null; // 這裡為什麼要先unlock一下呢？到這一行程式碼為止，我們沒有進行任何的任務處理 // Worker的建構函式中，setState(-1);這一行程式碼抑制了執行緒中斷，所以這裡需要unlock從而允許中斷 w.unlock(); // allow interrupts // 是否是異常終止的標識，預設為true。有2中情況為true // 1.執行任務丟擲了異常 // 2.worker被中斷 boolean completedAbruptly = true; try { // 獲取任務，如果getTask()方法返回null，那麼隨之worker也要-1，之後有getTask()方法分析 // 只有在等待從workQueue佇列裡獲取任務的時候才能中斷。 // 第一次執行傳入的任務，之後從workQueue佇列裡獲取任務，如果佇列為空則等待keepAliveTime這麼久 while (task != null || (task = getTask()) != null) { // 加鎖的目的在於防止在執行任務的時候，中斷當前worker 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 // 這個方法比較重要，當執行緒池正在關閉，確保worker被中斷 // 有2次runStateAtLeast(ctl.get(), STOP)方法呼叫是因為double-check // 第2次檢查Thread.interrupted()，該方法會直接擦除執行緒的interrupt標識 if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted()) wt.interrupt(); try { // 執行任務之前的操作，如統計日誌等，子類自己實現 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) { // 將異常包裝成Error丟擲 thrown = x; throw new Error(x); } finally { // 執行任務之前的操作，如統計日誌等，子類自己實現 afterExecute(task, thrown); } } finally { task = null; w.completedTasks++; // 解鎖，一次任務的執行結束 w.unlock(); } } completedAbruptly = false; } finally { // 結束worker的清理工作 processWorkerExit(w, completedAbruptly); } }
3.4 getTask()
/** * 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, and if the queue is * non-empty, this worker is not the last thread in the pool. * * @return task, or null if the worker must exit, in which case * workerCount is decremented */ 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. // 當執行緒池狀態是STOP或者SHUTDOWN並且workQueue佇列是空的，返回null if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } int wc = workerCountOf(c); // timed用來判斷該工作執行緒是否有超時控制？ // allowCoreThreadTimeOut引數是是否允許核心執行緒也有keepAliveTime這麼一個屬性 // 核心執行緒預設是沒有超時限制 boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; // 條件1：如果工作執行緒大於最大執行緒數或者超時了 // 條件2：如果工作執行緒大於1或者workQueue佇列為空 // 滿足以上2個條件則返回null if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } try { // 一個是阻塞方法，一個是非阻塞方法，關鍵還是看timed這個變數，見上 Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
3.5 shutdown
執行緒池將不會再接收新的任務，將先前放在佇列中的任務執行完成。
/** * 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() { // 獲取顯式鎖 final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { // 檢查shutdown許可權 checkShutdownAccess(); // 將執行緒池狀態改為SHUTDOWN advanceRunState(SHUTDOWN); // 中斷空閒worker // 如果該執行緒正在工作，則不中斷 interruptIdleWorkers(); onShutdown(); // hook for ScheduledThreadPoolExecutor } finally { mainLock.unlock(); } // 保證workQueue裡的剩餘任務可以執行完 tryTerminate(); }
參考資料：
《Java concurrence in practice》