ThreadPoolExecutor原始碼分析
摘要：
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...
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》