java原始碼-ThreadPoolExecutor（2）
開篇
這篇文章的主要目標是為了講解清楚ThreadPoolExecutor的提交任務的過程，非常推薦靜下心來仔細閱讀。
java原始碼-ThreadPoolExecutor（1）
java原始碼-ThreadPoolExecutor（2）
java原始碼-ThreadPoolExecutor（3）
ThreadPoolExecutor狀態介紹
ThreadPoolExecutor針對執行緒池一共維護了五種狀態，實現上用用高3位表示ThreadPoolExecutor的執行狀態，低29位維持執行緒池執行緒個數，分別是：
- RUNNING = -1 << COUNT_BITS = -1<<29 高三位為111
- SHUTDOWN = 0 << COUNT_BITS = 0<<29 高三位為000
- STOP = 1 << COUNT_BITS = 1<<29 高三位為001
- TIDYING = 2 << COUNT_BITS = 2<<29 高三位為010
- TERMINATED = 3 << COUNT_BITS = 3<<29 高三位為011
public class ThreadPoolExecutor extends AbstractExecutorService {
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// Integer.SIZE=32,Integer.SIZE-3=29,COUNT_BITS=29
private static final int COUNT_BITS = Integer.SIZE - 3;
// 執行緒池最大執行緒數=536870911（2^29-1）,CAPACITY二進位制中低29為為1，高3位為0
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// 用高3位表示ThreadPoolExecutor的執行狀態
// RUNNING=111
private static final int RUNNING = -1 << COUNT_BITS;
// SHUTDOWN=000
private static final int SHUTDOWN = 0 << COUNT_BITS;
// STOP=001
private static final int STOP = 1 << COUNT_BITS;
// TIDYING=010
private static final int TIDYING = 2 << COUNT_BITS;
// TERMINATED=110
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
// runStateOf通過獲取高3位來對比
private static int runStateOf(int c) { return c & ~CAPACITY; }
// workerCountOf通過比較低29位來獲取執行緒數
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
private static boolean runStateLessThan(int c, int s) {
return c < s;
}
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
private static boolean isRunning(int c) {
return c < SHUTDOWN;
}
ThreadPoolExecutor任務提交過程
ThreadPoolExecutor提交任務程式碼是在AbstractExecutorService當中通過submit()方法實現的，按照兩個步驟來實現：
- 通過newTaskFor()方法建立待提交任務，該方法內部的實現後面再分析。
- 通過execute()方法提交task，execute的在ThreadPoolExecutor類中實現重寫。
- 進一步跟進ThreadPoolExecutor的execute方法。
public abstract class AbstractExecutorService implements ExecutorService {
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;
}
}
整個ThreadPoolExecutor的execute其實在原始碼自帶的註釋中已經寫的很清楚了，怕自己翻譯的不是特別所以這次直接把註釋也貼在程式碼當中了，整個過程分為三個過程：
- 1、當前的執行緒數是否小於corePoolSize，新建core執行緒並執行第一個任務。
- 2、如果第一步不滿足條件，那麼就把任務提交到workQueue代表的佇列當中。
- 3、如果第二步不滿足條件，那麼就就新建不屬於corePoolSize計數的執行緒（也就是新建core以外的執行緒）來進行處理。
- 4、如果都失敗那麼就直接通過rejectHandler拒絕任務，步驟123當中任何檢測到執行緒池關閉的情況直接執行任務拒絕。
public class ThreadPoolExecutor extends AbstractExecutorService {
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);
}
}
ThreadPoolExecutor的addWorker方法有兩個引數，Runnable firstTask代表待執行任務, boolean core代表是否啟動核心執行緒，整個啟動過程主要分為三個步驟：
- 前置檢查：檢查執行緒池是否處於關閉狀態，在正常執行的情況下增加工作執行緒計數。
- 正常處理：建立Worker物件並在加鎖的條件下將新建worker新增到workers集合當中，並通過呼叫t.start()方法啟動執行緒。
- 後置處理：判斷啟動執行緒是否失敗，如果失敗那麼就嘗試中止執行緒池。
public class ThreadPoolExecutor extends AbstractExecutorService {
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
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);
// 判斷是否超出執行緒限制，corePoolSize和core執行緒數，
// maximumPoolSize代表超出core部分的執行緒數
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
}
}
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;
}
private void addWorkerFailed(Worker w) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (w != null)
workers.remove(w);
decrementWorkerCount();
tryTerminate();
} finally {
mainLock.unlock();
}
}
}
ThreadPoolExecutor的worker介紹
ThreadPoolExecutor的worker實現Runnable介面，在worker的內部run()方法中通過執行runWorker()方法來啟動task，啟動方式會呼叫task.run()方法，所以從這個角度來看，task的執行執行緒其實ThreadPoolExecutor執行緒池中的worker。
- Worker類內部包含：Thread thread工作執行緒用於執行task、Runnable firstTask標識待執行任務。
- runWorker()方法內部負責執行來自提交的firstTask或者阻塞從任務佇列通過getTask()方法取得待執行任務
- runWorker()方法內部通過執行task.run()負責真正執行任務。
public class ThreadPoolExecutor extends AbstractExecutorService {
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
private static final long serialVersionUID = 6138294804551838833L;
/** 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;
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
}
runWorker內部主要做兩件事情，分別是：
- 獲取任務：通過直接傳進來firstTask或者通過getTask從任務佇列中獲取任務
- 執行任務：task.run()執行真正的task任務
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()方法外層是一個for迴圈，然後內部從workQueue獲取任務，區分設定超時或者阻塞等待。
- 阻塞等待直至執行緒獲取到可消費任務。
- 超時等待使用的是keepAliveTime，用於超時後設定執行緒超時標記然後執行緒退出工作。
- 執行緒退出迴圈是通過返回task=null，外層迴圈直接結束實現。
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.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
// 標記執行緒退出工作部分的邏輯，通過返回task=null，從而在外層呼叫方實現退出while迴圈
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
}
ThreadPoolExecutor的task介紹
ThreadPoolExecutor的newTaskFor()方法負責建立task，建立的FutureTask的例項本身實現了Runnable、Future的介面。
- FutureTask內部可以建立入參為Runnable的物件的時候會建立一個代理器
- RunnableAdapter，建立入參為Callable的物件就比較直接了。
- FutureTask的執行函式run()負責執行Callable物件的call()方法並將返回值通過set()方法設定到outcome物件。
- FutureTask的get()方法負責獲取返回值，就是我們submit()後返回的future的get()呼叫。
public abstract class AbstractExecutorService implements ExecutorService {
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
}
public class Executors {
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}
}
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
public class FutureTask<V> implements RunnableFuture<V> {
/**
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;
private static final int EXCEPTIONAL = 3;
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;
private Callable<V> callable;
private Object outcome; // non-volatile, protected by state reads/writes
private volatile Thread runner;
private volatile WaitNode waiters;
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
// 核心的邏輯，負責呼叫物件的call方法並賦值返回值
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
runner = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
private static final long runnerOffset;
private static final long waitersOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = FutureTask.class;
stateOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("state"));
runnerOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("runner"));
waitersOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("waiters"));
} catch (Exception e) {
throw new Error(e);
}
}
}
參考文章
ThreadPoolExecutor解析-主要原始碼研究
ThreadPoolExecutor(五)——執行緒池關閉相關操作
ThreadPoolExecutor(六)——執行緒池關閉之後
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