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java 中ThreadPoolExecutor原理分析
#執行緒池簡介
Java執行緒池是開發中常用的工具，當我們有非同步、並行的任務要處理時，常常會用到執行緒池，或者在實作一個伺服器時，也需要使用執行緒池來接收連線處理請求。
執行緒池使用
JDK中提供的執行緒池實作位於java.util.concurrent.ThreadPoolExecutor。在使用時，通常使用ExecutorService介面，它提供了submit,invokeAll,shutdown等通用的方法。
在執行緒池配置方面，Executors類別中提供了一些靜態方法能夠提供一些常用場景的執行緒池，如newFixedThreadPool,newCachedThreadPool,newSingleThreadExecutor#等等，這些方法最終都是呼叫到了ThreadPoolExecutor的建構子。
ThreadPoolExecutor的包含所有參數的建構子是
/**
* @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
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;
}corePoolSize設定執行緒池的核心執行緒數，當新增任務時，如果執行緒池中的執行緒數小於corePoolSize，則不管目前是否有執行緒閒置，都會建立一個新的執行緒來執行任務。
maximunPoolSize是執行緒池中允許的最大的執行緒數
workQueue用於存放排隊的任務
#keepAliveTime是大於corePoolSize的執行緒閒置的逾時時間
handler用於任務逸出、執行緒池關閉時的任務處理，執行緒池的執行緒成長策略為，當前線程數小於corePoolSize時，新增線程，當線程數=corePoolSize且corePoolSize時，只有在workQueue不能存放新的任務時創建新線程，超出的線程在閒置keepAliveTime後銷毀。
實作(基於JDK1.8)
ThreadPoolExecutor中儲存的狀態有
#目前執行緒池狀態, 包括RUNNING,SHUTDOWN,STOP,TIDYING,TERMINATED。
目前有效的運行執行緒的數量。
將這兩個狀態放到一個int變數中，前三位作為執行緒池狀態，後29位元作為執行緒數。
例如0b11100000000000000000000000000001， 表示RUNNING, 一個執行緒。
透過HashSet來儲存工作者集合，存取該HashSet前必須先取得保護狀態的mainLock:ReentrantLock
submit、execute
#execute的執行方式為，首先檢查目前worker數量，如果小於corePoolSize，則嘗試add一個core Worker。執行緒池在維護執行緒數量以及狀態檢查上做了大量檢測。
public void execute(Runnable command) {
int c = ctl.get();
// 如果当期数量小于corePoolSize
if (workerCountOf(c) < corePoolSize) {
// 尝试增加worker
if (addWorker(command, true))
return;
c = ctl.get();
}
// 如果线程池正在运行并且成功添加到工作队列中
if (isRunning(c) && workQueue.offer(command)) {
// 再次检查状态，如果已经关闭则执行拒绝处理
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
// 如果工作线程都down了
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}addWorker方法實作
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);
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) {
// 如果添加成功，则启动该线程，执行Worker的run方法，Worker的run方法执行外部的runWorker(Worker)
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}Worker類別繼承了AbstractQueuedSynchronizer獲得了同步等待這樣的功能。
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. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
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);
}
// 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) {
}
}
}runWorker(Worker)是Worker的輪詢執行邏輯，不斷地從工作佇列中取得任務並執行它們。 Worker每次執行任務前需要進行lock，並防止在執行任務時被interrupt。
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);
}
}ThreadPoolExecutor的submit方法中將Callable包裝成FutureTask後交給execute方法。
FutureTask
FutureTask繼承於Runnable和Future，FutureTask定義的幾個狀態為
NEW, 尚未執行
COMPLETING, 正在執行
NORMAL, 正常執行完成得到結果
EXCEPTIONAL, 執行拋出異常
CANCELLED， 執行被取消
INTERRUPTING，執行正在被中斷
INTERRUPTED, 已經中斷。
其中關鍵的get方法
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}先取得目前狀態，如果還未執行完成且正常，則進入等待結果流程。在awaitDone不斷循環取得當前狀態，如果沒有結果，則將自己透過CAS的方式加入等待鍊錶的頭部，如果設定了逾時，則LockSupport.parkNanos到指定的時間。
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}FutureTask的run方法是執行任務並設定結果的位置,首先判斷當前狀態是否為NEW並且將當前線程設定為執行線程，然後在呼叫Callable的call取得結果後設定結果修改FutureTask狀態。
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 must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
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