前言
線程池實作原理-1
addWorker實作
在看addWorker方法之前，我們先看一個例子，了解一下retry的使用
- break retry 跳到retry處，且不再進入循環
- continue retry 跳到retry處，且再次進入循環
public static void main(String[] args) {
breakRetry();
continueRetry();
}
private static void breakRetry() {
int i = 0;
retry:
for (; ; ) {
System.out.println("start");
for (; ; ) {
i++;
if (i == 4)
break retry;
}
}
//start 進入外層循環
//4
System.out.println(i);
}
private static void continueRetry() {
int i = 0;
retry:
for(;;) {
System.out.println("start");
for(;;) {
i++;
if (i == 3)
continue retry;
System.out.println("end");
if (i == 4)
break retry;
}
}
//start 第一次進入外層循環
//end i=1輸出
//end i=2輸出
//start 再次進入外層循環
//end i=4輸出
//4 最後輸出
System.out.println(i);
} 複制
這裡說一下Runnable 參數的含義
- firstTask != null 說明任務被添加了，我們需要啟動一個線程去執行它
- fistTask == null 說明我隻想啟動一個線程去消費阻塞隊列中的任務
// core為ture表示是核心線程，否則非核心線程
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
/**
* 将條件改為如下形式，友善了解
* rs >= SHUTDOWN && (rs != SHUTDOWN || fistTask != null || workQueue.isEmpty)
* 1.如果目前線程池的狀态>SHUTDOWN，addWorker傳回false，添加任務失敗
* 2.如果目前線程池的狀态=SHUTDOWN，分為如下2種情況
* (1)workQueue為空，fistTask == null 和fistTask != null的任務都不能
* (2)workQueue不為空，可以添加fistTask != null的任務
*/
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
// 1.是核心線程 >= corePoolSize
// 2.非核心線程 >= maximumPoolSize
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// 成功将線程數+1，跳到retry處，并且不再進入死循環
if (compareAndIncrementWorkerCount(c))
break retry;
// 否則重新讀取ctl
c = ctl.get(); // Re-read ctl
// 線程狀态發生改變，跳到retry處，并且進入死循環
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
// 線程是否啟動的标志位
boolean workerStarted = false;
// 線程封裝成Worker對象，是否添加到線程池中的标志位
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());
// 1.rs < SHUTDOWN 即 rs = RUNNING
// 2.rs == SHUTDOWN && firstTask == null
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();
// 重新整理了largestPoolSize
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
// 留心一下這裡，後面會從這裡開始講起
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
} 複制
仔細了解一下這段代碼，其實就能了解，當線程池處于RUNNING 接受新任務，并且處理進入隊列的任務，處于SHUTDOWN 不接受新任務，處理進入隊列的任務，剩餘狀态都不會處理任務，上面代碼中的注釋有詳細解釋
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false; 複制
線程池在執行任務的時候，會把任務對象包裝成一個Worker對象，Worker對象是ThreadPoolExecutor的一個内部類，繼承了AbstractQueuedSynchronizer，實作了一個獨占鎖，status值為0表示未鎖定狀态，status值為1表示鎖定狀态，實作了Runnable接口，在執行run方法的時候，它執行完初始化的firstTask後，還會從workQueue中取出任務執行，這樣就不用建立一個線程執行任務，而是在一個線程中執行了好幾個任務
Worker内部類
// 省略了一部分對鎖的操作，簡單的對AQS的一個實作
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;
/**
* Creates with given first task and thread from ThreadFactory.
*/
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);
}
} 複制
runWorker實作
當t.start()被執行後，run方法會執行runWorker方法，來看runWorker方法
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
// 允許中斷
w.unlock(); // allow interrupts
// 辨別線程是不是異常終止的
boolean completedAbruptly = true;
try {
// 先執行初始化的fistTask，執行完成後還會無限循環擷取workQueue裡的任務來執行
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
// 配合shutdownNow 方法
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
// 線程開始開始執行之前執行此方法
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 {
// 線程執行後執行
afterExecute(task, thrown);
}
} finally {
// 運作過的置為null
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
} 複制
這個方法需要注意的就是
- getTask()從阻塞隊列中擷取任務，如果隊列中沒有任務會被阻塞，并不會占用CPU資源
- 可以根據業務需要自定義beforeExecute和afterExecute方法
getTask實作
private Runnable getTask() {
// 标記擷取頭結點并且移除頭結點的方法是否逾時
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.
// 1.rs >= STOP
// 2.rs == SHUTDOWN && workQueue.isEmpty()
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
// 用CAS将線程池中的數量-1，直到成功才會退出
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
// 1.核心線程允許被銷毀
// 2.核心線程數 > corePoolSize
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
// 1.timeOut為true，表示逾時擷取，workQueue沒有任務，說明線程應該被銷毀，但是還是要 && timed
// 2.wc > maximumPoolSize肯定要删除線程了
// 3.workQueue為空可以銷毀線程，此時有可能所有線程都被銷毀了
// 4.workQueue不為空，隻有wc > 1才能被删除
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
// timed為true，超過keepAliveTime還是沒有任務，傳回null
// timed為false，則一直阻塞等待任務
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
} 複制
processWorkerExit實作
線程執行完畢執行的方法
// processWorkerExit在runWorker結束之後被調用
private void processWorkerExit(Worker w, boolean completedAbruptly) {
// 如果是異常終止，或者被中斷，減少workerCount
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
// Transitions to TERMINATED state if either (SHUTDOWN and pool
// and queue empty) or (STOP and pool empty)
tryTerminate();
int c = ctl.get();
// 狀态為RUNNING或者SHUTDOWN
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
// 目前核心線程已經夠用了，不用再建立
if (workerCountOf(c) >= min)
return; // replacement not needed
}
// 增加一個消費的線程
addWorker(null, false);
}
} 複制
shutdown實作
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// 檢查能否操作線程
checkShutdownAccess();
// 確定狀态 >= SHUTDOWN
advanceRunState(SHUTDOWN);
// 中斷所有的空閑線程
interruptIdleWorkers();
// ScheduledThreadPoolExecutor會重寫這個方法，做一些其他的運算
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
} 複制
// 中斷空閑線程
private void interruptIdleWorkers() {
interruptIdleWorkers(false);
} 複制
// onlyOne為true則隻中斷一個空閑線程，否則全部中斷
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
Thread t = w.thread;
// 周遊Worker并執行中斷操作，w.tryLock()保證了正在執行的Worker不會被中斷
// 因為正在運作的Worker再次擷取鎖會失敗
if (!t.isInterrupted() && w.tryLock()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
} 複制
這裡需要注意的是不會中斷正在運作的線程，因為正在運作的線程w.tryLock()會傳回false
shutdownNow實作
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
// 確定狀态 >= STOP
advanceRunState(STOP);
// 中斷所有線程
interruptWorkers();
// 擷取所有沒有執行完成的task
// 即将阻塞隊列中的任務放到tasks中
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
} 複制
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfStarted();
} finally {
mainLock.unlock();
}
} 複制
// 這個是Worker内部類的方法
void interruptIfStarted() {
Thread t;
// state的初始值為-1，運作到runWorker才允許中斷
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
} 複制
shutdownNow會中斷所有的線程，因為和shutdown相比在中斷之前，不用擷取鎖
tryTerminate實作
// 将狀态轉換到TERMINATED
final void tryTerminate() {
for (;;) {
int c = ctl.get();
// 以下幾種狀态不能轉換為TERMINATED
// 1.RUNNING狀态
// 2.TIDYING或TERMINATED
// 3.SHUTDOWN狀态，但是workQueue不為空
if (isRunning(c) ||
runStateAtLeast(c, TIDYING) ||
(runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
return;
if (workerCountOf(c) != 0) { // Eligible to terminate
interruptIdleWorkers(ONLY_ONE);
return;
}
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
try {
// 讓子類去實作，做一些操作
terminated();
} finally {
ctl.set(ctlOf(TERMINATED, 0));
termination.signalAll();
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
} 複制
從上面可看出狀态轉換的條件
- SHUTDOWN想轉化為TIDYING，需要workQueue為空，同時workerCount為0
- STOP轉化為TIDYING，需要workerCount為0