目錄
Worker類是定義在ThreadPoolExecutor中的內部類,要了解Worker,先要知道ThreadPoolExecutor是什麼。
一,從ThreadPoolExecutor說起
ThreadPoolExecutor是JDK1.5加入的,用來生成線程池的類,並且使用execute(Runnable)或者submit()方法向線程池添加任務,比如這樣:
static void test() {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
threadPoolExecutor.execute(() -> {
System.out.println("test ThreadPoolExecutor");
System.out.println(Thread.currentThread().getName());
});
threadPoolExecutor.execute(() -> {
System.out.println("test ThreadPoolExecutor");
System.out.println(Thread.currentThread().getName());
});
}
execute()方法在ThreadPoolExecutor類中有重寫,當調用execute()方法時,ThreadPoolExecutor向線程池中添加了任務,並使用Worker類來處理任務,包括新建線程和任務處理等工作。
submit()方法在ThreadPoolExecutor中沒有,實現方法在ThreadPoolExecutor的父類AbstractExecutorService中,而且有幾個不同的方法重載:
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
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;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
可以看到submit()方法實際上是對任務進行了簡單的封裝,然後又調用了ThreadPoolExecutor的execute()方法,其中RunnableFuture類實現了Runnable接口。
二,Worker類的源碼
Worker類是ThreadPoolExecutor中定義的內部類,其代碼是這樣的:
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) {
}
}
}
}
可以看到Worker類實現了Runnable接口,說明Worker本身也是一個任務,有run()方法,可以被執行。
Worker類主要維護了4個參數:
1,final Thread thread;
thread屬性是Worker維護的線程,每個Worker對象一個,這個就是用來執行任務用的線程,也就是說,Worker對象的數量也就代表了線程池中活動線程的數量。
2,Runnable firstTask;
Worker對象的初始任務,多數情況下每個Worker對象創建時都伴隨一個初始任務,是這個Worker對象優先會執行的任務,當執行完firstTask後,Worker對象還會從線程池中繼續獲取任務並執行。
3,volatile long completedTasks;
該Worker對象執行完成的任務數。
4,private static final long serialVersionUID = 6138294804551838833L;
序列化UID,沒什麼用,註釋寫的也很坦誠,用不上,加這個參數就是爲了不想看到Java的警告。
三,execute()方法和新建Worker
Worker對象是在ThreadPoolExecutor執行execute()方法時產生的,execute()方法的代碼如下:
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
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);
}
總的來說這個方法分了三步:
1,當前Worker數小於corePoolSize時,說明活動線程數沒有達到允許的核心線程數上限,此時調用addWorker()方法。注意此時方法的第二個參數是true,代表本次添加Worker的前提是活動線程數不超過corePoolSize。
2,當前Worker數大於corePoolSize時,判斷線程池是否是RUNNING狀態,然後試圖向workQueue中添加任務。添加任務成功後需要進行二次檢查,如果線程池狀態不是RUNNING則從workQueue中刪除剛剛添加的任務,並且執行拒絕策略,另外如果活動線程爲0則可以調用addWorker()方法新增一個無任務的Worker。
3,如果來到這一步,也就是Worker數大於corePoolSize,而且向workQueue中添加任務失敗,則再次執行addWorker()方法。和第一步不同,此時調用addWorker()方法的第二個參數是false,代表本次添加Worker的前提是線程數不超過maximumPoolSize。這意味着這一步把線程池中的活動線程上限從corePoolSize提高到了maximumPoolSize。如果此時添加Worker失敗,則執行拒絕策略。
以上是執行execute()方法時添加Worker的邏輯,下面看一下addWorker()方法的具體代碼實現:
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) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
方法基本可以分爲兩部分:
第一部分由兩層循環判斷了一些不能添加Worker的情況,比如連接池狀態是SHUTDOWN,或者活動線程數超過閾值,閾值根據參數是true或false決定是corePoolSize或maximumPoolSize。
第二部分開始正式新增Worker,初始化Worker對象:
w = new Worker(firstTask);
此處Worker的構造方法如下:
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
初始化Worker對象的同時新增了一個線程作爲Worker的參數。
其中getThreadFactory()方法得到的是一個java.util.concurrent.ThreadFactory接口的實例,這個接口的newThread()方法:
Thread newThread(Runnable r);
傳了一個Runnable參數,此處實際上是把Worker對象本身作爲參數傳入了newThread()方法,這種生成線程的方式,當執行線程的start()方法時,會調用參數(也就是Worker對象)的run()方法。
如果我們創建ThreadPoolExecutor時使用默認的ThreadFactory,也就是Executors.defaultThreadFactory(),使用的就是Executors類中的DefaultThreadFactory類,實現代碼如下:
static class DefaultThreadFactory implements ThreadFactory {
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final ThreadGroup group;
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;
DefaultThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() :
Thread.currentThread().getThreadGroup();
namePrefix = "pool-" +
poolNumber.getAndIncrement() +
"-thread-";
}
public Thread newThread(Runnable r) {
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}
可以看到此類的newThread(Runnable r)方法,新建線程用的是:
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
其中參數r就是Worker對象,當執行線程的start()方法時,會調用Worker對象的run()方法。
至此Worker對象初始化完成。
四,Worker的run()方法
初始化Worker對象完成後,下面要啓動Worker的線程,下面的代碼使用了一個ReentrantLock鎖,把初始化好的Worker對象加入workers列表,workers是ThreadPoolExecutor中定義的一個HashSet,如果添加成功,則調用Worker線程的start()方法,也就是這一段代碼:
if (workerAdded) {
t.start();
workerStarted = true;
}
如前文所說,當調用t.start()方法時,會調用Worker的run()方法:
public void run() {
runWorker(this);
}
其中runWorker(this)方法:
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);
}
}
此方法就是Worker執行線程池任務的方法,注意到其中while循環的條件:
while (task != null || (task = getTask()) != null)
其中task由firstTask賦值,或者由getTask()方法獲得,可見,Worker線程首先執行自己的firstTask任務,執行完後再從線程池中獲取任務來執行。
getTask()方法的源碼後面有。
任務獲取完成後繼續回到runWorker(this)方法看Worker執行任務的流程,從任務一開始就調用了Work.lock()方法加鎖,然後分別執行了:
beforeExecute(wt, task);
task.run();
afterExecute(task, thrown);
三個方法。
其中beforeExecute(wt, task)是個空方法,裏面沒有任何代碼邏輯,是給我們自定義連接池的時候用的,我們可以自己定義一個連接池類,繼承ThreadPoolExecutor,並重寫beforeExecute(wt, task)方法。
task.run()方法就是執行任務的run()方法了。
afterExecute(task, thrown)是在執行完任務之後執行的方法,也是個空方法,等待開發者自己重寫。另外,這個方法是寫在finally裏的,也就是說,即使在執行任務的時候因異常而中斷,該方法也會執行。
循環的最後,把task設爲null,增加Worker的完成任務數,並調用w.unlock();解鎖,接着進入下一次循環,獲取新的任務。
當線程池已經不再有任務需要執行,則調用:
processWorkerExit(w, completedAbruptly);
方法退出Worker線程,processWorkerExit()方法的代碼:
private void processWorkerExit(Worker w, boolean completedAbruptly) {
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();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
方法裏又用了一個鎖,鎖中的內容是把Worker對象的完成任務數加到線程池的總完成任務數中,並從workers列表中移除這個Worker對象。
tryTerminate()方法的作用是嘗試把線程池狀態置爲TERMINATED,以下兩種情況可以設置成功:
1,線程池狀態爲SHUTDOWN,活動線程數爲0,任務隊列爲空。
2,線程池狀態爲STOP,任務隊列爲空。
其中修改狀態的操作有加鎖且爲CAS操作。
tryTerminate()方法之後,如果當前活動線程數小於核心線程數,而且線程池還沒有被關,則新建一個Worker,這個Worker沒有初始任務。
至此runWorker()方法完成。
另外單獨關注一下runWorker()方法中,用於獲得任務的getTask()方法的代碼:
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;
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;
}
}
}
在getTask()方法中,首先判斷連接池狀態,如果狀態已經是STOP,或者SHUTDOWN狀態但任務隊列沒有任務了,那就不用再獲取任務了,而且這種狀態下線程池也不可能再新增任務,所以嘗試減少Worker線程數量,也就是這段:
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
接下來定義了一個boolean參數timed,用來判斷Worker將會用哪種方式來獲取任務:
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
allowCoreThreadTimeOut代表核心線程是否需要考慮獲取的超時時間,默認是false,後面的wc > corePoolSize代表活動線程數是否大於核心線程數的閾值,原則上如果活動線程數大於核心線程數,從隊列獲取任務時需要考慮超時時間。
如果timed爲true,表示需要考慮超時時間,使用poll的方式,否則會用take的方式獲取任務,take方法在隊列爲空時會阻塞線程。
以上