總結(jié)：線程池ThreadPoolExecutor是通過控制Worker對象的數(shù)量來維護工作的工人集合，并且通過任務隊列workerQueue來存儲提交到線程池的任務。通過配置相關(guān)的容量，以及拒絕策略來更方便使用以及處理容量飽滿的情況。Worker使用了同步器來解決任務執(zhí)行前執(zhí)行時執(zhí)行后的同步問題。
值得注意的是submit()和execute()的區(qū)別主要是submit()方法會將任務用FutureTask進行包裝，包裝之后在用execute()執(zhí)行
java線程池內(nèi)容較多，簡單的架構(gòu)如下。
線程池結(jié)構(gòu)
頂層接口定義了execute方法
public interface Executor {
/**
* Executes the given command at some time in the future. The command
* may execute in a new thread, in a pooled thread, or in the calling
* thread, at the discretion of the {@code Executor} implementation.
*
* @param command the runnable task
* @throws RejectedExecutionException if this task cannot be
* accepted for execution
* @throws NullPointerException if command is null
*/
void execute(Runnable command);
}
ExecutorService定義了對像城池的一些操作，submit()和shutdown()在這里定義了
public interface ExecutorService extends Executor {
void shutdown();
List<Runnable> shutdownNow();
boolean isShutdown();
boolean isTerminated();
boolean awaitTermination(long var1, TimeUnit var3) throws InterruptedException;
<T> Future<T> submit(Callable<T> var1);
<T> Future<T> submit(Runnable var1, T var2);
Future<?> submit(Runnable var1);
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> var1) throws InterruptedException;
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> var1, long var2, TimeUnit var4) throws InterruptedException;
<T> T invokeAny(Collection<? extends Callable<T>> var1) throws InterruptedException, ExecutionException;
<T> T invokeAny(Collection<? extends Callable<T>> var1, long var2, TimeUnit var4) throws InterruptedException, ExecutionException, TimeoutException;
}
除了這兩個接口意外還定義了ScheduledExecutorService接口，并額外定義了定時執(zhí)行的功能
public interface ScheduledExecutorService extends ExecutorService {
ScheduledFuture<?> schedule(Runnable var1, long var2, TimeUnit var4);
<V> ScheduledFuture<V> schedule(Callable<V> var1, long var2, TimeUnit var4);
ScheduledFuture<?> scheduleAtFixedRate(Runnable var1, long var2, long var4, TimeUnit var6);
ScheduledFuture<?> scheduleWithFixedDelay(Runnable var1, long var2, long var4, TimeUnit var6);
}
再往下就是具體的實現(xiàn)了ThreadPoolExecutor作為一個具體的實現(xiàn)。來看他的構(gòu)造方法的參數(shù)。
-
corePoolSize：核心數(shù) -
maximumPoolSize：最大核心數(shù) -
keepAliveTime：當線程空閑時間達到keepAliveTime，該線程會退出，直到線程數(shù)量等于corePoolSize。（超出核心數(shù)的線程最大存活時間） -
unit：時間單位 -
workQueue：任務隊列 -
threadFactory：線程工廠，可以設(shè)置線程的名字 -
handler：拒絕策略，圖示**Policy的就是handler對應的策略ThreadPoolExecutor內(nèi)部實現(xiàn)了這些策略，可選配。
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)
- 再來看看線程池的源碼入口
execute()或者submit()
- 值得注意的是
submit()調(diào)用了execute()方法，具有返回值，并且對task進行了包裝。 -
execute()方法就是線程池執(zhí)行的核心了。結(jié)合配置的線程和核心數(shù)，拒絕策略，有三種場景
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
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.
*/
//獲取線程池狀態(tài)，從而判斷場景
//場景一：worker小于核心線程數(shù)
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//場景二：線程在運行，且任務隊列大于核心線程數(shù)
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);
}
//場景三：非核心工作線程邏輯
else if (!addWorker(command, false))
//執(zhí)行拒絕策略
reject(command);
}
通過對源碼的分析發(fā)現(xiàn)無論如何workQueue和addWorker（cammand,boolean）是關(guān)鍵`。
-
workQueue是一個阻塞隊列，用于存放所有被提交到線程池的任務（線程）。 -
addWorker()：線程池里面都是工人的概念，增加一個工人來執(zhí)行線程任務。工人可復用。
/**
* The queue used for holding tasks and handing off to worker
* threads. We do not require that workQueue.poll() returning
* null necessarily means that workQueue.isEmpty(), so rely
* solely on isEmpty to see if the queue is empty (which we must
* do for example when deciding whether to transition from
* SHUTDOWN to TIDYING). This accommodates special-purpose
* queues such as DelayQueues for which poll() is allowed to
* return null even if it may later return non-null when delays
* expire.
*/
private final BlockingQueue<Runnable> workQueue;
addWorker()的工作主要還是初始化工人并且讓工人開始工作，執(zhí)行任務。并且維護一個HashSet來維護工人workers集合
private final HashSet<Worker> workers = new HashSet<Worker>();
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 {
//初始化了一個工人，Worker持有一個thread
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();
//增加一個工人
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
//執(zhí)行任務
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
Worker工人是ThreadPoolExecutor的一個內(nèi)部類，實現(xiàn)了Runnable說明他本身是一個任務，繼承了AbstractQueuedSynchronizer說明有使用AQS保證一些執(zhí)行動作是同步
private final class Worker extends AbstractQueuedSynchronizer implements Runnable {
private static final long serialVersionUID = 6138294804551838833L;
//持有線程，該線程有配置的線程工廠產(chǎn)生
final Thread thread;
//持有需要執(zhí)行的任務本身
Runnable firstTask;
volatile long completedTasks;
Worker(Runnable var2) {
this.setState(-1);
this.firstTask = var2;
this.thread = ThreadPoolExecutor.this.getThreadFactory().newThread(this);
}
//實現(xiàn)了run方法
public void run() {
ThreadPoolExecutor.this.runWorker(this);
}
protected boolean isHeldExclusively() {
return this.getState() != 0;
}
protected boolean tryAcquire(int var1) {
if (this.compareAndSetState(0, 1)) {
this.setExclusiveOwnerThread(Thread.currentThread());
return true;
} else {
return false;
}
}
protected boolean tryRelease(int var1) {
this.setExclusiveOwnerThread((Thread)null);
this.setState(0);
return true;
}
public void lock() {
this.acquire(1);
}
public boolean tryLock() {
return this.tryAcquire(1);
}
public void unlock() {
this.release(1);
}
public boolean isLocked() {
return this.isHeldExclusively();
}
void interruptIfStarted() {
Thread var1;
if (this.getState() >= 0 && (var1 = this.thread) != null && !var1.isInterrupted()) {
try {
var1.interrupt();
} catch (SecurityException var3) {
}
}
}
}
通過源碼分析我們知道Worker的run方法調(diào)用了runWorker()方法，其中的同步操作就是task.run(),執(zhí)行具體任務的run方法，而task的來源除了worker持有的firtTask，就是getTask()方法從workQueue中獲取得到了。
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 {
//回收工人
processWorkerExit(w, completedAbruptly);
}
}
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;
}
}
}
最后還會通過processWorkerExit然當前工人執(zhí)行回收策略，回收工人。循環(huán)使用了workers
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);
}
}
