Java提供了一整套的线程管理机制，就是我们常常提起的线程池。今天我们来研究一下它的实现原理，重点研究的是我们提交的任务是如何得到执行的。
我们在构建ExecutorService时，都是调用Executors的方法。我们来查看一下Executors的方法。
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
我们以几个常用的为例，可以看到，它们都是调用了ThreadPoolExecutor这个类的构造。也就是说当我们得到的ExecutorService实际是ThreadPoolExecutor的实例。
下边我们来查看一下ThreadPoolExecutor的构造方法。
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), handler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
//构造方法是相互调用的，这个是最终执行的方法。
//首先进行异常判断。
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
//线程池线程数的大小。
this.corePoolSize = corePoolSize;
//线程池最大线程数
this.maximumPoolSize = maximumPoolSize;
//任务队列，非常重要，一会分析
this.workQueue = workQueue;
//当线程空闲的最大时间。
this.keepAliveTime = unit.toNanos(keepAliveTime);
//创建线程的工厂，使用默认的即可。
this.threadFactory = threadFactory;
//执行任务失败的回调。
this.handler = handler;
}
我们关心的其实是把任务提交之后，它的执行流程。所以来查看execute方法。
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.
*/
<!--获得当前运行的线程数，使用原子操作，线程安全。使用按位与的方式，保证这个c可以表示不同状态栏的线程，-->
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
<!--如果小于线程池的核心线程数，就调用addWorker，addWorker请看下边分析-->
if (addWorker(command, true))
return;
c = ctl.get();
}
<!--没有创建一个新的线程来执行当前command-->
<!--把command加入到当前的workQueue队列中-->
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
<!--如果线程池没有运行的线程，同时也可以把command成功出队，就调用reject，其实就是用那个处理错误的handler来处理-->
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
<!--如果线程数目为0，就重新创建，这次不限制只使用corePoolSize-->
addWorker(null, false);
}
else if (!addWorker(command, false))
<!--再创建线程执行一次,失败就通知handler来处理-->
reject(command);
}
<!--创建一个新的线程来执行当前任务，成功返回true-->
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
<!--如果当前线程池的状态是SHUTDOWN但是 firstTask不为空，或者workQueue还有任务，表示出现了异常，返回false-->
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
<!--开启循环去创建线程-->
for (;;) {
int wc = workerCountOf(c);
<!--如果当前线路程数已经大于或等于限制，创建失败，返回false-->
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
<!--尝试去创建线程，如果成功，就跳出循环 compareAndIncrementWorkerCount下边分析-->
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
<!--如果线程状态已经发生了变化，就跳出当前循环，继续外边的循环。-->
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 {
<!--Worker可以简单理解为就是一个封装咱们提交的Runnable的任务。-->
w = new Worker(firstTask);
<!--woker在new的同时会创建一个thread,后边分析-->
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();
<!--创建成功后把Worker加入到它的管理集合中-->
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;
}
这里我们已经明白了，当一个任务提交之后，当前线程数小于corePoolSize是会去创建一个Woker来执行它。有些情况下会把它加入到workQueue中，当加入到集合中它是如何执行的呢。
这就要说到Worker了，我们前边已经说了，有一个专门来执行任务的Woker对象，还有一个专门管理Worker的集合Workers。
<!--可以看到WorKer本身实现了 Runnable-->
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;
<!--这就是前边说的Worker里边的线程，它又把this放进去，所以前边addWorker中的t.start最后执行的其实就是worker里边run方法-->
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker. */
public void run() {
<!--最终是调用runWorker为实现的-->
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) {
}
}
}
}
<!--最终去执行任务的方法-->
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
<!--首先去把当前Worker身上绑定的任务取出来，然后把worker身上的任务置为null-->
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
<!--这里是关键，开启一个循环，当前任务不为空或者还能从getTask得到任务，就一直循环执行。-->
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
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 {
<!--最终没有任务时就会退出-->
processWorkerExit(w, completedAbruptly);
}
}
看似worker执行完任务是会退出的，实际上却很难，因为getTask()是个阻塞式的方法，只有在特殊情况下才会返回null。
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
<!--是一个死循环-->
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
只有当线程池关闭了并且 线程池停止或者队列为空的时候才会返回null, 正常使用的时候都不会进来。
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())) {
如果worker的数量比maximumPoolSize还大，或者 等待超时并且队列为空的时候 才会返回null
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;
}
}
}
到此为止，我们简单的线程池的实现原理分析了一下。流程是这样的，首先我们提交一个任务。线程池可能会new一个worker也就是线程来执行这个任务，也可能把它加入到任务队列中(创建worker失败)，然后等待执行。一旦一个worker启动之后，正常情况下它就不会停下来，执行完一个任务就执行下一个任务，因为取得任务的方法是阻塞式的。
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