一、线程的复用方法
创建一个Thread对象,start一次之后,等到执行完成再次start,会抛出异常,执行失败。
但是有这样的一个模型,可以使得一个线程先后执行多个任务。即,在新线程中使用一个循环,直接调用各个Runnable的run方法,而不是把这些Runnable封装成单独线程。
伪代码:
class PooledThread implements Runnable {
private Queue<Runnable> taskQueue;
private boolean stop;
@Override
public void run() {
while (!stop) {
Runnable task = taskQueue.get();
if (task == null) Thread.sleep(10);
else task.run();
}
}
}
二、ThreadPoolExecutor类初始化方法
从ThreadPoolExecutor类参数最多的构造方法来看:
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 |
“核心”线程数,线程池工作时的最少线程数 |
maximumPoolSize |
最大线程数。线程池中的线程总数不超过这个数 |
keepAliveTime |
当线程池中的线程数量“核心”线程数时,多余线程的最长空闲时间。超时后,这些线程将被销毁 |
workQuueue |
当线程数达到“核心线程数”时,用于暂存任务的阻塞队列。类型与大小都可以自定义 |
threadFactory |
线程工厂,可以用来指定线程池中线程的名称等属性 |
handler |
当阻塞队列满、线程数量也达到最大时的兜底处理策略 |
三、ThreadPoolExecutor类大致工作状态
ThreadPoolExecutor线程池对象刚构建起来时,内部的worker数(一个线程的包装)为0。随着任务被提交至线程池,线程池内的动作如下:
情形 |
任务提交后,线程池的处理方式 |
---|---|
worker数量未达到corePoolSize |
新建一个worker用于处理被提交的任务 |
worker数量已经达到corePoolSize |
任务进入阻塞队列workQueue中,随后被已存在的worker读取并执行 |
workerQueue队列中的任务达到队列容量上限 |
继续新建worker用于处理任务,直到worker数量达到maximumPoolSize |
workerQueue队列满,且worker数量达到maximumPoolSize |
提交的任务被拒绝执行,进入handle定义的兜底逻辑 |
线程池本身的状态有以下五种。代码中的注释如下:
/* The runState provides the main lifecycle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
*
* RUNNING -> SHUTDOWN
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING
* When both queue and pool are empty
* STOP -> TIDYING
* When pool is empty
* TIDYING -> TERMINATED
* When the terminated() hook method has completed
*/
线程池状态只会从上面的值变为下面的值。但中间可能跳过某些状态。
四、源代码
1、execute方法
execute方法用于提交一个任务。当线程池处于不同状态时,任务被处理的方法也不一样。
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.
*/
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) { //路径一
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) { //路径二
int recheck = ctl.get();
if (!isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false)) //路径三
reject(command); //路径四
}
ctl是一个AutomicInteger对象,前三位是线程池的runState,后29位表示线程池中的worker数量。
路径一:
如果当前worker数少于corePoolSize,则创建一个作为核心线程的worker。
addWorker的第二个参数,表明新建worker是受corePoolSize数量限制,还是受maximumPoolSize数量限制(而不是新建的worker是核心的还是非核心的,worker没有这个标记)。
路径二:
若路径一失败,将任务放入阻塞队列。进入队列的任务,除非遇到线程池强制关闭等特殊情况,应当被执行。
即使将任务成功放入阻塞队列,也需要二次检查,线程池是否还在运行(增加了方法返回值的准确性),或者worker数是否已经降到0(此时新增一个worker。add一个null任务的worker是一种特殊用法,仅创建worker而不创建任务,见下面的解析)
路径三:
若路径二仍然失败,新增一个worker。
路径四:
若路径三仍然失败,拒绝该任务并调用兜底策略处理之。
2、addWorker方法
addWorker方法用于新增一个worker。worker是ThreadPoolExecutor的一个内部类,是对一个运行线程的包装。
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 若已经关闭了,拒绝这个任务,但是有一种同时满足三个条件的特殊情况
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
// 检查当前worker数量。如果worker数量收到限制(可见core只是选择受哪一个限制,而不是给worker打上某种标记),则失败
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//院子操作并跳出这个循环
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 {
//制造一个新worker
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 {
//如果worker没起起来,回滚各项操作
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
如果线程池已经不是Running状态,则不允许新建worker。
另外的&&条件是一种特殊情况,说明在Shutdown状态,使用null来创建一个新worker,处理workQueue(阻塞队列)中残留的任务。同时满足这三个条件,是允许创建worker的。
此方法的第二个参数core仅仅用于容量检查,而不是给worker打上“核心”或者“非核心”的标记。worker上并没有这种标记。
compareAndIncrementWorkerCount方法操作的是c的本体ctl,是一个AtomicInteger对象,其内部实际调用ctl.compareAndSet(c,c+1)。也就是说,从得到c的值到此处,c的值不能被修改(worker数没有增减)。
成功则将worker计数加1,打破retry循环。
失败则继续向下执行,得到最新的ctl值。如果runState没有变,continue小循环,再试着检查池中线程数并加1;如果变了,continue外层循环,再次获得rs。
将任务传递给一个新建的Worker,w.thread是被worker包装的线程。检查runState,并以此判断是否把worker加入集合。第二个条件与上面的连续&&条件起的作用是一样的。t实际上是worker的包装(t和worker互相包装),执行的是runWorker方法
3、runWorker方法
上一节的t.start,t就是Worker类中thread字段。thread字段是ThreadFactory对自己的包装,因此,t.start实际运行的是Worker.run,也就是ThreadPoolExecutor.runWorker方法。
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.
* @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);
}
...
...
}
再看runWorker的代码,
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);
}
}
completedAbruptly用来表示,线程是否是异常退出的。
firstTask仅仅用一次,后面的task都是调用getTask方法,从阻塞队列中取出来的。
这个if条件不容易看明白。这是需要:
1、(runStateAtLeast(ctl.get(), STOP),或者Thread.interrupted() && runStateAtLeast(ctl.get(), STOP),满足其中一个
2、!wt.isInterrupted()一定要满足
分析:
如果线程池已经Stop,且还没有中断的,中断这个线程;
如果线程池还没有Stop,调用Thread.interrupted清除掉这个标志位,保证线程池不中断。恰巧清除之后的当间儿,线程池Stop了,则还是中断这个线程。
beforeExecute方法和afterExecute方法可以被重写,在每个任务前后执行。
执行完成一个任务之后,completedTasks计数加1。
整个worker结束之后,收尾方法processWorkerExit被调用。其中第二个参数completedAbruptly,说明了是循环自然结束的,还是因为抛出异常导致的。
4、getTask方法
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;
}
}
}
如果线程池已经Shutdown且阻塞队列空,或者线程池Stop,减去一个worker计数并返回null。这将导致上一个小节runWorker中止循环,一个worker消亡。
如果线程数超过corePoolSize,或者允许核心线程超时消亡,则将timed置为true,表示有超时返回null的需求
两组条件同时满足:
1、线程数超过maximumPoolSize,或者有超时需求且真的有超时
2、线程数大于1,或者阻塞队列已经空了
也扣减线程计数并返回null。但是为啥用的方法不一样?
timed表示是否有超时的需求,如果有的话,使用带有超时时间的poll方法;如果没有,使用无限阻塞的take方法。
看来超时撤销worker,需要执行到第二遍循环的时候,timeOut置为true,才能够做到。
当然,take阻塞调用有被中断的可能。因此有InterruptedException异常需要处理。
5、processWorkerExit方法
worker结束后用于处理的方法。completedAbruptly为true,表示worker是异常退出的。
假定worker正常退出时,线程池worker计数已经减1。上一节的getTask方法也确实是这样做的。
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly)
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);
}
}
统计completedTaskCount,原来是在worker结束的时候才加上的。
tryTerminate方法试图结束线程池。只要有worker结束,就将“试图”结束线程池,然而能否成功结束还要看很多条件。
若线程池还没达到Stop状态,则要小心,不能把所有的worker都给关完了。worker数量小于min(最小需要维持的数量)的话,还要给补上一个。