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在多线程编程中，线程池是一种非常重要的技术。它可以帮助我们有效地管理线程资源，避免频繁地创建和销毁线程，从而提高系统的性能和稳定性。Java提供了java.util.concurrent包来支持线程池的实现，其中最核心的类是ThreadPoolExecutor。本文将深入分析ThreadPoolExecutor的实现原理，并通过源码解析其工作流程。
线程池是一种多线程处理形式，它通过预先创建一定数量的线程，并将任务提交到线程池中执行，从而避免了频繁创建和销毁线程的开销。线程池中的线程可以重复使用，执行完一个任务后，线程不会被销毁，而是继续执行下一个任务。
ThreadPoolExecutor类ThreadPoolExecutor是Java线程池的核心实现类，它提供了线程池的基本功能。ThreadPoolExecutor继承自AbstractExecutorService，并实现了ExecutorService接口。
ThreadPoolExecutor使用一个AtomicInteger类型的变量ctl来表示线程池的状态和线程数量。ctl的高3位表示线程池的状态，低29位表示线程池中的线程数量。
线程池的状态包括：
ThreadPoolExecutor的构造方法ThreadPoolExecutor提供了多个构造方法，最常用的构造方法如下：
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;
}
该构造方法初始化了线程池的核心参数，包括核心线程数、最大线程数、空闲时间、任务队列、线程工厂和拒绝策略。
execute方法execute方法是线程池的核心方法，用于提交任务到线程池中执行。其源码如下：
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);
}
execute方法的主要逻辑如下：
null，如果是则抛出NullPointerException。addWorker方法addWorker方法用于创建新线程并执行任务。其源码如下：
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;
}
addWorker方法的主要逻辑如下：
false。false。Worker对象，并将其添加到workers集合中。Worker线程，如果启动成功则返回true，否则调用addWorkerFailed方法进行清理。runWorker方法runWorker方法是Worker线程的执行逻辑。其源码如下：
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);
}
}
runWorker方法的主要逻辑如下：
null，则执行任务。null，则调用getTask方法从任务队列中获取任务。beforeExecute方法进行前置处理。afterExecute方法进行后置处理。completedAbruptly为true。processWorkerExit方法处理线程退出。getTask方法getTask方法用于从任务队列中获取任务。其源码如下：
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;
}
}
}
getTask方法的主要逻辑如下：
null。poll方法从任务队列中获取任务，如果超时则返回null。take方法从任务队列中获取任务，如果任务队列为空则阻塞等待。processWorkerExit方法processWorkerExit方法用于处理线程退出。其源码如下：
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);
}
}
processWorkerExit方法的主要逻辑如下：
workers集合中移除该线程。tryTerminate方法尝试终止线程池。ThreadPoolExecutor提供了四种内置的拒绝策略：
RejectedExecutionException异常。除了使用内置的拒绝策略外，我们还可以自定义拒绝策略。自定义拒绝策略需要实现RejectedExecutionHandler接口，并重写rejectedExecution方法。
public class CustomRejectedExecutionHandler implements RejectedExecutionHandler {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
// 自定义拒绝策略的逻辑
}
}
线程池的监控主要包括以下几个方面：
isShutdown、isTerminated等方法监控线程池的状态。getPoolSize、getActiveCount等方法监控线程池中的线程数量。getQueue方法获取任务队列，并监控其大小。getCompletedTaskCount方法获取已完成任务的数量。线程池的调优主要包括以下几个方面：
SynchronousQueue；如果任务较少且执行时间较长，可以选择LinkedBlockingQueue。CallerRunsPolicy；如果任务的重要性较低，可以选择DiscardPolicy。本文详细分析了Java线程池的实现原理，并通过源码解析了ThreadPoolExecutor的工作流程。线程池是多线程编程中非常重要的技术，合理地使用线程池可以显著提高系统的性能和稳定性。在实际开发中，我们应根据系统的负载情况和任务的特性，合理地设置线程池的参数，并进行监控和调优，以确保线程池的高效运行。
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