线程池原理
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2022-05-22 12:34:03
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线程池原理
- 以ThreadPoolExecutor为例
private static final ThreadPoolExecutor EXECUTOR_SERVICE = new ThreadPoolExecutor(100,120,60, TimeUnit.SECONDS,
new ArrayBlockingQueue<>(1000),
new ThreadFactoryBuilder().setNameFormat("thread-pool-%d").build(),
new ThreadPoolExecutor.CallerRunsPolicy());
为什么要使用线程池
- 使用线程池主要有以下好处:
- 降低资源消耗。通过复用已存在的线程和降低线程关闭的次数尽可能降低系统性能损耗
- 提升系统响应速度。通过复用线程，省去创建线程的过程，因此整体上提升了系统的响应速度
- 提高线程的可管理性。线程是稀缺资源，如果无限制的创建，不仅会消耗系统资源，还会降低系统的稳定性，因此，需使用线程池来管理线程。
实现原理
- 线程池内部常量
//ct1的低29位表示线程池中的线程数，高3为表示当前线程状态
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
//运行状态，高3位为111
private static final int RUNNING = -1 << COUNT_BITS;
//关闭状态，高3位为000，在此状态下，线程池不再接受新任务，但是仍然处理阻塞队列中的任务
private static final int SHUTDOWN = 0 << COUNT_BITS;
//停止状态，高3位为001，在此状态下，线程池不再接受新任务，也不会处理阻塞队列中的任务，正在运行的任务也会停止
private static final int STOP = 1 << COUNT_BITS;
//高3位为010
private static final int TIDYING = 2 << COUNT_BITS;
//终止状态，高3位为011
private static final int TERMINATED = 3 << COUNT_BITS;
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
- Worker继承AQS框架，方便实现工作线程的中止、线程安全等操作
- Worker实现Runnable接口，将自身作为一个task在工作线程中执行
核心函数execute分析
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//workerCountOf计算当前线程池的线程数，如果小于corePoolSize，创建新线程
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//进入这里说明核心线程数满了
//判断当前线程是运行状态并且阻塞队列成功插入command，继续执行
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//二次检查，判断
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//阻塞队列已满，再尝试创建一次线程，如果失败就执行饱和策略
else if (!addWorker(command, false))
reject(command);
}
addWorker分析
private final HashSet<Worker> workers = new HashSet<Worker>();
//core代表是否是核心线程
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//判断当前线程状态是否正常，不正常则直接返回
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
//CAPACITY=2^29-1
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//CAS操作workerCount，成功则继续后面操作
if (compareAndIncrementWorkerCount(c))
break retry;
//如果CAS操作失败，再次循环
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
}
}
//下面代码就是尝试往workers插入新worker
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//将任务包装成worker
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
//使用ReentrantLock保证线程安全
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
//判断线程运行状态是否符合要求
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
//判断线程是否已启动
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
//设置池已使用容量
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
//如果worker正常插入，线程执行
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
- t.start()实际上执行的是Worker中的run方法，而run方法中执行的是runWorker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 {
//如果worker自己的task为null，那么就调用getTask从阻塞队列中获取等待任务执行，否则阻塞该方法
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);
}
}
- getTask
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);
// 如果线程池允许线程超时或者当前线程数大于核心线程数，则会进行超时处理
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
//如果在keepAliveTime时间内阻塞队列有任务，返回该任务并执行
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
//如果阻塞队列为空，当前线程阻塞，当队列有任务时，线程被唤醒，执行take返回的任务
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}