摘要:从手写线程池开始,逐步的分析这些代码在Java的线程池中是如何实现的。
本文分享自华为云社区《手写线程池,对照学习ThreadPoolExecutor线程池实现原理!》,作者:小傅哥。
谢飞机,小记!,上次吃亏在线程上,这可能一次坑掉两次吗!
谢飞机:你问吧,我准备好了!!!
面试官:嗯,线程池状态是如何设计存储的?
谢飞机:这!下一个,下一个!
面试官:Worker 的实现类,为什么不使用 ReentrantLock 来实现呢,而是自己继承AQS?
谢飞机:我…!
面试官:那你简述下,execute 的执行过程吧!
谢飞机:再见!
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 10, 0L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<>(10));
threadPoolExecutor.execute(() -> {
System.out.println("Hi 线程池!");
});
threadPoolExecutor.shutdown();
// Executors.newFixedThreadPool(10);
// Executors.newCachedThreadPool();
// Executors.newScheduledThreadPool(10);
// Executors.newSingleThreadExecutor();
这是一段用于创建线程池的例子,相信你已经用了很多次了。
线程池的核心目的就是资源的利用,避免重复创建线程带来的资源消耗。因此引入一个池化技术的思想,避免重复创建、销毁带来的性能开销。
那么,接下来我们就通过实践的方式分析下这个池子的构造,看看它是如何处理线程的。
为了更好的理解和分析关于线程池的源码,我们先来按照线程池的思想,手写一个非常简单的线程池。
其实很多时候一段功能代码的核心主逻辑可能并没有多复杂,但为了让核心流程顺利运行,就需要额外添加很多分支的辅助流程。就像我常说的,为了保护手才把擦屁屁纸弄那么大!
关于图 21-1,这个手写线程池的实现也非常简单,只会体现出核心流程,包括:
public class ThreadPoolTrader implements Executor {
private final AtomicInteger ctl = new AtomicInteger(0);
private volatile int corePoolSize;
private volatile int maximumPoolSize;
private final BlockingQueue<Runnable> workQueue;
public ThreadPoolTrader(int corePoolSize, int maximumPoolSize, BlockingQueue<Runnable> workQueue) {
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
}
@Override
public void execute(Runnable command) {
int c = ctl.get();
if (c < corePoolSize) {
if (!addWorker(command)) {
reject();
}
return;
}
if (!workQueue.offer(command)) {
if (!addWorker(command)) {
reject();
}
}
}
private boolean addWorker(Runnable firstTask) {
if (ctl.get() >= maximumPoolSize) return false;
Worker worker = new Worker(firstTask);
worker.thread.start();
ctl.incrementAndGet();
return true;
}
private final class Worker implements Runnable {
final Thread thread;
Runnable firstTask;
public Worker(Runnable firstTask) {
this.thread = new Thread(this);
this.firstTask = firstTask;
}
@Override
public void run() {
Runnable task = firstTask;
try {
while (task != null || (task = getTask()) != null) {
task.run();
if (ctl.get() > maximumPoolSize) {
break;
}
task = null;
}
} finally {
ctl.decrementAndGet();
}
}
private Runnable getTask() {
for (; ; ) {
try {
System.out.println("workQueue.size:" + workQueue.size());
return workQueue.take();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
private void reject() {
throw new RuntimeException("Error!ctl.count:" + ctl.get() + " workQueue.size:" + workQueue.size());
}
public static void main(String[] args) {
ThreadPoolTrader threadPoolTrader = new ThreadPoolTrader(2, 2, new ArrayBlockingQueue<Runnable>(10));
for (int i = 0; i < 10; i++) {
int finalI = i;
threadPoolTrader.execute(() -> {
try {
Thread.sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("任务编号:" + finalI);
});
}
}
}
// 测试结果
任务编号:1
任务编号:0
workQueue.size:8
workQueue.size:8
任务编号:3
workQueue.size:6
任务编号:2
workQueue.size:5
任务编号:5
workQueue.size:4
任务编号:4
workQueue.size:3
任务编号:7
workQueue.size:2
任务编号:6
workQueue.size:1
任务编号:8
任务编号:9
workQueue.size:0
workQueue.size:0
以上,关于线程池的实现还是非常简单的,从测试结果上已经可以把最核心的池化思想体现出来了。主要功能逻辑包括:
好,那么以上呢,就是这个简单线程池实现的具体体现。但如果深思熟虑就会发现这里需要很多完善,比如:线程池状态呢,不可能一直奔跑呀!?、线程池的锁呢,不会有并发问题吗?、线程池拒绝后的策略呢?,这些问题都没有在主流程解决,也正因为没有这些流程,所以上面的代码才更容易理解。
接下来,我们就开始分析线程池的源码,与我们实现的简单线程池参考对比,会更加容易理解 !
以围绕核心类 ThreadPoolExecutor 的实现展开的类之间实现和继承关系,如图 21-2 线程池类关系图。
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;
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
在 ThreadPoolExecutor 线程池实现类中,使用 AtomicInteger 类型的 ctl 记录线程池状态和线程池数量。在一个类型上记录多个值,它采用的分割数据区域,高3位记录状态,低29位存储线程数量,默认 RUNNING 状态,线程数为0个。
图 22-4 是线程池中的状态流转关系,包括如下状态:
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);
}
在阅读这部分源码的时候,可以参考我们自己实现的线程池。其实最终的目的都是一样的,就是这段被提交的线程,启动执行、加入队列、决策策略,这三种方式。
private boolean addWorker(Runnable firstTask, boolean core)
第一部分、增加线程数量
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
}
}
第一部分、创建启动线程
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 {
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;
添加执行任务的流程可以分为两块看,上面代码部分是用于记录线程数量、下面代码部分是在独占锁里创建执行线程并启动。这部分代码在不看锁、CAS等操作,那么就和我们最开始手写的线程池基本一样了
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // 允许中断
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null)
w.lock();
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();
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
其实,有了手写线程池的基础,到这也就基本了解了,线程池在干嘛。到这最核心的点就是 task.run() 让线程跑起来。额外再附带一些其他流程如下;
如果你已经开始阅读源码,可以在 runWorker 方法中,看到这样一句循环代码 while (task != null || (task = getTask()) != null)。这与我们手写线程池中操作的方式是一样的,核心目的就是从队列中获取线程方法。
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;
}
}
}
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