版权声明：本文为博主原创文章，未经博主允许不得转载。技术交流可邮:cjh94520@outlook.com https://blog.csdn.net/cjh94520/article/details/70545202
线程池
之前一直有这个疑问：我们平时使用线程都是各种new Thread(),然后直接在run()方法里面执行我们要做的各种操作，使用完后需要做什么管理吗？线程池为什么能维持住核心线程不释放，一直接收任务进行处理呢？
线程
线程无他，主要有两个方法，我们先看看start()方法介绍：
/**
* Causes this thread to begin execution; the Java Virtual Machine
* calls the run method of this thread.
*
* The result is that two threads are running concurrently: the
* current thread (which returns from the call to the
* start method) and the other thread (which executes its
* run method).
*
* It is never legal to start a thread more than once.
* In particular, a thread may not be restarted once it has completed
* execution.
*
* @exception IllegalThreadStateException if the thread was already
* started.
* @see #run()
* @see #stop()
*/
public synchronized void start() {
if (threadStatus != 0)
throw new IllegalThreadStateException();
/* Notify the group that this thread is about to be started
* so that it can be added to the group's list of threads
* and the group's unstarted count can be decremented. */
group.add(this);
started = false;
try {
nativeCreate(this, stackSize, daemon);
started = true;
} finally {
try {
if (!started) {
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
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从这个方法解释上看，start()这个方法，最终会交给VM 去执行run()方法，所以一般情况下，我们在随便一个线程上执行start(),里面的run()操作都会交给VM 去执行。
而且还说明，重复启用线程是不合法的，当一个线程完成的时候，may not be restarted once。
那么这种情况下，线程池是怎么做的？他为什么就能够重复执行各种任务呢？
带着各种疑问，我们去看看线程池自己是怎么实现的。
线程池
线程池常用的创建方法有那么几种：
1. newFixedThreadPool()
2. newSingleThreadExecutor()
3. newCachedThreadPool()
4. newScheduledThreadPool()
这4个方法创建的线程池实例具体就不一一介绍，无非是创建线程的多少，以及回收等问题，因为其实这4个方法最后都会调用统一的构造方法：
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
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具体来说只是这几个值的不同决定了4个线程池的作用：
1. corePoolSize 代表核心线程池的个数，当线程池当前的个数大于核心线程池的时候，线程池会回收多出来的线程
2. maximumPoolSize 代表最大的线程池个数，当线程池需要执行的任务大于核心线程池的时候，会创建更多的线程，但是最大不能超过这个数
3. keepAliveTime 代表空余的线程存活的时间，当多余的线程完成任务的时候，需要多长时间进行回收，时间单位是unit 去控制
4. workQueue 非常重要，这个工作队列会存放所有待执行的Runnable对象
@param workQueue the queue to use for holding tasks before they areexecuted. This queue will hold only the {@code Runnable} tasks submitted by the {@code execute} method.
1
我们平时在使用线程池的时候，都是直接 实例.execute(Runnable)，一起跟进去，看看这个方法具体做了什么
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.
*/
//结合上文的注释，我们得知，第一次，先判断当前的核心线程数，
//如果小于初始化的值，马上创建；然后第二个if,将这个任务插入到工作线程，双重判断任务，
//假定如果前面不能直接加入到线程池Worker集合里，则加入到workQueue队列等待执行。
//里面的if else判断语句则是检查当前线程池的状态。如果线程池本身的状态是要关闭并清理了，
//我们则不能提交线程进去了。这里我们就要reject他们。
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);
}
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所以其实主要起作用的还是addWorker()方法，我们继续跟踪进去：
private boolean addWorker(Runnable firstTask, boolean core) {
···多余代码
try {
w = new Worker(firstTask); 1.重点
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(); 2. 重点
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
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我们看重点部分，其实最重要的是firstTask这个Runnable，我们一直跟踪这个对象就可以了，这个对象会new Worker()，那么这个wroker()就是一个包装类，里面带着我们实际需要执行的任务，后面进行一系列的判断就会执行t.start(); 这个t 就是包装类worker类里面的Thread,所以整个逻辑又转化进去Worker内部。
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;
/**
* 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);
}
...省略代码
}
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这个Worker包装类，重要的属性两个，thread 就是刚才上面那个方法执行的start()对象，这个thread又是把这个worker对象本身作为一个Runnable对象构建出来的，那么当我们调用thread.start()方法时候，实际调用的就是Worker类的run()方法。现在又要追踪进去，看这个runWorker(this)，做的是什么鬼东西
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);
}
}
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这个方法还是比较好懂的：
1. 一个大循环，判断条件是task != null || (task = getTask()) != null，task自然就是我们要执行的任务了，当task空而且getTask()取不到任务的时候，这个while()就会结束，循环体里面进行的就是task.run();
2.这里我们其实可以打个心眼，那基本八九不离十了，肯定是这个循环一直没有退出，所以才能维持着这一个线程不断运行，当有外部任务进来的时候，循环体就能getTask()并且执行。
3.下面最后放getTask()里面的代码，验证猜想
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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真相大白了，里面进行的也是一个死循环，主要看 Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
工作队列workQueue会一直去拿任务，属于核心线程的会一直卡在 workQueue.take()方法，直到拿到Runnable 然后返回，非核心线程会 workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) ，如果超时还没有拿到，下一次循环判断compareAndDecrementWorkerCount就会返回null,Worker对象的run()方法循环体的判断为null,任务结束，然后线程被系统回收
总结
一句话可以概述了，线程池就是用一堆包装住Thread的Wroker类的集合，在里面有条件的进行着死循环，从而可以不断接受任务来进行。
---------------------
作者：cjh94520
来源：CSDN
原文：https://blog.csdn.net/cjh94520/article/details/70545202
版权声明：本文为博主原创文章，转载请附上博文链接！