Android线程池源码阅读记录介绍
作者：一颗暗色星辰
大家好，本篇文章主要讲的是Android线程池源码阅读记录介绍，感兴趣的同学赶快来看一看吧，对你有帮助的话记得收藏一下，方便下次浏览
今天面试被问到线程池如何复用线程的？当场就懵掉了...于是面试完毕就赶紧打开源码看了看，在此记录下:
我们都知道线程池的用法，一般就是先new一个ThreadPoolExecutor对象，再调用execute(Runnable runnable)传入我们的Runnable，剩下的交给线程池处理就行了，于是这次我就从ThreadPoolExecutor的execute方法看起：
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();
//1.如果workerCountOf(c)即正在运行的线程数小于核心线程数，就执行addWork
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//2.如果线程池还在运行状态并且把任务添加到任务队列成功
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//3.如果线程池不在运行状态并且从任务队列移除任务成功，执行线程池饱和策略（默认直接抛出异常）
if (! isRunning(recheck) && remove(command))
reject(command);
//4.否则如果此时运行线程数==0，就直接调用addWork方法
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//5.如果2条件不成立，继续判断如果addWork返回false，执行线程池饱和策略
else if (!addWorker(command, false))
reject(command);
}
大致过程就是如果核心线程未满，则直接addWorker（该方法下面会再分析）；如果核心线程已满，则尝试将任务加进消息队列中，并再判断如果此时运行线程数==0则调addWorker方法，否则不做任何处理（因为运行的线程处理完自己的任务后会去消息队列中取任务来执行，下面会分析）；如果任务队列添加任务失败，那么直接addWorker()，如果addWorker返回false，执行饱和策略，下面我们就来看看addWorker里面做了什么
/**
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
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);
//1.如果正在运行的线程数大于corePoolSize 或 maximumPoolSize（core代表以核心线程数还是最大线程数为边界），return false，表示addWorker失败
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//2.否则将运行线程数+1，并跳出这个for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 {
//3.创建一个Worker对象，传入我们的runnable
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) {
//4.开始启动线程
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
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);
}
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//1.当firstTask不为空或getTask不为空时一直循环
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 {
//2.执行任务
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);
}
}
可以看到addWorker方法主要就是先判断正在运行线程数是否超过了最大线程数（具体根据边界取），如果未超过则创建一个worker对象，其中firstTask是我们传入的Runnable，当然根据上面的execute方法可知当4条件满足时，传入的firstTask是null，Thread是用ThreadFactory创建的线程，传入的Runnable是Worker自己，最后开启线程，于是执行Worker这里的run、runWorker方法，在runWorker方法里，开启一个while循环，当firstTask不为空或getTask不为空时，执行task，下面我们接着看看getTask里面做了什么：
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?
//1.会不会淘汰空闲线程
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//2.return null意味着回收一个Worker即淘汰一个线程
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//3.等待指定时间
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
可以看1、2注释，allowCoreThreadTimeOut代表存活一定时间是否对核心线程有效（默认为false），先看它为ture的情况，此时不管是核心线程还是非核心线程在3处都会等待一定时间（就是我们传入的线程保活时间），等待时间内如果从任务队列取到任务，则返回执行，否则timeout为true，继续走到2，由于(timed && timedOut)和workQueue.isEmpty()均为true，返回null，代表回收一个线程；如果allowCoreThreadTimeOut为false，代表不回收核心线程，此时如果在3处没有取到任务，继续执行到2处，只有当wc > corePoolSize或wc > maximumPoolSize时才会执行return null，否则一直循环，相当于该线程一直处于运行状态，直到从任务队列拿到新的任务
到此这篇关于Android线程池源码阅读记录介绍的文章就介绍到这了,更多相关Android线程池内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！