ThreadPoolExecutor（二）
ThreadPoolExecutor实现流程
ThreadPoolExecutor中实现的具体方法
execute方法
简介：execute方法是线程池添加任务的核心方法，由方法内判断是添加还是执行拒绝策略
//ThreadPoolExecutor的核心方法
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//工作线程小于核心线程数 添加Woeker。
//如果失败进入下一步判断，成功则是直接添加进入workers线程集合内开始执行
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//判断线程池是否是RUNNING状态，并且添加至队列
if (isRunning(c) && workQueue.offer(command)) {
//双重校验
//1.如果添加至缓存队列之后，线程池状态变成非RUNNING状态，那么就队列移除任务，且执行拒绝策略
//如果线程池状态变成非RUNNING状态，且移除任务失败则添加一个空任务
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//1.线程池是RUNNING状态，添加至队列失败 或者 线程池是非RUNNING状态，去添加任务
else if (!addWorker(command, false))
//如果添加任务失败执行拒绝策略
reject(command);
}
addWorker方法
addWorker方法参数介绍：
- Runnable firstTask： 需要执行的任务
- boolean core：判断当前线程数与核心线程数比较还是最大线程数比较
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 校验线程池状态不为RUNNING，或者SHUTDOWN的时候缓存队列不为空返回失败
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//无限循环
for (;;) {
int wc = workerCountOf(c);
//工作线程大于核心或者最大线程返回失败
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//工作线程加 1 如果成功跳出循环
if (compareAndIncrementWorkerCount(c))
break retry;
//二次校验线程状态，CAS失败的原因一般是因为工作线程数量的改变
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//生成新的worker
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.
// 获得锁之后进行二次校验
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;
}
runWorker
简介：runWorker负责运行worker任务。其实runWorker(Worker)方法的整体逻辑是比较简单的，内部使用了一个while循环不断地获取任务对象，当能够获取到任务对象时，直接调用task.run()来执行任务，同时在任务执行前后都会调用相应的钩子方法
扩展：可以通过继承ThreadPoolExecutor重写beforeExecute 和 afterExecute 方法来对线程运行前后进行处理
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);
}
}