结构和参数
| 字段 | 代码 | 含义 |
|---|---|---|
| COUNT_BITS | Integer.SIZE - 3 | 30 |
| CAPACITY | (1 << COUNT_BITS) - 1 | 00011111_11111111_11111111_11111111 |
| RUNNING | -1 << COUNT_BITS | 11100000_00000000_00000000_00000000 |
| SHUTDOWN | 0 << COUNT_BITS | 00000000_00000000_00000000_00000000 |
| STOP | 1 << COUNT_BITS | 00100000_00000000_00000000_00000000 |
| TIDYING | 2 << COUNT_BITS | 01000000_00000000_00000000_00000000 |
| TERMINATED | 3 << COUNT_BITS | 01100000_00000000_00000000_00000000 |
//取最高的3位
private static int runStateOf(int c) { return c & ~CAPACITY; }
//取低29位
private static int workerCountOf(int c) { return c & CAPACITY; }
//控制位状态 + 线程数量得出来的 ctl值
private static int ctlOf(int rs, int wc) { return rs | wc; }
//c < s
private static boolean runStateLessThan(int c, int s) {
return c < s;
}
//c >= s
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
//是否是RUNNING状态
private static boolean isRunning(int c) {
return c < SHUTDOWN;
}
//尝试增加worker数
private boolean compareAndIncrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect + 1);
}
//尝试减少worker数
private boolean compareAndDecrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect - 1);
}
//减少worker数
private void decrementWorkerCount() {
do {} while (! compareAndDecrementWorkerCount(ctl.get()));
}
过程
execute
提交任务
- 如果运行的线程数比核心线程数少，开启新的线程
- 如果核心线程数满了提交到队列， 提交之后检查线程池是否已经关闭、工作线程的数量；如果已经关闭，则拒绝发送请求；如果工作线程数量为0，则创建新的线程。
- 如果提交到队列也失败，那么尝试添加新的线程。如果线程数大于最大的线程数，拒绝提交。
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//1
//if条件成功之后，addWorker过程可能遇到cas操作失败，核心线程已经被填满的情况
//这时候就会失败，变成提交队列
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//2
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);
}
//3
else if (!addWorker(command, false))
reject(command);
}
addWorker
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 1. RUNNING状态可以提交任何任务
// 2. 大于SHUTDOWN不能提交任务
// 3. 正在SHUTDOWN并且工作队列有任务，可以提交空任务
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
//当前任务数已经超过系统限制
//(核心任务)当前任务数超过核心线程数，则失败
//(非核心任务)当前任务数超过最大任务数，则失败
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//cas增加任务数量
if (compareAndIncrementWorkerCount(c))
break retry;
//cas增加任务数失败，导致失败的原因可能是状态变化，或者任务数量变化;
//状态变化，要重新执行retry过程
//任务数量变化，直接执行添加任务过程
c = ctl.get();
if (runStateOf(c) != rs)
continue retry;
}
}
//这时已经添加了worker数
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//创建worker(同时也新建了线程)
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());
//RUNNING状态可以提交任务
//SHUTDOWN状态可以提交空任务
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();
}
//添加之启动这个线程
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
worker线程运行
- 核心线程不断拉取queue中的内容 (可设置超时)
- queue中无任务时核心线程阻塞
- 超过queue大小时，创建额外线程，额外线程运行完任务后就结束，保证线程只有核心线程在循环
runWorker
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//获取不到task就退出
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();
} 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;
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
//SHUTDOWN并且workQueue为空
//>SHUTDOWN
//两种情况需要退出
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
//是否允许线程超时
//核心线程超时，需要设置allowCoreThreadTimeOut
//非核心线程自带超时功能
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//非核心线程，执行完就退出
//超时并且队列为空，则退出
//超时并且有至少一个工作线程，则退出
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//设置了超时 => 超时获取
//未设置超时 => 阻塞获取
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
//超时
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
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