简介
在并发编程中，线程池是不可或缺的工具。允许我们复用线程资源，减少线程创建和销毁的开销，提高程序响应速度。在Java中，ThreadPoolExecutor是核心实现。
线程池预先创建一定数量的线程存放于容器中，当需要执行新任务时，从容器中取出一个线程执行任务，而不是每次都创建新的线程。
ThreadPoolExecutor 重要属性及方法
低29位保存线程池线程数，高3位保存线程池状态
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;
// runState is stored in the high-order bits
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;
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
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线程池线程数
private static int workerCountOf(int c) { return c & CAPACITY; }
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线程池状态
private static int runStateOf(int c) { return c & ~CAPACITY; }
RUNNING
初始状态，此状态下可接收新任务和执行已添加任务
SHUTDOWN
当调用**shutdown()**方法，由RUNNING 变为SHUTDOWN。
此状态，不接收新任务，能继续处理已添加的任务。
STOP
调用shutdownNow(),由RUNNING或SHUTDOWN变为STOP。
此状态，不接收任务，不处理已添加任务，尝试中断正在执行的任务。
TIDYING
所有任务已终止，且任务数量为0，线程池就会变为TIDYING。
此状态下，线程池会执行函数terminated()
状态转换：
SHUTDOWN下，阻塞队列为空且线程池中执行的任务也为空，由SHUTDOWN变为TIEDYING
STOP下，执行的任务为空，由STOP变为TIEDYING
TERMINATED
执行完钩子函数，彻底终止，并进入TERMINATED
最终状态，不在进行操作
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构造函数
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0) throw new IllegalArgumentException(); if (workQueue == null || threadFactory == null || handler == null) throw new NullPointerException(); this.acc = System.getSecurityManager() == null ? null : AccessController.getContext(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }
corePoolSize：基本线程数，空闲时也不会销毁（除非设置allowCoreThreadTimeOut，并且空闲时间超过keepAliveTime）
maximumPoolSize：线程池中允许最大线程数
keepAliveTime：线程数大于核心线程数时，空闲线程允许等待新任务最长时间
unit：keepAliveTime单位
workQueue：用于保存等待执行任务的阻塞队列
threadFactory：创建线程工厂
handler：无法处理新任务所用策略
CallerRunsPolicy：直接在调用execute方法的线程中运行该任务
AbortPolicy（默认）：抛出RejectedExecutionException异常
DiscardPolicy：直接丢弃，不抛异常
DiscardOldestPolicy：丢弃队列中等待最长的任务，重新提交当前任务
源码分析
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
// 核心线程
if (addWorker(command, true))
return;
c = ctl.get();
}
// 添加队列
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);
}
// 非核心线程
else if (!addWorker(command, false))
reject(command);
}
从源码可以看出执行流程
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线程数小于核心线程数，创建核心线程，返回；
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RUNNING状态下，直接添加到队列中
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非running状态，任务移除队列，并执行拒绝策略
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再次检查线程数，为0，创建非核心线程
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添加队列失败，创建非核心线程；
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非核心线程创建失败，执行拒绝策略；
下面分析源码具体逻辑
addWorker(Runnable firstTask, boolean core)：创建线程
private boolean addWorker(Runnable firstTask, boolean core) {
// 1. 线程数+1
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 校验
// 非running && (shutdown && 任务为空或者队列为空)，可以接收新任务，其他情况直接返回
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
// 线程数+1
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
}
}
// 2. 创建线程
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 {
// 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
// t为工作线程，仍然存活，但已shutdown并且没有执行的任务，这种情况不被允许的
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;
}
分2部分
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增加线程数
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校验：非running && (shutdown && 任务为空或者队列为空)，可以接收新任务，其他情况直接返回
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线程数+1
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创建线程
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创建线程添加到容器中，并启动当前线程
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异常处理：代码已注释，此时应该调用addWorkerFailed（）
启动创建的线程，也就是Worker中的run方法
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())
// stop 尝试中断任务
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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执行当前任务或从队列中取任务执行
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最后处理当前这个线程：processWorkerExit
getTask()：从队列中获取任务
private Runnable getTask() {
// 是否因为超时而没获取到任务
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())) {
// stop 及之后不在处理任务 或 队列为空
// 线程数-1
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
// 允许核心线程超时或者大于核心线程数
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
// 线程数大于 允许线程数 或 允许超时同时超时未获取到 现成数大于1或队列为空
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
// 线程数-1
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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处于SHUTDOWN或STOP，并且队列为空，直接返回null
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超过核心线程数，允许超时，队列轮训获取任务，获取不到返回null
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如果没有超时限制，阻塞等待（take）获取一个任务
processWorkerExit：工作线程退出
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
// 工作线程异常，线程数-1
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);// 移除工作线程
} finally {
mainLock.unlock();
}
tryTerminate();// 尝试终止线程池
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
// 工作线程正常退出，是否需要创建新的线程
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);// 创建工作线程
}
}
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异常退出：completedAbruptly（true），工作线程-1
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更新完成任务计数，移除工作线程
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调用tryTerminate，是否可关闭线程池
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是否需要添加线程
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状态低于STOP（接收新任务或处理队列任务）
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工作线程正常退出
如果队列不为空，核心线程数为0，需要添加线程处理队列任务
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