线程池的好处
- 频繁的创建和销毁线程，会带来性能的问题。线程的创建和销毁都需要时间，当有大量的线程创建和销毁时，那么这些时间的消耗则比较明显，将导致性能上的缺失。
- 线程池方便管理线程，定时执行线程，间隔执行线程。线程池能控制线程的最大并发数量，避免大量抢占资源导致的阻塞现象。
ThreadPoorExecutor构造方法
ThreadPoorExecutor是线程池的真正实现，它的构造方法的参数会影响线程池的功能。ThreadPoorExecutor的构造方法如下：
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)
- corePoolSize:核心线程数，即使在空闲状态，也会在线程池中存在的线程数量，除非设置了allowCoreThreadTimeOut。
- maximumPoolSize:允许在线程池中的最大线程数量。
- keepAliveTime：非核心线程最长空闲时间，超过这个时间，空闲的非核心线程会被回收，设置allowCoreThreadTimeOut=true，同样也会作用在核心线程中。
- unit：时间单位。
- workQueue：存储将被execute方法执行的Runnable任务的队列。
- threadFactory：线程工厂为线程池创建新线程的功能。
- handler：不常用，当线程容量到顶执行被阻塞时，handler被用来通知调用者。
Android中常用几种线程线程池
FixedThreadPool
ExecutorService executor1 = Executors.newFixedThreadPool(3);
核心线程数和最大线程数相同，一个无限的任务队列，就是说线程池一直有固定的线程数处理任务。
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
10个线程加入线程池，查看执行循序：
for(int i = 0 ; i < 10 ; i++){
final int index = i;
executor1.execute(new Runnable() {
@Override
public void run() {
Log.i(TAG, "ExecutorActivity run: 任务 = " + index + "，线程 = " + Thread.currentThread().getName());
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
fixthreadpool.gif
CachedThreadPool
Executors.newCachedThreadPool();
没有核心线程，最大线程数为2^31-1，线程空闲60秒后被回收，任务队列SynchronousQueue是一个不存储的，所以这个线程的特点是只要任务一来，马上就有线程去执行。
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
10个线程加入线程池，查看执行循序：
cachetheadpool.gif
SingleThreadPool
Executors.newSingleThreadExecutor()
线程池中只有一个核心线程，按顺序执行队列中的任务
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
10个线程加入线程池，查看执行循序：
singleThreadpool.gif
ScheduledThreadPool
ScheduledExecutorService executor = Executors.newScheduledThreadPool(3)
该线程最大的特点就可以延迟执行
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE,
DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
new DelayedWorkQueue());
}
ScheduledThreadPool的常用方法：
public ScheduledFuture<?> schedule(Runnable command,
long delay, TimeUnit unit);
public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
long initialDelay,
long period,
TimeUnit unit);
ThreadPoorExecutor执行任务的顺序
（corePoolSize -> workQueue -> maximumPoolSize）
- 当未超过核心线程数时，就直接创建一个核心线程去执行任务。
- 当超过核心线程数，就将任务加入到workQueue的任务队列中等待
- 当任务队列中任务添满时候，在不超过最大线程数的情况下启动线程去处理任务
- 当线程数量超过最大线程数时，RejectedExecutionHandler对象通知调用者
执行方法
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.
* 如果当前的线程数小于核心线程池的大小，根据现有的线程作为第一个Worker运行的线程，
* 新建一个Worker，addWorker自动的检查当前线程池的状态和Worker的数量，
* 防止线程池在不能添加线程的状态下添加线程
*
* 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.
* 如果线程入队成功，然后还是要进行double-check的，因为线程池在入队之后状态是可能会发生变化的
*
* 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.
*
* 如果task不能入队(队列满了)，这时候尝试增加一个新线程，如果增加失败那么当前的线程池状态变化了或者线程池已经满了
* 然后拒绝task
*/
int c = ctl.get();
//当前的Worker的数量小于核心线程池大小时，新建一个Worker。
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))//recheck防止线程池状态的突变，如果突变，那么将reject线程，防止workQueue中增加新线程
reject(command);
else if (workerCountOf(recheck) == 0)//上下两个操作都有addWorker的操作，但是如果在workQueue.offer的时候Worker变为0，
//那么将没有Worker执行新的task，所以增加一个Worker.
addWorker(null, false);
}
//如果workQueue满了，那么这时候可能还没到线程池的maxnum，所以尝试增加一个Worker
else if (!addWorker(command, false))
reject(command);//如果Worker数量到达上限，那么就拒绝此线程
}
核心方法：addWorker#####
Worker的增加和Task的获取以及终止都是在此方法中实现的，也就是这一个方法里面包含了很多东西。在addWorker方法中提到了Status的概念，Status是线程池的核心概念，这里我们先看一段关于status的注释：
/**
* 首先ctl是一个原子量，同时它里面包含了两个field，一个是workerCount，另一个是runState
* workerCount表示当前有效的线程数，也就是Worker的数量
* runState表示当前线程池的状态
* The main pool control state, ctl, is an atomic integer packing
* two conceptual fields
* workerCount, indicating the effective number of threads
* runState, indicating whether running, shutting down etc
*
* 两者是怎么结合的呢？首先workerCount是占据着一个atomic integer的后29位的，而状态占据了前3位
* 所以，workerCount上限是(2^29)-1。
* In order to pack them into one int, we limit workerCount to
* (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
* billion) otherwise representable. If this is ever an issue in
* the future, the variable can be changed to be an AtomicLong,
* and the shift/mask constants below adjusted. But until the need
* arises, this code is a bit faster and simpler using an int.
*
* The workerCount is the number of workers that have been
* permitted to start and not permitted to stop. The value may be
* transiently different from the actual number of live threads,
* for example when a ThreadFactory fails to create a thread when
* asked, and when exiting threads are still performing
* bookkeeping before terminating. The user-visible pool size is
* reported as the current size of the workers set.
*
* runState是整个线程池的运行生命周期，有如下取值：
* 1. RUNNING：可以新加线程，同时可以处理queue中的线程。
* 2. SHUTDOWN：不增加新线程，但是处理queue中的线程。
* 3.STOP 不增加新线程，同时不处理queue中的线程。
* 4.TIDYING 所有的线程都终止了（queue中），同时workerCount为0，那么此时进入TIDYING
* 5.terminated()方法结束，变为TERMINATED
* The runState provides the main lifecyle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
* 状态的转化主要是：
* RUNNING -> SHUTDOWN（调用shutdown()）
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP(调用shutdownNow())
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING（queue和pool均empty）
* When both queue and pool are empty
* STOP -> TIDYING（pool empty，此时queue已经为empty）
* When pool is empty
* TIDYING -> TERMINATED(调用terminated())
* When the terminated() hook method has completed
*
* Threads waiting in awaitTermination() will return when the
* state reaches TERMINATED.
*
* Detecting the transition from SHUTDOWN to TIDYING is less
* straightforward than you'd like because the queue may become
* empty after non-empty and vice versa during SHUTDOWN state, but
* we can only terminate if, after seeing that it is empty, we see
* that workerCount is 0 (which sometimes entails a recheck -- see
* below).
*/
下面是状态的代码：
//利用ctl来保证当前线程池的状态和当前的线程的数量。ps：低29位为线程池容量，高3位为线程状态。
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
//设定偏移量
private static final int COUNT_BITS = Integer.SIZE - 3;
//确定最大的容量2^29-1
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
//几个状态，用Integer的高三位表示
// runState is stored in the high-order bits
//111
private static final int RUNNING = -1 << COUNT_BITS;
//000
private static final int SHUTDOWN = 0 << COUNT_BITS;
//001
private static final int STOP = 1 << COUNT_BITS;
//010
private static final int TIDYING = 2 << COUNT_BITS;
//011
private static final int TERMINATED = 3 << COUNT_BITS;
//获取线程池状态，取前三位
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
//获取当前正在工作的worker,主要是取后面29位
private static int workerCountOf(int c) { return c & CAPACITY; }
//获取ctl
private static int ctlOf(int rs, int wc) { return rs | wc; }
接下来贴上addWorker方法看看：
/**
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked, which requires a
* backout of workerCount, and a recheck for termination, in case
* the existence of this worker was holding up termination.
*
* @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.
/**
* rs!=Shutdown || fistTask！=null || workCount.isEmpty
* 如果当前的线程池的状态>SHUTDOWN 那么拒绝Worker的add 如果=SHUTDOWN
* 那么此时不能新加入不为null的Task，如果在WorkCount为empty的时候不能加入任何类型的Worker，
* 如果不为empty可以加入task为null的Worker,增加消费的Worker
*/
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
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
}
}
Worker w = new Worker(firstTask);
Thread t = w.thread;
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 c = ctl.get();
int rs = runStateOf(c);
/**
* rs!=SHUTDOWN ||firstTask!=null
*
* 同样检测当rs>SHUTDOWN时直接拒绝减小Wc，同时Terminate，如果为SHUTDOWN同时firstTask不为null的时候也要Terminate
*/
if (t == null ||
(rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null))) {
decrementWorkerCount();
tryTerminate();
return false;
}
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
} finally {
mainLock.unlock();
}
t.start();
// It is possible (but unlikely) for a thread to have been
// added to workers, but not yet started, during transition to
// STOP, which could result in a rare missed interrupt,
// because Thread.interrupt is not guaranteed to have any effect
// on a non-yet-started Thread (see Thread#interrupt).
//Stop或线程Interrupt的时候要中止所有的运行的Worker
if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
t.interrupt();
return true;
}
addWorker中首先进行了一次线程池状态的检测：
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//判断当前线程池的状态是不是已经shutdown，如果shutdown了拒绝线程加入
//(rs!=SHUTDOWN || first!=null || workQueue.isEmpty())
//如果rs不为SHUTDOWN，此时状态是STOP、TIDYING或TERMINATED，所以此时要拒绝请求
//如果此时状态为SHUTDOWN，而传入一个不为null的线程，那么需要拒绝
//如果状态为SHUTDOWN，同时队列中已经没任务了，那么拒绝掉
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
其实是比较难懂的，主要在线程池状态判断条件这里：
如果是running，那么跳过if。
如果rs>=SHUTDOWN,同时不等于SHUTDOWN，即为SHUTDOWN以上的状态，那么不接受新线程。
如果rs>=SHUTDOWN，同时等于SHUTDOWN，同时first != null，那么拒绝新线程，如果first==null，那么可能是新增加线程消耗Queue中的线程。但是同时还要检测workQueue是否isEmpty()，如果为Empty，那么队列已空，不需要增加消耗线程，如果队列没有空那么运行增加first=null的Worker。
从这里是可以看出一些策略的首先
在rs>SHUTDOWN时，拒绝一切线程的增加，因为STOP是会终止所有的线程，同时移除Queue中所有的待执行的线程的，所以也不需要增加first=null的Worker了。
其次，在SHUTDOWN状态时，是不能增加first！=null的Worker的，同时即使first=null，但是此时Queue为Empty也是不允许增加Worker的，SHUTDOWN下增加的Worker主要用于消耗Queue中的任务。
SHUTDOWN状态时，是不允许向workQueue中增加线程的，isRunning(c) && workQueue.offer(command) 每次在offer之前都要做状态检测，也就是线程池状态变为>=SHUTDOWN时不允许新线程进入线程池了。
for (;;) {
int wc = workerCountOf(c);
//如果当前的数量超过了CAPACITY，或者超过了corePoolSize和maximumPoolSize（试core而定）
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//CAS尝试增加线程数，如果失败，证明有竞争，那么重新到retry。
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
//判断当前线程池的运行状态
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
这段代码做了一个兼容，主要是没有到corePoolSize 或maximumPoolSize上限时，那么允许添加线程，CAS增加Worker的数量后，跳出循环。
接下来实例化Worker,实例化Worker其实是很关键的，后面会说。
因为workers是HashSet线程不安全的，那么此时需要加锁，所以mainLock.lock(); 之后重新检查线程池的状态，如果状态不正确，那么减小Worker的数量，为什么tryTerminate（）目前不大清楚。如果状态正常，那么添加Worker到workers。最后：
if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
t.interrupt();
注释说的很清楚，为了能及时的中断此Worker，因为线程存在未Start的情况，此时是不能响应中断的，如果此时status变为STOP，则不能中断线程。此处用作中断线程之用。
接下来我们看Worker的方法：
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
这里可以看出Worker是对firstTask的包装，并且Worker本身就是Runnable的，看上去真心很流氓的感觉~~~
通过ThreadFactory为Worker自己构建一个线程。
因为Worker是Runnable类型的，所以是有run方法的,上面也看到了会调用t.start() 其实就是执行了run方法：
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
调用了runWorker:
/**
* Main worker run loop. Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
* 1 Worker可能还是执行一个初始化的task——firstTask。
* 但是有时也不需要这个初始化的task（可以为null）,只要pool在运行，就会
* 通过getTask从队列中获取Task，如果返回null，那么worker退出。
* 另一种就是external抛出异常导致worker退出。
* 1. We may start out with an initial task, in which case we
* don't need to get the first one. Otherwise, as long as pool is
* running, we get tasks from getTask. If it returns null then the
* worker exits due to changed pool state or configuration
* parameters. Other exits result from exception throws in
* external code, in which case completedAbruptly holds, which
* usually leads processWorkerExit to replace this thread.
*
*
* 2 在运行任何task之前，都需要对worker加锁来防止other pool中断worker。
* clearInterruptsForTaskRun保证除了线程池stop，那么现场都没有中断标志
* 2. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and
* clearInterruptsForTaskRun called to ensure that unless pool is
* stopping, this thread does not have its interrupt set.
*
* 3. Each task run is preceded by a call to beforeExecute, which
* might throw an exception, in which case we cause thread to die
* (breaking loop with completedAbruptly true) without processing
* the task.
*
* 4. Assuming beforeExecute completes normally, we run the task,
* gathering any of its thrown exceptions to send to
* afterExecute. We separately handle RuntimeException, Error
* (both of which the specs guarantee that we trap) and arbitrary
* Throwables. Because we cannot rethrow Throwables within
* Runnable.run, we wrap them within Errors on the way out (to the
* thread's UncaughtExceptionHandler). Any thrown exception also
* conservatively causes thread to die.
*
* 5. After task.run completes, we call afterExecute, which may
* also throw an exception, which will also cause thread to
* die. According to JLS Sec 14.20, this exception is the one that
* will be in effect even if task.run throws.
*
* The net effect of the exception mechanics is that afterExecute
* and the thread's UncaughtExceptionHandler have as accurate
* information as we can provide about any problems encountered by
* user code.
*
* @param w the worker
*/
final void runWorker(Worker w) {
Runnable task = w.firstTask;
w.firstTask = null;
//标识线程是不是异常终止的
boolean completedAbruptly = true;
try {
//task不为null情况是初始化worker时，如果task为null，则去队列中取线程--->getTask()
while (task != null || (task = getTask()) != null) {
w.lock();
//获取woker的锁，防止线程被其他线程中断
clearInterruptsForTaskRun();//清楚所有中断标记
try {
beforeExecute(w.thread, task);//线程开始执行之前执行此方法，可以实现Worker未执行退出，本类中未实现
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);//线程执行后执行，可以实现标识Worker异常中断的功能，本类中未实现
}
} finally {
task = null;//运行过的task标null
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
//处理worker退出的逻辑
processWorkerExit(w, completedAbruptly);
}
}
从上面代码可以看出，execute的Task是被“包装 ”了一层，线程启动时是内部调用了Task的run方法。
接下来所有的核心集中在getTask()方法上：
/**
* Performs blocking or timed wait for a task, depending on
* current configuration settings, or returns null if this worker
* must exit because of any of:
* 1. There are more than maximumPoolSize workers (due to
* a call to setMaximumPoolSize).
* 2. The pool is stopped.
* 3. The pool is shutdown and the queue is empty.
* 4. This worker timed out waiting for a task, and timed-out
* workers are subject to termination (that is,
* {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
* both before and after the timed wait.
*
* @return task, or null if the worker must exit, in which case
* workerCount is decremented
*
*
* 队列中获取线程
*/
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//当前状态为>stop时，不处理workQueue中的任务，同时减小worker的数量所以返回null，如果为shutdown 同时workQueue已经empty了，同样减小worker数量并返回null
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
boolean timed; // Are workers subject to culling?
for (;;) {
int wc = workerCountOf(c);
timed = allowCoreThreadTimeOut || wc > corePoolSize;
if (wc <= maximumPoolSize && ! (timedOut && timed))
break;
if (compareAndDecrementWorkerCount(c))
return null;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
这段代码十分关键，首先看几个局部变量：
boolean timedOut = false;
主要是判断后面的poll是否要超时
boolean timed;
主要是标识着当前Worker超时是否要退出。wc > corePoolSize时需要减小空闲的Worker数，那么timed为true，但是wc <= corePoolSize时，不能减小核心线程数timed为false。
timedOut初始为false，如果timed为true那么使用poll取线程。如果正常返回，那么返回取到的task。如果超时，证明worker空闲，同时worker超过了corePoolSize，需要删除。返回r=null。则 timedOut = true。此时循环到wc <= maximumPoolSize && ! (timedOut && timed)时，减小worker数，并返回null，导致worker退出。如果线程数<= corePoolSize，那么此时调用 workQueue.take()，没有线程获取到时将一直阻塞，知道获取到线程或者中断，关于中断后面Shutdown的时候会说。
关于终止线程池
我个人认为，如果想了解明白线程池，那么就一定要理解好各个状态之间的转换，想理解转换，线程池的终止机制是很好的一个途径。对于关闭线程池主要有两个方法shutdown()和shutdownNow():
首先从shutdown()方法开始：
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
*
* <p>This method does not wait for previously submitted tasks to
* complete execution. Use {@link #awaitTermination awaitTermination}
* to do that.
*
* @throws SecurityException {@inheritDoc}
*/
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
//判断是否可以操作目标线程
checkShutdownAccess();
//设置线程池状态为SHUTDOWN,此处之后，线程池中不会增加新Task
advanceRunState(SHUTDOWN);
//中断所有的空闲线程
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
//转到Terminate
tryTerminate();
}
shutdown做了几件事：
- 检查是否能操作目标线程
- 将线程池状态转为SHUTDOWN
- 中断所有空闲线程
这里就引发了一个问题，什么是空闲线程？
这需要接着看看interruptIdleWorkers是怎么回事。
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
//这里的意图很简单，遍历workers 对所有worker做中断处理。
// w.tryLock()对Worker加锁，这保证了正在运行执行Task的Worker不会被中断，那么能中断哪些线程呢？
try {
for (Worker w : workers) {
Thread t = w.thread;
if (!t.isInterrupted() && w.tryLock()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
这里主要是为了中断worker，但是中断之前需要先获取锁，这就意味着正在运行的Worker不能中断。但是上面的代码有w.tryLock()，那么获取不到锁就不会中断，shutdown的Interrupt只是对所有的空闲Worker（正在从workQueue中取Task，此时Worker没有加锁）发送中断信号。
while (task != null || (task = getTask()) != null) {
w.lock();
//获取woker的锁，防止线程被其他线程中断
clearInterruptsForTaskRun();//清楚所有中断标记
try {
beforeExecute(w.thread, task);//线程开始执行之前执行此方法，可以实现Worker未执行退出，本类中未实现
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);//线程执行后执行，可以实现标识Worker异常中断的功能，本类中未实现
}
} finally {
task = null;//运行过的task标null
w.completedTasks++;
w.unlock();
}
}
在runWorker中，每一个Worker getTask成功之后都要获取Worker的锁之后运行，也就是说运行中的Worker不会中断。因为核心线程一般在空闲的时候会一直阻塞在获取Task上，也只有中断才可能导致其退出。这些阻塞着的Worker就是空闲的线程（当然，非核心线程，并且阻塞的也是空闲线程）。在getTask方法中：
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//当前状态为>stop时，不处理workQueue中的任务，同时减小worker的数量所以返回null，如果为shutdown 同时workQueue已经empty了，同样减小worker数量并返回null
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
boolean timed; // Are workers subject to culling?
for (;;) {
//allowCoreThreadTimeOu是判断CoreThread是否会超时的，true为会超时，false不会超时。默认为false
int wc = workerCountOf(c);
timed = allowCoreThreadTimeOut || wc > corePoolSize;
if (wc <= maximumPoolSize && ! (timedOut && timed))
break;
if (compareAndDecrementWorkerCount(c))
return null;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
会有两阶段的Worker：
刚进入getTask()，还没进行状态判断。
block在poll或者take上的Worker。
当调用ShutDown方法时，首先设置了线程池的状态为ShutDown，此时1阶段的worker进入到状态判断时会返回null，此时Worker退出。
因为getTask的时候是不加锁的，所以在shutdown时可以调用worker.Interrupt.此时会中断退出，Loop到状态判断时，同时workQueue为empty。那么抛出中断异常，导致重新Loop，在检测线程池状态时，Worker退出。如果workQueue不为null就不会退出，此处有些疑问，因为没有看见中断标志位清除的逻辑，那么这里就会不停的循环直到workQueue为Empty退出。
这里也能看出来SHUTDOWN只是清除一些空闲Worker，并且拒绝新Task加入，对于workQueue中的线程还是继续处理的。
对于shutdown中获取mainLock而addWorker中也做了mainLock的获取，这么做主要是因为Works是HashSet类型的，是线程不安全的，我们也看到在addWorker后面也是对线程池状态做了判断，将Worker添加和中断逻辑分离开。
接下来做了tryTerminate()操作，这操作是进行了后面状态的转换，在shutdownNow后面说。
接下来看看shutdownNow：
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks
* that were awaiting execution. These tasks are drained (removed)
* from the task queue upon return from this method.
*
* <p>This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
*
* <p>There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks. This implementation
* cancels tasks via {@link Thread#interrupt}, so any task that
* fails to respond to interrupts may never terminate.
*
* @throws SecurityException {@inheritDoc}
*/
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(STOP);
interruptWorkers();
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
shutdownNow和shutdown代码类似，但是实现却很不相同。首先是设置线程池状态为STOP，前面的代码我们可以看到，是对SHUTDOWN有一些额外的判断逻辑，但是对于>=STOP,基本都是reject，STOP也是比SHUTDOWN更加严格的一种状态。此时不会有新Worker加入，所有刚执行完一个线程后去GetTask的Worker都会退出。
之后调用interruptWorkers：
/**
* Interrupts all threads, even if active. Ignores SecurityExceptions
* (in which case some threads may remain uninterrupted).
*/
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
try {
w.thread.interrupt();
} catch (SecurityException ignore) {
}
}
} finally {
mainLock.unlock();
}
}
这里可以看出来，此方法目的是中断所有的Worker，而不是像shutdown中那样只中断空闲线程。这样体现了STOP的特点，中断所有线程，同时workQueue中的Task也不会执行了。所以接下来drainQueue：
/**
* Drains the task queue into a new list, normally using
* drainTo. But if the queue is a DelayQueue or any other kind of
* queue for which poll or drainTo may fail to remove some
* elements, it deletes them one by one.
*/
private List<Runnable> drainQueue() {
BlockingQueue<Runnable> q = workQueue;
List<Runnable> taskList = new ArrayList<Runnable>();
q.drainTo(taskList);
if (!q.isEmpty()) {
for (Runnable r : q.toArray(new Runnable[0])) {
if (q.remove(r))
taskList.add(r);
}
}
return taskList;
}
获取所有没有执行的Task，并且返回。
这也体现了STOP的特点：
拒绝所有新Task的加入，同时中断所有线程，WorkerQueue中没有执行的线程全部抛弃。所以此时Pool是空的，WorkerQueue也是空的。
这之后就是进行到TIDYING和TERMINATED的转化了：
/**
* Transitions to TERMINATED state if either (SHUTDOWN and pool
* and queue empty) or (STOP and pool empty). If otherwise
* eligible to terminate but workerCount is nonzero, interrupts an
* idle worker to ensure that shutdown signals propagate. This
* method must be called following any action that might make
* termination possible -- reducing worker count or removing tasks
* from the queue during shutdown. The method is non-private to
* allow access from ScheduledThreadPoolExecutor.
*/
final void tryTerminate() {
for (;;) {
int c = ctl.get();
if (isRunning(c) ||
runStateAtLeast(c, TIDYING) ||
(runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
return;
if (workerCountOf(c) != 0) { // Eligible to terminate
interruptIdleWorkers(ONLY_ONE);
return;
}
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
try {
terminated();
} finally {
ctl.set(ctlOf(TERMINATED, 0));
termination.signalAll();
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
}
上面的代码其实很有意思有几种状态是不能转化到TIDYING的:
RUNNING状态
TIDYING或TERMINATED
SHUTDOWN状态，但是workQueue不为空
也说明了两点：
- SHUTDOWN想转化为TIDYING，需要workQueue为空，同时workerCount为0。
- STOP转化为TIDYING，需要workerCount为0
如果满足上面的条件（一般一定时间后都会满足的），那么CAS成TIDYING，TIDYING也只是个过度状态，最终会转化为TERMINATED。
至此，ThreadPoolExecutor一些核心思想就介绍完了，想分析清楚实在是不容易，对于ThreadPoolExecutor我还是有些不懂地方，以上只是我对源码的片面的见解，如果有不正确之处，希望大神能不吝赐教。同时也希望给正在研究ThreadPoolExecutor的童鞋提供一点帮助。
