线程池的原理
上一篇介绍了为什么要使用线程，以及创建线程的的几种方式。接下来将详细介绍线程的实现原理。
由源码可知，线程池实际上调用的还是ThreadPoolExecutor方法。下面将看一下ThreadPoolExecutor的源代码和注释解析。
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters and default thread factory and rejected execution handler.
* It may be more convenient to use one of the {@link Executors} factory
* methods instead of this general purpose constructor.
*
* @param corePoolSize 线程池核心线程数量
* @param maximumPoolSize 线程池最大线程数量
* @param keepAliveTime 当活跃线程数大于核心线程数时，空闲的多余线程最大存活时间
* @param unit. 存活时间的单位
* @param workQueue 存放任务的队列
* @param threadFactory 工厂使用执行器时创建一个新线程
* @param handler 超出线程范围和队列容量的任务的处理程序
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
}
由源码可知，ThreadPoolExecutor实际上调用的是execute方法。下面将看一下execute的源代码和注释解析。
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*
* @param command 要执行的任务
*/
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.
* 翻译：工作线程数小于核心线程数时，直接新建核心线程执行任务。
* 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.
* 翻译：大于核心线程数时，将任务添加进等待队列。队列满时，创建非核心线程执行任务。
* 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.
* 翻译：工作线程数大于最大线程数时，拒绝任务。
*/
int c = ctl.get();
// 工作线程数小于核心线程数时，直接新建核心线程执行任务。
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
// 当大于核心线程数时，将任务添加进等待队列。队列满时，创建非核心线程执行任务
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);
}
else if (!addWorker(command, false))
// 创建线程失败，则采取阻塞处理的方式
reject(command);
}
由源码可知，execute实际上调用的是addWorker方法。下面将看一下addWorker的源代码和注释解析。
addWorker的作用：1.才用循环CAS操作来将线程数加1。2. 新建一个线程并启用。
/**
* 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. If the thread
* creation fails, either due to the thread factory returning
* null, or due to an exception (typically OutOfMemoryError in
* Thread.start()), we roll back cleanly.
*
* @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) {
//循环CAS操作，将线程池中的线程数+1
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 仅在必要时检查队列是否为空
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
//core true代表是往核心线程池中增加线程 false代表往最大线程池中增加线程
//线程数超标，不能再添加了，直接返回
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//CAS修改clt的值+1，在线程池中为将要添加的线程流出空间，成功退出cas循环，失败继续
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
//如果线程池的状态发生了变化回到retry外层循环
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
//新建线程，并加入到线程池workers中
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
//对workers操作要通过加锁来实现
final ReentrantLock mainLock = this.mainLock;
//细化锁的力度，防止临界区过大，浪费资源
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
//判断线程池的状态
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
//判断添加的任务状态,如果已经开始丢出异常
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
//将新建的线程加入到线程池中
workers.add(w);
int s = workers.size();
/largestPoolSize的值
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
//线程添加线程池成功，则开启新线程
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
//线程添加线程池失败或者线程start失败，则需要调用addWorkerFailed函数，
//如果添加成功则需要移除，并回复clt的值
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
Worker类继承自AbstractQueuedSynchronizer，具有锁的功能，实现Runable接口，具有线程的功能。
/**
* Class Worker mainly maintains interrupt control state for
* threads running tasks, along with other minor bookkeeping.
* This class opportunistically extends AbstractQueuedSynchronizer
* to simplify acquiring and releasing a lock surrounding each
* task execution. This protects against interrupts that are
* intended to wake up a worker thread waiting for a task from
* instead interrupting a task being run. We implement a simple
* non-reentrant mutual exclusion lock rather than use
* ReentrantLock because we do not want worker tasks to be able to
* reacquire the lock when they invoke pool control methods like
* setCorePoolSize. Additionally, to suppress interrupts until
* the thread actually starts running tasks, we initialize lock
* state to a negative value, and clear it upon start (in
* runWorker).
*/
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** 线程池中正真运行的线程。通过我们指定的线程工厂创建而来, 如果工厂失败，则为空 */
final Thread thread;
/** 线程包装的任务。thread 在run时主要调用了该任务的run方法,要运行的初始任务。可能是零*/
Runnable firstTask;
/** 线程任务计数器 */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* 使用线程工厂，创建第一个任务和线程
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
//利用我们指定的线程工厂创建一个线程
this.thread = getThreadFactory().newThread(this);
}
/** 将主运行循环委托给runWorker */
public void run() {
runWorker(this);
}
// Lock methods
//
// 值0表示解锁状态。
// 值1表示锁定状态。
protected boolean isHeldExclusively() {
return getState() != 0;
}
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
由源码可知，Worker类的run方法实际上调用的还是ThreadPoolExecutor的runworker方法。下面将看一下ThreadPoolExecutor的runworker源代码和注释解析。
/**
* Main worker run loop. Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
*
* 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. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and then we
* 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) {
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();
//线程池处于stop状态或者当前线程被中断时，线程池状态是stop状态。
//但是当前线程没有中断，则发出中断请求
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
// 开始执行任务前的Hook
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 {
// 任务执行后的Hook，类似回调函数
afterExecute(task, thrown);
}
} finally {
// 执行完毕后task重置，completedTasks计数器++，解锁
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
// 线程空闲达到我们设定的值时，Worker退出销毁。
processWorkerExit(w, completedAbruptly);
}
}
/**
* 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, and if the queue is
* non-empty, this worker is not the last thread in the pool.
*
* @return task, 如果工作者必须退出，则为null，在这种情况下workerCount将递减
*/
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 如果线程池处于shutdown状态，
// 并且队列为空，或者线程池处于stop或者terminate状态，
// 在线程池数量-1，返回null，回收线程
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
//如果allowCoreThreadTimeOut 为ture
//或者当前线程数量大于核心线程池数目，
//则需要超时回收
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//如果线程数目小于最大线程数目，
//且不允许超时回收或者未超时，
//则跳出循环，继续去阻塞队列中取任务
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
//如果上面if没有成立，则当前线程数-1，返回null，回收该线程
//如果上面if没有成立，则CAS修改ctl失败，重读，cas循环重新尝试修改
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//如果允许空闲回收，则调用阻塞队列的poll，
//否则take，一直等到队列中有可取任务
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
//取到任务，返回任务，
//否则超时timedOut = true;进入下一个循环,
//并且在 if ((wc > maximumPoolSize || (timed && timedOut))处会不成立，进而进入到cas修改ctl的程序中
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}