[Java] 深入理解Java线程池从设计思想到源码解读

根据穆氏哲学，投入越多，收获越大。我作此文时，披肝沥胆，汝读此文时，一目十行，我们的收获当然不同。
…
那怎么有更大的收获呢？根据科学研究，当你为一个事物付出时（包括情绪付出），你就会对它更专注。最直接的付出是什么呢？当然是点赞和收藏啦。

getTask()是一个方法，将任务队列中的任务调度给空闲线程，在解读线程池有详细介绍

public ThreadPoolExecutor(int corePoolSize,
  int maximumPoolSize,
  long keepAliveTime,
  TimeUnit unit,
  BlockingQueue<Runnable> workQueue) {
 this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
  Executors.defaultThreadFactory(), defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
  int maximumPoolSize,
  long keepAliveTime,
  TimeUnit unit,
  BlockingQueue<Runnable> workQueue,
  ThreadFactory threadFactory) {
 this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
  threadFactory, defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
  int maximumPoolSize,
  long keepAliveTime,
  TimeUnit unit,
  BlockingQueue<Runnable> workQueue,
  RejectedExecutionHandler handler) {
 this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
  Executors.defaultThreadFactory(), handler);
}
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.corePoolSize = corePoolSize;
 this.maximumPoolSize = maximumPoolSize;
 this.workQueue = workQueue;
 this.keepAliveTime = unit.toNanos(keepAliveTime);
 this.threadFactory = threadFactory;
 this.handler = handler;
}

/**
 * The default thread factory
 */
static class DefaultThreadFactory implements ThreadFactory {
 private static final AtomicInteger poolNumber = new AtomicInteger(1);
 private final ThreadGroup group;
 private final AtomicInteger threadNumber = new AtomicInteger(1);
 private final String namePrefix;
 DefaultThreadFactory() {
 SecurityManager s = System.getSecurityManager();
 group = (s != null) ? s.getThreadGroup() :
  Thread.currentThread().getThreadGroup();
 namePrefix = "pool-" +
   poolNumber.getAndIncrement() +
   "-thread-";
 }
 public Thread newThread(Runnable r) {
 Thread t = new Thread(group, r,
  namePrefix + threadNumber.getAndIncrement(),
  0);
 if (t.isDaemon())
  t.setDaemon(false);
 if (t.getPriority() != Thread.NORM_PRIORITY)
  t.setPriority(Thread.NORM_PRIORITY);
 return t;
 }
}

volatile int runState;
// 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;

public boolean prestartCoreThread() {
 return addIfUnderCorePoolSize(null); //注意传进去的参数是null
}
public int prestartAllCoreThreads() {
 int n = 0;
 while (addIfUnderCorePoolSize(null))//注意传进去的参数是null
 ++n;
 return n;
}

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class MyTest {
public static void main(String[] args) {
// 创建线程池
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(3, 5, 5, TimeUnit.SECONDS,
new ArrayBlockingQueue<Runnable>(5));
// 向线程池提交任务
for (int i = 0; i < threadPool.getCorePoolSize(); i++) {
threadPool.execute(new Runnable() {
@Override
public void run() {
for (int x = 0; x < 2; x++) {
System.out.println(Thread.currentThread().getName() + ":" + x);
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
});
}
// 关闭线程池
threadPool.shutdown(); // 设置线程池的状态为SHUTDOWN，然后中断所有没有正在执行任务的线程
// threadPool.shutdownNow(); // 设置线程池的状态为STOP，然后尝试停止所有的正在执行或暂停任务的线程，并返回等待执行任务的列表，该方法要慎用，容易造成不可控的后果
}
}

pool-1-thread-2:0
pool-1-thread-1:0
pool-1-thread-3:0
pool-1-thread-2:1
pool-1-thread-3:1
pool-1-thread-1:1

// 使用默认线程工厂
public static ExecutorService newFixedThreadPool(int nThreads) {
 return new ThreadPoolExecutor(nThreads, nThreads,
   0L, TimeUnit.MILLISECONDS,
   new LinkedBlockingQueue<Runnable>());
}
// 需要自定义线程工厂
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
 return new ThreadPoolExecutor(nThreads, nThreads,
   0L, TimeUnit.MILLISECONDS,
   new LinkedBlockingQueue<Runnable>(),
   threadFactory);
}

// 1. 创建线程池对象，设置核心线程和最大线程数为5
ExecutorService fixedThreadPool = Executors.newFixedThreadPool(5);
// 2. 创建Runnable（任务）
Runnable task =new Runnable(){
 public void run() {
 System.out.println(Thread.currentThread().getName() + "--->运行");
 }
};
// 3. 向线程池提交任务
fixedThreadPool.execute(task);

public static ExecutorService newSingleThreadExecutor() {
 return new FinalizableDelegatedExecutorService
 (new ThreadPoolExecutor(1, 1,
  0L, TimeUnit.MILLISECONDS,
  new LinkedBlockingQueue<Runnable>()));
}
// 为节省篇幅，省略了自定义线程工厂方式的源码

// 1. 创建单线程线程池
ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
// 2. 创建Runnable（任务）
Runnable task = new Runnable(){
 public void run() {
 System.out.println(Thread.currentThread().getName() + "--->运行");
 }
};
// 3. 向线程池提交任务
singleThreadExecutor.execute(task);

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
 return new ScheduledThreadPoolExecutor(corePoolSize);
}
// 继承了 ThreadPoolExecutor
public class ScheduledThreadPoolExecutor extends ThreadPoolExecutor
 implements ScheduledExecutorService {
 // 构造函数，省略了自定义线程工厂的构造函数
public ScheduledThreadPoolExecutor(int corePoolSize) {
 super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
  new DelayedWorkQueue());
}
// 延时执行任务
public ScheduledFuture<?> schedule(Runnable command,
   long delay,
   TimeUnit unit) {
 ...
 }
// 定时执行任务
public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
     long initialDelay,
     long period,
     TimeUnit unit) {...}
}

// 1. 创建定时线程池
ExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5);
// 2. 创建Runnable（任务）
Runnable task = new Runnable(){
 public void run() {
 System.out.println(Thread.currentThread().getName() + "--->运行");
 }
};
// 3. 向线程池提交任务
scheduledThreadPool.schedule(task, 2, TimeUnit.SECONDS); // 延迟2s后执行任务
scheduledThreadPool.scheduleAtFixedRate(task,50,2000,TimeUnit.MILLISECONDS);// 延迟50ms后、每隔2000ms执行任务

public static ExecutorService newCachedThreadPool() {
 return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
   60L, TimeUnit.SECONDS,
   new SynchronousQueue<Runnable>());
}

// 1. 创建缓存线程池
ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
// 2. 创建Runnable（任务）
Runnable task = new Runnable(){
 public void run() {
 System.out.println(Thread.currentThread().getName() + "--->运行");
 }
};
// 3. 向线程池提交任务
cachedThreadPool.execute(task);

/**
 * 在将来的某个时候执行给定的任务。任务可以在新线程中执行，也可以在现有的池线程中执行。
 * 如果由于此执行器已关闭或已达到其容量而无法提交任务以供执行，则由当前的{@code RejectedExecutionHandler}处理该任务。
 * 
 * @param command the task to execute 待执行的任务命令
 */
public void execute(Runnable command) {
 if (command == null)
 throw new NullPointerException();
 /*
 * Proceed in 3 steps:
 * 
 * 1. 如果运行的线程少于corePoolSize，将尝试以给定的命令作为第一个任务启动新线程。
 *
 * 2. 如果一个任务可以成功排队，那么我们仍然需要仔细检查两点，其一，我们是否应该添加一个线程
 * （因为自从上次检查至今，一些存在的线程已经死亡），其二，线程池状态此时已改变成非运行态。因此，我们重新检查状态，如果检查不通过，则移除已经入列的任务，如果检查通过且线程池线程数为0，则启动新线程。
 * 
 * 3. 如果无法将任务加入任务队列，则将线程池扩容到极限容量并尝试创建一个新线程，如果失败则拒绝任务。
 */
 int c = ctl.get();
 
 // 步骤1：判断线程池当前线程数是否小于线程池大小
 if (workerCountOf(c) < corePoolSize) {
 // 增加一个工作线程并添加任务，成功则返回，否则进行步骤2
 // true代表使用coreSize作为边界约束，否则使用maximumPoolSize
 if (addWorker(command, true))
  return;
 c = ctl.get();
 }
 // 步骤2：不满足workerCountOf(c) < corePoolSize或addWorker失败，进入步骤2
 // 校验线程池是否是Running状态且任务是否成功放入workQueue（阻塞队列）
 if (isRunning(c) && workQueue.offer(command)) {
 int recheck = ctl.get();
 // 再次校验，如果线程池非Running且从任务队列中移除任务成功，则拒绝该任务
 if (! isRunning(recheck) && remove(command))
  reject(command);
 // 如果线程池工作线程数量为0，则新建一个空任务的线程
 else if (workerCountOf(recheck) == 0)
  // 如果线程池不是Running状态，是加入不进去的
  addWorker(null, false);
 }
 // 步骤3：如果线程池不是Running状态或任务入列失败，尝试扩容maxPoolSize后再次addWorker，失败则拒绝任务
 else if (!addWorker(command, false))
 reject(command);
}

private boolean addWorker(Runnable firstTask, boolean core) {
 // 外层循环：判断线程池状态
 retry:
 for (;;) {
 int c = ctl.get();
 int rs = runStateOf(c);
 /** 
  * 1.线程池为非Running状态（Running状态则既可以新增核心线程也可以接受任务）
  * 2.线程为shutdown状态且firstTask为空且队列不为空
  * 3.满足条件1且条件2不满足，则返回false
  * 4.条件2解读：线程池为shutdown状态时且任务队列不为空时，可以新增空任务的线程来处理队列中的任务
  */
 if (rs >= SHUTDOWN &&
  ! (rs == SHUTDOWN &&
  firstTask == null &&
  ! workQueue.isEmpty()))
  return false;
// 内层循环：线程池添加核心线程并返回是否添加成功的结果
 for (;;) {
  int wc = workerCountOf(c);
  // 校验线程池已有线程数量是否超限：
  // 1.线程池最大上限CAPACITY 
  // 2.corePoolSize或maximumPoolSize（取决于入参core）
  if (wc >= CAPACITY ||
  wc >= (core ? corePoolSize : maximumPoolSize)) 
  return false;
  // 通过CAS操作使工作线程数+1，跳出外层循环
  if (compareAndIncrementWorkerCount(c)) 
  break retry;
  // 线程+1失败，重读ctl
  c = ctl.get(); // Re-read ctl
  // 如果此时线程池状态不再是running，则重新进行外层循环
  if (runStateOf(c) != rs)
  continue retry;
  // 其他 CAS 失败是因为工作线程数量改变了，继续内层循环尝试CAS对线程数+1
  // else CAS failed due to workerCount change; retry inner loop
 }
 }
 /**
 * 核心线程数量+1成功的后续操作：添加到工作线程集合，并启动工作线程
 */
 boolean workerStarted = false;
 boolean workerAdded = false;
 Worker w = null;
 try {
 final ReentrantLock mainLock = this.mainLock;
 w = new Worker(firstTask);
 final Thread t = w.thread;
 if (t != null) {
  // 下面代码需要加锁：线程池主锁
  mainLock.lock(); 
  try {
  // 持锁期间重新检查，线程工厂创建线程失败或获取锁之前关闭的情况发生时，退出
  int c = ctl.get();
  int rs = runStateOf(c);
// 再次检验线程池是否是running状态或线程池shutdown但线程任务为空
  if (rs < SHUTDOWN ||
   (rs == SHUTDOWN && firstTask == null)) {
   // 线程已经启动，则抛出非法线程状态异常
   // 为什么会存在这种状态呢？未解决
   if (t.isAlive()) // precheck that t is startable
   throw new IllegalThreadStateException();
   workers.add(w); //加入线程池
   int s = workers.size();
   // 如果当前工作线程数超过线程池曾经出现过的最大线程数，刷新后者值
   if (s > largestPoolSize)
   largestPoolSize = s; 
   workerAdded = true;
  }
  } finally {
  mainLock.unlock(); // 释放锁
  }
  if (workerAdded) { // 工作线程添加成功，启动该线程
  t.start();
  workerStarted = true;
  }
 }
 } finally {
 //线程启动失败，则进入addWorkerFailed
 if (! workerStarted) 
  addWorkerFailed(w);
 }
 return workerStarted;
}

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;
 
 /** Thread this worker is running in. Null if factory fails. */
 final Thread thread; 
 
 /** Initial task to run. Possibly null. */
 Runnable firstTask;
 
 /** Per-thread task counter */
 volatile long completedTasks;
 
 /**
 * Creates with given first task and thread from ThreadFactory.
 * @param firstTask the first task (null if none)
 */
 // 通过构造函数初始化，
 Worker(Runnable firstTask) {
 //设置AQS的同步状态
 // state：锁状态，-1为初始值，0为unlock状态，1为lock状态
 setState(-1); // inhibit interrupts until runWorker 在调用runWorker前，禁止中断
 
 this.firstTask = firstTask;
 // 线程工厂创建一个线程
 this.thread = getThreadFactory().newThread(this); 
 }
 
 /** Delegates main run loop to outer runWorker */
 public void run() {
 runWorker(this); //runWorker()是ThreadPoolExecutor的方法
 }
 
 // Lock methods
 // The value 0 represents the unlocked state. 0代表“没被锁定”状态
 // The value 1 represents the locked state. 1代表“锁定”状态
 
 protected boolean isHeldExclusively() {
 return getState() != 0;
 }
 
 /**
 * 尝试获取锁的方法
 * 重写AQS的tryAcquire()，AQS本来就是让子类来实现的
 */
 protected boolean tryAcquire(int unused) {
 // 判断原值为0，且重置为1，所以state为-1时，锁无法获取。
 // 每次都是0->1，保证了锁的不可重入性
 if (compareAndSetState(0, 1)) {
  // 设置exclusiveOwnerThread=当前线程
  setExclusiveOwnerThread(Thread.currentThread()); 
  return true;
 }
 return false;
 }
 
 /**
 * 尝试释放锁
 * 不是state-1，而是置为0
 */
 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(); }
 
 /**
 * 中断（如果运行）
 * shutdownNow时会循环对worker线程执行
 * 且不需要获取worker锁，即使在worker运行时也可以中断
 */
 void interruptIfStarted() {
 Thread t;
 //如果state>=0、t!=null、且t没有被中断
 //new Worker()时state==-1，说明不能中断
 if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
  try {
  t.interrupt();
  } catch (SecurityException ignore) {
  }
 }
 }
}

final void runWorker(Worker w) {
 Thread wt = Thread.currentThread();
 Runnable task = w.firstTask;
 w.firstTask = null;
 // allow interrupts
 // new Worker()是state==-1，此处是调用Worker类的tryRelease()方法，将state置为0，而interruptIfStarted()中只有state>=0才允许调用中断
 w.unlock(); 
  
 // 线程退出的原因，true是任务导致，false是线程正常退出
 boolean completedAbruptly = true; 
 try {
 // 当前任务和从任务队列中获取的任务都为空，方停止循环
 while (task != null || (task = getTask()) != null) {
  //上锁可以防止在shutdown()时终止正在运行的worker，而不是应对并发
  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
  /**
  * 判断1：确保只有在线程处于stop状态且wt未中断时，wt才会被设置中断标识
  * 条件1：线程池状态>=STOP,即STOP或TERMINATED
  * 条件2：一开始判断线程池状态<STOP，接下来检查发现Thread.interrupted()为true，即线程已经被中断，再次检查线程池状态是否>=STOP（以消除该瞬间shutdown方法生效，使线程池处于STOP或TERMINATED），
  * 条件1与条件2任意满意一个，且wt不是中断状态，则中断wt，否则进入下一步
  */
  if ((runStateAtLeast(ctl.get(), STOP) ||
   (Thread.interrupted() &&
   runStateAtLeast(ctl.get(), STOP))) &&
  !wt.isInterrupted())
  wt.interrupt(); //当前线程调用interrupt()中断
  
  try {
  //执行前（空方法，由子类重写实现）
  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 {
   //执行后（空方法，由子类重写实现）
   afterExecute(task, thrown); 
  }
  } 
  finally {
  task = null; 
  w.completedTasks++; //完成任务数+1
  w.unlock(); //释放锁
  }
 }
 // 
 completedAbruptly = false;
 } 
 finally {
 //处理worker的退出
 processWorkerExit(w, completedAbruptly);
 }
}

private Runnable getTask() {
 // 最新一次poll是否超时
 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.
 /**
  * 条件1：线程池状态SHUTDOWN、STOP、TERMINATED状态
  * 条件2：线程池STOP、TERMINATED状态或workQueue为空
  * 条件1与条件2同时为true，则workerCount-1，并且返回null
  * 注：条件2是考虑到SHUTDOWN状态的线程池不会接受任务，但仍会处理任务
  */
 if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
  decrementWorkerCount();
  return null;
 }
 int wc = workerCountOf(c);
 // Are workers subject to culling?
 /**
  * 下列两个条件满足任意一个，则给当前正在尝试获取任务的工作线程设置阻塞时间限制（超时会被销毁？不太确定这点），否则线程可以一直保持活跃状态
  * 1.allowCoreThreadTimeOut：当前线程是否以keepAliveTime为超时时限等待任务
  * 2.当前线程数量已经超越了核心线程数
  */
 boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
  
 // 两个条件全部为true，则通过CAS使工作线程数-1，即剔除工作线程
 // 条件1：工作线程数大于maximumPoolSize，或（工作线程阻塞时间受限且上次在任务队列拉取任务超时）
 // 条件2：wc > 1或任务队列为空
 if ((wc > maximumPoolSize || (timed && timedOut))
  && (wc > 1 || workQueue.isEmpty())) {
  // 移除工作线程，成功则返回null，不成功则进入下轮循环
  if (compareAndDecrementWorkerCount(c))
  return null;
  continue;
 }
 // 执行到这里，说明已经经过前面重重校验，开始真正获取task了
 try {
  // 如果工作线程阻塞时间受限，则使用poll(),否则使用take()
  // poll()设定阻塞时间，而take()无时间限制，直到拿到结果为止
  Runnable r = timed ?
  workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
  workQueue.take();
  // r不为空，则返回该Runnable
  if (r != null)
  return r;
  // 没能获取到Runable，则将最近获取任务是否超时设置为true
  timedOut = true;
 } catch (InterruptedException retry) {
  // 响应中断，进入下一次循环前将最近获取任务超时状态置为false
  timedOut = false;
 }
 }
}

/**
 * Performs cleanup and bookkeeping for a dying worker. Called
 * only from worker threads. Unless completedAbruptly is set,
 * assumes that workerCount has already been adjusted to account
 * for exit. This method removes thread from worker set, and
 * possibly terminates the pool or replaces the worker if either
 * it exited due to user task exception or if fewer than
 * corePoolSize workers are running or queue is non-empty but
 * there are no workers.
 *
 * @param w the worker
 * @param completedAbruptly if the worker died due to user exception
 */
private void processWorkerExit(Worker w, boolean completedAbruptly) {
 /**
 * 1.工作线程-1操作
 * 1）如果completedAbruptly 为true，说明工作线程发生异常，那么将正在工作的线程数量-1
 * 2）如果completedAbruptly 为false，说明工作线程无任务可以执行，由getTask()执行worker-1操作
 */
 if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
 decrementWorkerCount();
 // 2.从线程set集合中移除工作线程，该过程需要加锁
 final ReentrantLock mainLock = this.mainLock;
 mainLock.lock();
 try {
 // 将该worker已完成的任务数追加到线程池已完成的任务数
 completedTaskCount += w.completedTasks;
 // HashSet<Worker>中移除该worker
 workers.remove(w);
 } finally {
 mainLock.unlock();
 }
 
// 3.根据线程池状态进行判断是否结束线程池
 tryTerminate();
/**
 * 4.是否需要增加工作线程
 * 线程池状态是running 或 shutdown
 * 如果当前线程是突然终止的，addWorker()
 * 如果当前线程不是突然终止的，但当前线程数量 < 要维护的线程数量，addWorker()
 * 故如果调用线程池shutdown()，直到workQueue为空前，线程池都会维持corePoolSize个线程，然后再逐渐销毁这corePoolSize个线程
 */
 int c = ctl.get();
 if (runStateLessThan(c, STOP)) {
 if (!completedAbruptly) {
  int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
  if (min == 0 && ! workQueue.isEmpty())
  min = 1;
  if (workerCountOf(c) >= min)
  return; // replacement not needed
 }
 addWorker(null, false);
 }
}